Proton spin relaxation of butenes and butane in CaNaA zeolites

Using nmr pulse techniques the temperature dependence of proton spin relaxation times T₁ and T₂ of n-butene and butane molecules adsorbed on CaNaA zeolites with different content of Ca⁺⁺ ions has been investigated. The observed diminution of correlation times with rising degree of Ca⁺⁺ exchange can only be explained if translational motions, i.e. jumps between the supercages, dominate the proton spin relaxation of the adsorbates. Peculiarities in the microdynamical behaviour of the various n-butenes are in accordance with the model of electrostatic interactions between molecular dipole moments and electric fields in A zeolites as proposed by Tempère and Imelik in order to explain the stereoselectivity of the isomerization of n-butenes in A zeolites. Applying Torrey's well-known theory of nmr relaxation as dominated by translational jumps with arbitrary lengths, the mean time between two subsequent jumps has been estimated. In combination with measurements of self-diffusion coefficients by Kärger and Renner, these values lead to mean jump lengths which are reasonable compared with the distance of two neighbouring large zeolitic cavities. In order to interpret the methyl reorientation which dominates the longitudinal proton spin relaxation of the adsorbed hydrocarbons at lower temperatures, a model for calculating the intermolecular contribution to the relaxation rate has been discussed.