The protonated methane, CH5+, is produced in large abundance in typical mass spectrometers and are stable in sufficiently inert atmosphere. Though no attractive industrial use of this molecule has been proposed it did generate a lot of theoretical interest as what its structure could be. With the early theoretical studies pointing in different directions we had more questions than answers. The story from experimental approach was also not very different. After several years of concerted efforts in 1995 Oka and co-workers were able to measure the IR spectrum of the species over a small window near the typical C-H stretching frequency. The high resolution spectrum still remain unassigned due to its richness. This is a classic example of how so tiny a molecule can be hugely complex!
More recent theoretical studies suggest this prototypical carbo-cation is highly fluxional (floppy) in character due to the small energy barriers separating its many isomeric structures. The most stable structure of the molecule proposed may be visualized as a CH3 tripod to which an H2 moiety is attached with a rare kind of chemical bond operative. The low energy barriers separating the isomeric structures would cause a perpetual scrambling dynamics of its hydrogens even at very low temperatures due to tunneling effects. The molecule thus appears unamenable to a straight forward application of the present state-of-the-art simulation techniques. However the observation that finite temperature could mimic the quantum effects remarkably well in this particular case left a very optimistic note.
We employed Born-Oppenheimer molecular dynamics technique to generate high quality dynamical trajectory of the molecule and predicted the infrared spectrum of the species. Simultaneous to our theoretical effort Schlemmer and co-workers at Leiden were working on an indigenous experiment to measure the spectrum of the molecule. To our excitement the measured spectrum was a near identic of the prediction. The time correlation functions of select internal coordinates together with the topology of the potential energy surface that is mapped on to a reduced-two-dimensional space explains the rather simple internal rotation to the complex hydrogen scrambling dynamics of CH5+, and the impact of these modes on the IR spectrum of the molecule. The story of CH5+ is certainly to continue...
For details see,
- Science, 309, 1219-1222 (2005), "Understanding the Infrared Spectrum of Bare CH5+", Oskar Asvany, Padma Kumar, P., Britta Redlich, Ilka Hagemann, Stephan Schlemmer, Dominik Marx.
- Phys. Chem. Chem. Phys., 8, 573-586 (2006), "Understanding Hydrogen Scrambling and Infrared Spectrum of Bare CH5+ Based on ab initio Simulations", Padma Kumar, P., Dominik Marx.
- J. Chem. Phys., 121, 3973-3983 (2004), "Quantum Corrections to Classical Time-Correlation Functions:Hydrogen Bonding and Anharmonic Floppy Modes", Rafael Ramirez, Telesforo Lo'pez-Ciudad, Padma Kumar, P., Dominik Marx.