Ab initio molecular geometry and anharmonic vibrational spectra of thiourea and thiourea-d4.

Potential energy surface (PES) of thiourea, SC(NH(2))(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 ν(N-H) stretching and δ(NH(2)) deformation modes (15-5 cm(-1)). Application of internal coordinates in the VSCF calculation results in slight underestimation of δ(NH(2)) deformation mode fundamentals and correct description of out-of-plane large-amplitude τ(SCNH) modes.

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

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.

Anharmonic vibrational spectra of hydroxylamine and its 15N, 18O, and deuterium substituted analogs.

Vibrational self-consistent field (VSCF) and correlation-corrected vibrational self-consistent field (CC-VSCF) methods were used to compute the anharmonic frequencies of fundamentals, overtones, and combination transitions of natural abundance hydroxylamine, 15NH2OH, NH2(18)OH, ND2OD, ND2OH, and NH2OD isotopomers at second order Møller-Plesset perturbation theory (MP2) in basis sets of triple-zeta quality. Frequencies of the fundamental transitions observed in the gas phase spectrum were reproduced by CC-VSCF treatment within 20 cm(-1) in TZV(d,p) and TZV(2d,2p) basis sets, and the change of basis set composition had only minor effect on the frequencies of the computed fundamentals. CC-VSCF computed wave numbers of overtone and combination transitions were typically within 1-40 cm(-1) of the gas phase band positions, except for those resulting from multiple excitations of v2, v3, and v7 fundamentals, because of a strong mutual coupling between these modes. Integral transition intensities calculated at MP2 level closely followed those of experimental spectrum, including intensity decrease in v1, 2v1, 3v1 progression, and 30-fold intensity increase of 2v8 in respect to that of v8 fundamental. The frequency of the OH torsional fundamental was found to be strongly dependent on the mode-mode interaction potential among v9 and v1, v7, v2, v4, v5 modes. Band shifts resulting from 15N, 18O and complete 2H substitutions were reproduced almost quantitatively by CC-VSCF computation in TZV(d,p) basis. Computed anharmonic isotope frequency shifts were different from those obtained in the harmonic approximation and no scaling procedure seemed capable of performing their interchange.