ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
In this paper we further explore the applicability of a vibrational T2 process based on the extended-exponential modulation model [Rothschild, Perrot, and Guillaume, J. Chem. Phys. 87, 7293 (1987)] to Raman correlation data of concentrated aqueous solutions of LiSCN and KSCN [Kato¯, Mol. Phys. 48, 1119 (1983); Kato¯ and Takenaka, Mol. Phys. 46, 257 (1982)]. In general, the values of dispersion parameter α in the modulation function exp[−(t/τ)α], obtained from the fit of the theory to the isotropic correlation data of the CN oscillator, predict the prevalence of interrelated, collective dynamic processes in the medium that are the cause of the instantaneous oscillator transition frequency shifts (motional narrowing). In particular we predict, from the observed concentration dependence of α, strong short-time (fraction to several ps) cation–water–anion interactions that, in the more concentrated LiSCN–H2O systems at 303 K, are above a site percolation threshold with a value of α∼0.3 (close to that found in glasses). The expectation value of t, 〈t〉=τΓ(1+1/α), becomes critical near a concentration of 5 mol/l and shows a pronounced Vogel–Fulcher-type temperature dependence (T0=250 K) in the 10 mol/l LiSCN–H2O system over a range 0.45–76 ps. However, since α approaches its limiting value=1 at the highest temperature reported (353 K), the large-cluster cation–water–anion distributions in LiSCN–H2O must be rather tenuous. In contrast, the characteristics of α and of 〈t〉 for the KSCN–H2O systems agree with the relatively weak cation–water forces; the (inverse) concentration dependence of α is linear, its temperature dependence is flat, and the Vogel–Fulcher-type temperature behavior of 〈t〉 for the 10 mol/l solution stretches merely from 0.7 to 1.4 ps.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.455414
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