Probing the structure of CH5+ ions and deuterated variants via collisions

Abstract

Numerous recent calculations have provided a rather detailed picture how the protonated methane, CH₅⁺, really may look like at very low temperatures; however, there is not yet any experiment, providing information on the correlation of a structure of this fluxional ion with a state to state transition induced by a photon or a collision. Various efforts in spectroscopy and mass spectrometry have contributed important pieces to the puzzle but there are no real final conclusions, e.g. infrared spectra in the region of the C–H stretching vibration are waiting for assignment since several years. This contribution reviews and discusses the potential and the limitations of a variety of detailed collision experiments for learning more about protonated methane and deuterated variants, either via creation, modification or destruction of CH₅⁺. There has been a controversial discussion about an experiment which seemed to indicate that deuteron transfer from CD₄⁺ to CH₄ can create stable isotopomers with chemically distinguishable hydrogen atoms, CH₃-HD⁺. Detailed integral and differential cross sections measured with sophisticated ion beam techniques revealed interesting dynamics; but, unfortunately, CH₅⁺ formation is certainly more complicated than just a simple proton transfer into a stable isomer. In collisions of CH₄⁺ with CH₄ or CD₄, there is significant scrambling and one would need to use differential scattering selection for getting ions produced exclusively via a specific mechanism. There have been several low temperature ion trap studies leading to CH₅⁺, e.g., simply via radiative association of CH₃⁺ with H₂ or via hydrogen abstraction in CH₄⁺ + H₂ collisions. Very interesting and in some cases unforeseen observations have been made by using deuterated variants in low temperature collisions. A general conclusion is that H–D exchange is not only influenced by the differences in zero point energies but that symmetry selection rules can significantly restrict the scrambling. For example, conservation of nuclear spin may allow one to synthesize specific CD₃H₂⁺ ions with a local ortho hydrogen rotator via radiative association. Cold trapped CH₅⁺ ions have been probed by collisions with HD. Despite the high sensitivity of the trapping technique, no H–D exchange could be observed at all while a few isotopic equivalents of CH₇⁺ grow via radiative association. Finally, our most recent activities are briefly mentioned, probing cold CH₅⁺ ions via collisions with slow H or D atoms. This survey ends with a conclusion and an outlook. It is very sure that all traditional methods of collisional probing are unable to provide evidence for different CH₅⁺ isomers. If, however, spectroscopy, low temperature collision dynamics, multi-electrode traps and supersonic or effusive beams are combined in a suitable way, one may succeed in probing single states of unperturbed cold ions at low temperatures.