A theoretical study of complex formation, isotope effects, and energy partitioning in the O(1D) + H2 (D2, HD) reaction

Michael S. Fitzcharles, George C Schatz

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Abstract

This paper presents a detailed study of the O(1D) + H2 → OH + H reaction, with an emphasis on how isotope effects probe the mechanism of complex formation and product-state energy partitioning. Most of our results were generated by using quasi-classical trajectories, but we also include comparisons using microcanonical statistical theory, phase space theory, and the adiabatic capture/infinite order sudden approximation model. Most of our calculations have used either the recent potential surface of Murrell and Carter (MC) or the one developed by Schinke and Lester (SL) (their surface no. 1). Two other surfaces from Schinke and Lester were also considered in selected calculations. Our most important results concern the intramolecular isotope ratio for O + HD, where we find that the OD/OH ratio depends strongly on potential surface, with a value of about 1.1 on the MC surface and 1.8 on the SL surfaces. The MC result is in good agreement with a recent measurement. A detailed analysis of the reaction mechanism is performed, and we find that the MC isotope ratio is determined by a combination of (1) long-range potential orientation effects that favor collisions at collinear OHD and ODH geometries and (2) formation of a loose complex in which the first-formed bond becomes the product diatomic. The Schinke and Lester isotope ratio arises from a very different long-range orientation effect (favoring perpendicular collisions) which leads to formation of a tight complex in which a collision between H and D leads to dissociation into OD + H predominantly. We have also studied product vibrational and rotational distributions for all four isotopic reactions. The MC distributions are generally quite similar to those from the SL surfaces, and the comparison of these with recently measured distributions for both OH and OD products is generally very good. A simple kinematic model is developed for the rotational distributions which indicates that these distributions arise from repulsion between the two hydrogen atoms when the H2O complex breaks up. Characterization of the vibrational distributions is more subtle, with no simple correlation seen between the shape of the vibrational distribution (i.e., inverted or not) and either the presence of collinear entrance channel barriers or the fraction of reaction that occurs by an insertion mechanism.

Original languageEnglish
Pages (from-to)3634-3644
Number of pages11
JournalJournal of Physical Chemistry
Volume90
Issue number16
Publication statusPublished - 1986

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Isotopes
isotope effect
isotope ratios
Surface potential
products
energy
collisions
Electron energy levels
Hydrogen
Kinematics
Trajectories
entrances
Atoms
insertion
hydrogen atoms
Geometry
kinematics
trajectories
dissociation
probes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

A theoretical study of complex formation, isotope effects, and energy partitioning in the O(1D) + H2 (D2, HD) reaction. / Fitzcharles, Michael S.; Schatz, George C.

In: Journal of Physical Chemistry, Vol. 90, No. 16, 1986, p. 3634-3644.

Research output: Contribution to journalArticle

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AB - This paper presents a detailed study of the O(1D) + H2 → OH + H reaction, with an emphasis on how isotope effects probe the mechanism of complex formation and product-state energy partitioning. Most of our results were generated by using quasi-classical trajectories, but we also include comparisons using microcanonical statistical theory, phase space theory, and the adiabatic capture/infinite order sudden approximation model. Most of our calculations have used either the recent potential surface of Murrell and Carter (MC) or the one developed by Schinke and Lester (SL) (their surface no. 1). Two other surfaces from Schinke and Lester were also considered in selected calculations. Our most important results concern the intramolecular isotope ratio for O + HD, where we find that the OD/OH ratio depends strongly on potential surface, with a value of about 1.1 on the MC surface and 1.8 on the SL surfaces. The MC result is in good agreement with a recent measurement. A detailed analysis of the reaction mechanism is performed, and we find that the MC isotope ratio is determined by a combination of (1) long-range potential orientation effects that favor collisions at collinear OHD and ODH geometries and (2) formation of a loose complex in which the first-formed bond becomes the product diatomic. The Schinke and Lester isotope ratio arises from a very different long-range orientation effect (favoring perpendicular collisions) which leads to formation of a tight complex in which a collision between H and D leads to dissociation into OD + H predominantly. We have also studied product vibrational and rotational distributions for all four isotopic reactions. The MC distributions are generally quite similar to those from the SL surfaces, and the comparison of these with recently measured distributions for both OH and OD products is generally very good. A simple kinematic model is developed for the rotational distributions which indicates that these distributions arise from repulsion between the two hydrogen atoms when the H2O complex breaks up. Characterization of the vibrational distributions is more subtle, with no simple correlation seen between the shape of the vibrational distribution (i.e., inverted or not) and either the presence of collinear entrance channel barriers or the fraction of reaction that occurs by an insertion mechanism.

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