Gas phase spectroscopic studies on hydroquinone–water complex

Abstract

In this paper, we examine the effect of an electron donating group in the para position, on the relative stabilization of the S1 state, in the hydroquinone–water complex. The 1 ∶ 1 complex of hydroquinone and water (HQ–W1) formed in the supersonic jet was studied using REMPI spectroscopy. Hydroquinone exists as cis and trans isomers in the gas phase and therefore the 1 ∶ 1 water complex is expected to have at least four different conformations, viz., H-bond donor and acceptor forms for each of the two isomers. However, only two HQ–W1 conformers were identified using hole-burning spectroscopy. The stable structures of all possible conformers in the ground electronic (S0) state were calculated ab initio at the Hartree–Fock (HF) and density functional theory (DFT, with a hybrid functional B3LYP) levels using the 6-31G** basis set. The experimentally observed conformers were assigned as the trans-donor HQ–W1 and cis-donor HQ–W1 conformers based on the spectroscopic results and the calculations. The optimized structures and the vibronic spectra for these isomers in the excited electronic state (S1) were calculated at the configuration interaction with singlet excitation (CIS) level using the 6-31G** basis set. A correlation was drawn between the red shift in the band origin of the 1 ∶ 1 water complexes of various phenols with the relative changes in the negative charge density on the phenolic oxygen atom upon excitation to the S1 state.