A COMPARISON OF CALCULATED AND EXPERIMENTAL HYPERPOLARIZABILITIES FOR ACETONITRILE IN GAS AND LIQUID-PHASES

The nonlinear susceptibility of acetonitrile has been measured in both the gas and liquid phases by dc electric field-induced second harmonic generation (dc-SHG or EFISH). The EFISH signal for this molecule is dominated by the first hyperpolarizability beta. It is shown that local field factors do not adequately describe the effect of the solvent environment. Thus it is not possible to extract ''gas phase'' values of hyperpolarizabilities from measurements made in solution. First and second hyperpolarizabilities, beta and gamma, have been calculated for acetonitrile using ab initio techniques. These calculations are compared to the gas phase experimental measurements. Excellent agreement is obtained when electron correlation effects are included.