A study of solvent effects on hyperpolarizabilities: The reaction field model applied to acetonitrile

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Abstract
The reaction field method has been applied in the calculation of the nonlinear optical properties of acetonitrile (CH3CN) in the liquid phase. Both self-consistent field (SCF) and second order perturbation theory (MP2) methods are used to calculate the dipole moment, polarizability, first and second static hyperpolarizabilities. Based on previous gas-phase studies which stress the importance of electron correlation in the calculation of hyperpolarizabilities, the MP2 results should be more reliable than the corresponding SCF results. The choice of a cavity radius is of considerable importance. For the two choices made, one based on the liquid density and the other on van der Waals’ parameters, the total second hyperpolarizability changed by as much as a factor of 3 and these calculated values bracket the two differing experimental results. The form of the reaction field factor suggests that the radius dependence should be less for acetonitrile in a solvent of reduced dielectric constant. Although this is true, as demonstrated by calculations involving acetonitrile dissolved in chloroform, significant dependence is still observed. The effect of using an ellipsoidal cavity as opposed to a spherical one is also examined. In this case the hyperpolarizability is less sensitive to the change in cavity parameters from those corresponding to the liquid density to those based on van der Waals’ radii. The hyperpolarizability determined is intermediate between the two values obtained with the spherical cavity and closer to one of the experimental values. Further work using more sophisticated solvation models is required in order to establish whether the reaction field model, with an appropriate choice of cavity parameters, can be used to investigate the hyperpolarizabilities of molecules in solution reliably.