An Evolved Explanation for the Molecular Geometry and Electronic Structure of Diphenyl-Substituted Cyclic Trimethylenemethane in the Ground State: A Nearly Planar Conformation with a Considerably Localized Electronic State

摘要

We reinvestigated the molecular geometry and electronic structure of the diphenyl-substituted, five-membered cyclic trimethylenemethane (TMM) diradical (Berson's TMM, 3‥) using UV/VIS absorption and emission spectroscopy combined with density functional theory (DFT) and time-dependent (TD)-DFT calculations. Two intense absorption bands, A and B, with λ_(ab) at 298 and 328 nm, respectively, a weak absorption band C, with λ_(ab) at 472 nm, and an intense emission band D, with λ_(em) at 491 nm, were observed for 3‥. By comparing the spectrum of 3‥ with those of the 1,1-diphenylethyl (7·) and cyclopent-2-en-1-yl (9·) radicals, it was found that bands B, C, and D originated from the diphenylmethyl radical moiety (subunit I), while band A should most likely be assigned to an electronic transition related to an interaction between subunit I and residual subunit Ⅱ, the cyclopentenyl radical moiety. An UB3LYP/cc-pVDZ calculation indicated that, in the ground state, the two unpaired electrons of 3‥ are mainly localized in subunits I and II, respectively, and the interaction between them is inefficient, despite the nearly planar conformation (θ = +23.5°). Furthermore, a TD-UB3LYP/cc-pVDZ calculation suggested that absorption band A is assigned to an electronic transition involved with enhancement of the electron density of the C-2-C-3 bond. Substituent effects on the absorption and emission spectra of 3‥ using 11‥ and 13‥support the conclusion based on the experiments and calculations. Therefore, we propose an evolved explanation for the molecular geometry and electronic structure of the ground state of 3‥ in a low-temperature matrix, a nearly planar conformation with a considerably localized electronic state, which alone accounts for the spectroscopic characteristics.