Propyne confinement in solid parahydrogen: Methyl rotation and site effects

Samples of propyne trapped in solid parahydrogen show multiple peak structures in their infrared spectra. These structures are attributed to molecules in two distinct kinds of matrix sites. The most intense lines are assigned to propyne molecules executing a slightly hindered methyl rotation, as was extensively studied in our earlier publication from our two groups, and the other set of peaks to propyne trapped in a secondary site where the methyl rotation is quenched and replaced by methyl torsion within the matrix site. The assignment of the various rovibrational transitions is made possible by the observation of nuclear spin conversion (NSC) within the methyl group at long timescales. The NSC rate depends on the site and is much slower in the sites where the methyl rotation is quenched.