LASER-INDUCED FLUORESCENCE EXCITATION SPECTROSCOPY OF JET-COOLED TROPOLONE CARBON-MONOXIDE VAN-DER-WAALS COMPLEXES

The 1:1 and 1:2 van der Waals complexes of tropolone (TRN) and tropolo ne-OD with CO [TRN .(CO)(n) (n = 1, 2)] have been synthesized in a sup ersonic expansion, and their S-1-S-0 laser-induced fluorescence excita tion spectra have been measured. Two distinct isomers of the 1:1 compl ex, TRN . CO(I) and TRN CO(II), with their origins displaced 54.5 and 79.5 cm(-1) respectively to the red of the origin of bare tropolone, h ave been identified. In TRN CO(I), the CO is located over the seven-me mbered ring and is primarily dispersively bound. In TRN CO(II), the CO is located in the plane of the ring and forms a hydrogen-bonded compl ex with the hydroxyl group of the chromophore. TRN (CO), has a spectra l shift which is exactly twice that of the 1:1 TRN CO(I) complex, indi cating that the 1:2 complex has a symmetrical sandwich structure with one CO residing on each side of the chromophore. A large decrease in t he proton tunnelling splitting, from 19.4 cm(-1) in bare tropolone to 3.5 cm(-1) in TRN CO(I) is observed, indicating that CO strongly pertu rbs the reaction coordinate by significantly increasing the barrier he ight in the excited state. Strong coupling of the tunnelling mode to t he in-plane CO torsional mode is suggested to be the likely source of this effect. No tunnelling doublets are observed in TRN . CO(II), sugg esting that hydrogen bonding quenches excited state proton transfer. T he results of parallel studies on tropolone-OD are completely consiste nt with this model. Empirical calculations of the geometries and bindi ng energies of the complexes using Lennard-Jones 6-12 atom-atom pair p otentials support the interpretations.