Simple method for calculation of net atomic charges: rapid access to charge-dependent properties of chemicals

Abstract

Data on charge distribution in molecules provide information on the properties of primary interest to chemists — reactivity, dipole moments, chemical shifts, etc. The charge distribution is now easily obtained by quantum chemical computer programs, but there is a drastic increase in computation times with increasing numbers of electrons. This meant that numerous efforts to derive simplified empirical methods (based on electronegativity equalization, for example) to calculate atomic charges were developed. In this work we present a method to calculate charges in molecules based on the knowledge of molecular topology (connectivity) and atomic electronegativities. The method consists in using empirical formula to simulate the following electronic effects that are well known to chemists: 1. inductive effect along ordinary (sigma) bonds, i.e. r −1*exp(−a* r); 2. pi inductive effect; 3. back donation of lone pairs into the pi-systems. As an approximation r is the shortest distance on the molecular graph between two atoms including hydrogen. Compared to other methods which require electronegativities and molecular topology, this method correctly provides the above-mentioned properties having very good agreement with many experimentally measured charge-dependent properties (dipole moments, chemical shifts even for 19F, Hammet and Taft's sigma constants, etc.). For example, very good correlation (corr. coeff=0.990, n=22) is found between ESCA C1s electron binding energy shifts and the charge calculated as above (using well known data from J. Gasteiger).