Proton Magnetic Resonance of the CH3 Group. V. Temperature Dependence of T1 in Several Molecular Crystals

Abstract

The proton T1 has been observed by rf pulse techniques in acetonitrile, neopentane, t-butyl chloride, 2,2-dichloropropane, and methyl chloroform from their melting points to temperatures about —150°C. Activation energies of 5 to 6 kcal/mole were obtained for self diffusion in the high-temperature solid phase of neopentane, t-butyl chloride, and 2,2-dichloropropane. Activation energies ranging from 1 to 4.5 kcal/mole were obtained for molecular tumbling and from 3 to 4 kcal/mole for CH3 group reorientation in the series of tetrasubstituted methanes, neopentane to methyl chloroform. The effects of intermolecular forces on CH3 group reorientation are shown by the activation energy of 2.1 kcal/mole found for acetonitrile, which has no internal barrier. The various results confirm that a tunneling mechanism is plausible for the CH3 group reorientations. Other results include: the Bloch decay after a 90° pulse in solid methyl chloroform has a ``slow beat'' which is the Fourier transform of the triplet line shape found in steady-state experiments. The dependence of T1 upon resonance frequency in the low-temperature phase of methyl chloroform exhibits systematic effects attributable to proton-chlorine spin exchange induced by reorientations of the CCl3 group. The T1 is constant in the super-cooled high-temperature solid phase of methyl chloroform. There is an anomalous increase in the T1 at the low-temperature transition in t-butyl chloride, indicating a higher molecular mobility during the phase transition than in either stable solid phase. A 2:1 mixture of CH2DCCl3 and CH3CCl3 gave proton T1 values only 1.3 to 1.6 times as long as those in normal CH3CCl3, whereas the tunneling mechanism suggests a 20-fold increase. Spin exchange between protons in neighboring CH2DCCl3 and CH3CCl3 molecules accounts for the results and it is proposed that spin exchange may be investigated directly by observing T1 in appropriate isotopic mixtures of known composition.