Anomalous Contact-Ion Pairing in the Glassy States of “Dilute” Aqueous Lithium and Sodium Perchlorate Solution

Abstract

Aqueous D2O solutions of lithium (0.5, 1, and 3 M) and sodium (1 and 3 M) perchlorate have been studied by FT-IR spectroscopy in their glassy states at 80 K, and their IR spectra compared with those of 1.0 M D2O solutions recorded from 300 to 235 K. Evidence for strongly increasing contact-ion pairing in going from ambient temperature to the glassy state is based on the development of a second band in the ν1 band region centered at high frequency of the band from “free” perchlorate and complete loss of degeneracy in the ν3 and ν4 band region. On the basis of curve resolution of the ν1 band region, increasing contact-ion pairing is much more pronounced for sodium than for lithium perchlorate. The spectral features of contact-ion paired perchlorate are consistent with at most C2v symmetry with bidentate binding. Anomalous increase in contact-ion pairing on hyperquenching into the glassy state is attributed to water's density maximum and the anomalies of supercooled water and “dilute” aqueous solutions. Spectral features of ion-pairing are weakly observable already at 300 K in the ν1 and ν3 band region and become more pronounced on cooling to 235 K. Significance of the results is discussed with respect to (i) differences in local geometry of contact-ion pairs in aprotic donor solvents, (ii) vapor-deposited alkali metal perchlorates isolated in a water matrix, (iii) thermodynamic continuity of states between vapor-deposited amorphous water, hyperquenched glassy water and normal supercooled water, and (iv) cryofixation of polyelectrolyte-type biomolecules such as nucleic acid sodium salts.