Optically detected magnetic resonance studies of photoexcited ¹⁷O−benzophenone. Orbital rotation in the lowest triplet state

The magnetically active isotope of oxygen 17O has been used to probe the changes in the electron charge and spin density distributions in oxygen valence orbitals which occur when benzophenone is excited to its lowest triplet state. The data obtained include the optically detected magnetic resonance (ODMR) and electron‐nuclear double resonance spectra at both zero and high magnetic fields. New methods of analysis of zero‐field ODMR spectra, appropriate when the second‐order hyperfine splitting exceeds the quadrupole coupling, are described. This analysis yields the principal values of the electron fine‐structure (D), oxygen hyperfine (A), and oxygen quadrupole (Q) tensors, and the orientation of their principal axes with respect to the molecular frame. It is found, consistent with expectations for an nπ* state, that the direction of the largest component of Q is different from that of the ground state. It is also found, by two independent methods, that the principal transverse axes of A and Q do not conform to the local C2v symmetry axes of the carbonyl group. This result is interpreted to mean that the axis of the n‐type oxygen 2p orbital is rotated out of the carbonyl plane, a rotation which appears to be direct consequence of nπ*/ππ* configurational mixing. In agreement with this, the principal values of D, A, and Q are different from those expected for a ’’pure’’ nπ* state. Other consequences of nπ*/ππ* mixing, not only in benzophenone but also in the lowest triplet states of other aromatic carbonyls, are discussed briefly