6910
J. Phys. Chem. 19!32,96, 6910-6916
(38) Ruggiero, A. J.; Todd, D. C.; Fleming, G.R. J . Am. Chem. Soc. 1990, 112, 1003. (39) Hansen, J. E.; Rosenthal, S.J.; Fleming, G.R. J . Phys. Chem. 1992, 96, 3034. (40) Pines, E.; Fleming, G.R. J . Am. Chem. SOC.,submitted for publi-
cation. (41) Eaton, D. Tetrahedron 1987, 43, 1551. (42) Hercdia, A.; Reguena, G.; Sanchez, F. G.J . Chem. SOC.,Chem. Commun. 1985, 24, 1814.
(43) Cox, G.S.;Turro, N. J.; Yang, N. C.; Chen, M.J. J. Am. Chem. Soc. 1984, 106, 422. (44) Bronsted, J. N.; Pedersen, K.J. Z . J . Phys. Chem. 1924, 108, 185. (45) Agbaria, R. A.; Uzan, B.; Gill, D. J . Phys. Chem. 1989, 93, 3855. (46) Agmon, N.; Huppert, D.; Masad, A.; Pines, E. J . Phys. Chem. 1991, 95, 10407. (47) Lee, J.; Robinson, G.W.; Webb, S.P.; Phillips, L. A.; Clark, J. H. J . Am. Chem. SOC.1986, 108, 6538.
Resonance Raman Intensity Analysis of the Excited-State Proton Transfer in 2-hydroxy acetophenone Linda A. Peteanu and Richard A. Mathie* Chemistry Department, University of California, Berkeley, California 94720 (Received: April 1, 1992; In Final Form: May 1 1 , 1992)
Resonance Raman spectra have been obtained of 2-hydroxyacetophenone (OHAP) using an excitation wavelength (337 nm) which is resonant with the proton-transfer electronic absorption band. Fourteen modes are found to be significantly enhanced, the most intense of which is the 1324-cm-l symmetric stretch of the substituted benzene ring. No displacement is observed along the hydroxy stretching coordinate in the Franck-Condon region of the excited state. Furthermore, the modes which exhibited significant excited-state displacements were found to be insensitive to the replacement of the exchangeable proton for a deuteron. Absolute Raman intensity analysis was performed to determine the excited-state geometry changes for each of the modes observed in the resonance Raman spectrum as well as the inhomogeneous (470 cm-I hwhm) and Lorentzian homogeneous (101 cm-' hwhm) line widths. This analysis indicates that, immediately following excitation, OHAP evolves rapidly along a large number of skeletal coordinates which surprisingly do not include proton motion.
Introduction Excited-state proton tautomerism was first recognized in methyl salicylate by We1ler.I Since then, numerous other molecules having nearby proton donor and acceptor groups have been shown to tautomerize in the excited state, the signature of that process being emission of strongly Stokes shifted fluorescence following absorption of a UV photon. Subsequent steady-state and timeresolved spectroscopic measurements have elucidated many aspects of the kinetics and mechanism of this important photophysical process in a variety of chemical species.2 One example is a recent femtosecond stimulated emission measurement on 242hydroxypheny1)benzothiazole (HBT) which has shown that, in dry nonpolar solvents, the production of the tautomeric state is exceedingly rapid (170 fs) and proceeds via a nearly barrierless excited-state potential surface.' Another frequently studied proton-transfer molecule, 3-hydroxyflavone,tautomerizes on nearly the same time scale (230 fs) in nonpolar environments, but even more rapidly (
