ATR FT-IR H2O spectra of acidic aqueous solutions. Insights about proton hydration

Proton hydration in aqueous solutions has been recently characterised in our laboratory by means of vibrational spectra of HDO isotopically diluted in H2O [M. Śmiechowski, J. Stangret, J. Chem. Phys. 125 (2006) 204508]. Here, we attempt to study quantitatively H2O spectra of acidic aqueous solutions. In principle, H2O spectra provide more information about the structural state of water molecules, resulting from oscillator couplings in the system, but they are much more difficult in interpretation, when compared with HDO spectra. The spectra of aqueous solutions of monoprotic acids (HCl, HClO4, HPF6) have been measured by Attenuated Total Reflectance (ATR) FT-IR spectroscopy. Spectral data have been analysed in a way that led to removal of the contribution of bulk water, in order to separate the spectra of solute-affected water only. The analysis has been focused on the infinite dilution limit behaviour of the spectrum. Changes induced in the affected spectra by temperature have been studied for HPF6 solutions at 25–45 °C. The stretching vibration fundamental has been found to be primarily affected by counter-anion. Proton-affected H2O spectrum shows the presence of very wide absorption bands in the range, where bulk water shows negligible own absorption, rather than “absorption continua”. They could be adequately resolved into analytical components. These bands have been unaffected by temperature and loosely correlated with the stretching fundamental, as indicated by 2D IR correlation spectra. All spectral effects of the studied acids on H2O in solution have been quantitatively evidenced and discussed. They seem to be in accordance with the main conclusions about proton hydration derived from recent studies of HDO spectra mentioned above.