Communication: H-atom reactivity as a function of temperature in solid parahydrogen: The H + N2O reaction

Abstract

We present low temperature kinetic measurements for the H + N2O association reaction in solid parahydrogen (pH2) at liquid helium temperatures (1-5 K). We synthesize (15)N2(18)O doped pH2 solids via rapid vapor deposition onto an optical substrate attached to the cold tip of a liquid helium bath cryostat. We then subject the solids to short duration 193 nm irradiations to generate H-atoms produced as byproducts of the in situ N2O photodissociation, and monitor the subsequent reaction kinetics using rapid scan FTIR. For reactions initiated in solid pH2 at 4.3 K we observe little to no reaction; however, if we then slowly reduce the temperature of the solid we observe an abrupt onset to the H + N2O → cis-HNNO reaction at temperatures below 2.4 K. This abrupt change in the reaction kinetics is fully reversible as the temperature of the solid pH2 is repeatedly cycled. We speculate that the observed non-Arrhenius behavior (negative activation energy) is related to the stability of the pre-reactive complex between the H-atom and (15)N2(18)O reagents.

Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling

“…The barrier to forming CH 3 O product via TS-H is narrower, with an imaginary frequency of v̅ = 2958 cm –1 , compared with that forming CH 2 OH via TS-M ( v̅ = 1420 cm –1 ). Similar behavior has been observed for other reactions in the solid state. , …”

“…Similar behavior has been observed for other reactions in the solid state. 23,24 There have been several theoretical studies of the OH + methanol reaction, 25,26 most focusing on behavior above 200 K, but also at lower T's relevant to the Laval nozzle experiments. 25 The open source master equation solver MESMER (Master Equation Solver for Multi Energy well Reactions) 27 was used to calculate individual rate coefficients, k(E), as a function of energy within the complex both for redissociation to reactants and tunneling to form products.…”

Quantum Diffusion-Controlled Chemistry: Reactions of Atomic Hydrogen with Nitric Oxide in Solid Parahydrogen

“…For example, in the case of the H + N 2 O → cis-HNNO reaction we observed a very strange temperature dependence. 30 This reaction only begins to occur at temperatures below ∼2.4 K, and at higher temperatures the reaction does not occur. We therefore wanted to quantify the temperature dependence of the H + NO reaction for comparison with these other H atom reaction studies.…”

“…Therefore, we decided to reinvestigate this system at two different temperatures, 1.8 and 4.3 K, respectively, to measure the effect of temperature on the reaction rate. Another reason to revisit this system is the anomalous results we have recently measured − for other H atom reactions in solid pH 2 . For example, in the case of the H + N 2 O → cis -HNNO reaction we observed a very strange temperature dependence .…”

“…Another reason to revisit this system is the anomalous results we have recently measured − for other H atom reactions in solid pH 2 . For example, in the case of the H + N 2 O → cis -HNNO reaction we observed a very strange temperature dependence . This reaction only begins to occur at temperatures below ∼2.4 K, and at higher temperatures the reaction does not occur.…”