High-Resolution Infrared Spectroscopy and Nuclear Spin Conversion of CH3D in Solid Parahydrogen: Crystal Field Effects in Nuclear Spin Conversion.

The nuclear spin conversion of CH3D isolated in solid parahydrogen (pH2) was investigated by high-resolution Fourier transform infrared (FTIR) spectroscopy. From the analysis of the temporal changes in the CH3D/pH2 rovibrational absorption spectra, the nuclear spin conversion rates associated with rotational relaxation from the J = 1, K = 1 state to the J = 0, K = 0 state were determined over the 1.5-4.3 K temperature range. As-deposited CH3D/pH2 samples contain two different crystal structures allowing the CH3D nuclear spin conversion rates to be measured for two different trapping sites, which revealed that CH3D trapped in hexagonal close-packed (hcp) crystal sites relax more than twice as fast as CH3D isolated in face centered cubic (fcc) crystal sites. The nuclear spin conversion rates of CH3D trapped in single substitution hcp crystal sites increase rapidly above 2.5 K, but the rates were almost temperature independent below 2 K leading to a limiting nonzero conversion rate of k = 2.76(8) × 10-3 min-1 at 1.58(1) K. Comparison of the temperature dependence of the CH3D nuclear spin conversion rate measured here with analogous measurements for CH4 and CD4 trapped in solid pH2 shows that CH3D relaxes with a rate constant intermediate between CH4 and CD4, and the faster relaxation for species containing deuterium atoms can be qualitatively explained by the quadrupole interaction that is absent in all hydrogen containing CH4 isotopomers.