Theoretical analysis of heteroaromatic thioaminyl radicals. Part 2: A comparison of ab initio and density functional methods in the description of redox processes

Gas-phase electron-transfer processes for 12 known thioaminyl radicals belonging to 7 general classes of heterocycles have been investigated using the HF and B3LYP methods with 6-31G(d) basis set augmented with diffuse functions. The basis set was chosen based on favorable comparison of the theoretical and experimental normal modes for 1a. The calculated ionization potentials (I-p) and free energy changes were subsequently compared with the experimental I-p and electrochemical redox potentials. It was found that the DFT method performs better than the HF method giving excellent correlations for vertical I-p and E-1/2(red) (R-2 > 0.98), but not for E-1/2(ox) or E-cell (R-2 = 0.7). The calculated disproportionation energies were compared with the available conductivity data for radicals showing qualitative correlation. Generally, low disproportionation energies and hence high conductivity are obtained for large pi systems, such as 11, containing the -N-S-S- array of heteroatoms. The established correlations provide a set of empirical scaling factors relating the calculated quantities with experimental observables for cyclic thioaminyl radicals. They were used in analysis of several radicals in the context of conductivity of molecular solids.