Increasing contact ion pairing in the supercooled and glassy states of "dilute" aqueous magnesium, calcium, and strontium nitrate solution: implications for biomolecules

Abstract

FTIR spectra of aqueous D 2 O solutions of magnesium (0.5 and 0.05 M), calcium (0.5, 0.2, 0.05, and 0.02 M) and strontium (0.5 M) nitrate in their glassy states at 78 K and of 0.5 M calcium nitrate in supercooled solution from 300 to 235 K are reported and compared with those recorded at 300 K. All four fundamental vibrational regions of the nitrate ion and a combination band could be evaluated by careful subtraction of the D 2 O solvent. Evidence for increasing contact-ion pairing in going from ambient temperature to the glassy state is based on (i) development of a new band in the v 4 band region at 727 cm -1 (Mg) and increasing intensity of the band at ≃740 cm -1 (Ca and Sr), (ii) doubling of bands in the v 1 and v 2 band regions, and (iii) fourth-derivative curves of the v 3 band region and its curve fitting (Ca)