Assessment of the polarizabilities (α, β) of a nonlinear optical compound [<i>N</i>-(4-nitrophenyl)-(<i>L</i>)-prolinol] from an experimental electronic density study

Abstract

The underlying ambition of this work in addition to the x-ray diffraction electronic density determination is the estimation of the nonlinear optical properties of NPP [N-(4-nitrophenyl)-(L)-prolinol] from a model due to Robinson allowing us to connect the polarizabilities, under some approximations, to the different multipolar moments of the electronic charge distribution. The calculations of the atomic net charges demonstrate the character of the donor-acceptor (DA) pair linked to the phenyl transmitter. The dipolar molecular moments obtained by several methods have similar values, except the one from the \ensuremath{\kappa} method. This analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. The validity of the Uns\"old approximation, using the results obtained by a semiempirical method implemented in the electronic part of MOPAC (AM1 Hamiltonian), have been evidenced. The Uns\"old approximation gives relatively good results for \ensuremath{\alpha} but falls off by two orders of magnitude in the estimation of \ensuremath{\beta}. The results from the experimental electronic density study indicate that there is some added fluctuation in the modulus of the components, with respect to those derived from the so-called point charge model, but the signs are in good agreement. Those fluctuations are certainly related to the molecular interactions, which screen to some extent the nonlinear efficiency of the molecule. \textcopyright{} 1996 The American Physical Society.