EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN JET-COOLED 3-HYDROXYFLAVONE - DEUTERATION STUDIES, VIBRONIC DOUBLE-RESONANCE EXPERIMENTS, AND SEMIEMPIRICAL (AM1) CALCULATIONS OF POTENTIAL-ENERGY SURFACES

3-Hydroxyflavone (3-HF) and 3-deuteroxyflavone (3-DF) were examined by fluorescence excitation spectroscopy in a supersonic free jet. Compar ed to 3-HF, vibronic bands of 3-DF are significantly narrower. Substit ution of H by D also appears to split vibronic bands into at least thr ee bands leading to a congested spectrum. Fluorescence-dip double-reso nance spectroscopy revealed that the complicated spectrum of 3-DF cons ists of at least three independent partial spectra which are superimpo sed. The vibrational pattern of every partial spectrum is identical to that of 3-HF, but partial spectra differ in their spectral position. Semiempirical calculations (MNDO/AM1 with limited CI) were used to gen erate ground- and singlet excited-state potential energy surfaces as a function of phenyl and hydroxyl torsional angles. The deuteration eff ects suggest the existence of potential energy barriers to low-frequen cy hydroxyl hydrogen motion, with barrier height on the order of the v ibrational zero-point energy.