Dielectric study on pure and KOH-doped tetrahydrofuran clathrate hydrates

Complex dielectric permittivities of pure and KOH-doped (x = 1.8 x 10^–4) tetrahydrofuran clathrate hydrates were measured in the temperature range 20–260 K and in the frequency range 20 Hz-1 MHz. The relaxation time of the water reorientational motion was found to decrease drastically as a result of the doping; e.g., the relaxation time of the doped sample was 10^–9 times shorter than that of the pure sample at 70 K. The activation enthalpy of the motion was reduced to 7.4 kJ mol^–1. On cooling the crystal, the value of decreased suddenly at the 62 K phase transition to the ₂ value of the pure sample and at the same time disappeared. No dispersion effect due to the guest reorientation was observed below the transition. These data indicate that both the host and guest molecules become ordered or, at least, change their mobility drastically. In the pure sample, a relaxation phenomenon of ₀₂ was found around the glass transition region. The relaxation times agreed well with those derived from the enthalpy of relaxation in a calorimetric study.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Thermodynamic studies of clathrate hydrates

Thermodynamic studies of clathrate hydrates, mainly of structures I and II, are considered in this review which is based on 147 references. There are two main subjects. The first is the host lattice energy and the guest-host interaction energy, both of these quantities being related to the enthalpy of dissociation and composition of the hydrates. The second subject concerns orientational ordering phenomena occurring in both host and guest, as reflected in the low temperature heat capacity. The classical theoretical treatment of clathrate formation has been reconsidered on the basis of recent experimental results. Particular emphasis has been given to orientational ordering since this topic is undoubtedly central to clarifying the nature of clathrate hydrates.Ausgehend von 147 Literaturangaben wurden in diesem Review thermodynamische Untersuchungen von Klathrathydraten hauptsächlich der Struktur I und II betrachtet. Es gibt zwei Hauptaugenmerke. Als erstes die Wirtsgitterenergie und die Gast-Wirt-Wechselwirkungsenergie, beide bezogen auf die Dissoziationsenthalpie und die Bildungsenthalpie der Hydrate. Das zweite Hauptaugenmerk betrifft Orientierungs-Konditionierungserscheinungen sowohl in Wirt als auch Gast, wie in den Wärmekapazitäten bei niedrigen Temperaturen widergespiegelt wird. Auf der Basis jüngster experimenteller Ergebnisse wurde die klassische theoretische Betrachtung über die Bildung von Klathraten überprüft. Der Orientierung-Konditionierung wurde besonderer Nachdruck verliehen, da dies zweifellos eine entscheidende Rolle bei der Klärung der Natur der Klathrathydrate spielt. 147 I II. . «» « — », . «» « », . . , .

---

Contribution No. 155 from the Chemical Thermodynamics Laboratory.     

Local Subexponentiality and Self-decomposability

The class of exponential tilts of convolution equivalent distributions is determined. As a corollary, the local subexponentiality of one-sided infinitely divisible distributions is characterized. It is applied to the subexponentiality of the densities of a self-decomposable distribution and its Lévy measure. Bondesson’s conjecture on the density of the Lévy measure of a lognormal distribution is solved as an example. Results of Denisov et al. on the distributions of random sums are extended to the two-sided case. Finally, the local subexponentiality of the distribution of the supremum of a random walk is characterized.     

Isotope-dependent crystalline phases at ambient temperature: Spectroscopic and calorimetric evidence for a deuteration-induced phase transition at 320 K in α-DCrO2

The heat capacities of HCrO₂ and DCrO₂ have been measured at temperatures between 5 and 360 K. A lambda anomaly occurred at 320 K for DCrO₂ but not for HCrO₂. Infrared spectra were recorded at temperatures between 5 and 405 K. The spectrum of DCrO₂ changed at 320 K with respect to the peak wave-number and splitting of some of the vibrations, while those of HCrO₂ depended smoothly on temperature. These experimental results are discussed in terms of a deuteration-induced phase transition.     

Recurrence-Transience for Self-similar Additive Processes Associated with Stable Distributions

For any stable distribution on the line, recurrence-transience of the selfsimilar additive process {X _t ,t0} with (X ₁)= is determined. Comparison with the stable Lévy process {Y _t ,t0} with (Y ₁)= is made: if is not strictly stable, then {Y _t } is transient but {X _t } is recurrent except the obviously transient case of monotone sample functions.     

Two Non-closure Properties on the Class of Subexponential Densities

Relations between subexponential densities and locally subexponential distributions are discussed. It is shown that the class of subexponential densities is neither closed under convolution roots nor closed under asymptotic equivalence. A remark is given on the closure under convolution roots for the class of convolution equivalent distributions.     

Union and symmetry preserving homomorphisms of semigroups of closed relations

The symbol β_X denotes the semigroup of all binary relations on a nonempty set X under composition which is defined by α_oβ={(x,y)} ∈ X×X: (x,z) ∈ β and (z,y) ∈ α for some z∈X} for all α,β ∈ β_x . In a recent paper [1, Theorem 3, p. 310], A. H. Clifford and D. D. Miller initiated a study of the endomorphisms of β_X when they completely determined those which preserve unions and take symmetric relations into symmetric relations. The purpose here is to place the theorem of Clifford and Miller in a topological setting and to discuss some of the problems which then arise naturally. The full results will appear in [9]. Partial financial support from Australian National University and the research foundation of State University of New York is gratefully acknowledged.     

Incoherent quasielastic neutron scattering study on the polymorphism of tristearin: dynamical properties of hydrocarbon chains

Dynamical properties of acyl chains in the three polymorphic phases alpha, beta', and beta of tristearin [C(3)H(5)(OCOC(17)H(35))3] have been studied by means of incoherent quasielastic neutron scattering (IQNS) using selectively deuterated samples. The mean square displacement of hydrogen atoms, , was estimated from the scattering vector Q dependence of the elastic scattering component under the harmonic approximation. It was shown that the temperature dependence of was significantly different between the three phases. There was no marked difference in between these phases up to 193 K, and the value increased linearly with temperature. Although the beta phase remained linear up to 293 K, the alpha and beta' phases started to show a nonlinear increase around 200 K, suggesting an anharmonic nature of molecular motions. The alpha phase exhibited the most conspicuous temperature dependence. These characteristics were ascribable to the difference in the lateral packing of acyl chains. Compared with the beta phase which has a tightly packed T// subcell, the beta' and alpha phases have looser O perpendicular and H subcells, respectively. The molecular motion in the alpha phase was analyzed using the model of uniaxial rotational diffusion in a onefold cosine potential. It has been clarified that the rotational fluctuation about the chain axis in the alpha phase is rather restricted compared with that in the rotator phase of n-alkanes.