Solvation dependence of valence electronic states of water diluted in organic solvents probed by soft X-ray spectroscopy

Abstract

Knowledge of hydrogen bonds is important in a wide range of scientific fields. However, information on the electronic state of hydrogen bonded molecules in liquids and solutions is still limited. We applied X-ray emission and absorption spectroscopy to observe the electronic state around oxygen of water and investigate the solvent dependence of occupied and unoccupied valence electronic states of water in acetonitrile, 3-methylpyridine, and ethylenediamine at room temperature under atmospheric pressure. Based on the direct comparison of the spectra, the pronounced variation of the unoccupied 2b₂ state of water in acetonitrile and 3-methylpyridine was assigned to the difference of the dipole–dipole interaction and the donor hydrogen bond. For ethylenediamine solution, an enhancement of the peak structure in the post-edge region of X-ray absorption spectra and an evident shoulder structure on the higher energy side of the occupied 1b₁ state in X-ray emission spectra were observed. Although ethylenediamine has two amino groups that can form the hydrogen bonds as a proton donor, the obtained results indicate that the amino groups of ethylenediamine also act as a proton acceptor in the hydrogen bond between water molecules. A systematic peak intensity variation of the pre-edge peak corresponding to the lowest unoccupied molecular orbital, 4a₁, was also found through the comparison of three organic solvents. Since the peak intensity tends to decrease with increasing number of hydrogen bonds, the results can be interpreted by the covalency of hydrogen bonds.