First-principles calculation of geometry and anharmonic vibrational spectra of thioformamide and thioformamide-d2

Abstract

The equilibrium geometry of thioformamide HCSNH2 has been determined at the MP2 and CCSD(T) electron correlation levels under C(s) symmetry constraints using triple-zeta basis sets up to cc-pVTZ. All optimized planar structures are true minima on the potential-energy surface and are characterized by the C-N bond length within 1.353-1.343 A, C-S distances of 1.656-1.628 A, and NCS angle between 125.7 degrees and 125.9 degrees . The wave number of the NH2 out-of-plane wagging mode computed in the harmonic approximation shows stronger dependence on the basis set rather than the electron correlation level and varies from 85.9 cm(-1) at CCSD(T)cc-pVDZ level to 335 cm(-1) at MP2/aug-cc-pVTZ level. Anharmonic vibrational spectra of HCSNH2 and HCSND2 have been determined directly from the potential-energy surfaces computed at MP2 level in triple-zeta valence (TZV)(2df,2p) and TZV+(2df,2p) basis sets using vibrational self-consistent-field (VSCF) and correlation-corrected VSCF (CC-VSCF) methods. CC-VSCF wave numbers of fundamental, first overtone, and most intense combination transitions are reported for thioformamide and those of fundamentals for thioformamide-d2. The NH2 wagging (nu12) mode is strongly anharmonic and its fundamentals have been computed at 406.9 cm(-1) [TZV(2df,2p)] and 399.5 cm(-1) [TZV+(2df,2p)], which is remarkably close to the experimental energy of 393 cm(-1). Anharmonically computed fundamentals of this mode in thioformamide-d2, 299.7 cm(-1) [TZV(2df,2p)] and 299.6 cm(-1) [TZV+(2df,2p)], are only approximately 7 cm(-1) higher than the transition energy (293 cm(-1)) observed in the gas phase spectrum of HCSND2. The first overtone of the NH2 wagging mode of thioformamide (nu12 (02)) has been calculated by CC-VSCF procedure at 830.8 cm(-1) [TZV(2df,2p)] and 880.0 cm(-1) [TZV+(2df,2p)], which implies "negative" (nu12 (02)>2*nu12 (01)) anharmonicity of this mode.