Proton hydration in aqueous solution: Fourier transform infrared studies of HDO spectra

Abstract

This paper attempts to elucidate the number and nature of the hydration spheres around the proton in an aqueous solution. This phenomenon was studied in aqueous solutions of selected acids by means of Fourier transform infrared spectroscopy of semiheavy water (HDO), isotopically diluted in H(2)O. The quantitative version of difference spectrum procedure was applied for the first time to investigate such systems. It allowed removal of bulk water contribution and separation of the spectra of solute-affected HDO. The obtained spectral data were confronted with ab initio calculated structures of small gas-phase and polarizable continuum model (PCM) solvated aqueous clusters, H+(H2O)n, n=2-8, in order to help in establishing the structural and energetic states of the consecutive hydration spheres of the hydrated proton. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. The structure of proton hydration shells outside the first hydration sphere essentially follows the model structure of other hydrated cations, previously revealed by affected HDO spectra. The first hydration sphere complex in diluted aqueous solutions was identified as an asymmetric variant of the regular Zundel cation [The Hydrogen Bond: Recent Developments in Theory and Experiments, edited by P. Schuster, G. Zundel, and C. Sandorfy (North-Holland, Amsterdam, 1976), Vol. II, p. 683], intermediate between the ideal Zundel and Eigen structures [E. Wicke et al., Z. Phys. Chem. Neue Folge 1, 340 (1954)]. Evidence was found for the existence of strong and short hydrogen bonds, with oxygen-oxygen distance derived from the experimental affected spectra equal 2.435 A on average and in the PCM calculations about 2.41-2.44 A. It was also evidenced for the first time that the proton possesses four well-defined hydration spheres, which were characterized in terms of hydrogen bonds' lengths and arrangements. Additionally, an outer hydration layer, shared with the anion, as well as loosely bound water molecules interacting with free electron pairs of the central complex were detected in the affected spectra.