Subscriber access provided by TULANE UNIVERSITY
Article 3
Spin-Forbidden Channels in Reactions of Unsaturated Hydrocarbons with O(P) Pavel Pokhilko, Robin Shannon, David Glowacki, Hai Wang, and Anna I. Krylov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10225 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Spin-Forbidden Channels in Reactions of Unsaturated Hydrocarbons with O(3P) Pavel Pokhilkoa , Robin Shannonb , David Glowackib , Hai Wangc , and Anna I. Krylov∗a a
Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, U.S.A. b c
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
Department of Mechanical Engineering, Stanford University, Stanford, California 94305-3032, U.S.A. ∗
E-mail:
[email protected] December 19, 2018 Abstract The electronic structure of four prototypical Cvetanovi´c diradicals, species derived by addition of O(3 P) to unsaturated compounds, is investigated by high-level electronic structure calculations and kinetics modeling. The main focus of this study is on the electronic factors controlling the rate of intersystem crossing (ISC): minimal energy crossing points (MECPs) and spin-orbit couplings (SOCs). The calculations illuminate significant differences in the electronic structure of ethylene- and acetylene-derived compounds and explain the effect of methylation. The computed MECP heights and SOCs reveal different mechanisms of ISC in ethylene- and acetylene-derived species, thus explaining variations in the observed branching ratios between singlet and triplet products and a puzzling effect of methyl substitution. In the ethylene- and propylene-derived species, the MECP is very low and the rate is controlled by variations of SOC, whereas in the acetylene- and propyne-derived species the MECP is high and the changes in the ISC rate due to methyl substitutions are driven by the variations of MECP heights.
1
Introduction
Among various reaction products, triplet oxygen atoms reacting with unsaturated hydrocarbons (RH) can produce Cvetanovi´c diradicals,1, 2 species in which O(3 P) is added to a double or triple bond. Because oxygen is a triplet, the diradical is formed in the triplet state. It can then undergo series of reactions on the triplet potential energy surface (PES) as well as intersystem crossing (ISC) to the singlet manifold. The reactions of Cvetanovi´c diradicals are important in hydrocarbon combustion, and, to an extent, in atmospheric and interstellar chemistry.1–5 For example, the reaction of ethylene with O(3 P) can lead to effective secondary chain-branching and thus an enhanced fuel oxidation rate. Although numerous studies have investigated the role of Cvetanovi´c diradicals and ISC in the reactions of O(3 P) with unsaturated hydrocarbons,3, 4, 6–15 the key details of ISC are still unresolved. One particularly interesting question 1
ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 24
is what aspects of the electronic structure of the Cvetanovi´c diradicals derived from different unsaturated hydrocarbons control the rate of ISC. Fig. 1 shows the yields of singlet reaction products in the reaction of O(3 P) with ethylene,16 acetylene,17 propene,18 and propyne.19 In these four unsaturated hydrocarbon species, the branching ratios between the singlet and triplet pathways vary from 10% to 84%. The efficiency of ISC in alkenes and alkynes is quite different: while in ethylene the branching ratio for the singlet products is above 50%, in acetylene it is less than 10%. Most intriguing are the variations due to substituting one of the hydrogens by the methyl group: in alkenes, this substitution results in a moderate drop in the yield of singlet products, whereas in alkynes the branching ratio increases eightfold. The goal of the present study is to characterize the electronic structure of the prototypical Cvetanovi´c diradicals derived by O(3 P) addition to ethene, propene, acetylene, and propyne, with an emphasis on factors relevant to ISC. While the quantitative determination of the yield of the reaction products formed via ISC to the singlet state requires full multi-well modeling of the reaction20, 21 at the conditions that match the available experiments, the rate of the actual ISC step is determined by two quantities15, 22–25 — the spin-orbit coupling constant (SOCC) and the location of minimal-energy crossing point (MECP). Generally, larger SOCs and lower MECPs lead to faster ISC rates, however, the interplay between these two parameters in specific molecules can lead to distinctly different mechanisms. Consequently, the origin of the variations in the observed branching ratios in homologically similar compounds could be attributed to different factors. For example, in species with very low MECPs, the variations in ISC rates are due to SOCCs, whereas in systems with high MECPs, the variations in SOCC values become less important. Concluding'Remarks' Concluding'Remarks ' ' ' Concluding'Remarks Concluding'Remarks
a Ethene (>50%) b d a b b c(40%)c Allene d Acetylene Allene (90%) Propyne (81%) (50%) Propene (40%)c a (>50%) b Propene Acetylene (
