Ab Initio Molecular Geometry and Anharmonic Vibrational Spectra of Thiourea and Thiourea-d4

Potential energy surface (PES) of thiourea, SC(NH2)(2), has been searched for stable conformers under C-1, C-s, C-2, and C-2v symmetry constraints by post-Hartree Fock ab initio methods with electron correlation level varying from second-order Moeller Plesset perturbation theory (MP2) to quadratic configuration interaction with single and double excitations (QCISD) and basis sets of double- and triple-zeta quality within 6-31+G(d,p) to aug-cc-pVTZ range. Thiourea conformers of C-2 and C-s symmetry have been found as stationary points on the PES with no imaginary frequencies at MP2/6-31+G(d,p) level, whereas only the C-2 conformer seems as true minimum when basis sets containing more polarization and/or diffuse functions were used. At QCISD/cc-pVTZ level, only the C-2 thiourea conformer has been found as true minimum on the PES. Anharmonic vibrational spectra of C-2 conformers of thiourea and thiourea-d(4) have been computed by vibrational self-consistent field (VSCF) and correlation-corrected VSCF methods using quartic force field approximation at MP2/TZV+(2d,2p), and MP2/6-311+G(3df,2p) level and direct approach at MP2/6-31+G(2d,p) level. Both quartic force field and direct VSCF methods used PES expansion in curvilinear (internal) coordinates. Wavenumbers of fundamental, first overtone, and combination transitions of C-2 symmetry conformer have been calculated for natural abundance thiourea and thiourea-d(4) isotopomer. Anharmonic corrections originating from mean field and mode coupling effects vary from 5 to 60 cm(-1), whereas mode mode correlation contribution seems significant in the case of v(N-H) stretching and delta(NH2) deformation modes (15-5 cm(-1)). Application of internal coordinates in the VSCF calculation results in slight underestimation of delta(NH2) deformation mode fundamentals and correct description of out-of-plane large-amplitude tau(SCNH) modes. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 718-729, 2011