Trans-2-butene Ozonolysis: Mechanism and Atmospheric Implications
Matthew J. Carlson and Keith T. Kuwata Department of Chemistry, Macalester College, Saint Paul, MN 55105 Introduction Research Goals and Methods Results: Decomposition and Isomerization of Syn and An important atmospheric reaction is ozonolysis of alkenes. There is evidence •Using Gaussian 03, optimize the geometry of each of the first order saddle points and Anti Acetaldehyde Oxide minima of Reactions 1-4 with B3LYP/6-31+G(d,p). Refine the geometries using that these reactions produce hydroxyl radical (·OH), a major species involved Because of the exothermicity of the formation of the primary ozonide, the syn 1,2 QCISD/6-31G(d) and again using QCISD/MG3. From the QCISD/MG3 optimized in the oxidation of atmospheric hydrocarbons . The cycloaddition of ozone and anti acetaldehyde oxide molecules that are formed from its decomposition geometries and QCISD/6-31G(d) frequencies of the structures with less than 15 atoms to trans-2-butene leads to a primary ozonide. become chemically activated. The relatively small size of the alkene allows for and the QCISD/6-31G(d) optimized geometries and frequencies of the structures with some of these molecules to have enough energy to undergo unimolecular 15 atoms, correct the relative energies with the MCG3 composite method. reactions. •With the MCG3 relative energies of each step of the Reactions 1 and 2, run MultiWell simulations were run to determine the yield of each of the products MultiWell simulations to determine the yield of syn acetaldehyde oxide, anti of these reactions as well as the yield of collisionally stabilized syn and anti Previous studies3,4,5 indicate that the primary ozonide then decomposes to acetaldehyde oxide, the primary ozonide, trans-2-butene, and ozone. Run a separate acetaldhyde oxide. It was found that there was not enough energy to overcome form a carbonyl and a carbonyl oxide (Criegee Intermediate) product. In this set of MultiWell simulations on each syn acetaldehyde oxide and anti acetaldehyde the reaction barrier for isomerization, neither isomer generated the other case, the products are acetaldehyde and syn- and anti-acetaldehyde oxide. oxide to determine the yield of each product of Reaction 3, as well as the yield of syn through a unimolecular reaction at any pressure. and anti acetaldehyde oxide. Run 105 trials for each simulation at eleven pressures ranging from 1 Torr to 760 Torr. The amount of stabilized anti acetaldehyde oxide increased with increasing pressure. The dioxirane was the major product and very little OH was produced.
Results: Ozonolysis of Trans-2-butene The amount of stabilized syn acetaldehyde oxide also increased with increasing The ozonolysis of trans-2-butene leads to a primary ozonide. This primary ozonide pressure, but much more dramatically. Vinyl hydroperoxide was the major decomposes into acetaldehyde and syn and anti acetaldehyde oxide. MultiWell product of the reaction. TS simulations predict that all of the primary ozonide would decompose into either syn or 42.28 Syn and anti acetaldehyde oxide can each rearrange via hydrogen shift and anti acetaldehyde oxide and none would revert back to trans-2-butene and ozone. This O O ring closure. OH is generated directly as a product of the hydrogen shift in anti is what was expected since ozonides are fairly unstable and the activation barrier is too acetaldehyde oxide and indirectly via a vinyl hydroperoxide intermediate from TS large for a a significant amount to be able to convert back to the reactants. TS 33.58 syn acetaldehyde oxide. In this work, only anti acetaldehyde oxide was 38.51 studied as a source of OH. TS O O 25.09 TS TS 18.34 17.85 3.45 OH 0.00 + O OH O O -1.92 71-24% <0.01% O O O -17.77 29-75% 11-29%
O <1% O 89-71% O O
Figure 2: Unimolecular Reactions of syn and anti acetaldehyde Zero-point corrected relative energies (in blue) in kcal/mol The isomerization reaction from syn to anti acetaldehyde oxide is also possible, Product yields (in red) varied with pressure. with two potential transition structures. An open-shell singlet diradical transition structure involves rotation about the C=O bond. A closed-shell transition structure keeps the carbon and oxygen atoms in the same plane and Acknowledgments there is inversion around the center oxygen atom. • American Chemical Society Petroleum Research Fund (Type B Award: 44764-B6) •Midwest Undergraduate Computational Chemistry Consortium • NSF-Supported Computing at U of Illinois (CHE070008) and Hope College (CHE0520704) Figure 1: Mechanism of the Ozonolysis Reaction Zero-point corrected relative energies (in blue) in kcal/mol References 1) Fenske, J. D.; Hasson, A. S.; Ho, A. W.; Paulson, S. E. J. Phys. Chem. A The relative energies of the species involved in these reactions were used in Product yields (in red) were found to be independent of pressure MultiWell simulations to predict the yield of each of the reactants and 2000, 104, 9221. 2) Rathman, W. C. D.; Claxton, T. A.; Rickard, A. R.; Marston, G. Phys. Chem. products. These results will help determine the significance of ozonolysis of Chem. Phys. 1999, 1, 3981. trans-2-butene as a source of ·OH in the atmosphere. 3) Johnson, D.; Lewin, A. G.; Marston, G. J. Phys. Chem. A 2001, 105, 2933. 4) Zhang, D.; Zhang, R. J. Am. Chem. Soc. 2002, 124, 2692. 5) Kuwata, K. T.; Valin, L. C.; Converse, A. D. J. Phys. Chem. A 2005, 109, 10710.