STRUCTURE AND DISSOCIATION-ENERGY OF THE WEAKLY-BOUND COMPLEX CH5+(H2)

The possible low-lying stationary points of CH5+(H2) have been investigated using high level ab initio quantum mechanical techniques. All structures were optimized up to the TZ2P CISD level of theory. The global minimum structure (1) was also optimized at the TZ2P CCSD(T) and TZ2P+d CCSD levels. Four C(s) structures, which have H-2 bound to one of two protons of the three-center-two-electron bond of the CH5+ subunit, have essentially the same energy. Consequently, several internal degrees of freedom in CH5+(H2) are virtuallY completely unrestricted. We determined the dissociation energy D0 of CH5+(H2) to be 1.46 kcal/mol [TZ2P-+d CCSD(T)]. Harmonic vibrational frequencies have been evaluated for all structures at the TZ2P SCF level of theory; for the energetically lowest-lying structure (1), we also determined the vibrational frequencies up to the TZ2P CCSD level. The H-H stretching frequency shift (-83 cm-1) predicted theoretically is in excellent agreement with the recently determined experimental value (-83.6 cm-1). The average value of theoretical rotational constants for the energetically lowest lying stationary points (1-4) and one other minimum (7) compares moderately well with the experimental result.