Ab initio determination of the vibrational and electronic first hyperpolarizabilities of reference compounds for non-linear 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, respectively. The geometrical parameters, charge distributions, first hyperpolarizabilities and vibrational normal modes and frequencies of POM and NPP are compared with available experimental and theoretical investigations. The vibrational first hyperpolarizability is analysed in terms of the most contributing vibrational normal modes. The ratio between the vibrational and electronic first hyperpolarizabilities is considered as a function of the chemical nature of the π-conjugated system, the NLO process and the response time.