Chemical chameleon loses its camouflage

5 March 2010
Researchers of the FOM Institute Rijnhuizen and German colleagues have resolved the structure of a molecule in which the atoms constantly change places. The 'chemical chameleon' CH5⁺ is a model system for a class of molecules which play an important role in the chemistry of hydrocarbons. Using the infrared free electron laser FELIX and advanced quantumsimulations, the researchers determined the structure of CH5⁺. The team published its discovery in Nature Chemistry. 

In most molecules, the atoms take ordered positions and the molecule has a well-defined structure. An important exception is CH5⁺, created by adding an extra hydrogen atom to a methane molecule. Even at very low temperatures, the five hydrogen atoms are permanently in motion. CH5⁺ constantly  changes its structure, but the dance of the protons around the central carbon atom is choreographed such, that one structure dominates. The researchers determined this structure a few years ago: a tripod of CH3­, to which a 'dangling' H₂-group is attached. The new investigation shows what happens when the lighter hydrogen nuclei are replaced one by one by heavier deuterium nuclei. Using the infrared free electron laser FELIX, infrared spectra were recorded from the entire series of molecules: CH5⁺, CH4⁺D, CH3⁺D2, ..., CD5⁺. Each spectrum contains fingerprint information about the internal motions of the molecule under investigation; exchanging a hydrogen for a deuterium nucleus results in dramatic changes in the infrared spectra.

By comparing theoretical and experimental spectra, the researchers discovered the preferential positions of the H- and D-atoms. In the tripod/dangling group-structure of the molecule, the heavier D-atoms are found preferentially in the tripod, whereas the lighter H-atoms prefer to be in the dangling group of two atoms. This discovery is of fundamental importance for other researchers, because CH5⁺ is a model system for a class of molecules in which constituent atoms constantly change positions. The techniques developed here can therefore lead to a better comprehension of these dynamical molecules.

Picture of the low temperature ion trap in which the ions under investigation are captured. They are exposed to light of the infrared free electron laser FELIX and reacted in the trap. Superimposed is as an example the infrared spectrum of the CHD4⁺ molecule. Each of the CH5⁺ variations – with increasing number of hydrogen atoms replaced by deuterium – yields a significantly different spectrum. From these variations, the preferential positions of hydrogen and deuterium in the molecule has been determined.

credit: Köln University