Vibrational conical intersections in CH3SH: implications for spectroscopy and dynamics in the CH stretch region

The adiabatic separation in methyl mercaptan of the high-frequency asymmetric CH stretch vibrations from the low-frequency torsional ($\gamma$) and CSH bend ($\rho$) coordinates yields a set of 7 vibrational conical intersections (CIs). The three CIs in the staggered conformation at $\rho$ = $79^{\circ}$ are close to the global minimum energy geometries ($\rho$$_{e}$ = $83.3^{\circ}$), accounting for the observed near-degeneracy of the two asymmetric CH stretch vibrations. The vibrational frequencies were computed at the CCSD(T)/aug-cc-pVTZ level. A new high-order Exe Jahn-Teller model, which involves a spherical harmonic expansion in $\rho$ and $\gamma$, fits the calculated electronic and vibrational energies over the whole range of $\gamma$ and for $\rho$ between $0^{\circ}$ and $100^{\circ}$ to within a standard deviation of 0.2 \wn. The pattern of the CIs contrasts with that in methanol where the CIs occur only in the eclipsed conformation near the top of the torsional barrier. An examination of three alternative diabatization schemes for the two molecules points to rather different nuclear dynamics. In \chem{CH_3SH} crossings between the upper and lower adiabatic surfaces are predicted to occur predominantly with motion along the CSH bending coordinatewhereas in \chem{CH_3OH}, such crossings are predicted to occur predominantly with torsional motion