Intramolecular proton or hydrogen-atom transfer in the ground- and excited-states of 2-hydroxybenzophenone: A theoretical study

The internal rotational motion and the ground- and excited-state intramolecular proton transfer (GISPT, EISPT) reaction of 2-hydroxybenzophenone (HBP) are studied at the B3LYP/6-31G**, CIS/6-31G** and TD B3LYP/6-31++G** level. The calculated results show that there is only one minimum at the So and S, state, the enol form (E) is the single minimum at the So state, and its rotamers and tautomers are with more than 8.24 and 10.43 kcal/mol less stable, respectively, however, the keto tautomer (K*) is the single minimum at the S-1 state. Therefore, the ground- and excited-state intramolecular proton transfer reacts without a barrier between E and K or E* and K*. The energy gap between the first excited electronic state S-1 1(pi, pi*)(1) and the second excited electronic state S-2 2(n, pi*)(1) of E is only 0.019 eV at CIS/6-31G** level. So the S-0 --> S-1 and S-0 --> S-2 excitation may occur at the same time, and the S-1 state may occur the EISPT process and result a large Stokes shifted fluorescence. And the S2 state may through internal conversion to the S-1 state or intersystem crossing to the T-2 state. These theoretical results contrast with the conclusion of Nakayama and his coworkers [M. Hagiri, N. Ichinose, J. Kinugasa, T. Iwasa, T. Nakayama, Chem. Lett. 33 (2004) 326] stating the present of two ground-state conformers of HBP. (C) 2005 Elsevier B.V. All rights reserved.