Nuclear Pure Quadrupole Relaxation and Its Temperature Dependence in Solids

Abstract

The Cu63 and Cl35 pure quadrupole relaxation times have been measured by rf pulse techniques in cuprous oxide, paradichlorobenzene, 2,2-dichloropropane, t-butyl chloride, and methylene chloride at temperatures from 77°K to room temperature. The Cu63 T1 data agree with the ionic lattice model. The Cl35 T1 data for paradichlorobenzene agree with a torsional molecular-oscillator model. For 2,2-dichloropropane the Cl35 T1 values agree with a model based on field-gradient fluctuations produced by reorienting CH3 groups. The more complicated T1 temperature dependences observed in t-butyl chloride and methylene chloride appear to be the result of multiple thermal motions. The same can be said of our limited data on the Cl35 relaxation in 1,2-dichloroethane. No discernible difference was found for the relaxation times of the two Cl35 resonance lines in methyl chloroform at 77°K. In general, the available proton T1 and linewidth data correlate well with our chlorine results; this agreement is discussed. The chlorine spin phase memory times are governed by the local magnetic fields except when decreased by spin—lattice lifetime broadening. The inverse linewidth parameters exhibit broadening by a distribution of field gradients. The fadeout of the Cl35 quadrupole resonance with increasing temperature in 2,2-dichloropropane is the result of T1 broadening, while in t-butyl chloride and methylene chloride it is produced by phase transitions. The Bayer model for spin—lattice relaxation by molecular torsional oscillations is treated in some detail, including a new approach which dispenses with some of his simplifying assumptions. A brief analysis is given of T1 for the case in which field-gradient fluctuations are produced by random, large-angle reorientations of groups near to the relaxed nucleus. The theory is presented for a null method of measuring the pure quadrupole T1. The method, which was used in most of our measurements, is similar to the Carr—Purcell 180°—90° pulse method for measuring nuclear magnetic T1's.