Assignment of the Lowest Triplet State of the Carbonyl Group

A study of the electronic spectrum of the 3nπ*←1A1 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 (Ib / Ia) 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 σz and σx spin designated states are the active ones; this result shows that the 3nπ* state is of A2 (in C2υ) type and that the σx 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.