Observation of the Influence of Centrifugal Distortion of the Methane Molecule on Nuclear Spin Relaxation in the Gas

Abstract

The spin–lattice relaxation time T1 was measured in gaseous CH4 as a function of density at room temperature between 0.006 and 7.0 amagats. T1 was found to pass through a minimum near 0.04 amagats in agreement with previous, less precise measurements. The spin–rotation interaction is the dominant relaxation mechanism in gaseous CH4. Since the spin–rotation constants are accurately known for CH4, the relaxation experiments provide a check on the theory of spin–lattice relaxation for spherical top molecules. In the conventional theory, it is assumed that the correlation function of the spin–rotation interaction is a simple exponential function of time. These experiments show that this assumption is not true for CH4 gas. The observed fine structure in the plot of relaxation rate versus density is attributed to the influence of centrifugal distortion of the CH4 molecule, which removes the degeneracy of rotational states having the same value of the quantum number J by an amount somewhat greater than the nuclear Larmor frequency of 30 MHz.

Proton spin relaxation in dilute methane gas: A symmetrized theory and its experimental verification

“…The spectrometer used is an improved version of that described previously (Beckmann 1971;Beckmann et al 1972). The RF transmitter is similar to that discussed by Hardy (1966).…”

“…The outputs of both the voltmeter and the time-interval plugin are recorded digitally on paper tape using a custom interface constructed by Technical Associates Limited (Vancouver). While the signal-to-noise is only moderately improved over that obtained by Beckmann et al (1972), the new data acquisition system allows systematic computer reduction of large amounts of data, thereby reducing the final errors in R1.…”

“…Only one change was made in the room temperature cell described previously (Beckmann et al 1972); the glass cylinder surrounding the coil was removed. Thus in both sample cells, we had the coil immersed in the sample gas rather than having the gas confined to the interior of the coil.…”

“…3D, E, and F for R1ρ against ρ. The data presented for 295 K include the measurements of Beckmann et al (1972). (123 of the 293 room temperature points are new.…”

“…By going to lower densities at room temperature, Dong and Bloom (1970) were able to observe the characteristic maximum in the density dependence of R1 and were able to interpret their results successfully without taking into account the splitting of the J levels by centrifugal distortion effects. In a more detailed room temperature study with improved techniques over a wider range in density, Beckmann et al (1972) showed that transitions between those levels with the same J but different distortion energies make a significant contribution to R1. These effects were introduced into their analysis in a phenomenological way.…”