Hole-burning spectra of tropolone–(CO2)n (n=1,2) van der Waals complexes and density functional study Original Research Article

The hole-burning, fluorescence excitation, and dispersed fluorescence spectra of jet-cooled tropolone (TRN)–(CO2)n (n=1,2) complexes are measured to investigate the structures of the complexes and the effects of intermolecular interaction on proton tunneling in TRN. The electronic transitions of TRN–(CO2)n (n=1,2) are well separated in the hole-burning spectrum. Only the transitions of one species have been identified for the 1:1 and 1:2 complexes. Structures of TRN–(CO2)n (n=1,2) are optimized by the density functional theory calculations at the B3LYP/cc-pVDZ level. Three local minima have been obtained for both the 1:1 and 1:2 complexes. In the 1:1 complex CO2 is bonded in the molecular plane close to a solvation site, CO (Isomer I), CO⋯H–O (Isomer II), or CO (Isomer III). These complexes are stabilized mainly by the dipole–quadrupole interaction, and the binding energies for these complexes are estimated to be much smaller than those for the hydrogen-bonded complexes. Isomers I and II are plausible candidates for the observed species. The calculations imply that asymmetry of the double-minimum potential well along the tunneling coordinates of TRN–(CO2)1 is large enough to quench proton tunneling. This prediction is consistent with the nonobservation of the tunneling splittings even for the excitation of the tunneling promoting mode ν13(a1) or ν14(a1) of TRN.