Rovibrational analysis of c-SiCH: Further evidence for out-of-plane bending issues in correlated methods.

While the issue of properly describing the out-of-plane bends (OPBs) in sp hybridized carbon atoms has reappeared for c-SiCH, the present quantum chemical study provides a new characterization of this molecule in order to aid in its potential detection in astrophysically relevant studies. Combining the previous, high-level approach with MP2-F12/aug-cc-pVDZ gives exceptionally accurate results for the comparison of experimental rotational constants and seemingly reliable vibrational frequencies. Most notably, the brightest fundamental vibrational frequency in c-SiCH, the b OPB, is predicted to lie at 673.4 cm, within 4.0 cm of the previous matrix isolation experiment. As with c-CH, CCSD(T)-F12/aug-cc-pVTZ appears to be quite susceptible to over estimating the OPB anharmonic correction in c-SiCH and may also do such for in-plane bends, as well. MP2-F12/aug-cc-pVDZ is less susceptible to these errors, and increasing the step size reduces this positive anharmonicity issue in both the cases. The OPB underestimation, however, likely still remains. Finally, estimates for some anharmonic vibrational frequencies are provided for the methylated form, c-SiCHCH, which is likely also a product of gas phase reactions of ·SiH with various alkynes.