2-pyridone:: The role of out-of-plane vibrations on the S1⇆S0 spectra and S1 state reactivity

The S-1 <-> S-0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 0(0)(0)+98 cm(-1) and the bands at +218 and +252 cm(-1) are identified as overtones of the S-1 state out-of-plane vibrations nu(1)' and nu(2)', as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further nu(1)' and nu(2)' overtones and combinations, reproducing similar to 80% of the vibronic bands up to 600 cm(-1) above the 0(0)(0) band. The fluorescence spectra excited at the electronic origins and the nu(1)' and nu(2)' out-of-plane overtone levels confirm these assignments. The S-1 nonplanar minima and S-1 <- S-0 out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 0(0)(0) and 0(0)(0)+98 cm(-1) upper states by Held [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S-1 state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200 cm(-1) excess vibrational energy, while the in-plane vibrations nu(5)', nu(8)', and nu(9)' have longer lifetimes, although they are above or interspersed with the "dark" out-of-plane states. This is interpreted in terms of an S-1' state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S-0' state fundamentals, overtones of the S-0 state nu(1)" and nu(2)" out-of-plane vibrations, and > 30 other overtones and combination bands. The B3LYP/6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Delta(rms)=1.3%. (c) 2006 American Institute of Physics.