Anharmonic IR and Raman spectra and electronic and vibrational (hyper)polarizabilities of barbituric, 2-thiobarbituric and 2-selenobarbituric acids.

Infrared, Raman and electronic absorption spectra, electronic and vibrational (hyper)polarizabilities, of barbituric, 2-thiobarbituric and 2-selenobarbituric acids were studied in gas using ab initio and density functional theory levels. The vibrational spectra were computed using harmonic and anharmonic methods. Anharmonic contributions improve the agreement between calculated and available experimental wavenumbers, especially in the highest-energy spectral region (wavenumbers >1700 cm(-1)). Vibrational and electronic transitions potentially useful to identify the investigated compounds were explored. The electronic and vibrational hyperpolarizabilities for the IDRI nonlinear optical (NLO) process at the λ value of 790 nm were computed. Supported by spectroscopic results, electronic and vibrational polarizabilities and second-order hyperpolarizabilities increase progressively in the order barbituric acid<2-thiobarbituric acid<2-selenobarbituric acid. The seleno-derivative is predicted to be ca. three/four times more hyperpolarizable than the barbituric acid. The Se→O or Se→S substitutions can be practical strategies to enhances the NLO properties of barbituric and thiobarbituric acid-based materials.