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

Abstract The hole-burning, fluorescence excitation, and dispersed fluorescence spectra of jet-cooled tropolone (TRN)–(CO 2 ) 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–(CO 2 ) 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–(CO 2 ) 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 CO 2 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–(CO 2 ) 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 ν 13 (a 1 ) or ν 14 (a 1 ) of TRN.