Knowledge Bank

We report detailed measurements of the optically-detected electron nuclear double resonance (ENDOR) spectra of several isotopically-labeled $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}(n\Pi \ast )$ benzophenones in high magnetic field. The species investigated include benzophenone, perdeuterobenzophenone, carbonyl-$\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$-benzophenone, and carbonyl-$\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$-perdeuterobpnzophenone ($\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C-BP-d10$) in single crystals of $4,4\prime$-dibroraodiphenylecher at $1.4\circ K$. The identification of double-quantum radiofrequency transitions and their utilisation in enhancing the resolution of ENDOR spectra will be described. We also report a new method of determining the absolute signs of the hyperfine parameters of oriented triplet status. The results, which include the complete angular dependence of the optically-detected $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$-ENDOR spectrum of $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C-BF-d10$, have been analyzed to obtain both the principal values and axes of the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ hyperfine tensor in $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}(n\Pi \ast )$ benzophenone. These shed considerable light on the geometry and spin distribution in this prototype molecule.