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.