A Nuclear Spin Relaxation Study of the Spin–Rotation Interaction in Symmetric Top Molecules

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Abstract
The spin–lattice relaxation times T1 of 1H, 19F, and 31P nuclei were measured in gaseous samples of BF3, CHF3, CH3F, PH3, and NH3 at room temperature for densities from 0.03 to 10 amagat. In several cases the behavior of T1 at the lowest densities snowed deviations from the linear variation characteristic of the extreme narrowing region. The spin–rotation interaction provides the dominant relaxation mechanism in all cases. The data are analyzed on the basis of the assumption that the collision modulated spin–rotation interaction may be described by a single correlation function which is a simple exponential function of time. Values of an effective spin–rotation constant and a cross section for molecular reorientation are obtained for each gas. The results obtained are compared with those available from other types of experiments. This comparison indicates that the theory for spin–lattice relaxation in dilute gases of symmetric top molecules needs to be carefully reassessed.

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