Theoretical Analysis of Heteroaromatic Thioaminyl Radicals. Part 1:  A Comparison of Ab Initio and Density Functional Methods in Calculations of Molecular Geometry and Isotropic Hyperfine Coupling Constants

Molecular parameters of 23 heterocyclic radicals containing the thioaminyl fragment were systematically investigated with ab initio and DFT methods. Several basis sets were used to establish correlations between calculated and experimental geometries and isotropic hyperfine coupling constants. Distribution of spin and charge densities in the radicals was analyzed using the UB3LYP/6-31G* method. The data were subsequently analyzed with an emphasis on the quality of correlation rather than on the absolute accuracy of the calculated values. This resulted in a set of empirical scaling factors relating the calculated and experimental hfcc for cyclic thioaminyl radicals and provided a tool for routine design, structural elucidation, and characterization of new radicals. The protocol is used in the critical analysis of hfcc for several previously reported radicals. A companion paper (ref 1) follows in this issue [Kaszynski, P. J. Phys. Chem. A 2001, 105, 7626].