Proton Magnetic Resonance of the CH3 Group. I. Investigation of Six Tetrasubstituted Methanes

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Abstract
Measurements have been made of the proton magnetic resonance absorption in methyl chloroform, 2,2‐dinitropropane, 2‐chloro−2‐nitropropane, t‐butyl chloride, t‐butyl bromide, and neopentane, in the solid state. In all the materials at −196°C, except neopentane, the absorption corresponds to CH3 groups effectively fixed in the lattice. The narrowing of the absorption lines with rising temperature in methyl chloroform, 2,2‐dinitropropane, and 2‐chloro−2‐nitropropane is attributable to the increasing rate of motion of the CH3 groups about their C3 symmetry axis. In methyl chloroform at higher temperatures, and in t‐butyl chloride, t‐butyl bromide, and neopentane an additional motion of the CH3 groups is required to explain the observed absorption, and possible motions are suggested. In all the polar materials the line becomes very narrow at the thermal transition at which the dielectric constant rises to a high value, marking the onset of over‐all molecular rotation. In neopentane the line is narrower than expected for the rigid lattice at −196°C. With rising temperature, the line narrows to a very small value between the thermal transition and melting points. Theoretical analyses are given of the effects of external broadening upon the line shapes of fixed and rotating CH3 systems, and the results are used in interpreting the experimental data.