Toward Understanding the Dissociation of Weak Acids in Water: 1. Using IR Spectroscopy to Identify Proton-Shared Hydrogen-Bonded Ion-Pair Intermediates†.

Cryogenic conditions favor the formation of ion-pair dissociation intermediates in amorphous mixtures of HF and H2O, making possible their characterization by means of infrared spectroscopy. The experimental infrared spectra show a structurally rich “continuous” absorption ranging from 1000 to 3400 cm−1, which, in principle, contains important information regarding the microscopic structure of the aforementioned dissociation intermediates. Herein, we demonstrate that this microscopic information can be extracted by comparing and contrasting experimental spectra with those obtained by means of carefully designed first-principles molecular dynamics calculations. Very good, systematic agreement between theoretical and experimental spectra can be obtained for HF/H2O mixtures of various compositions, revealing the presence of proton-shared, dissociation intermediates Fδ−···H···δ+OH2. The existence of similar proton-shared, hydrogen-bonded intermediates of ionization, that are stable in solution, but not in the gas phase, has been previously suggested by other groups, using, among other techniques, low temperature NMR data and aprotic, dipolar, solvents. Our investigation reveals that similar structures are also stable in aqueous solutions of HF. We discuss some of the implications of the present findings as far as the mechanism of dissociation of weak acids is concerned. [ABSTRACT FROM AUTHOR]