Carbon-13 spin relaxation of butanol molecules adsorbed on the surface of NaY-type zeolite

Abstract

Using molecules partially enriched with carbon-13 nuclei longitudinal carbon-13 relaxation times and Overhauser factors of butanol molecules adsorbed on zeolites of type NaY were measured as a function of temperature between 220 and 420 K. Transverse relaxation times were derived from the linewidths at lower temperatures and by using Hahn's spin-echo technique above 270 K. The temperature dependence of the relaxation times and of the Overhauser factor can be explained in terms of a purely dipolar coupling between protons and carbons, with the model of a log-normal distribution of correlation times. Numerical values for the longitudinal and transverse carbon-13 relaxation times were calculated for different distribution parameters β and plotted as a function of log ( ω I τ cm). Experimental values for a sample containing 4.4 molecules per large cavity of the NaY-type zeolite can be interpreted if one takes a value of β = 3, a mean correlation time τ cm = 4 × 10 −9 sec (at 286 K), and an activation energy for τ cm of 19.5 kJ/mole. Physical models for explaining the distribution of correlation times are discussed.