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Title: Microcanonical statistical study of ortho-para conversion in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} at very low energies

Abstract

The ortho-para conversion of H{sub 3}{sup +} and H{sub 2} in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} in interstellar space is possible by scrambling the five protons via (H{sub 5}{sup +}){sup *} complex formation. The product distribution of the ortho-para conversion reaction can be given by ratios of cumulative reaction probabilities (CRP) calculated by microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin, and parity. A statistical method to calculate the state-to-state reaction probabilities for given initial nuclear spin species, rotational states, and collision energies is developed using a simple semiclassical approximation of tunneling and above-barrier reflection. A new calculation method of branching ratios for given total nuclear spins and scrambling mechanisms is also developed. The anisotropic long-range electrostatic interaction potential of H{sub 2} in the Coulomb field of H{sub 3}{sup +} is taken into account using the first-order perturbation theory in forming the complex. The CRPs and the product distribution of the ortho-para conversion reaction at very low energies with reactants in their ground vibronic and lowest rotational states for given initial nuclear spin species are presented as a function of collision energy assuming complete proton scrambling or incomplete protonmore » scrambling. The authors show that the product distribution at very low energies (or very low temperatures) differs substantially from the high energy (or high temperature) limit branching ratios.« less

Authors:
;  [1]
  1. Department of Chemistry, University of Chicago, Chicago, Illinois 60637 and James Franck Institute, University of Chicago, Chicago, Illinois 60637 (United States)
Publication Date:
OSTI Identifier:
20991209
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 126; Journal Issue: 4; Other Information: DOI: 10.1063/1.2430711; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CONVERSION; HYDROGEN; HYDROGEN IONS 3 PLUS; ION-MOLECULE COLLISIONS; PARITY; PERTURBATION THEORY; PROTONS; REACTION KINETICS; ROTATIONAL STATES; SEMICLASSICAL APPROXIMATION; SPIN; STATISTICAL MODELS; TUNNEL EFFECT

Citation Formats

Park, Kisam, and Light, John C. Microcanonical statistical study of ortho-para conversion in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} at very low energies. United States: N. p., 2007. Web. doi:10.1063/1.2430711.
Park, Kisam, & Light, John C. Microcanonical statistical study of ortho-para conversion in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} at very low energies. United States. doi:10.1063/1.2430711.
Park, Kisam, and Light, John C. Sun . "Microcanonical statistical study of ortho-para conversion in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} at very low energies". United States. doi:10.1063/1.2430711.
@article{osti_20991209,
title = {Microcanonical statistical study of ortho-para conversion in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} at very low energies},
author = {Park, Kisam and Light, John C.},
abstractNote = {The ortho-para conversion of H{sub 3}{sup +} and H{sub 2} in the reaction H{sub 3}{sup +}+H{sub 2}{yields}(H{sub 5}{sup +}){sup *}{yields}H{sub 3}{sup +}+H{sub 2} in interstellar space is possible by scrambling the five protons via (H{sub 5}{sup +}){sup *} complex formation. The product distribution of the ortho-para conversion reaction can be given by ratios of cumulative reaction probabilities (CRP) calculated by microcanonical statistical theory with conservation of energy, motional angular momentum, nuclear spin, and parity. A statistical method to calculate the state-to-state reaction probabilities for given initial nuclear spin species, rotational states, and collision energies is developed using a simple semiclassical approximation of tunneling and above-barrier reflection. A new calculation method of branching ratios for given total nuclear spins and scrambling mechanisms is also developed. The anisotropic long-range electrostatic interaction potential of H{sub 2} in the Coulomb field of H{sub 3}{sup +} is taken into account using the first-order perturbation theory in forming the complex. The CRPs and the product distribution of the ortho-para conversion reaction at very low energies with reactants in their ground vibronic and lowest rotational states for given initial nuclear spin species are presented as a function of collision energy assuming complete proton scrambling or incomplete proton scrambling. The authors show that the product distribution at very low energies (or very low temperatures) differs substantially from the high energy (or high temperature) limit branching ratios.},
doi = {10.1063/1.2430711},
journal = {Journal of Chemical Physics},
number = 4,
volume = 126,
place = {United States},
year = {Sun Jan 28 00:00:00 EST 2007},
month = {Sun Jan 28 00:00:00 EST 2007}
}
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  • The chemistry in the diffuse interstellar medium (ISM) initiates the gradual increase of molecular complexity during the life cycle of matter. A key molecule that enables build-up of new molecular bonds and new molecules via proton donation is H{sub 3}{sup +}. Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H{sub 2} and H{sub 3}{sup +} in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H{sub 3}{sup +}, unlike H{sub 2}, seems to be outmore » of thermal equilibrium, contrary to the predictions of modern astrochemical models. We conduct the first time-dependent modeling of the para-fractions of H{sub 2} and H{sub 3}{sup +} in the diffuse ISM and compare our results to a set of line-of-sight observations, including new measurements presented in this study. We isolate a set of key reactions for H{sub 3}{sup +} and find that the destruction of the lowest rotational states of H{sub 3}{sup +} by dissociative recombination largely controls its ortho/para ratio. A plausible agreement with observations cannot be achieved unless a ratio larger than 1:5 for the destruction of (1, 1)- and (1, 0)-states of H{sub 3}{sup +} is assumed. Additionally, an increased cosmic-ray ionization rate to 10{sup –15} s{sup –1} further improves the fit whereas variations of other individual physical parameters, such as density and chemical age, have only a minor effect on the predicted ortho/para ratios. Thus, our study calls for new laboratory measurements of the dissociative recombination rate and branching ratio of the key ion H{sub 3}{sup +} under interstellar conditions.« less
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