SYNTHESIS, POLYMORPHIC CHARACTERIZATION AND STRUCTURAL COMPARISONS OFTHE NONLINEAR OPTICALLY-ACTIVE AND INACTIVE FORMS OF POLYMORPHS OF 3-(NITROANILINO) CYCLOALK-2-EN-1-ONES

Three 3-(nitroanilino)cycloalk-2-en-1-ones have been synthesized and t heir non-linear optical(NLO) properties investigated. Two of these com pounds have been found to exist in two polymorphic forms (alpha and be ta) that exhibit different second-order NLO properties. These polymorp hic forms were prepared and characterized by second-harmonic generatio n measurements as well as the more conventional methods of X-ray powde r diffraction and infrared spectroscopy. Of these polymorphs, the alph a- and beta-forms of 3-(4-nitroanilino)cyclohex-2-en-1-one (4NACHD) ha ve been further characterized by X-ray single-crystal diffraction and the crystal structures obtained have been compared with each other to rationalize why these two crystalline forms exhibit different second-o rder NLO properties. The alpha-form of 4NACHD crystallizes in a centro symmetric space group (P2(1)/c) with a = 6.863(7), b = 12.767(3), c = 13.410(4) Angstrom, beta = 104.59 (5)degrees, Z = 4, D-c = 1.356 g cm( -3) and R = 0.046, whereas the beta-form crystallizes in a noncentrosy mmetric space group (P2(1)2(1)2(1)) with a = 7.228(3), b = 12.064(3), c = 13.104(3) Angstrom, Z = 4, D-c = 1.350 g cm(-3) and R = 0.076. Exc ept for the slight difference in bond distances, both the alpha- and b eta-forms have the same orientation of the carbonyl group and hydrogen -bonding interactions. The carbonyl group is anti to the N-H group in both the two forms that result in the lambda (Lambda) conformation. Th e whole molecule of 4NACHD is more twisted in the beta-form than in th e alpha-form. Based on structural comparisons of the polymorphs of 4NA CHD and other compounds, the more twisted conformation in the beta-for m may bias molecules to pack into a non-centrosymmetric structure. Pre liminary results suggest that 3-(nitroanilino)cycloalk-2-en-1-one comp ounds may have a higher chance of forming non-centrosymmetric crystal structures than normal achiral organic molecules.