Knowledge Bank

Previous optical detection of magnetic resonance experiments on $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}(n\pi \ast )$ benzophenone have established that this prototype photochemical system has (1) a highly localized n-electron distribution, (2) a highly delocalized $\pi \ast$-electron distribution, and (3) an essentially planar $C1-C(0)-C1\prime$ $framework.1-3$ Other structural features, however, remain to be exposed. In this report, we describe detailed measurements of the angular dependence of the optically-detected $\mathrm{<mrow\; data-mjx-texclass="ORD">19</mrow>}F$ ENDOR spectrum of $3(n\pi \ast )4,4\prime$-difluorobenzophenone in the single crystal host $4,4\prime$-dibromodiphynylether. An analysis of the data yields accurate values for the angles of ring twist in the lowest triplet state and confirms the significant $\pi \ast$-electron delocalization model for the parent molecule.