Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

Abstract

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.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen

“…The term “as-deposited” refers to p-H 2 samples right after deposition, which have not been exposed to sample temperatures above ∼2.5 K. As documented in numerous studies, ,,− the RVD method produces polycrystalline as-deposited p-H 2 solids that contain metastable face-centered-cubic (fcc) and hexagonal-close-packed (hcp) crystal structures. Warming the sample to 4.0 K for several minutes irreversibly anneals the solid to a nearly pure hcp crystal structure and results in a high degree of alignment of the c -axis of the hcp crystallites with the substrate surface normal. , This annealing process is quantified − in this study by measuring the fraction of hcp crystal structures, X hcp = N hcp /( N hcp + N fcc ), using the zero phonon U 0 (0) solid p-H 2 single transition near 1167.1 cm –1 . Only p-H 2 molecules in hcp crystal sites that lack a center of inversion can contribute to the integrated intensity of the U 0 (0) transition such that the integrated intensity can be used to monitor X hcp using the following equation, X hcp = 2.303 c V 0 α̃ l ν̃ 0 N A ∫ A 10 false( ν̃ false) d ν̃ where c is the speed of light in vacuo , α∼ is the U 0 (0) absorption coefficient (51.0(2.5) × 10 –17 cm 3 s –1 molecule –1 ), N A is Avogadro’s constant, and ν∼ 0 is the peak frequency of the transition in cm –1 .…”

“…times greater than a H atom and, thus, O atom tunneling should be roughly 4 times less probable, and (iii) the rate of quantum diffusion should be inversely proportional to the guest concentration 48. While we did not test the effect of the O 2 concentration on the measured kinetics thoroughly in this study, the one experiment (L02) on an as-deposited sample with an initial O 2 concentration of 157(3) ppm gave a rate constant k = 22.0(9) cm 3 mol −1 s −1 compared to the average rate constant for the four other as-deposited samples (k = 9.3(10) cm 3 mol −1 s −1 ) with an average initial O 2 concentration of 399(16) ppm.…”

Hydrogen‐atom tunneling reactions in solid para‐hydrogen and their applications to astrochemistry

“…[38] Fushitani and Momose reported the first investigation on the H-atom quantum diffusion reaction H + NO ! HNO T A B L E 1 Comparison of the de Boer parameters for various crystals [26] Crystal σ(Å) ϵ (K) Λ Intermolecular potential parameters from Ref. [27].…”

“…From several precursors that produced H 2 O upon photolysis in situ, [26,51,53,55,56,58] two intense transient lines (denoted S1 and S2) on either side of line R(0) of ν 3 atype transition (1 01 0 00 ) of H 2 O appeared; their intensities were linearly correlated [58] . They were most intense immediately after photolysis and decayed when photolysis terminated.…”

“…This result is in contrast to some systems studied by the same group that showed atypical temperature behavior. For example, reaction H + HCOOH → H 2 + HOCO occurs only below ~2.7 K, but ceases above 2.7 K. [ ] Similarly, reaction H + N 2 O → cis ‐HNNO/ trans ‐HNNO [ 26,55 ] proceeds only below 2.4 K. Reaction H + CH 3 OH also showed this unique non‐Arrhenius characteristic [ 56 ] . The reason for this distinct temperature behavior is still unclear, but the details of H‐atom diffusion are expected to play a critical role.…”