Knowledge Bank

Isotope effects on the fine-structure parameters of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}(n\pi \ast )$ benzophenones containing $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}H, 2H, 12C, 13C, 16O$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O$ nuclei have been examined by zero-field optically-detected magnetic resonance (zf-ODMR) spectroscopy. A portion of these results have recently been reported by Hochstrasser, Scott and $Zewail.1$ The changes produced by $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}H$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ substitution are different in magnitude and opposite in sign, those produced by $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O$ substitution are similar in magnitude as well as sign, and those produced by simultaneous $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}H$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ substitution are additive. Concomitant changes are observed in the phosphorescence spectra of these species. A comparison of the data provides direct experimental evidence for the importance of Franck-Condon factors in determining the extent to which the lowest triplet state is mixed with nearby states via the spin-orbit interaction.