4-Substituted 1-Acyloxypyridine-2(1H)-thiones: Experimental and Computational Studies of the Substituent Effect on Electronic Absorption Spectra

Abstract

A series of eight 4-substituted 1-(adamantane-1-carbonyloxy)pyridine-2(1H)-thiones (1, X = H, OC(7)H(15), Me, CF(3), SC(3)H(7), CN, COOMe, and Cl) was prepared and characterized by UV-vis spectroscopy in MeCN and cyclohexane. The observed lowest energy transition, designated as pi(CS) --> pi*(ring), exhibits a substantial substituent effect and lambda(max) ranges from 333 (X = OC(7)H(15)) to 415 nm (X = CN). Experimental lambda(max) values for all esters except for 1b (X = OC(7)H(15)) correlate with the sigma(p)(-) parameter (rho = 0.41 +/- 0.03, r(2) = 0.95). In contrast, the energy of the absorption band at about 295 nm, designated as pi(CS) --> pi*(CS), is practically substituent independent. Both absorption bands exhibit a modest negative solvatochromic effect. The experimental absorption energies correlate better with excitation energies calculated for N-acetyloxy analogues 2 with the ZINDO//DFT than with the TD-DFT//DFT method. Calculations for a series of 12 N-acetates 2 predict the most blue-shifted pi --> pi* transition for the alkoxy substituent and most red-shifted for the NO(2) group relative to the parent 2a (X = H).