The extension of a previously described method for the evaluation of atomic charges in organic structures is presented. The application to structures containing charged atoms is considered. A search for the atoms involved in the initial distribution of the charge is afforded extending a method previously described. Bond orders are changed according to the existence of canonical forms. The problem of backdonating atoms is analyzed and discussed and a possible solution is presented. The method is tested through calculation on several molecules and results are discussed. This program is included on disk in this issue.
I N T R O D U C T I O N
A computer program (Lilith 1) for Computer Assisted Organic Syntesis is developing at our group. As described in our previous papers 2 a method for the evaluation of atomic charges in organic structures is being introduced. Molecules that are not neutral, where the presence of one or more whole charges characterizes the input structure, and molecules containing heteroatoms with lone electron pairs conjugated with multiple bonds, where it is possible to find a partial redistribution of the electron density over the entire system, will be analyzed. The method will be compared with some other empirical methods 4-7 that are as fast and efficient as ours, disregarding quantum mechanical ones. 3