Rotational excitations of methane molecules in porous media

Abstract

Rotational tunneling of CH 4 molecules in microporous and mesoporous silicate structures has been studied by inelastic neutron scattering. The rotational excitation energies of the spherical quantum rotor CH 4 are a sensitive probe of the strength and symmetry of local potentials. Measurements of methane molecules isolated in micropores of dodecasil 3C and of solid methane condensates in mesoporous silicagel and in the mesoporous channels of MCM-41 show broad distributions of rotational energies. A simple model which is based on a distribution of local potentials was applied to the data. In dodecasil 3C time-resolved measurements did not show spin conversion behaviour in a measuring time of 15 h. Even in pores with diameters as large as 100 Å no tendency towards the ordering of the CH 4 phase II was found.