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 <i>J</i> = 1, <i>K</i> = 1 state to the <i>J</i> = 0, <i>K</i> = 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 <i>k</i> = 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.