Structure, infrared spectrum, and dissociation energy of SiH₇⁺

Ab initio quantum mechanical methods, including the self-consistent field (SCF), single- and double-excitation configuration interaction (CISD), single- and double-excitation coupled cluster (CCSD), and the single-, double-, and perturbative triple-excitation coupled cluster [CCSD(T)] have been applied to three stationary points on the SiH₇⁺ potential energy hypersurface. Double-ζ plus polarization (DZP) and triple-ζ plus double-polarization [TZ2P and TZ2P(f,d)] basis sets were employed. The C₂ structure, where two symmetry-equivalent H₂ subunits complex the SiH₃⁺ cation, was found to be the global minimum, in agreement with the findings of Liu and Zhou (J. Phys. Chem. 1993, 97, 9555). The bound vs free H2 harmonic vibrational frequency shift obtained at the TZ2P CCSD level (259 cm^-1) is 36 cm^-1 less than the experimental frequency shift (295 cm^-1), compared with the shift obtained by Liu and Zhou with second-order perturbation theory, which was 33 cm^-1 higher than the value from experiment. The theoretical rotational constants are compared with the experiments of Okumura's group. The dissociation energy D₀ of SiH₇⁺ to yield SiH₅⁺ and H₂ is sizable, 4.6 kcal/mol [TZ2P(f,d)] CCSD-(T) + ZPVE(TZ2P CCSD)], much larger than the analogous value for CH₇⁺.