FELIX Highlights
2010 2009 2008 2007 2006 2005
Highlights 2010
Quantum-induced symmetry breaking explains infrared spectra of CH5+ isotopologues
Sergei D. Ivanov, Oskar Asvany, Alexander Witt, Edouard Hugo, Gerald Mathias, Britta Redlich, Dominik Marx & Stephan Schlemmer
For decades, protonated methane, CH5+, has provided new surprises and challenges for both experimentalists and theoreticians. This is because of the correlated large-amplitude motion of its five protons around the carbon nucleus, which leads to so-called hydrogen scrambling and causes a fluxional molecular structure. Here, the infrared spectra of all its H/D isotopologues have been measured using the ‘Laser Induced Reactions’ technique. Their shapes are found to be extremely dissimilar and depend strongly on the level of deuteration (only CD5+ is similar to CH5+). All the spectra can be reproduced and assigned based on ab initio quantum simulations. The occupation of the topologically different sites by protons and deuterons is found to be strongly non-combinatorial and thus non-classical. This purely quantum-statistical effect implies a breaking of the classical symmetry of the site occupations induced by zero-point fluctuations, and this phenomenon is key to understanding the spectral changes studied here. © 2010 Nature Publishing Group
Nature Chemistry 28 February 2010
Internal Proton Transfer Leading to Stable Zwitterionic Structures in a Neutral Isolated Peptide
Anouk M. Rijs, Gilles Ohanessian, Jos Oomens, Gerard Meijer, Gert von Helden, and Isabelle Compagnon
No solution for the zwitterion: An acid- and base-containing pentapeptide was designed to explore the possibility of zwitterion formation in the gas phase, in the absence of a net charge. Internal proton transfer between peptide side chains in vacuo gave a zwitterion (highlighted in yellow; the canonical form is highlighted in blue), which was identified by gas-phase IR spectroscopy. © 2010 WILEY-VCH Verlag GmbH
Angew. Chem. Int. Ed. 49 (2010)
Infrared Induced Reactivity on the Surface of Isolated Size-Selected Clusters: Dissociation of N2O on Rhodium Clusters
Suzanne M. Hamilton, W. Scott Hopkins, Dan J. Harding, Tiffany R. Walsh, Philipp Gruene, Marko Haertelt, Andre Fielicke, Gerard Meijer and Stuart R. Mackenzie
Infrared multiple photon dissociation spectroscopy (IR-MPD) has recently emerged as a powerful technique in determining the structures of small gas-phase transition metal clusters. A form of indirect or action spectroscopy, IR-MPD relies on the loss of some moiety from the cluster to serve as a mass-spectrometric signature that infrared radiation has been absorbed. Monitoring either the depletion of the parent cluster signal or the enhancement of the fragment signal as a function of wavelength provides a highly sensitive method by which to record the vibrational spectrum of naked and/or decorated metal clusters. Here, we present evidence of infrared driven surface chemistry on RhnN2O+ clusters. IR-pumping of any vibrational mode of a molecularly adsorbed N2O molecule results in the dissociation of the adsorbate with the concomitant loss of N2 and the production of a partially oxidized rhodium cluster. © American Chemical Society
J. Am. Chem. Soc. 132, 1448 (2010)
Infrared Spectroscopy of Hydrated Bicarbonate Anion Clusters: HCO3−(H2O)1−10
Etienne Garand, Torsten Wende, Daniel J. Goebbert, Risshu Bergmann, Gerard Meijer, Daniel M. Neumark and Knut R. Asmis
Infrared multiple photon dissociation spectra are reported for HCO3−(H2O)1−10 clusters in the spectral range of 600−1800 cm−1. In addition, electronic structure calculations at the MP2/6-311+G(d,p) level have been performed on the n = 1−8 clusters to identify the structure of the low-lying isomers and to assign the observed spectral features. General trends in the stepwise solvation motifs of the bicarbonate anion can be deduced from the overall agreement between the calculated and experimental spectra. The most important of these is the strong preference of the water molecules to bind to the negatively charged CO2 moiety of the HCO3− anion. However, a maximum of four water molecules interact directly with this site. The binding motif in the most stable isomer of the n = 4 cluster, a four-membered ring with each water forming a single H-bond with the CO2 moiety, is retained in all of the lowest-energy isomers of the larger clusters. Starting at n = 6, additional solvent molecules are found to form a second hydration layer, resulting in a water−water network bound to the CO2 moiety of the bicarbonate anion. Binding of a water to the hydroxyl group of HCO3− is particularly disfavored and apparently does not occur in any of the clusters investigated here. Similarities and differences with the infrared spectrum of aqueous bicarbonate are discussed in light of these trends. © American Chemical Society
J. Am. Chem. Soc. 132, 849 (2010)
Structure Elucidation of Dimethylformamide-Solvated Alkylzinc Cations in the Gas Phase
Frank Dreiocker, Jos Oomens, Anthony J. H. M. Meijer, Barry T. Pickup, Richard F. W. Jackson and Mathias Schäfer
Organozinc iodides, useful for the synthesis of nonproteinogenic amino acids, are investigated in the gas phase by a combination of electrospray (ESI)-MS/MS, accurate ion mass measurements, and infrared multiphoton dissociation (IRMPD) spectroscopy employing a free electron laser. ESI allowed the full characterization of a set of dimethylformamide (DMF)-solvated alkylzinc cations formed by formal loss of I− in the gas phase. Gas phase ion structures of the organozinc cations were identified and optimized by computations at the B3LYP/6-311G** level of theory. The calculations indicate that the zinc cation in gas phase alkylzinc-DMF species preferentially adopts a tetrahedral coordination sphere with four ligands, namely the alkyl group, any internal coordinating group, and DMF (the number of which depends on the number of internal coordinating groups present). Besides the sequential loss of coordinated DMF, collision-induced dissociation (CID) patterns demonstrate that the zinc-DMF interaction in tetrahedral four-coordinate mono-DMF-zinc complex ions can be even stronger than covalent bonds. The IRMPD spectra of the alkylzinc-DMF species examined show a rich pattern of indicative bands in the range of 1000−1800 cm−1. All major features of the recorded IRMPD spectra are consistent with the computed IR spectra of the respective gas phase ion structures predicted by theory, allowing identification and assignment. © American Chemical Society
Amide-I and -II Vibrations of the Cyclic β-Sheet Model Peptide Gramicidin S in the Gas Phase
Peter Kupser, Kevin Pagel, Jos Oomens, Nick Polfer, Beate Koksch, Gerard Meijer and Gert von Helden
In the condensed phase, the peptide gramicidin S is often considered as a model system for a β-sheet structure. Here, we investigate gramicidin S free of any influences of the environment by measuring the mid-IR spectra of doubly protonated (deuterated) gramicidin S in the gas phase. In the amide I (i.e., C═O stretch) region, the spectra show a broad split peak between 1580 and 1720 cm−1. To deduce structural information, the conformational space has been searched using molecular dynamics methods and several structural candidates have been further investigated at the density functional level. The calculations show the importance of the interactions of the charged side-chains with the backbone, which is responsible for the lower frequency part of the amide I peak. When this interaction is inhibited via complexation with two 18-crown-6 molecules, the amide I peak narrows and shows two maxima at 1653 and 1680 cm−1. A comparison to calculations shows that for this complexed ion, four C═O groups are in an antiparallel β-sheet arrangement. Surprisingly, an analysis of the calculated spectra shows that these β-sheet C═O groups give rise to the vibrations near 1680 cm−1. This is in sharp contrast to expectations based on values for the condensed phase, where resonances of β-sheet sections are thought to occur near 1630 cm−1. The difference between those values might be caused by interactions with the environment, as the condensed phase value is mostly deduced for β-sheet sections that are embedded in larger proteins, that interact strongly with solvent or that are part of partially aggregated species. © American Chemical Society
J. Am. Chem. Soc. 132, 2085 (2010)
Highlights 2009
Effect of Peptide Fragment Size on the Propensity of Cyclization in Collision-Induced Dissociation: Oligoglycine b2-b8
Xian Chen, Long Yu, Jeffrey D. Steill, Jos Oomens and Nick C. Polfer
The chemistry of peptide fragmentation by collision-induced dissociation (CID) is currently being reviewed, as a result of observations that the amino acid sequence of peptide fragments can change upon activation. This rearrangement mechanism is thought to be due to a head-to-tail cyclization reaction, where the N-terminal and C-terminal part of the fragment are fused into a macrocycle (= cyclic peptide) structure, thus “losing” the memory of the original sequence. We present a comprehensive study for a series of b fragment ions, from b2 to b8, based on the simplest amino acid residue glycine, to investigate the effect of peptide chain length on the appearance of macrocycle fragment structures. The CID product ions are structurally characterized with a range of gas-phase techniques, including isotope labeling, infrared photodissociation spectroscopy, gas-phase hydrogen/deuterium exchange (using CH3OD), and computational structure approaches. The combined insights from these results yield compelling evidence that smaller bn fragments (n = 2, 3) exclusively adopt oxazolone-type structures, whereas a mixture of oxazolone and macrocycle b fragment structures are formed for midsized bn fragments, where n = 4−7. As each of these chemical structures exchanges at different rates, it is possible to approximate the relative abundances using kinetic fits to the H/D exchange data. Under the conditions used here, the “slow”-exchanging macrocycle structure represents 30% of the b ion population for b6−b7, while the “fast”-exchanging oxazolone structure represents the remainder (70%). Intriguingly, for b8 only the macrocycle structure is identified, which is also consistent with the “slow” kinetic rate in the HDX results. In a control experiment, a protonated cyclic peptide with 6 amino acid residues, cyclo(Gln-Trp-Phe-Gly-Leu-Met), is confirmed not to adopt an oxazolone structure, even upon collisional activation. These results demonstrate that in some cases larger macrocycle structures are surprisingly stable. While more studies are required to establish the general propensity for cyclization in b fragments, the implications from this study are troubling in terms of faulty sequence identification. © American Chemical Society
J. Am. Chem. Soc. 131, 18272 (2009)
INFRARED SPECTRA OF ISOLATED PROTONATED POLYCYCLIC AROMATIC HYDROCARBON MOLECULES
Harald Knorke, Judith Langer, Jos Oomens and Otto Dopfer
Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H+PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H+PAHs contribute as carriers of the UIR bands. © 2009 The American Astronomical Society
Gas-Phase Deprotonation of p-Hydroxybenzoic AcidInvestigated by IR Spectroscopy:
Solution-Phase Structure is Retained upon ESI
Jeffrey D. Steill and Jos Oomens
The gas-phase structure of the conjugate base of p-hydroxybenzoic acid (and related compounds) and the influence of the solvant used in its generation by electrospray ionization has recently been under debate. While the phenoxide structure of know to be lower in energy in the gas phase, the carboxylate structure of favored in aqueous solution, fuelling the controversy. Here we probe the structure of this gas-anion by IR spectroscopy and show that its structure is determined by the protic or aprotic nature of the solvent, which suggests that it is the solution-phase structure that is transferred to the gas phase. © American Chemical Society
J. Am. Chem. Soc. 131, 13570-13571 (2009)
Structures of Protonated Dipeptides: The Role of Arginine in Stabilizing Salt Bridges
James S. Prell, Jeremy T. O’Brien, Jeffrey D. Steill, Jos Oomens, and Evan R. Williams
Structures of protonated dipeptides containing N-terminal Gly, Val, Pro, Lys, His, or Arg and C-terminal Arg are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm−1 and theory. The IRMPD spectra clearly indicate that, when Gly, Val, Pro, Lys, or His are N-terminal to Arg, these protonated dipeptides adopt gas-phase structures with a single formal charge site (SCS), whereas ArgArg·H+ has a salt-bridge (SB) structure in which the C-terminus is deprotonated and two basic sites are protonated. There are only subtle differences in the IRMPD spectra for dipeptides containing Gly, Val, Pro, and Lys. A sharp, intense peak at 1080 cm−1 is observed for HisArg·H+ that is attributed to the neutral histidine side chain, an assignment that is confirmed by comparison to the IRMPD spectrum of (HisArg·H2)2+. Lowest-energy B3LYP/6-31+G(d,p) structures and energies for the SCS and SB forms of these protonated dipeptides indicate that stability of the SB form relative to the SCS form generally increases with increasing gas-phase basicity of the N-terminal amino acid, but only ArgArg·H+ is calculated to have a SB ground state at 298 K, in agreement with the results from IRMPD spectroscopy. This is the first direct experimental evidence for a salt-bridge structure in a gaseous protonated peptide, and ArgArg·H+ is the smallest protonated peptide for which a SB structure has been reported. These results suggest that SB structures should be common for protonated peptides containing at least two arginine residues and may also occur for large protonated peptides or proteins with at least one arginine residue and other basic residues, such as lysine or histidine. © 2009 American Chemical Society
J. Am. Chem. Soc. 131, 11442-1149 (2009)
Peptide Length, Steric Effects, and Ion Solvation Govern Zwitterion Stabilization in Barium-Chelated Di- and Tripeptides
Robert C. Dunbar, Jeffrey D. Steill, Nick C. Polfer and Jos Oomens
Infrared multiple-photon dissociation (IRMPD) spectroscopy has given infrared spectra of complexes of di- and tripeptides (AlaAla, AlaAlaAla, AlaPhe, PheAla) with singly and doubly charged metal ions (K+, Ca2+, Sr2+, and Ba2+). The switch between charge-solvated (CS) and salt-bridged zwitterion (SB) conformations is displayed through highly diagnostic features in the mid-infrared. Systematic trends are found correlating with the length of the peptide chain (tripeptides favoring CS conformations), metal ion size (larger metals favoring SB conformations), metal ion charge (doubly charged ions favoring SB conformations), and sterically available Lewis-basic side-chain interactions with the metal ion (for example a cation-p interaction with Ba2+ stabilizes CS for PheAla but not for AlaPhe). The principle is that CS conformations are favored for small metal ions with high charge density and extensive microsolvation of the charge by Lewis-basic groups, especially amide carbonyls; SB conformations are favored by metal ions of high charge but low charge density, which are better stabilized by salt-bridge Coulomb interactions. © 2009 American Chemical Society
J. Phys. Chem.B 113, 10552 (2009)
Conformation Switching in Gas-Phase Complexes of Histidine with Alkaline Earth Ions
Robert C. Dunbar, Alan C. Hopkinson, Jos Oomens, Chi-Kit Siu, K. W. Michael Siu, Jeffrey D. Steill, Udo H. Verkerk and Junfang Zhao
Infrared multiple photon dissociation spectroscopy of gas-phase doubly charged alkaline earth complexes of histidine reveals a transition from dominance of the zwitterion (salt bridge, SB) conformation with Ba2+ to substantial presence of the canonical (charge-solvated, CS) conformation with Ca2+. This result is a clear illustration of the importance of metal-ion size in governing the delicate balance between these two modes of complexation of gas-phase amino acids. The two conformational motifs are clearly distinguished by characteristic spectral features, confirmed by density functional theory simulated IR spectra of the low-energy conformers. As a further illustration of histidine complexation possibilities, the spectrum of the Na+His complex shows purely CS character and emphasizes the greater tendency toward SB character induced by the higher charge in the alkaline earth complexes. Calculation of the complete series of alkaline earth/histidine complexes confirms the increasing stability of the SB conformations relative to CS with increasing metal ion size, as well as showing that among SB conformations the most highly chelated conformation (SB3) is favored for small metals, whereas the most extended conformation (SB1) is favored for large metals. A decomposition of the binding thermochemistry shows that these thermochemical trends versus metal-ion size are due to differences in electrostatic binding energies, with relatively little contribution from the deformation and rearrangement energy costs of distorting the ligand framework. © 2009 American Chemical Society
J. Phys. Chem.B 113, 10403 (2009)
H2 Ejection from Polycyclic Aromatic Hydrocarbons: Infrared Multiphoton Dissociation Study of Protonated 1,2-Dihydronaphthalene
Martin Vala, Jan Szczepanski, Jos Oomens and Jeffrey D. Steill
1,2-Dihydronaphthalene (DHN) has been studied by matrix isolation infrared absorption spectroscopy, multiphoton infrared photodissociation (IRMPD) action spectroscopy, and density functional theory calculations. Formed by electrospray ionization, protonated 1,2-dihydronapthalene was injected into a Fourier transform ion cyclotron resonance mass spectrometer coupled to an infrared-tunable free electron laser and its IRMPD spectrum recorded. Multiphoton infrared irradiation of the protonated parent (m/z 131) yields two dissociation products, one with m/z 129 and the other with m/z 91. Results from density functional theory calculations (B3LYP/6-31++G(d,p)) were compared to the low-temperature matrix isolation infrared spectrum of neutral DHN, with excellent results. Calculations reveal that the most probable site of protonation is the 3-position, producing the trihydronaphthalene (THN) cation, 1,2,3-THN+. The observed IRMPD spectrum of vapor-phase protonated parent matches well with that computed for 1,2,3-THN+. Extensive B3LYP/6-31G(d,p) calculations of the potential energy surface of 1,2,3-THN+ have been performed and provide insight into the mechanism of the two-channel photodissociation. These results provide support for a new model of the formation of H2 in the interstellar medium. This model involves hydrogenation of a PAH cation to produce one or more aliphatic hydrogen-bearing carbons on the PAH framework, followed by photolytic formation and ejection of H2. © 2009 American Chemical Society
J. Am. Chem. Soc. 131, 5784 (2009)
VIBRATIONAL SPECTROSCOPY OF BARE AND SOLVATED IONIC COMPLEXES OF BIOLOGICAL RELEVANCE
Nick C. Polfer, and Jos Oomens
The low density of ions in mass spectrometers generally precludes direct infrared (IR) absorption measurements. The IR spectrum of an ion can nonetheless be obtained by inducing photodissociation of the ion using a high-intensity tunable laser. The emergence of free electron lasers (FELs) and recent breakthroughs in bench-top lasers based on nonlinear optics have now made it possible to routinely record IR spectra of gasphase ions. As the energy of one IR photon is insufficient to cause dissociation of molecules and strongly bound complexes, two main experimental strategies have been developed to effect photodissociation. In infrared multiple-photon dissociation (IR-MPD) many photons are absorbed resonantly and their energy is stored in the bath of vibrational modes, leading to dissociation. In the ‘‘messenger’’ technique a weakly bound van der Waals atom is detached upon absorption of a single photon. Fundamental, historical, and practical aspects of these methods will be presented. Both of these approaches make use of very different methods of ion preparation and manipulation. While in IR-MPD ions are irradiated in trapping mass spectrometers, the ‘‘messenger’’ technique is generally carried out in molecular beam instruments. The main focus of this review is the application of IR spectroscopy to biologically relevant molecular systems (amino acids, peptides, proteins). Particular issues that will be addressed here include gas-phase zwitterions, the (chemical) structures of peptides and their collision-induced dissociation (CID) products, IR spectra of gas-phase proteins, and the chelation of metal–ligand complexes. Another growing area of research is IR spectroscopy on solvated clusters, which offer a bridge between the gas-phase and solution environments. The development of state-of-the-art computational approaches has gone hand-inhand with advances in experimental techniques. The main advantage of gas-phase cluster research, as opposed to condensed-phase experiments, is that the systems of interest can be understood in detail and structural effects can be studied in isolation. It will be shown that IR spectroscopy of mass-selected (bio)molecular systems is now well-placed to address specific questions on the individual effect of charge carriers (protons and metal ions), as well as solvent molecules on the overall structure. © 2009 Wiley Periodicals, Inc.
Mass Spectrometry Reviews 28, 468 (2009)
Changes in Binding Motif of Protonated Heterodimers Containing Valine and Amines Investigated Using IRMPD Spectroscopy between 800 and 3700 cm-1 and Theory
Jeremy T. O’Brien, James S. Prell, Jeffrey D. Steill, Jos Oomens and Evan R. Williams
Proton-bound dimers consisting of valine and basic primary and secondary amines of varying gas-phase basicity (GB) were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy between 800 and 3700 cm-1, collisionally activated dissociation, and theory. The low-energy dissociation of these dimers results in a sharp transition from formation of primarily protonated valine to protonated base for dimers with ethylamine and propylamine, respectively, from which a GB of 880 kJ/mol is deduced for valine, a value that is slightly higher than previously reported. The IRMPD spectra clearly indicate that, for bases with GB values within 20 kJ/mol of that of valine, the base coordinates to the N-terminus of a nonzwitterionic form of valine. In contrast, calculations indicate that valine is zwitterionic for complexes where the base is less basic. For bases with GB values at least 20 kJ/mol greater than that of valine, the spectra indicate a transition in structure, and for diethylamine (?GB = 40 kJ/mol), the dominant structure is one in which the base coordinates to the carbonyl oxygen of a nonzwitterionic form of valine and the carboxylic acid donates an intramolecular hydrogen bond to the N-terminus. These results are consistent with the destabilization of the N-terminally coordinated structure due to the increasing difference in proton affinities of the constituent molecules and the increasing importance of a stabililizing hydrogen bond formed in the C-terminally coordinated structure. Even when the GB of the base is 40 kJ/mol higher than that of valine, the form of the amino acid is nonzwitterionic, indicating that careful application of the kinetic method should provide reliable information about the basicity of valine and other aliphatic amino acids. © 2009 American Chemical Society
J. Am. Chem. Soc. 131, 3905 (2009)
Continuous Voltage Tunability of Intersubband Relaxation Times in Coupled SiGe QuantumWell Structures Using Ultrafast Spectroscopy
P. Rauter, T. Fromherz, N. Q. Vinh, B. N. Murdin, G. Mussler, D. Grützmacher, and G. Bauer
We demonstrate continuous voltage control of the nonradiative transition lifetime in semiconductor heterostructures. The results were obtained by picosecond time-resolved experiments on biased SiGe valence band quantum well structures using a free electron laser. By varying the applied voltage, the intersubband hole relaxation times for quantum well structures were varied by a factor of 2 as the wavefunctions and their overlaps were tuned. The range of magnitudes for the lifetime indicates a possible route to silicon-based quantum cascade lasers.
© 2009 The American Physical Society
Phys. Rev. Lett. 102, 147401 (2009)
Gas-Phase IR Spectroscopy of Deprotonated Amino Acids
Jos Oomens, Jeffrey D. Steill and Britta Redlich
Gas-phase infrared multiple photon dissociation (IRMPD) spectra have been recorded for the conjugate bases of a series of amino acids (Asp, Cys, Glu, Phe, Ser, Trp, Tyr). The spectra are dominated by strong symmetric and antisymmetric carboxylate stretching modes around 1300 and 1600 cm-1, respectively. Comparison of the experimental spectra with spectra calculated at the DFT level suggests a carboxylate structure for all species investigated, which is in contrast with what has recently been suggested in this journal for deprotonated cysteine [J. Am. Chem. Soc. 2007, 129, 5403-5407]. In addition, the IR spectrum of the conjugate base of tyrosine is also unambiguously that of a carboxylate ion and not that of a phenoxide ion. In sharp contrast with the conjugate bases of other amino acids investigated here, the aspartate and glutamate anions show very broad, hardly resolved spectral features. We present qualitative experimental evidence indicating that this can be attributed to the formation of a proton bridge between the backbone and side chain carboxylate groups. The large amplitude motion of this shared proton, coupling to virtually all other vibrational modes, causes extensive spectral broadening. © 2009 American Chemical Society
J. Am. Chem. Soc. 131, 4310 (2009)
Stiff, and Sticky in the Right Places: Binding Interactions in Isolated Mechanically Interlocked Molecules Probed by Mid-Infrared Spectroscopy
Anouk M. Rijs, Isabelle Compagnon, Jos Oomens, Jeffrey S. Hannam, David A. Leigh and Wybren J. Buma
We report the results of high-resolution spectroscopic studies on isolated, jet-cooled [2]rotaxanes. Employing IR absorption spectroscopy, we show how these noncovalently bound, multicomponent molecular systems that so far have been out of reach from high-resolution techniques, can now be characterized at an unprecedented level. IR absorption spectra of prototypical hydrogen-bond assembled rotaxanes as well as their associated threads and macrocycles are shown to provide a direct view on the effects of interlocking the macrocycle and thread, and to offer a straightforward approach for the study of their structural and dynamical properties. © 2009 American Chemical Society
J. Am. Chem. Soc. 131, 2428 (2009)
Fast detector of the ellipticity of infrared and terahertz radiation based on HgTe quantum well structures
S. N. Danilov, B. Wittmann, P. Olbrich, W. Eder, W. Prettl, L. E. Golub, E. V. Beregulin, Z. D. Kvon, N. N. Mikhailov, S. A. Dvoretsky, V. A. Shalygin, N. Q. Vinh, A. F. G. van der Meer, B. Murdin, and S. D. Ganichev
We report a fast, room temperature detection scheme for the polarization ellipticity of laser radiation, with a bandwidth that stretches from the infrared to the terahertz range. The device consists of two elements, one in front of the other, that detect the polarization ellipticity and the azimuthal angle of the ellipse. The elements, respectively, utilize the circular photogalvanic effect in a narrow gap semiconductor and the linear photogalvanic effect in a bulk piezoelectric semiconductor. For the former we characterized both a HgTe quantum well and bulk Te, and for the latter, bulk GaAs. In contrast with optical methods we propose is an easy to handle all-electric approach, which is demonstrated by applying a large number of different lasers from low power, continuous wave systems to high power, pulsed sources. ©2009 American Institute of Physics
J. Appl. Phys. 105, 013106 (2009)
Spectroscopic Evidence for an Oxazolone Structure of the b2 Fragment Ion from Protonated Tri-Alanine
Jos Oomens, Sarah Young, Sam Molesworth and Michael van Stipdonk
Infrared multiple photon dissociation (IRMPD) spectroscopy is used to identify the structure of the b2+ ion generated from protonated tri-alanine by collision induced dissociation (CID). The IRMPD spectrum of b2+ differs markedly from that of protonated cyclo-alanine-alanine, demonstrating that the product is not a diketopiperazine. Instead, comparison of the IRMPD spectrum of b2+ to spectra predicted by density functional theory provides compelling evidence for an oxazolone structure protonated at the oxazolone N-atom.
© 2009 Elsevier B.V.
J. Am. Soc. Mass. Spectrom. 20, 334 (2009)
Structures of Silicon Cluster Cations in the Gas Phase
Jonathan T. Lyon, Philipp Gruene, Andre Fielicke, Gerard Meijer, Ewald Janssens, Pieterjan Claes, and Peter Lievens
We present gas-phase infrared spectra for small silicon cluster cations possessing between 6 and 21 atoms. Infrared multiple photon dissociation (IR-MPD) of these clusters complexed with a xenon atom is employed to obtain their vibrational spectra. These vibrational spectra give for the first time experimental data capable of distinguishing the exact internal structures of the silicon cluster cations. By comparing the experimental spectra with theoretical predictions based on density functional theory (DFT), unambiguous structural assignments for most of the Si n+ clusters in this size range have been made. In particular, for Si 8+ an edge-capped pentagonal bypriamid structure, hitherto not considered, was assigned. These structural assignments provide direct experimental evidence for a cluster growth motif starting with a pentagonal bipyramid building block and changing to a trigonal prism for larger clusters.
© 2008 American Chemical Society
J. Am. Chem. Soc. 131, 1115 (2009)
Role of Sequence in Salt-Bridge Formation for Alkali Metal Cationized GlyArg and ArgGly Investigated with IRMPD Spectroscopy and Theory
James S. Prell, Maria Demireva, Jos Oomens and Evan R. Williams
The roles of hydrogen bonding, electrostatic interactions, sequence, gas-phase basicity, and molecular geometry in determining the structures of protonated and alkali metal-cationized glycyl-l-arginine (GlyArg) and l-arginylglycine (ArgGly) were investigated using infrared multiple photon dissociation spectroscopy in the spectral range 900-1800 cm-1 and theory. The IRMPD spectra clearly indicate that GlyArg•M+, M = Li, Na, and Cs, form similar salt-bridge (SB) structures that do not depend significantly on metal ion size. In striking contrast, ArgGly•Li+ exists in a charge-solvated (CS) form, whereas ArgGly•M+, M = K and Cs, form SB structures. SB and CS structures are similarly populated for ArgGly•Na+. Computed energies of low-energy structures are consistent with these results deduced from the experimental spectra. By comparison to Arg•M+, GlyArg•M+ and ArgGly•M+ have a greater and lesser propensity, respectively, to form SB structures. The greater propensity for GlyArg to adopt SB structures in complexes with smaller metal cations than for ArgGly is due to the ability of alkali metal-cationized GlyArg to adopt a nearly linear arrangement of formal charge sites, a structure unfavorable for ArgGly complexes due to geometric constraints induced by its different amino acid sequence. The amide carbonyl oxygen solvates charge in both the SB and CS form of both dipeptides. ArgGly is calculated to be slightly more basic than GlyArg, indicating that differences in intrinsic basicity do not play a role in the relative SB stabilization of these ions. Loss of a neutral water molecule from complexes in which SB structures are most stable indicates that CS structures are intermediates in the dissociation pathway, but these intermediates do not contribute to the measured IRMPD spectra. © 2008 American Chemical Society
J. Am. Chem. Soc. 131, 1232 (2009)
Highlights 2008
Structure of the Observable Histidine Radical Cation in the Gas Phase: A Captodative a-Radical Ion
Jeffrey Steill, Junfang Zhao, Chi-Kit Siu, Yuyong Ke, Udo H. Verkerk, Jos Oomens, Robert C. Dunbar, Alan C. Hopkinson, and K.W. Michael Siu
The first infrared multiple photon dissociation (IRMPD) spectroscopic experiments on a prototypical amino acid radical cation, the Histidine radical cation, and its ternary complex ion, are reported. The study provides the first direct evidence of the captodative structure for the radical ion of an amino acid and confirms deductions based on DFT calculations and tandem MS experiments.
© 2008 WILEY-VCH Verlag GmbH
Angew. Chem. Int. Ed. 47, 9666 (2008)
Conformational Preferences of an Amyloidogenic Peptide: IR Spectroscopy of Ac-VQIVYK-NHMe
Timothy D. Vaden, Sally A. N. Gowers, Tjalling S. J. A. de Boer, Jeffrey D. Steill, Jos Oomens, and Lavina C. Snoek
The 306VQIVYK311 sequence in the tau peptide is essential for the formation of intracellular amyloid fibrils related to Alzheimer’s disease, where it forms interdigitating cross-beta-structures. The inherent conformational preferences of the capped hexapeptide Ac-VQIVYK-NHMe were characterized in the gas phase. IR/UV double-resonance spectroscopy of the peptide isolated in a cold molecular beam was used to probe the conformation of the neutral peptide. The influence of protonation at the lysine side chain was investigated at 298 K by characterizing the protonated peptide ion, Ac-VQIVYK(H+)-NHMe, with IRMPD spectroscopy in the fingerprint and amide I/II band region in an FTICR mass spectrometer. The conformations for both neutral and protonated peptides were predicted by an extensive conformational search procedure followed by cluster analysis and then DFT calculations. Comparison of the experimental and computed IR spectra, with consideration of the relative energies, was used to assign the dominant conformations observed. The neutral peptide adopts a beta-hairpin-like conformation with two loosely extended peptide chains, demonstrating the preference of the sequence for extended beta-strand-like structures. In the protonated peptide, the lysine NH3+ disrupts this extended conformation by binding to the backbone and induces a transition to a random-coil-like structure. © 2008 American Chemical Society
J. Am. Chem. Soc. 130, 14640 (2008)
Structures of neutral Au7, Au19, and Au20 clusters in the gas phase
Philipp Gruene, David M. Rayner, Britta Redlich, Alexander F. G. van der Meer, Jonathan T. Lyon, Gerard Meijer, Andre Fielicke
The unique catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. Here the geometries of several small neutral gold clusters in the gas phase are revealed by means of vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The lowering of the symmetry when a corner-atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.
Science 321, 674 (2008)
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Silicon as a model ion trap: Time domain measurements of donor Rydberg states
N. Q. Vinh, P. T. Greenland, K. Litvinenko, B. Redlich, A. F. G. van der Meer, S. A. Lynch, M. Warner, A. M. Stoneham, G. Aeppli, D. J. Paul, C. R. Pidgeon, and B. N. Murdin
One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr–Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus- and arsenic-doped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times—key to many well known schemes for quantum computing qubits—for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.
PNAS 105, 10649 (2008)
The Cationic C-F+ Stretching Vibration in the Gas Phase
Jos Oomens and Thomas Hellman Morton
The carbonyl stretch has been recognized as the most distinctive vibration of polyatomic organic molecules, after Coblentz reported the infrared (IR) absorptions of aromatic aldehydes more than a century ago. The present work explores the comparison between the C-O and C-F+ bonds, which are isoelectronic and isolobal. © 2008 WILEY-VCH Verlag GmbH
Angewandte Chemie 120, 2136 (2008)
The Effect of Charge on CO Binding in Rhodium Carbonyls: From Bridging to Terminal CO
Ingmar Swart, Frank M. F. de Groot, Bert M. Weckhuysen, David M. Rayner, Gerard Meijer, and André Fielicke
The structures of cationic rhodium carbonyl cluster compounds containing one to six Rh atoms are established by infrared multiple photon dissociation spectroscopy. Comparison with their well-known neutral analogues reveals that ionization of the neutral compounds destabilizes bridge bound CO ligands in Rh2(CO)8 and Rh4(CO)12, leading to cationic complexes with only terminally bound CO. The destabilization is associated with removal of charge from a highest occupied molecular orbital that is bonding with respect to bridge-bound CO. Density functional theory calculations support this conclusion. The results provide a possible insight into electronic promoter effects in catalysis. © 2007 American Chemical Society.
J. Am. Chem. Soc. 130, 2126 (2008)
Highlights 2007
Gas-Phase Zwitterion Stabilization by a Metal Dication
Robert C. Dunbar, Nick C. Polfer, and Jos Oomens
Abstract: Although solution-phase amino acids normally exist as zwitterions, this is not the case under gas-phase conditions, where the canonical structure is favored. Complexation to a metal ion can increase the relative zwitterion stability, but even then, the zwitterion (salt bridge, SB) form is not the most stable form of such singly charged complexes except for basic amino acids. Computation suggests enhanced SB stability for doubly charged complexes of weakly binding metal ions, but this has not hitherto been verified experimentally. Using infrared-spectroscopic characterization of the ion structure by multiple-photon infrared dissociation by the FELIX free electron laser, the Ba2+ complex of Trp has been shown to have the SB structure, and the presence of the nonzwitterionic (charge-solvated) form has been ruled out. The principal spectroscopic signatures of the SB structure are the appearance of the antisymmetric CO stretch of the carboxylate group at 1600 cm-1 and the umbrella mode of NH3 at 1400 cm-1. © 2007 American Chemical Society
J. Am. Chem. Soc. 129, 14562 (2007)
Gas-Phase Infrared Spectroscopy and Multidimensional Quantum Calculations of the Protonated Ammonia Dimer N2H7+
K. R. Asmis. Y. Yang, G. Santambrogio, M. Brümmer, J.R. Roscioli, L.R. McCunn, M.A. Johnson, O. Kühn
Introduction: The challenge of understanding the unusually high proton conductivity in water and in ice as well as water-mediated proton transfer across biomembranes has triggered considerable work on protonated water networks. Important limiting structures in describing these rapidly fluctuating networks are the Zundel (H5O2+) and Eigen (H9O4+) cations, which are characterized by broad but distinct infrared (IR) absorption spectra in the condensed phase. Interestingly, the infrared signature of the isolated Zundel cation, in which a proton is strongly bound and equally shared by two water molecules, has only very recently been fully elucidated. The vibrational frequency of the XH+X proton-transfer mode (X=closed-shell atom or molecule) in systems containing strong hydrogen bonds is dramatically red-shifted from the value of the free XH stretch; it often occurs below 1000 cm-1. Until recently, this spectral region has been inaccessible to tunable tabletop lasers, because these lasers were not sufficiently powerful to carry out action spectroscopy on the isolated systems. Theoretical analysis of the resulting band patterns may be even more challenging. The pronounced anharmonic character of such strong hydrogen bonds requires highly accurate, multidimensional quantum treatment of the vibrational-level structure that pushes the limits of current computers. © 2007 WILEY-VCH Verlag GmbH
Angew. Chem. Int. Ed. 46, 8691-8694 (2007)
Benchmarking of Electro-Optic Monitors for Femtosecond Electron Bunches
G. Berden, W.A. Gillespie, S.P. Jamison, E.-A. Knabbe, A. M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H. Schlarb, B. Schmidt, P. Schmüser, and B. Steffen
Abstract: The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties. © 2007 The American Physical Society.
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Phys. Rev. Lett. 99, 164801 (2007)
Donor-State-Enabling Er-Related Luminescence in Silicon: Direct Identification and Resonant Excitation
I. Izeddin, M.A.J. Klik, N.Q. Vinh, M.S. Bresler, and T. Gregorkiewicz
Abstract: We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 µm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as ED=218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor. © 2007 The American Physical Society.
Phys. Rev. Lett. 99, 077401 (2007)
Infrared Spectroscopy and Theoretical Studies on Gas-Phase Protonated Leu-enkephalin and Its Fragments: Direct Experimental Evidence for the Mobile Proton
Nick C. Polfer, Jos Oomens, Sndor Suhai, and Béla Paizs
Abstract: The gas-phase structures of the protonated pentapeptide Leu-enkephalin and its main collision-induced dissociation (CID) product ions, b4 and a4, are investigated by means of infrared multiple-photon dissociation (IR-MPD) spectroscopy and detailed molecular mechanics and density functional theory (DFT) calculations. Our combined experimental and theoretical approach allows accurate structural probing of the site of protonation and the rearrangement reactions that have taken place in CID. It is shown that the singly protonated Leu-enkephalin precursor is protonated on the N-terminus. The b4 fragment ion forms two types of structures: linear isomers with a C-terminal oxazolone ring, as well as cyclic peptide structures. For the former structure, two sites of proton attachment are observed, on the N-terminus and on the oxazolone ring nitrogen, as shown in a previous communication (Polfer, N. C.; Oomens, J.; Suhai, S.; Paizs, B. J. Am. Chem. Soc. 2005, 127, 17154-17155). Upon leaving the ions for longer radiative cooling delays in the ion cyclotron resonance (ICR) cell prior to IR spectroscopic investigation, one observes a gradual decrease in the relative population of oxazolone-protonated b4 and a corresponding increase in N-terminal-protonated b4. This experimentally demonstrates that the mobile proton is transferred between two sites in a gas-phase peptide ion and allows one to rationalize how the proton moves around the molecule in the dissociation process. The a4 fragment, which is predominantly formed via b4, is also confirmed to adopt two types of structures: linear imine-type structures, and cyclic structures; the former isomers are exclusively protonated on the N-terminus in sharp contrast to b4, where a mixture of protonation sites was found. The presence of cyclic b4 and a4 fragment ions is the first direct experimental proof that fully cyclic structures are formed in CID. These results suggest that their presence is significant, thus lending strong support to the recently discovered peptide fragmentation pathways (Harrison, A. G.; Young, A. B.; Bleiholder, B.; Suhai, S.; Paizs, B. J. Am. Chem. Soc. 2006, 128, 10364-10365) that result in scrambling of the amino acid sequence upon CID. © 2007 American Chemical Society.
J. Am. Chem. Soc. 129, 5887 (2007)
Hydrogen-Induced Transition from Dissociative to Molecular Chemisorption of CO on Vanadium Clusters
Ingmar Swart, André Fielicke, Britta Redlich, Gerard Meijer, Bert M. Weckhuysen, and Frank M. F. de Groot
Abstract: We report on the size-dependent interaction of carbon monoxide molecules with hydrogen covered vanadium clusters containing between 5 and 20 atoms. Structural information on these hydrogen covered vanadium clusters and their complexes with CO is obtained from infrared multiple photon dissociation spectroscopy, complemented with density functional theory calculations for the V5 to V9 cluster sizes. The non-dissociative or dissociative binding of CO on the metal clusters is detected by the presence or absence of the (CO) stretching band in the infrared spectra. It is found that the CO molecule dissociates on bare vanadium clusters, while it adsorbs intact on all saturated hydrogen covered V5-20+ clusters, with the distinctive exceptions of V5+, V9+, V11+, and V19+. We show that dissociative chemisorption is prevented when the potential binding sites of atomic C and O atoms are blocked by H atoms. © 2007 American Chemical Society.
J. Am. Chem. Soc. 129, 2516 (2007)
Highlights 2006
Isomorphous Substitution in Bimetallic Oxide Clusters
E. Janssens, G. Santambrogio, M. Brümmer, L. Wöste, P. Lievens, J. Sauer, G. Meijer, and K. R. Asmis
Abstract: The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V2O5)n-1(VTiO5)- (n=2–4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V2O5)n clusters. Upon systematic exchange of V by Ti atoms in V4-nTinO (n=1–4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti4O. © 2006 The American Physical Society
Phys. Rev. Lett. 96, 233401 (2006)
Isotope Dependence of the Lifetime of the 1136-cm-1 Vibration of Oxygen in Silicon
K.K. Kohli, G. Davies, N.Q. Vinh, D. West, S.K. Estreicher, T. Gregorkiewicz, I. Izeddin, and K.M. Itoh
Abstract: By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate tau of the asymmetric-stretch vibration (nu3 = 1136 cm-1) of oxygen (Oi) in silicon changes by a factor of about 2.5. These data establish that nu3 decays by creating one nu1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (nu3 - nu1) is less than the maximum frequency nu_m of the host lattice, as for 28Si-16O-28Si in natural silicon, then it is emitted as one lattice mode, and tau depends on the density of one-phonon states at nu3 - nu1. If (nu3-nu1) > nu_m, as for 16O in single-isotope 30Si silicon, two lattice modes are created in addition to nu1, increasing tau. Prediction of tau for a particular defect clearly requires a detailed knowledge of that defect. © 2006 The American Physical Society
Phys. Rev. Lett. 96, 225503 (2006)
Spin Relaxation by Transient Monopolar and Bipolar Optical Orientation
B.N. Murdin, K. Litvinenko, D.G. Clarke, C.R. Pidgeon, P. Murzyn, P.J. Phillips, D. Carder, G. Berden, B. Redlich, A.F.G. van der Meer, S. Clowes, J.J. Harris, L.F. Cohen, T. Ashley and L. Buckle
Abstract: We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps. © 2006 The American Physical Society
Phys. Rev. Lett. 96, 096603 (2006)
Stimulated Terahertz Stokes Emission of Silicon Crystals Doped with Antimony Donors
S.G. Pavlov, H.-W. Huebers, J.N. Hovenier, T.O. Klaassen, D.A. Carder, P.J. Phillips, B.Redlich, H. Riemann, R.Kh. Zhukavin and V.N. Shastin
Abstract: Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g phonon in silicon (2.92 THz). The emission frequency covers the range of 4.6-5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center. © 2006 The American Physical Society
Phys. Rev. Lett. 96, 037404 (2006)
Highlights 2005
Spectroscopic and Theoretical Evidence for Oxazolone Ring Formation in Collision-Induced Dissociation of Peptides
Nick C. Polfer, Jos Oomens, Sándor Suhai, and Béla Paizs
Abstract: Infrared spectroscopy of the b4+ fragment of Leu-enkephalin demonstrates that the oxazolone ring is formed during collision-induced dissociation of protonated peptides, whereas the linear acylium structure is not observed. Three distinct oxazolone structures are identified, based on the highly conformer-diagnostic C=O stretching mode of the oxazolone ring, clearly showing that proton transfer from the oxazolone ring to the N-terminus takes place. Note that the presence of a cyclic peptide b4+ isomer cannot be excluded. © 2005 American Chemical Society.
J. Am. Chem. Soc. 127, 17154 (2005)
Understanding the Infrared Spectrum of Bare CH5+
Oskar Asvany, Padma Kumar, Britta Redlich, Ilka Hegemann, Stephan Schlemmer, Dominik Marx
Abstract: Protonated methane, CH5+, continues to elude definitive structural assignment, as large-amplitude vibrations and hydrogen scrambling challenge both theory and experiment. Here, the infrared spectrum of bare CH5+ is presented, detected by reaction with carbon dioxide gas after resonant excitation by the free electron laser at the Felix Facility. Comparison of the experimental spectrum at ~110 Kelvin to finite-temperature infrared spectra, calculated by ab initio molecular dynamics, supports fluxionality of bare CH5+ under experimental conditions and provides a dynamical mechanism for exchange of hydrogens between CH3 tripod positions and the threecenter bonded H2 moiety, which eventually leads to full hydrogen scrambling. Furthermore the possibility of artificially freezing out scrambling and internal rotation in the simulations allowed assignment of the infrared spectrum despite this pronounced fluxionality.© 2005 Science.
Science, 309, 1219-1222 (2005)
Gold Cluster Carbonyls: Saturated Adsorption of CO on Gold Cluster Cations, Vibrational Spectroscopy, and Implications for Their Structures
André Fielicke, Gert von Helden, Gerard Meijer, David B. Pedersen, Benoit Simard, and David M. Rayner.
Abstract: We report on the interaction of carbon monoxide with cationic gold clusters in the gas phase. Successive adsorption of CO molecules on the Aun+ clusters proceeds until a cluster size specific saturation coverage is reached. Structural information for the bare gold clusters is obtained by comparing the saturation stoichiometry with the number of available equivalent sites presented by candidate structures of Aun+. Our findings are in agreement with the planar structures of the Aun+ cluster cations with n 7 that are suggested by ion mobility experiments [Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. M. J. Chem. Phys. 2001, 116, 4094]. By inference we also establish the structure of the saturated Aun(CO)m+ complexes. In certain cases we find evidence suggesting that successive adsorption of CO can distort the metal cluster framework. In addition, the vibrational spectra of the Aun(CO)m+ complexes in both the CO stretching region and in the region of the Au-C stretch and the Au-C-O bend are measured using infrared photodepletion spectroscopy. The spectra further aid in the structure determination of Aun+, provide information on the structure of the Aun+-CO complexes, and can be compared with spectra of CO adsorbates on deposited clusters or surfaces. © 2005 American Chemical Society.
J. Am. Chem. Soc. 127, 8418 (2005)
Polyhedral Vanadium Oxide Cages: Infrared Spectra of Cluster Anions and Size-Induced d Electron Localization
Knut R. Asmis, Gabriele Santambrogio, Mathias Brümmer, Joachim Sauer.
Abstract: we report the first experimental infrared spectra of transition metal oxide cluster anions in the gas phase. We combine infrared multiple photon dissociation (IRMPD) spectroscopy with density functional theory (DFT) to characterize the geometric and electronic structures of a representative series of vanadium oxide cluster anions, (V2O5)n- (n=2,3, or 4). Compelling evidence is produced that these anions have the polyhedral cage structures that have been predicted before, but have eluded spectroscopic detection until now. Evidence is also found for a size-induced localization of the extra electron in this series of anions. © 2005 Wiley-VCH Verlag GmbH.
Angew. Chem. Int. Ed. 44, 3122-3125 (2005)
Picosecond Thermometer in the Amide I Band of Myoglobin
Robert H. Austin, Aihua Xie, Lex van der Meer, Britta Redlich, Per-Anker Lindgård, Hans Frauenfelder, and Dan Fu
Abstract: The amide I and II bands in myoglobin show a heterogeneous temperature dependence, with bands at 6.17 and 6.43 µm which are more intense at low temperatures. The amide I band temperature dependence is on the long wavelength edge of the band, while the short wavelength side has almost no temperature dependence. We compare concepts of anharmonic solid-state crystal physics and chemical physics for the origins of these bands. We suggest that the long wavelength side is composed of those amino acids which hydrogen bond to the hydration shell of the protein, and that temperature dependent bands can be used to determine the time it takes vibrational energy to flow into the hydration shell. We determine that vibrational energy flow to the hydration shell from the amide I takes approximately 20 ps to occur. © 2005 The American Physical Society
Phys. Rev. Lett. 94, 128101 (2005)
Spectroscopic Evidence for Gas-Phase Formation of Successive beta-Turns in a Three-Residue Peptide Chain
Wutharath Chin, Isabelle Compagnon, Jean-Pierre Dognon, Clélia Canuel, François Piuzzi, Iliana Dimicoli, Gert von Helden, Gerard Meijer, and Michel Mons
Abstract: We report the first gas-phase spectroscopic study of a three-residue model of a peptide chain, Ac-Phe-Gly-Gly-NH2 (Ac = acetyl), using the IR/UV double resonance technique. The existence of at least five different conformers under supersonic expansion conditions is established, most of them exhibiting rather strong intramolecular H-bonds. One of the most populated conformers, however, exhibits a different H-bonding network characterized by two weak H-bonds. Comparison of the amide A and I/II experimental data with density functional theory calculations carried out on a series of selected conformations enables us to assign this conformer to two successive -turns along the peptide chain, the two H-bonds being of C10 type, i.e., each of them closing a 10-atom ring in the molecule. The corresponding form is found to be more stable than the 310 helix secondary structure (not observed), presumably because of specific effects due to the glycine residues. © 2005 American Chemical Society.
J. Am. Chem. Soc. 127, 1288 (2005)
Experimental Determination of the nu5 Cis-Bending Vibrational Frequency and Renner-Teller Structure in Ground State C2H2+ Using Laser Induced Reactions.
O. Asvany, T. Giesen, B. Redlich, and S. Schlemmer
Abstract: The spectrum of the nu5 cis-bending vibration of ground state (X2u) C2H2+ has been recorded applying the method of laser induced reactions in a low-temperature 22-pole ion trap. It is obtained by counting the number of products of the reaction C2H2+ (v5 = 1) +H2C2H3+ + H as a function of the laser wavelength. The vibronic transitions - and - with their corresponding spin-orbit and Renner-Teller substructure have been observed. Using a perturbative analysis, the vibrational frequency has been determined to 5 = (710±4) cm-1 and the Renner-Teller parameter 5 is on the order of 3×10-2. ©2005 The American Physical Society
Phys. Rev. Lett. 94, 073001 (2005)
Electro-Optic Technique with Improved Time Resolution for Real-Time, Nondestructive, Single-Shot Measurements of Femtosecond Electron Bunch Profiles.
G. Berden, S.P. Jamison, A.M. MacLeod, W.A. Gillespie, B. Redlich, and A.F.G. van der Meer
Abstract: Electro-optic detection of the Coulomb field of a relativistic electron bunch combined with single-shot cross correlation of optical pulses is used to enable single-shot measurements of the shape and length of femtosecond electron bunches. This method overcomes a fundamental time-resolution limit of previous single-shot electro-optic measurements, which arises from the inseparability of time and frequency properties of the probing optical pulse. Using this new technique we have made real-time measurements of a 50 MeV electron bunch, observing the profile of 650 fs FWHM (~275 fs rms) long bunches. ©2004 The American Physical Society .
Phys. Rev. Lett. 93, 114802 (2004)
Structure Determination of Isolated Metal Clusters via Far-Infrared Spectroscopy.
André Fielicke, Andrei Kirilyuk, Christian Ratsch, Jörg Behler, Matthias Scheffler, Gert von Helden, and Gerard Meijer
Abstract: We present a new method for the size selective structure determination of small isolated metal clusters in the gas phase. The technique is applied to cationic vanadium clusters containing 6 to 23 atoms, whose far infrared absorption spectra are measured in the 140–450 cm–1 spectral range. The spectra are unique for each cluster size and are true fingerprints of the cluster's structure. By comparing the experimental spectra to spectra obtained from density-functional theory, the geometric cluster structure can be identified. ©2004 The American Physical Society.
Phys. Rev. Lett. 93, 023401 (2004)
Probing the Glycosidic Linkage: UV and IR Ion-Dip Spectroscopy of a Lactoside.
Rebecca A. Jockusch, Romano T. Kroemer, Francis O. Talbot, Lavina C. Snoek, Pierre Çarçabal, John P. Simons,* Martina Havenith, Joost M. Bakker, Isabelle Compagnon, Gerard Meijer, and Gert von Helden
Abstract: The (14) glycosidic linkage found in lactose is a prevalent structural motif in many carbohydrates and glycoconjugates. Using UV and IR ion-dip spectroscopies to probe benzyl lactoside isolated in the gas phase, we find that the disaccharide unit adopts only a single, rigid structure. Its fully resolved infrared ion-dip spectrum is in excellent agreement with that of the global minimum structure computed ab initio. This has glycosidic torsion angles of H (H1-C1-O-C4') 180 and H (C1-O-C4'-H4') 0 which correspond to a rotation of ~150 about the glycosidic bond compared to the accepted solution-phase conformation. We discuss the biological implications of this discovery and the generality of the strategies employed in making it. © 2004 American Chemical Society.
J. Am. Chem. Soc., 126 (18), 5709-5714 (2004).
The Site of Cr+ Attachment to Gas-Phase Aniline from Infrared Spectroscopy.
Jos Oomens, David T. Moore, Gert von Helden, Gerard Meijer, and Robert C. Dunbar
Abstract: Infrared spectroscopy of gas-phase Cr+ complexes of aniline was studied using the FELIX free electron laser interfaced to a Fourier transform ion cyclotron resonance spectrometer. For both the monomer complex Cr+(aniline) and the dimer complex Cr+(aniline)2 the spectra showed features indicating binding of the metal ion to the aromatic cloud, as opposed to the nitrogen atom. Agreement with DFT-calculated infrared absorption spectra for the ring-bound complexes was good using the MPW1PW91 functional, but the B3LYP functional predicted the wrong binding site. The spectroscopic results resolve the ambiguity in computational prediction of the preferred binding site and support the use of the MPW1PW91 functional for these systems. © 2003 American Chemical Society.
J. Am. Chem. Soc., 126 (3), 724-725 (2004).
Infrared Spectroscopy of Neutral C7H7 Isomers: Benzyl and Tropyl.
Rob G. Satink, Gerard Meijer, and Gert von Helden
Abstract: The gas-phase infrared absorption spectra of neutral benzyl and tropyl, isomers of formula C7H7, have been measured in the 400-1800 cm-1 spectral region. In addition, a quantum chemical calculation has been performed to model the infrared spectra. For the benzyl radical, the theory shows satisfactory overlap with the experiment, although vibrations involving the CH2 group might be anharmonic. The tropyl radical, which is subject to the Jahn-Teller effect, seems well modeled for the out-of-plane vibrational modes, but less so for the in-plane vibrational modes. © 2003 American Chemical Society.
J. Am. Chem. Soc., 125 (51), 15714 -15715, 2003.
Gas-phase infrared spectrum of the protonated water dimer.
Knut R. Asmis, Nicholas L. Pivonka, Gariele Santambrogio, Mathias Brümmer, Christina Kaposta, Daniel M. Neumark, Ludger Wöste
Abstract: The protonated water dimer is a prototypical system for the study of proton transfer in aqueous solution. We report infrared photodissociation spectra of cooled H+·(H2O)2 (and D+·(D2O)2) ions, measured between 620 and 1900 wavenumbers (cm-1). The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600 cm-1–1 and one at 1740 cm-1–(for H502+). From a comparison to multi-dimensional quantum calculations, the three lower energy bands were assigned to stretching and bending fundamentals involving the O···H+···O moiety, and the highest energy band to a terminal water bend. These results highlight the importance of intermode coupling in shared proton systems. ©2003 Science.
URL: http://www.sciencemag.org/cgi/doi/10.1126/science.1081634
Dispersion of the second order susceptibility of GaAs
T. Dekorsy, V. A. Yakovlev, W. Seidel, M. Helm, and F. Keilmann
The investigation of Second Harmonic Generation (SHG) in the FIR remained largely unexplored due to the lack of high-power pulsed FIR sources. Here, SHG in GaAs crystals at input frequencies below the fundamental optical phonon frequency of the crystal were investigated using FELIX. According to theoretical predictions, the nonlinear coefficient of SHG changes sign in this frequency range, since higher-order contributions of the lattice potential cancel each other. The exact knowledge of this zero-crossing point of the nonlinear susceptibility provides a quantitative measure of these higher order contributions. A maximum of the SHG intensity at half the optical phonon frequency could be observed for the first time while the predicted minimum of the SHG was found at 177 cm-1, significantly above the theoretical value of 169 cm-1. These findings allowed a new determination of the higher order lattice potentials of GaAs.
Abstract: Nonlinear probing of the fundamental lattice vibration of polar crystals is shown to reveal insight into higher-order cohesive lattice forces. With a free-electron laser tunable in the far infrared we experimentally investigate the dispersion of the second-order susceptibility due to the phonon resonance in GaAs. We observe a strong resonance enhancement of second harmonic light generation at half the optical phonon frequency, and in addition a minimum at a higher frequency below the phonon frequency. Measuring this frequency and comparison to a theoretical model allows the determination of competing higher-order lattice forces. ©2003 The American Physical Society.
URL: http://link.aps.org/abstract/PRL/v90/e055508
Single-Shot Electron-Beam Bunch Length Measurements
I. Wilke, A. M. MacLeod, W. A. Gillespie, G. Berden, G. M. H. Knippels, and A. F. G. van der Meer.
Abstract: We report subpicosecond electro-optic measurements of the length of individual relativistic electron bunches. The longitudinal electron-bunch shape is encoded electro-optically on to the spectrum of a chirped laser pulse. The electron-bunch length is determined by analyzing individual laser-pulse spectra obtained with and without the presence of an electron bunch. Since the length of the chirped laser pulse can be easily changed, the electron bunch can be visualized on different time scales. This single-shot imaging technique is a promising method for real-time electron-bunch diagnostics. ©2002 The American Physical Society.
URL: http://link.aps.org/abstract/PRL/v88/e124801
Excited-State Lifetimes of Far-Infrared Collective Modes in Proteins
A. Xie, A. F. G. van der Meer, and R. H. Austin.
Abstract: Vibrational excitations of low frequency collective modes are essential for functionally important conformational transitions in proteins. Here we report the first direct measurement on the lifetime of vibrational excitations of the collective modes at 87 µm (115 cm-1) in bacteriorhodopsin, a transmembrane protein. The data show that these modes have extremely long lifetime of vibrational excitations, over 500 ps, accommodating 1500 vibrations. We suggest that there is a connection between this relatively slow anharmonic relaxation rate of approximately 109 sec-1 and the similar observed rate of conformational transitions in proteins, which require multilevel vibrational excitations. ©2001 The American Physical Society.
URL: http://link.aps.org/abstract/PRL/v88/e018102
Ultrafast Vibrational Dynamics and Stability of Deuterated Amorphous Silicon
Jon-Paul R. Wells, Ruud E. I. Schropp, Lex F. G. van der Meer, and Jaap I. Dijkhuis.
Abstract: Infrared four-wave mixing experiments performed upon deuterated amorphous silicon layers (a-Si:D) reveal profound differences in the dynamics of Si-D stretch vibrations compared to those of analogous Si-H vibrational modes in hydrogenated amorphous silicon (a-Si:H). Remarkably, transient-grating measurements of the population decay rate of the Si-D vibrations show single-exponential decay directly into collective modes of the a-Si host, bypassing the local bending modes of the defect into which the Si-H vibrations decay. Photon-echo measurements of the vibrational dephasing suggest at low temperature contributions from TO nonequilibrium phonons and at elevated temperatures elastic phonon scattering of TA phonons. ©2002 The American Physical Society.
URL: http://link.aps.org/abstract/PRL/v89/e125504
