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

Abstract

Aqueous D 2 O 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 D 2 O 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 ν 1 band region centered at high frequency of the band from “free” perchlorate and complete loss of degeneracy in the ν 3 and ν 4 band region. On the basis of curve resolution of the ν 1 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 C 2 v 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 ν 1 and ν 3 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.