MOLECULAR PACKING ANALYSIS .3. THE PREDICTION OF M-NITROANILINE CRYSTAL-STRUCTURE

Molecular packing analysis of m-nitroaniline was carried out in order to study whether it is possible to predict the crystal structure of systems that include hydrogen bonds. Off-ridge eigenvector minimization with annealing (OREMWA) was carried out with Biosym's potential parameters and potential-derived charges obtained from an ab initio Hartree-Fock calculation using the 6-31++G** basis set. Two types of initial structure were prepared for the OREMWA calculations (ten structures were created for each type of initial structure). The first-type initial structures were created by deforming the observed crystal structure up to 1 Angstrom in the unit cell lengths and molecular translations, and up to 10 degrees in the unit cell angles and molecular rotations, The second-type initial structures were created by four molecules satisfying space symmetry relationships of the observed space group Pbc2(1) in a 12 Angstrom x 12 Angstrom x 12 Angstrom cubic unit cell. In the second-type initial structures, the orientation of a reference molecule was determined randomly. Molecular translation variables of three molecules, molecular rotation valuables of all four molecules, and six unit cell parameters were varied in the OREMWA calculations starting from the first-type initial structures. Molecular translation variables and molecular rotation variables of a reference molecule, and cell lengths only, were varied in the OREMWA calculations starting from the second-type initial structures in order to retain the observed space group symmetry, because the Pbc2(1) space group is orthorhombic with 90 degrees cell angles, This space group is also polar in the c-axis direction, so that one molecular origin in this direction is fixed, The observed crystal structure of m-nitroaniline was reproduced as the most stable crystal structure starting from both types of initial structure.