NEW METHOD FOR THE DERIVATION OF NET ATOMIC CHARGES FROM THE ELECTROSTATIC POTENTIAL

Net atomic charges are derived from a least-squares fitting to electrostatic potentials at atomic sites. The method treats atoms in the molecule as having spherically averaged Hartree-Fock densities, the atomic densities overlapping with one another. The method has the advantage of best reproducing the electrostatic potentials at the atomic nuclei and avoiding the arbitrariness in choosing the points used in the fitting. We have written a FORTRAN program, CHELPN92 (Z. Su, Chemistry Department, SUNY at Buffalo, Buffalo, NY, 1992), based on the method and applied it to deuterated benzene, l-alanine, d,l-histidine, 2-methyl-4-nitroaniline, and deuterated pyridinium-1-dicyanomethylide using the molecular geometry and electrostatic potentials from analysis of accurate X-ray diffraction data. The derived charges are used to calculate the molecular dipole moments. While the charges from this method are in general significantly different from those from the kappa refinement [P. Coppens, T.N. Guru Row, P. Leung, E.D. Stevens, P.J. Becker, and Y.W. Yang, Acta Cryst. A, 35, 63 (1979)], the dipole moments obtained with the new method agree well with those from the kappa refinement. (C) 1993 by John Wiley & Sons, Inc.