Ab initio molecular geometry and anharmonic vibrational spectra of thiourea and thiourea‐d₄

Abstract

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.

Spectroelectrochemical and DFT Study of Thiourea Adsorption on Gold Electrodes in Acid Media

“…40 A further detailed study including anharmonicity was carried out using MP2 and QCISD methods. 44 We are aware of only one theoretical work dealing with the adsorption of thiourea and thioureate (on a Ag(111) surface), authored by Patrito et al, 35 where electric field and solvent effects were considered.…”

“…Other authors studied the vibrational behavior of TU and TU- d 4 by comparing the experimental spectra in argon and nitrogen matrices with their respective theoretical vibrational frequencies obtained at the B3LYP/6-31+G(2d,p) level . A further detailed study including anharmonicity was carried out using MP2 and QCISD methods . We are aware of only one theoretical work dealing with the adsorption of thiourea and thioureate (on a Ag(111) surface), authored by Patrito et al, where electric field and solvent effects were considered.…”

Molecular Structure of Thiourea

“…After its first structure determination by X-ray diffraction, only recently (in 2004) has a microwave spectroscopy study provided a partial gas-phase substitution structure . On the contrary, the vibrational spectrum of thiourea has been widely investigated and characterized (see refs − and references therein). While in most vibrational studies bands were assigned assuming for thiourea a C 2 v planar symmetry, ab initio calculations ranging from second-order Møller–Plesset perturbation theory (MP2) to density functional theory (DFT) have shown that only the heavy atom skeleton, NCSN, is planar, and the C 2 conformer, with nonplanar amino groups, is the lowest in energy.…”

Resonance Raman spectroscopic and density functional theory investigation of the excited state structural dynamics of 2‐mercapto‐1‐methylimidazole

“…[11,13] The ground and excited state geometry structures of thiourea, one of the simplest thiocarbonyl compounds, and its derivatives were determined from the theoretical calculations and experimental measurements. [1,[16][17][18][19][20][21][22][23] It has been concluded that the geometry structure of thiourea in ground electronic state keeps the coplanarity of N-C(N)=S moiety due to the interaction between the nitrogen lone pairs and the thiocarbonyl p system, while that of thiourea in S 2 (pp*) excited state undergoes the out-of-plane deformation motion or the pyramidalization of carbon in the N-C(N)=S moiety, and the coplanarity disappears. [23] 2-Mercapto-1-methylimidazole (MMI) is a five-member aromatic heterocyclic derivative of thiourea.…”

“…Strong chemical interaction was found between the excited states of thione conformer and hydrocarbon solvent in the process of intermolecular photochemistry and photophysics . The ground and excited state geometry structures of thiourea, one of the simplest thiocarbonyl compounds, and its derivatives were determined from the theoretical calculations and experimental measurements . It has been concluded that the geometry structure of thiourea in ground electronic state keeps the coplanarity of N‐C(N)=S moiety due to the interaction between the nitrogen lone pairs and the thiocarbonyl π system, while that of thiourea in S 2 (ππ*) excited state undergoes the out‐of‐plane deformation motion or the pyramidalization of carbon in the N‐C(N)=S moiety, and the coplanarity disappears .…”