Renner-Teller bending frequencies of the approximately A 2Pi state of OCS+

There are inconsistencies among previously reported Renner-Teller bending frequencies for the approximately A 2Pi state of OCS+. To resolve these controversies, we have computed vibrational frequencies using high-level excited electronic state ab initio equation-of-motion coupled cluster methods. On the basis of equation-of-motion coupled cluster theory including single, double, and iterative inclusion of partial triple excitations (EOM-CC3) paired with the correlation-consistent polarized valence quadruple-zeta basis set (cc-pVQZ), we predict harmonic bending frequencies of 364 and 401 cm-1 for the A' and A" components of approximately A 2Pi OCS+, respectively. Particularly for the upper Renner-Teller component, these results are lower than the theoretical predictions of 370 and 459 cm-1 reported by Chen, Hochlaf, Rosmus, He, and Ng [J. Chem. Phys. 2002, 116, 5612]. Instead, the presently computed bending frequencies are more consistent with the experimentally derived average value of 357 +/- 5 cm-1 recently reported by Sommavilla and Merkt [J. Phys. Chem. A 2004, 108, 9970], lending credence to the spectral assignments made in this later work. The two components of the Renner-Teller bending frequencies of approximately X 2Pi OCS+ are similarly predicted to be 396 and 453 cm-1. Anharmonicity constants arising from a quartic force field computed at the cc-pVQZ EOM-CC3 level of theory are given, to provide a more complete characterization of the potential energy surface of the approximately A 2Pi state of OCS+.