Electronic substituent effects on the EPR-spectral D parameter of trimethylenemethane (TMM) triplet diradicals compared with localized cyclopentane-1,3-diyls

Abstract

The EPR-spectral D parameter for a series of aryl-substituted trimethylenemethane (TMM) derivatives 2a–g has been determined in a 2-methyltetrahydrofuran (MTHF) matrix at 77 K. Although the substituent effects on the D parameter are relatively small compared with those of the localized 1,3-diradicals 4, the good correlation between the D values of TMMs 2 and those established for 4 signifies that electronic substituent effects may also be adequately monitored in this way for cross-conjugated triplet species. Thus, the p-NO₂ group acts as a spin acceptor, while the p-F substituent behaves as a spin donor. Significant steric effects in the aryl-substituted TMMs 2a–m impose a twisted conformation and, thus, reduce conjugation at the benzyl-radical center and are responsible for the relatively small substituent effects on the D parameter in these TMM triplets. Moreover, good linearity is observed between the AM1/AUHF-calculated spin densities at the benzyl position for the twisted conformation and the experimental D values of the TMM triplet diradicals 2.