A density functional study of hyperfine coupling constants in steroid radicals

We present the results of density functional calculations of H-1 hyperfine coupling constants (hfcc's) in four types of model steroid radicals (R1 thr ough R4). The calculations are performed using the B3LYP functional in comb ination with Pople basis sets (6-31G(d) and 6-311G(d)) and the EPR-III basi s set. In radicals R1, R2, and R3 the agreement between experimental and ca lculated isotropic hfcc's is excellent (differences < 5%) except for two pr otons in radical R3 where the calculations provide evidence for reconsideri ng the experimental assignment. The agreement is rather poor in the case of radical R4 and is indicative either of specific geometry effects that cann ot be modeled in the proposed isolated molecule approach or of a different radical identity.