CH3+ Is the Most Trivial Carbocation, but Are Its Heavier Congeners Just Lookalikes?

While the strongly bound D3h AH3+ cations are favored energetically for CH3+ and SiH3+, Cs HA+···H2 complexes are computed to be the global minima for A = Sn and Pb. The structures, relative energies, and dissociation limits (Do) of the AH3+ cations (A = C, Si, Ge, Sn, and Pb) were studied at the Becke3LYP density functional level [6-311++G(2d,2p) basis sets for C, Si, and Ge, (quasi)relativistic effective core potentials with TZ+2P valence basis sets for Sn and Pb]. The SiH3+ potential energy surface computed for calibration at the CCSD(T) level is in very good quantitative agreement with the Becke3LYP results. The AH3+ → HA+···H2 rearrangement illustrates the rare case where two transition states are connected directly (one high-lying, 45−58 kcal mol-1 vs D3h AH3+, and one lower in energy, 0−37 kcal mol-1) without an intervening minimum. Due to the appreciable Do values for both the AH3+ and the HA+···H2 species, they should be experimentally verifiable.