Guest Molecules In Clathrates Research Articles

Librational heat capacity of fullerenes in the Einstein model

In search of low-energy excitations in rigid molecular crystals, the available heat capacity, Cp, data of 85%C60–15%C70 (wt %) mixture at temperatures in the 2–30 K range have been analyzed. It is found that its Cp/T3 against T plot has an asymmetric peak centered at 4.5 K, which has the features of both the Boson peak and of the peak attributed to librations of guest molecules in clathrates. Appearance of a peak in the Cp/T3 against T plot is a characteristic of the Einstein equation for random phase atomic motions. Most of the Cp of fullerenes at T<10 K is attributed to random phase librations of C60 and C70 molecules at their crystal sites.

Low-energy excitations of guest molecules in clathrates and the Boson peak

The heat capacity, Cp, of seven ice clathrates, four β-quinol clathrates and one squarate clathrate containing rare gas atoms or molecules as guests in their cage-like structures at T<30 K has been analysed, and compared against that of empty β-quinol-clathrate, two channel-type thiourea clathrates containing polar molecules and four canonical glasses. Ethylene oxide–ice clathrate shows a peak in Cp/T3 at ca. 7.5 K, and argon–β-quinol clathrate at ca. 16 K. Other clathrates show an approach toward a Cp/T3 peak at 5<T<12 K. On the basis of their (i) Cp, (ii) Raman scattering, (iii) far infra-red absorption, and (iv) density of vibrational states data, it is shown that low-energy (2–8 meV) excitations are intrinsic to clathrates. Their features are remarkably similar to those of the Boson peak, which has been interpreted in terms of translational vibrations of 20–100 atoms group or of strongly cohesive nanometer size zones separated by softer zones in the heterogeneous glass structures. Since only translational (rattling) and/or rotational (torsional oscillations or librations) motions of atoms or molecules can occur in clathrate cages and channels, and nanometer size clusters cannot form, the similarity noted above indicates that the microstructural origin of the Boson peak in glasses may need reconsideration. Interaction between the guest and cage molecules explains satisfactorily neither the low-energy (2–8 meV) features of clathrates, nor its low-thermal conductivity relative to ice.

Nitrogen and oxygen guest molecules in clathrate hydrates: different sites revealed by Raman spectroscopy

N2 and O2 hydrates (clathrates) from a polar ice core (Vostok) were analysed by high-sensitivity Raman microspectrometry. Three new lines in the region of the stretching modes of N2 and O2 guest molecules were observed and were interpreted by considering both small (512) and large (51264) cages in type II clathrates, isotopic effects and the possibility of double occupancy of the large cages. The isotopic effect for N2 and O2 guest molecules in clathrates is demonstrated. © 1997 John Wiley & Sons, Ltd.

Nitrogen and oxygen guest molecules in clathrate hydrates: different sites revealed by Raman spectroscopy

N2 and O2 hydrates (clathrates) from a polar ice core (Vostok) were analysed by high-sensitivity Raman microspectrometry. Three new lines in the region of the stretching modes of N2 and O2 guest molecules were observed and were interpreted by considering both small (512) and large (51264) cages in type II clathrates, isotopic effects and the possibility of double occupancy of the large cages. The isotopic effect for N2 and O2 guest molecules in clathrates is demonstrated. © 1997 John Wiley & Sons, Ltd.

Computational investigation of the molecular orientation and dynamics of CCl 3Z guest molecules in clathrates of Dianin's compound and its mercapto analogue

Non-bonded potential energies have been calculated for CCl 3Z guest molecules (Z = H, F, Cl, Br, I, CH 3, CF 3, CN or NO 2) in the trigonal cavities of the clathrates formed by Dianin's compound 4- p-hydroxyphenyl-2,2,4-trimethylchroman ( 1) and its mercapto analogue 4- p-mercaptophenyl-2,2,4-trimethylchroman ( 2). The volume ratio of guest molecule to cavity, the packing density, predominates in the order-disorder effect and in the molecular reorientation of these molecules in the cavities. Consequently, a disordered structure with a high degree of translational-rotational freedom is obtained when the volume of Z is small or the cavity relatively large. Intramolecular reorientations are discussed. ▪

35Cl nuclear quadrupole resonance spectra of paradichlorobenzene as a guest molecule in clathrates formed by the werner complexes M (II)(L 4)X 2

The 35Cl nuclear quadrupole resonance (NQR) spectra of paradichlorobenzene included in the Werner complexes M (II) (L 4)X 2 (L=4-picoline, 4-ethylpyridine, 4-vinylpyridine or 4-acetylpyridine; M (II)=Mn, Fe, Co or Ni; X − =SCN − or SeCN −) are reported. In all these complexes only one 35Cl resonance is exhibited, thus showing that there is only one type of inclusion site and that the symmetry of the site preserves the equivalence of the two chlorine atoms in PDCB. The temperature dependence of the 35Cl NQR frequency provides a measure of the degree of mobility of the guest molecule and shows that this mobility increases in the order Ni> Co> Fe> Mn and in the order 4-vinylpyridine>4-ethylpyridine>4-methylpyridine. In general, a change of anion from SCN − to SeCN − also increases the guest mobility. An independent check on the space available to the guest molecule has been obtained by measuring the unit-cell dimensions for a number of these complexes.

Motion of Guest Molecules in Clathrates

AbstractFirst the term clathrates is explained and illustrated more precisely on the three examples Ice Clathrate, Urea Clathrate, and Quinol Clathrate. Then a review of experimental results, which exist in the literature in the following domains, is given: Thermodynamic studies, dielectric relaxation, far infrared, infrared, and Raman spectroscopic studies, nuclear magnetic and nuclear quadrupole resonance, electron paramagnetic resonance, magnetic susceptibility, and neutron scattering. Emphasis is laid on the informations of the dynamical behaviour. The results given by several methods for the same inclusion and the results given by one method for different systems are compared. Finally a model is developed, which describes the movements of the guest molecules, assuming in a first step, that there is no interaction between molecules of the neighbouring cages. After this the forces between molecules in neighbouring cages are taken into account.

Motion of the guest molecules in the Hofmann-type clathrates

Proton resonance spectra for Hofmann-type clathrates were measured at temperatures ranging from 80 to 296°K. The spectra of enclathrated benzene molecules showed a motional narrowing. A gradual change in the line width with temperature was observed for both aniline and pyrrole clathrates. Thiophene clathrate, however, exhibited no change in the line width. From these results, the motion of the guest molecules in clathrates was estimated. The strength of the “host-guest interaction” of benzene clathrate was compared with those of other benzene complexes.