Crystal structure features of nitro derivatives of methylcytizine and their relationship with second-order nonlinear optical susceptibility

The crystal packings of mono- (1) and dinitro derivatives (3) of methylcytizine are analyzed in terms of combined geometric and energy approaches and based on the evaluation of the degree of molecular deformation (a decrease in the molecular volume) in going from the free molecule to the crystal. An increase in the crystal density in going from the mono- to dinitro derivative (1.440 g cm−3 for 1 and 1.528 g cm−3 for 3 at 120 K) is associated with an increase in the density of the isolated molecule rather than with an increase in the packing density, which is almost equal in the crystals of compounds 1 and 3. The second-order nonlinear optical susceptibility of crystals was estimated in terms of a charge model, the prediction error of which is within 10%, being comparable with experimental accuracy. The similarity of the packing densities of compounds 1 and 3 enabled a detailed comparative analysis of the eff ect of the crystal fi eld on the nonlinear optical properties. Both compounds are characterized by relatively high nonlinear optical susceptibility, which is higher than that of urea, a commonly accepted standard, by a factor of 10 and 6.5 for compounds 1 and 3, respectively.