Experimental and theoretical study of the charge density in 2-methyl-4-nitroaniline

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Abstract
The in-crystal molecular dipole moment of the nonlinear optical material 2-methyl-4-nitroaniline has been determined from a charge density analysis of x-ray diffraction data. The results indicate a considerable enhancement of the free molecule dipole moment, due to the crystal field. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of hydrogen atoms, in the absence of complementary neutron diffraction data. An analysis of the deformation density and Laplacian of the charge density proves useful for revealing weak hydrogen bonding effects. Ab initio calculations at the Hartree–Fock double-ζ level are reported for the molecule 2-methyl-4-nitro-aniline, with and without an applied electric field. In the former case, the magnitude and direction of the applied field were determined by a dipole lattice sum, to assess the magnitude of crystal field effects. The effect was to considerably enhance the molecular dipole moment, from 9 to 20 D, in agreement with the experimentally observed enhancement. Structure factors were generated from the ab initio wave functions and subjected to multipole refinement, to effectively project the theoretical ρ(r) into the same atom-centered multipole expansion form obtained from experiment. Monopole and dipole populations obtained in this way show convincing agreement with experiment.

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