No abstract

FTIR spectra of aqueous DzO solutions of magnesium ( O S and 0.05 M), calcium (0.5,0.2,0.05, and 0.02 M) and strontium ( O S 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 D20 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 v4 band region at 727 cm-* (Mg) and increasing intensity of the band at ~7 4 0 cm-I (Ca and Sr), (ii) doubling of bands in the V I and v2 band regions, and (iii) fourth-derivative curves of the v j band region and its curve fitting (Ca). The latter also indicated, in addition to "free" nitrate and the contact-ion pair, the presence of a third species which is tentatively assigned to ion triplets. Relative intensities of bands due to "free" nitrate and contact-ion pair were determined by curve fitting the v4, v2, and v 1 band region.For the glassy 0.5 M nitrate solutions, the relative area of the band assigned to the contact-ion pair is Sr > Ca z Mg for the v4 band region and Sr > Ca > Mg for the V I band region. Evidence for increasing ion pairing is seen already in supercooled solution at 1260 K. We suggest that water's structural changes toward a more open, fully hydrogen-bonded tetrahedral network in its supercooled and glassy state are at the bottom of increasing contact-ion pairing with decreasing temperature. Implications for biomolecules stabilized by contact-ion pairs are discussed with respect to cold denaturation of proteins, cryoenzymology, and cryofixation.

Curve fitting of highly overlapping bands can be improved by comparison of the fourth derivative of the experimental composite band profile with that of the sum of the curve-fitted component bands. Optimization is achieved with fixed values of one band parameter, aiming thereby for optimal correspondence between the fourth-derivative curves. FT-IR spectra of nitrate's v1 band region in ≈1 M glassy Ca(NO3)2 solution, and of its v2 band region in 10 M LiNO3 solution, in ≈0.5 M glassy Mg(NO3)2 solution and in 5 M Ca(NO3)2 solution, all with D2O as solvent, were used. As a general rule, reliable curve fitting of two overlapping bands of unknown band shape is possible only when separation of their peak maxima is larger than their average full width at half-height (FWHH). We show here that by comparison of fourth-derivative curves many more overlapped composite bands can be reliably resolved into their component bands, and, for the favorable case of 10 M LiNO3 solution, separation of two bands by only about a third of their average FWHH is sufficient for reliable curve fitting. This method requires data with very high signal-to-noise ratios. Improved curve fitting of highly overlapping bands is demonstrated for both a sum and a product of Gaussian and Lorentzian peak shapes, and a sort of recipe is given for both types of curve fitting. Synthetic spectra of two overlapping bands, where the degree of overlap and their relative intensity were varied in a systematic way, are given together with their fourth-derivative curves. This information will be of help for interpreting fourth-derivative curves of experimental highly overlapping two-band systems.

Raman, ultraviolet resonance Raman (UVRR) and far-IR spectra are reported for the mercury-cysteamine complex, Hg(SCH(2)CH(2)NH(2))(2). Band assignments are made for Hg(SCH(2)CH(2)NH(2))(2), and also for [Hg(SBu(t))(3)](-) and [Hg(SMe)(3)](-) on the basis of ab initio calculations with the effective core potential approximation and also on the basis of comparison with vibrational data of corresponding thiols. The calculations show that geometry-optimized [Hg(SBu(t))(3)](-) and [Hg(SMe)(3)](-) have virtually the same Hg-S bond lengths, but very different nu(s) HgS frequencies, 196 and 268 cm(-)(1), in good agreement with the experimental data. The exceptionally low HgS frequency for [Hg(SBu(t))(3)](-) compared to [Hg(SMe)(3)](-) and to the Hg-MerR protein results from kinematic interactions of the Hg-S stretching and S-C-C bending coordinates when all three substituents at C(alpha) are carbon atoms. For Hg(SCH(2)CH(2)NH(2))(2), the HgS stretching coordinate is distributed over three modes, at 339, 273, and 217 cm(-)(1), all of which exhibit UVRR enhancement. The other contributors to these modes are angle bending and torsional coordinates of the chelate rings. Involvement of the CCN bending coordinates is supported by observed and calculated frequency shifts in D(2)O. The excitation profiles track the main UV absorption band, associated with S-->Hg charge transfer. Enhancement is attributable to the weakening of the Hg-S bonds in the excited state, and probably to changes in the SCC bond angle. Also enhanced, albeit weakly, is the nu(CS) mode at 658 cm(-)(1), reflecting C-S bond shortening in the excited state. The mingling of metal-sulfur and internal ligand coordinates is reminiscent of the mingling seen in RR spectra of type 1 Cu proteins. In both cases the phenomenon may be associated with elevated torsional contributions associated with the rigidity of the ligands.