Theoretical density functional and ab initio computational study of vertical ionization potentials, dipole moments and ¹³C and ¹⁴N-NMR shifts of the 2-mercaptopyridine system. A model for thiated nucleobases †

Abstract

Lower valence vertical ionization potentials (VIPs) of pyridine-2(1H)-thione (1), 1-methylpyridine-2(1H)-thione (3) and their tautomers (2 and 4) from B3LYP/6-311+G(2d,p) calculations were in very good agreement with experimental values when the SCF calculated first VIP was added to the relative energy of the corresponding Kohn–Sham orbitals. Except for the first VIP, the valence VIPs were poorly reproduced by HF or MP2 calculations following Koopmans’ theorem. Both MP2 and B3LYP electronic correlation methods using 6-311+G(2d,p) basis sets gave good predictions for the dipole moments of 3 and 4 in benzene solution. Dipole moments from calculations including solvent effects by SCRF methods were very large. Relative ¹⁴N and ¹³C-NMR shifts of 3 and 4 tautomers from the GIAO method applied at the B3LYP/6-311+G(2d,p) level were in good agreement with experimental values using a scaling factor of 0.96.

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