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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 closepacked (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. 

The vibrational dynamics of diborane have been extensively studied both theoretically and experimentally ever since the bridge structure of diborane was established in the 1950s. Numerous infrared and several Raman spectroscopic studies have followed in the ensuing years at ever increasing levels of spectral resolution. In parallel, ab initio computations of the underlying potential energy surface have progressed as well as the methods to calculate the anharmonic vibration dynamics beyond the double harmonic approximation. Nevertheless, even 70 years after the bridge structure of diborane was established, there are still significant discrepancies between experiment and theory for the fundamental vibrational frequencies of diborane. In this work we use para-hydrogen (pH2) matrix isolation infrared spectroscopy to characterize six fundamental vibrations of B2H6 and B2D6 and compare them with results from configuration-selective vibrational configuration interaction theory. The calculated frequencies and intensities are in very good agreement with the pH2 matrix isolation spectra, even several combination bands are well reproduced. We believe that the reason discrepancies have existed for so long is related to the large amount of anharmonicity that is associated with the bridge BH stretching modes. However, the calculated frequencies and intensities reported here for the vibrational modes of all three boron isotopologues of B2H6 and B2D6 are within ± 2.00 cm− 1 and ± 1.44 cm− 1, respectively, of the experimental frequencies and therefore a refined vibrational assignment of diborane has been achieved.