H-atom transfer and rotational processes in the ground and first singlet excited electronic states of 2-(2′-hydroxyphenyl)oxazole derivatives: Experimental and theoretical studies

The H-atom transfer and the rotational processes of 2-(2′-hydroxyphenyl)oxazole derivatives in both ground (S0) and first singlet (S1) excited electronic states have been respectively studied from experimental and theoretical points of view. Experiment and theory support the coexistence of two ground state rotamers, E and ER, with OH⋯N and OH⋯O hydrogen bonds, respectively, rotamer E being the most stable and the only one that experiences a photoinduced H-atom motion in the S1 state. The fluorescence of 2-(2′-hydroxyphenyl)-4-methyloxazole in a rigid polymeric medium suggests that in fluid media the phototautomer of the excited enol rotamer suffers a twisting motion around the C-C bond linking both moieties of the molecule. Ab initio calculations at the Hartree-Fock and CI-all-singles levels reveal (a) the existence of a high-energy barrier to the H-atom transfer in the S0 state, whereas in the S1 state this transfer has a small or null energy barrier, (b) a coupling between a charge transfer and the nuclear rearrangement (OH and N⋯O modes) that makes the system move from the enol to the keto form, and (c) the presence of excited state rotamers of the keto phototautomer in these oxazole derivatives. © 1996 American Chemical Society.