Electron density study by X-ray and neutron diffraction of an NLO compound: <i>N</i>-(4-nitrophenyl)-<scp>L</scp>-prolinol. Description of quadratic hyperpolarizability

Abstract

The study of the electron density and the determination of the dipolar moment of the molecule in the crystalline compound N-(4-nitrophenyl)-L-prolinol have been completed using X-ray and neutron diffraction. The deformation maps and the calculations of the atomic net charges demonstrate the character of the donor-acceptor couple (DA) linked to the phenyl transmitter. The dipolar molecular moments obtained by several methods have similar values, except the method of kappa refinement. The kappa refinement appears inadequate to solve the phase problem. Accurate knowledge of the first molecular moment reduces the ambiguity on the quadratic polarizability coefficient β, measured from electric field-induced second-harmonic generation (EFISHG).The coefficients of the cubic moment (third-order semi-invariant) of the electronic distribution in the crystal state are compared with those of the quadratic hyperpolarizability obtained by theoretical calculations. An estimate of relevant coefficients in octupolar-type material would provide guidance to the choice of systems for non-linear optics. Crystal data: C 11 H 14 N 2 O 3 , M r , = 222.2, monoclinic, P2 1 , Z = 2, F(000) = 236, T = 122 K, a = 5.152 (4), b = 14.790 (3), c = 7.134 (2) A, β = 106.14 (4)°, V = 522.18 A 3 , D x = 1.41 Mg mm -3 at 122 K, μ = 0.084 mm -1 for λ = 0.7107 A