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

Lower valence vertical ionization potentials (VIPs) of pyridine-2(IK)-thion e (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 experime ntal values when the SCF calculated first VIP was added to the relative ene rgy of the corresponding Kohn-Sham orbitals. Except for the first VIP, the valence VIPs were poorly reproduced by HF or MP2 calculations following Koo pmans' theorem. Both MP2 and B3LYP electronic correlation methods using 6-3 11 + G(2d,p) basis sets gave good predictions for the dipole moments of 3 a nd 4 in benzene solution. Dipole moments from calculations including solven t effects by SCRF methods were very large. Relative N-14 and C-13-NMR shift s 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 f actor of 0.96.