What have the XH4+ (X = C, Si, Ge) ions in common? An updated summary.

Abstract

A high level quantum mechanical study has been performed to explore the structural rearrangement and relative stability of the XH4+ (X = C, Si, Ge) radical cations at their X̃2T2 ground electronic states. All the stationary points located on the lowest adiabatic sheet of the Jahn-Teller (JT) split X̃2T2 state are fully optimized and characterized by performing harmonic vibrational frequency calculations. Five JT distorted stationary points with D2d(B22), C3v(A12), C2v(B22), and Cs(A'2) symmetries are located on the CH4+ ground state potential energy surface (PES), whereas four such structures are found on each of the SiH4+ and GeH4+ PESs. While the C2v(B22) isomer is found to be a global minimum and the Cs(A'2) one as a transition state for CH4+, the nature of them is reversed for SiH4+ and GeH4+. In particular, the Cs(A'2) stationary points are now global minima for the latter pair of radical cations, and C2v(B22) represents the transition state. Attempts are being made to understand such inconsistent findings via a combination of JT and epikernel principles. The barriers between equivalent C2v(B22) global minimum structures for CH4+ are found to be low, and thus CH4+ undergoes rapid interconversion along cyclic exchange of three hydrogen atoms via Cs transition state. The general features of the ground state PESs of SiH4+ and GeH4+ are similar. The pseudorotation between the Cs lowest energy structures undergoes along SiH2 and GeH2 wagging motions via C2v(B22) transition state for SiH4+ and GeH4+, respectively.