Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen.

Parahydrogen (H) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CHCCH) isolated in solid H is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm region. Detailed analyses of a variety of parallel (Δ = 0) and perpendicular (Δ=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but rotation of the methyl group around the symmetry axis still persists. However, this single-axis rotation is significantly hindered for propyne trapped in solid H such that the energies of the rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual - and -propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective ″ constant of propyne.