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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-zeta level are reported for the
molecule 2-methyl-4-nitroaniline, 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 rho(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.
