Steric Influence of E- or Z-Monosubstituted and Terminally Disubstituted Vinyl Groups in Matrix-Isolated Cyclopentane-1,3-diyl Triplet Diradicals as Probed by the Zero-Field EPR D Parameter

The zero-field D parameter of the localized E, Z, and disubstituted vinyl cyclopentane-1,3-diyl triplet diradicals V was determined at 77 K in a 2-methyltetrahydrofuran (2-MTHF) matrix. Good linear correlations were obtained with the semiempirically (PM3; r² = 0.991, n = 19) and DFT (B3LYP; r² = 0.998, n = 7) calculated spin densities of the triplet diradicals. The D values for the disubstituted triplet diradicals V are generally larger than the corresponding monosubstituted ones and, thus, the former are less well-delocalized and thereby more poorly stabilized. For the E- and Z-diastereomeric pairs V, only marginal changes in the theoretical assessed spin densities as well as in the D values have been found. Steric effects operate and distort the conformation of the vinyl substituent in the triplet diradical V. This is adequately reproduced by theoretical calculations. For the diphenyl-substituted triplet diradical Vl, for example, they show a more or less planar alignment of the E-phenyl group and the allylic π system (torsion angle 12°) and thus optimal delocalization of spin, whereas the Z-phenyl group is twisted about 78° out of plane and therefore is not involved in the delocalization and stabilization of spin. This results in a slightly higher D value (0.0368 cm^-1), and the spin is delocalized less than in the monosubstituted E-Vm one (0.0357 cm^-1).