The silyl anion (SiH−3): Cubic/quartic force field and anharmonic contributions to the fundamental vibrational frequencies

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Abstract
Fundamental vibrational frequencies for the silyl anion have been determined using two distinct vibrational theoretical methods, namely, the standard second‐order perturbation theory and Pulay’s variational theory. Several full quartic force fields were determined and used in both vibrational methods. Ab initio quantum mechanical methods used to generate energy derivatives include self‐consistent‐field (SCF), configuration interaction with single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD), in conjunction with basis sets including double zeta plus polarization (DZP), triple zeta plus double polarization (TZ2P), and TZ2P plus diffuse p functions on the silicon atom [TZ2P+diff(Si)]. SCF energy third derivatives were determined using analytic methods, while SCF fourth derivatives were determined from finite differences of third derivatives. CISD analytic energy gradients were used to generate second, third, and fourth derivatives by finite difference methods. Anharmonic corrections determined from both perturbation and variational theories are found to be quite insensitive to basis set size and electron correlation. The two theoretical methods for the treatment of vibrations have shown small but systematic differences in their prediction of anharmonic corrections. The fundamental vibrational frequencies obtained agree reasonably well with the limited available experimental results.