No abstract

No abstract

Infrared spectroscopy is used to investigate the process of molecular hydrogen ortho-to-para (o/p) conversion in solid hydrogen samples doped with small concentrations (10–50 ppm) of hydrogen atoms (H-atoms) as an impurity. The H-atoms are generated using the in situ 193 nm photolysis of N2O dopant molecules. For hydrogen crystals with relatively low initial ortho-H2 fractions (Xo ≤ 0.03), the o/p conversion kinetics at temperatures of 1.8 and 4.0 K follow kinetic equations developed previously for H-atom catalyzed o/p conversion. The measured atom catalyzed o/p conversion kinetics indicates the H-atoms are mobile under these conditions in agreement with previous ESR measurements. It has been proposed that the H-atoms diffuse by a quantum tunneling mechanism that is described as chemical diffusion. Detailed fits of the measured o/p conversion kinetic data allow the initial H-atom concentration after photolysis to be extracted assuming literature values for the H-atom recombination rate constant (H + H → H2). The measured o/p conversion kinetics show the observed o/p conversion is much less than expected based on the previously measured H-atom recombination rate constant and thus suggest that the H-atoms do not diffuse randomly through the crystal but rather diffuse preferentially in regions of high para-hydrogen content. The estimated H-atom concentrations from this study are consistent with previous ESR measurements but in conflict with kinetic studies of H-atom reactions with various dopants such as N2O.

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N2O, which induces a two-step tunneling reaction, H + N2O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N2O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N2O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Parahydrogen (pH 2) 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 (CH 3 CCH) isolated in solid pH 2 is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm-1 region. Detailed analyses of a variety of parallel (K=0) and perpendicular (K=1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C 3 symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH 2 such that the energies of the K 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 ortho-and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.