Laser-induced fluorescence excitation spectroscopy of jet-cooled tropolone-carbon monoxide van der Waals complexes

Abstract

The 1:1 and 1:2 van der Waals complexes of tropolone (TRN) and tropolone-OD with CO [TRN · (CO n ( n = 1, 2)] have been synthesized in a supersonic expansion, and their S 1−S 0 laser-induced fluorescence excitation spectra have been measured. Two distinct isomers of the 1:1 complex, TRN · CO(I) and TRN and CO(II), with their origins displaced 54.5 and 79.5 cm −1 respectively to the red of the origin of bare tropolone, have been identified. In TRN · CO(I), the CO is located over the seven-membered 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 complex with the hydroxyl group of the chromophore. TRN · (CO) 2 has a spectral shift which is exactly twice that of the 1:1 TRN · CO(I) complex, indicating that the 1:2 complex has a symmetrical sandwich structure with one CO residing on each side of the chromophore. A large decrease in the 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 perturbs the reaction coordinate by significantly increasing the barrier height in the excited state. Strong coupling of the tunnelling mode to the in-plane CO torsional mode is suggested to be the likely source of this effect. No tunnelling doublets are observed in TRN · CO(II), suggesting that hydrogen bonding quenches excited state proton transfer. The results of parallel studies on tropolone-OD are completely consistent with this model. Empirical calculations of the geometries and binding energies of the complexes using Lennard-Jones 6–12 atom-atom pair potentials support the interpretations.