Van der Waals Complexes of Tropolone with Carbon Dioxide

Abstract

van der Waals complexes of tropolone (TRN) with carbon dioxide have been synthesized by expanding mixtures of TRN and CO2 in He in a supersonic free-jet expansion, and have been examined by laser-induced fluorescence excitation spectroscopy and by ab initio structural calculations. At higher partial pressures of CO2 large clusters of TRN(CO2)n are formed in which TRN excited to S1 remains fluorescent. At sufficiently low partial pressures of CO2, well-resolved blue-shifted features in the spectra due to TRN(CO2) and TRN(CO2)2 may be identified. Their microscopic solvent shifts and the effects of solvation on the proton tunneling doublets and vibrational frequencies of the chromophore have been measured. The experimental results and calculations all suggest that the CO 2 binds to TRN in the 1:1 complex in a fashion which is similar to common hydrogen-bonding addends. The CO2 lies in the plane of the TRN ring and interacts with the keto oxygen and the hydroxyl hydrogen in such a way that the intramolecular hydrogen bond of TRN is partially disrupted, lengthening the R O- -- H distance and opening the H-O-C angle. Consistent with this interaction, the proton tunneling rate is reduced so that the tunneling doublets can no longer be resolved, and the frequency of the out-of-plane wagging vibration of the two oxygen atoms in the chromophore increases. The microscopic solvatochromic shifts of these and other hydrogen-bonded complexes are well-correlated with the calculated binding energies of the addends to the chromophore and with their proton affinities. The latter correlation suggests that CO2 acts as a weak proton acceptor in a hydrogen-bonding-like interaction with TRN.