Adsorption sites and rotational tunneling of methyl groups in cubic I methyl fluoride water clathrate

Abstract

Neutron spectroscopy in the meV and meV regime and quasielastic scattering is applied to characterize the dynamics of methyl groups of methyl fluoride guest molecules in cubic I CH 3 F-water clathrate. Only above T B 60 K quasielastic spectra are unaffected by quantum effects. They are well described by two Lorentzians representing the CH 3 F species in the small and large cages of the structure. The intensities show that both cages are completely filled. The linear broadenings with temperature follow the model of rotational diffusion. Two clearly separated tunneling bands were observed at T ¼ 4.2 K and are also assigned to the two types of water cages. Disorder of the environment (H-bonds) is reflected in the shape of the bands. For the less hindered species housing the large cages the tunneling band can be quantitatively converted into a potential distribution function within the model of single particle rotation. Transitions to excited rotational states show the dominance of a sixfold potential term V 6 ¼ 13 meV modified by a weak threefold term distributed around a characteristic value V 3 ¼ 0.9 meV. The potential distribution of V 3 influences the barrier for classical reorientation only weakly in agreement with the results from quasielastic data. Adsorption sites with the guest molecules oriented towards a hydrogen bond along one of twelve local twofold axes of the cage are proposed. Such sites are consistent with the sixfold rotational potential and earlier results from methyl iodide clathrate. Rotation-translation coupling as an alternative dynamical process is excluded.

Critical Size for Guest Molecules to Occupy Dodecahedral Cage of Clathrate Hydrates

“…Neutron diffraction analysis revealed that the CH 3 F hydrate formed SI. 18 Then, we considered that the CH 3 F molecules were enclathrated in both the 5 12 and 5 12 6 2 cages. The I L /I S values for the 2860 and 2960 cm -1 peaks were I L /I S ∼ 3.5 ((0.2) and 3.1 ((0.6) for, respectively.…”

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

“…Although the hydrogen atomso ft hese guestsa re not visible in these structures by means of PXRD,t he analytical resultsf or the rigid-body constraints suggestt hat the hydrogen atom of CH 3 F, CH 2 F 2 , and CHF 3 molecules point to the directions facing the hexagonal planes. Quantum rotational dynamics of methyl groups in CH 3 Fh ydratew ereo bserved by using neutrons pectroscopy under lower temperature conditions, below about 50 K. [32] The non-isotropic distribution of CH 3 Fw ithin the 5 12 …”

Les clathrates hydrates

“…De plus, ces molécules invitées possèdent un axe de symétrie C 3 (la liaison C-X), leur conférant un caractère rotationnel 1-D et rendant la description mathématique (voir section précédente) beaucoup plus simple que celle du méthane (3-D). L'étude de la dynamique rotationnelle par effet tunnel est disponible dans la littérature pour des structures de type I formées avec les molécules de bromo-méthane [49], de fluoro-méthane [50] et de chloro-méthane [51]. Comme dans le cas du méthane, l'analyse des spectres est rendue complexe par le fait que les deux types de cages de la structure de type I sont remplies.…”