Specificity of C-13 NMR Shielding Calculations in Thiocarbonyl Compounds
Isotropic C-13 nuclear magnetic shielding values (sigma(iso)) and corresponding chemical shift magnitudes (delta(C)) have been calculated for the large series of the thiocarbonyl compounds using DFT (RI-BP86 and B3LYP) and ab initio (RHF and MP2, with or without resolution of the identity approximation) levels of theory and different basis sets. As expected, the Hartree-Fock method and hybrid B3LYP approach essentially overestimate deshielding of the C=S carbon nuclei. The RI-BP86 level of approximation provides a noticeably better agreement with the experiment and can be used for calculations of the theoretical C-13 NMR spectra for the most classes of the thiocarbonyl compounds. Geometry optimizations and the nuclear magnetic shielding calculations using the MP2 level of theory provide the most reliable results. The resolution of the identity algorithm implemented into the TURBOMOLE program set does not affect the calculated sigma(iso) and delta C values compared to the strict MP2 nuclear magnetic shielding calculations (using the GAUSSIAN-03 program packet). A poor agreement with the experiment found in the case of the thiourea and thiosemicarbazide derivatives is referred to the different conformations of these species in the gas phase, solid state, and solution.
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1386-1405
Informa UK Limited