Quantum chemical and experimental studies on the structure and vibrational spectra of substituted 2-pyranones

A systematic study on the structural characteristics of the 2-pyranone ring containing mole- cules with bromine, nitrile, and amide substituents at the C-3 position in the ring is conducted in the electronic ground (S0) state by DFT calculations using the B3LYP/6-311++G** method. The geometrical structure of the bromine substituted compound, which shows potent hepato- protective activity, is studied both in the ground (S0) and first excited singlet (S1) states using RHF/6-311++G** and CIS/6-311++G** methods respectively. The molecules are found to ex- ist in two isomeric forms gauche and trans that have the enthalpy difference of less than 3.32 kcal/mol; the latter is the preferred orientation in the gaseous phase. The S1 state is a 1 (,*) state that arises -electron transfer from the region of a double bond in the pyranone ring to the region of the internuclear bond connecting the 2-pyranone and benzene rings. A complete vibrational analysis is conducted for the 3-bromo-6-(4-Chlorophenyl)-4-thiomethyl- 2H-pyran-2-one molecule based on the experimental infrared spectra in the 50—4000 cm -1 re- gion and DFT/6-311++G** computations of vibrational frequencies for the gauche and trans isomeric forms. Spectral assignments based on the potential energy distribution along the in- ternal coordinates confirm the non-planar structure of the molecule. !