Theoretical determination of the vibrational and electronic (hyper)polarizabilities of C4H4X (X = O, S, Se, Te) heterocycles

Electronic, pure vibrational and dispersion contributions to the linear polarizability, alpha, and first hyperpolarizability, beta, of the group 16 chalcogenides C4H4X (X=O, S, Se, Te) have been studied by ab initio calculations using the 6-31G*, the Sadlej POL, correlation consistent Dunning and aug-CEP-31G basis sets. Electron correlation was introduced through the MP2 theory. Vibrational (hyper)polarizabilities, evaluated within the double harmonic oscillator approximation, were analyzed in terms of single normal mode contributions. They were found to be a fraction of the electronic counterpart. beta(v) does not show a clear dependence on the heavy atom, contrary to beta(e), which steadily increases along the series. The beta(v)/beta(e) ratio is very dependent on the basis set, decreasing and also changing sign in the presence of diffuse functions. Frequency-dependent electronic (hyper)polarizabilities were evaluated in the 0.02-0.16 a.u. frequency range. In the low-frequency range they can be fitted into a power series expansion in omega. In accord with the theory, the second-order coefficients of the beta expansion are the same for both the SHG and EOPE non-linear optical processes, while in the alpha expansion they can be related to the lowest electronic excitation energy.