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Journal of Structural Chemistry

2011 year, number 2

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

P. Thul, V.P. Gupta, D. Chaturvedi, P. Tandon
Department of Physics University of Lucknow
Keywords: 2-pyranone, molecular conformation, infrared spectra, DFT
Pages: 259-268


A systematic study on the structural characteristics of the 2-pyranone ring containing molecules 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 hepatoprotective 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 exist 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(π,π*) state that arises π-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 region 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 internal coordinates confirm the non-planar structure of the molecule.