Assignment of the Lowest Triplet State of the Carbonyl Group

Abstract

A study of the electronic spectrum of the ${}^3{\mathrm{n\pi *\leftarrow }}^1{A}_1$ transition of benzophenone crystals is presented. The transition originates at 24 194 cm⁻¹, and it displays carbonyl stretching and ring torsion modes of the molecule. The observed polarization ratio ${\mathrm{(I}}_b{\mathrm{\hspace{0.17em}/\hspace{0.17em}I}}_a)$ of 2.5 ± 0.2 is indicative of the fact that at least two of the spin substates of the triplet can combine with the ground state in electric dipole radiation. The two transitions are $z$ (parallel to C–O) and $y$ (perpendicular to C–O in the R₂CO plane) polarized with the former dominant by a factor of 3. A polarized high‐field Zeeman study of the transition shows that the ${\sigma }_z$ and ${\sigma }_x$ spin designated states are the active ones; this result shows that the ${}^3\mathrm{n\pi *}$ state is of $A_2$ (in $C_{\text{2υ}}$ ) type and that the ${\sigma }_x$ state becomes active through vibronic coupling amongst higher states. The appearance of torsional modes in the spectrum is discussed in relation to this vibronic coupling.