FLUORESCENCE EXCITATION AND HOLE-BURNING SPECTRA OF JET-COOLED TROPOLONE-M (M = N-2, CO) VAN-DER-WAALS COMPLEXES - STRUCTURES AND PROTON TUNNELING IN THE S-1 STATE

Proton tunneling in the S-1 state of jet-cooled tropolone-(N-2)(n) (n = 1, 2) and tropolone-(CO)(1) van der Weals complexes is investigated by measuring the hole-burning and fluorescence excitation spectra in t he S-1-S-0 region. The hole-burning spectra enabled us to separate thr ee overlapping absorption systems due to transitions between low-waven umber and high-wavenumber tunneling doublet components of tropolone-(N -2)(1) and those of tropolone-(N-2)(2). The 0-0 tunneling doublet spli tting of tropolone-(N-2)(1) has been confirmed to be 9.7 cm(-1). It ha s been suggested that the magnitude of the tunneling splitting depends on the excited intermolecular vibrational level. In addition to the s andwich isomer I for tropolone-(N-2)(2) observed previously, a second isomer [II] has been identified. A much smaller microscopic red shift (-57.9 cm(-1)) than the corresponding value (-129.0 cm(-1)) for isomer I indicates that two N-2 molecules are on the same side of the tropol one ring in isomer Il. No tunneling splitting has been observed for tr opolone-(N-2)(2)[II]. The hole-burning spectrum of tropolone-(CO)(1) i ndicates that only one species is observed in the excitation spectrum, although the existence of two isomers was reported previously (Chem. Phys. 1996, 213, 397). The vibronic bands of tropolone-(CO)(1) show no tunneling splitting. The intensity of the origin band of tropolone-(C O)(1) is considerably weaker than vibronic bands, suggesting that the equilibrium structure in the S-1 state is substantially different from that in the S-0 state. The decreases in the tunneling splittings of t ropolone-(N-2)(n) (n = 1, 2) and tropolone-(CO)(1) are attributed to t he coupling of the intermolecular vibrations with intramolecular vibra tions, which may significantly increase the height of the potential en ergy barrier and distort the potential energy surface in the S-1 state , leading to the decreased tunneling splitting. This coupling is very strong when the adduct is bonded close to the O ... H-O moiety.