AB-INITIO DETERMINATION OF THE VIBRATIONAL AND ELECTRONIC FIRST-HYPERPOLARIZABILITIES OF REFERENCE COMPOUNDS FOR NONLINEAR-OPTICAL (NLO) APPLICATIONS - 3-METHYL-4-NITROPYRIDINE-1-OXIDE (POM) AND N-(4-NITROPHENYL)-(L)-PROLINOL (NPP)

The electronic and vibrational first hyperpolarizabilities of POM and NPP are determined at the HF/6-311G* level of approximation by using the coupled Hartree-Fock and double harmonic oscillator schemes, respe ctively. The geometrical parameters, charge distributions, first hyper polarizabilities and vibrational normal modes and frequencies of POM a nd NPP are compared with available experimental and theoretical invest igations. The vibrational first hyperpolarizability is analysed in ter ms of the most contributing vibrational normal modes. The ratio betwee n the vibrational and electronic first hyperpolarizabilities is consid ered as a function of the chemical nature of the n-conjugated system, the NLO process and the response time.