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.

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One of the main objectives in the study of weakly bound complexes is to provide a quantitative description of the (ro)vibrational dynamics which can be dominated by nuclear quantum effects. For example, water and carbon monoxide form a weakly bound complex where the H 2 O moiety can undergo a large-amplitude tunneling motion within the complex that exchanges the free and bound hydrogen atoms in the intermolecular bond. The exchange symmetry of identical particles entangles the spin and spatial quantum states of H 2 O such that in the ground vibrational state, the symmetric tunneling state A correlates with para-H 2 O (I=0), while the antisymmetric tunneling state B correlates with ortho-H 2 O (I=1). The gas phase water-CO complex has been extensively studied by microwave ab and IR spectroscopy cde and when compared with full-dimensional quantum bound state calculations on a nine-dimensional potential energy surface, the agreement is very good. f We have completed analogous IR studies of the tunneling splittings of the water-CO complex when the complex is isolated in a parahydrogen quantum solid. We can estimate the tunneling splittings in the ground and excited (CO stretch, water stretch, and water bend) vibrational states to see how the tunneling motion is perturbed by the presence of the quantum solid. Furthermore, we can examine the nuclear spin conversion kinetics between the two tunneling levels in the ground vibrational state by rapidly changing the temperature of the sample. Nuclear spin conversion has not been reported in the previous gas phase studies and thus these are the first results for this water-CO complex.

In the late 1960s Andreev and Lifshitz predicted that at sufficiently low temperatures defects in quantum crystals such as solid parahydrogen should move freely through the crystal possessing the property of superfluidity. a The hydrogen atom (H-atom) is an ideal candidate for such a defect owing to its small mass and neutral charge. In 2013 our group published a communication b on the kinetics of the H + N 2 O reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the in situ photodissociation of N 2 O and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze N 2 O doped parahydrogen solids with a short duration of 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample after photolysis we observe an abrupt onset to the reaction at temperatures below 2.4 K. This change in the reaction kinetics is fully reversible with temperature. We have subsequently improved our experimental apparatus such that we can record the sample temperature with millisecond time resolution while we measure the reaction kinetics using FTIR spectroscopy. We have now performed a number of additional kinetic experiments at constant temperatures of 1.5 K, 4.0 K, and intermediate temperatures within the range from 1.5 to 4.0 K. These measurements have shown that the reaction yield changes dramatically over this temperature range, but the kinetic rate coefficients do not change significantly. The remarkable change in the reaction kinetics with temperature is not as abrupt as originally thought, but now has been reproduced under a variety of conditions. This strange behavior is intimately linked to the motion and reactivity of H-atoms in solid parahydrogen and the most recent experiments and analysis will be presented.

In 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles. a The term "quantum crystal" was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal. b The main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communication c on the kinetics of the H + N 2 O reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the in situ photodissociation of N 2 O and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze N 2 O doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + N 2 O reaction shows an inverse dependence on the N 2 O concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogen d and more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions.