The molecular linear polarizability from X-ray diffraction study. The case of 3-Methyl 4-Nitropyridine N-oxide (POM)

The main purpose of this work, besides X-ray diffraction electronic density determination of a nonlinear molecular crystal, is to revisit the earlier Robinson model [Bell Syst. Tech. J. (1967) 913], which proposed to derive the nonlinear optical properties of molecule from its ground state charge distribution. The present study is mainly based on the ground state charge distribution inferred from X-ray diffraction data and the application of the Unso¨ld approximation through the Robinson model. This model has been applied to the POM a prototypical nonlinear organic crystal, whereby relations between polarisabilities and multipolar moments of the electronic charge distribution have been evidenced. The experimental electronic density analysis has been carried out by use aspherical model of Hansen and Coppens [Acta Cryst. A34 (1978), 909]. The electrostatic moments were then estimated by direct integration methods using both discreet and fuzzy boundary space partitioning. The electrostatic moments were also inferred from the charges derived from a semi-empirical calculation implemented in the electronic part of MOPAC (MNDO, PM3 Hamiltoniens). The results of this so called hereafter ‘Point Charge Model’ show relatively good agreements with those derived from X-ray electron density distribution except, however, for the component along X: The validity of the Unso¨ld approximation has been checked by comparing the values of the linear a property estimated from the Finite Field method with those deduced from Robinson model using the ground state moments of the point charge model. Comparison of the results obtained on the free molecule using the point charge model with those derived from the experimental charge distribution seems to reveal interesting information as to the influence of the crystal field effects on the molecular properties. q 2003 Elsevier B.V. All rights reserved.