ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
We describe the temperature dependence of the inhomogeneously broadened CN Raman profile I(ω) at ωc ∼2230 cm−1 of the title compounds (n=1, 6, 8, 9, 11, 12) in their isotropic liquid phase and solutions (CHCl3, CCl4) by simulating the oscillator amplitude correlation function by a vibrational equilibrium renewal process in terms of random fluctuations of the oscillator transition frequency ω(t)=ωc +ω1(t) about its central value ωc. To this effect, the autocorrelation function of the frequency shift ω1(t) is expressed as a probability density function (PDF) Fˆ(t) of recurrence times of the stochastic motional narrowing events in the local environment of the CN oscillators. System-related physical meaning and satisfactory data fit is obtained if Fˆ(t) is understood as an expansion in terms of parallel, independent exponential relaxation processes with characteristic times τ that are distributed by a PDF ρα(τ)=〈τ〉h(τ)/τ, where α is the dispersion parameter of the extended exponential and 〈τ〉 the expectation of τ. Width and ranges of h(τ) show strong molecule–molecule clustering, possibly indicating a trend with alkyl chain length. At temperatures just above the mesophase–liquid-phase transition, the range of the prevalent relaxation times τ in the local environment of the CN oscillators is of the order of 1–4 ps. Only at temperatures near 570 K or by high dilution in the solvents are the inter- and intracluster forces sufficiently diminished to approximate those of ordinary fluids. We consider our method to give a realistic description of the dynamics of types of macroscopically isotropic fluids where, nevertheless, the shape, size, and polarity of their molecules lead to a degree of aggregation that weakens the identity and the influence of constituent members. The temporary structure of the macroscopically isotropic fluids in the liquid-crystal systems is best understood by admitting a significant presence of randomly distributed local regions of dynamic nematicity, causing temperature-dependent relaxation pathways over 10–50 A(ring) distances.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.461007
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