An experimental charge density study of the effect of the noncentric crystal field on the molecular properties of organic NLO materials

The structure, packing, and charge distribution in molecules of nonlinear o ptical materials have been analysed with reference to their counterparts in centrosymmetric structures based on low temperature X-ray measurements. Th e systems studied are the centric and noncentric polymorphs of 5-nitrouraci l as well as the diamino, dithio, and thioamino derivatives of 1,1-ethylene dicarbonitrile; the latter possesses a noncentric structure. The molecular structure of 5-nitrouracil is invariant between the two forms, while the cr ystal packing is considerably different, leading to dimeric N-H . . . O rin gs in the centric polymorph and linear chains in noncentric one. There is a n additional C-H . . . O contort in the centric form with a significant ove rlap of the electrostatic potentials between the alkenyl hydrogen atom and an oxygen atom of the nitro group. The dipole moment of 5-nitrouracil in th e noncentric form is much higher (mu = 9 D) than in the centric form (appro ximate to 6 D). Among the 1,1-ethylenedicarbonitriles, there is an increase d charge separation in the noncentric thioamino derivative, feeding to an e nhanced dipole of 15 D compared to the centric diamino (5 D) and dithio (6 D) derivatives. The effect of the crystal field is borne out by semiempiric al AM1 calculations on the Two systems. Dipole moments calculated for the m olecules in the frozen geometries match closely with those obtained for cen tric crystals from the experimental charge densities. The calculated values of the dipole moment in the frozen or optimized geometries in the noncentr ic structures are, however, considerably lower than the observed value. Fur thermore, the conformation of the S-CH3 group in the noncentric crystal is anti with respect to the central C=C bond while the syn conformation is pre dicted for the free molecule in the optimized geometry.