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44764-B6
Computational Studies of the Atmospheric Chemistry of Alkene Ozonolysis Intermediates

Keith T. Kuwata, Macalester College

            My laboratory has focused on three projects with PRF support.  All have involved the use of quantum chemistry (in some cases combined with statistical rate theory) to predict quantities that can be tested against experiment.

            (1) Hydrogen Transfer Kinetics: The b-hydroxyethylperoxy (I) and b-hydroxyethoxy (III) radicals are prototypes of species that possess intramolecular hydrogen bonds which enable intramolecular hydrogen shift reactions.  These reactions may play an important role in both the atmosphere and in combustion systems.  We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2).  Our MCG3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol.  These predictions agree rather well with MPW1K and BB1K predictions, but disagree with predictions based on B3LYP energies or geometries.  Our RRKM/master equation simulations suggest that a majority of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain.  For Reaction 1 at 298 K, the variational TST rate constant is ~30% lower than the conventional TST result, and the mOMT method predicts that tunneling accelerates the reaction by a factor of 3.  TST calculations on Reaction 2 reveal no variational effect, and a 298 K mOMT transmission coefficient of 10⁵.  The Eckart method overestimates transmission coefficients for both reactions.

            (2) Alkene Oxidation: Methacrolein is a major product of isoprene ozonolysis, and methacrolein oxide is an important intermediate of the same reaction.  We use CBS-QB3 and RRKM/master equation calculations to characterize all known methacrolein formation pathways and all the unimolecular reactions of methacrolein oxide.  Our predicted methacrolein yield is significantly lower than experiment, suggesting the existence of another methacrolein formation pathway.  The vinyl group of methacrolein oxide allows the species to cyclize to a dioxole with a reaction barrier lower than the barriers to either hydroperoxide or dioxirane formation.  Two dioxole derivatives, 1,2-epoxy-2-methyl-3-propanal and 2-methyl-3-oxopropanal, should be measurable products of isoprene ozonolysis.

            (3) Spectroscopy of Transition Metal Oxides: The expectation value of for the ground electronic state of TaO was deduced from the hyperfine coupling tensor calculated using unrestricted density functional theory.  The BHandHLYP hybrid functional and the all-electron well-tempered basis set (WTBS) were used both to optimize the geometry of TaO and compute the hyperfine constants.  Hybrid functionals, which contain some Hartree-Fock (HF) exchange, provide more accurate descriptions of core-shell spin polarization than pure density functionals, and avoid the severe spin contamination of unrestricted HF theory.  Both properties foster the prediction of rather accurate hyperfine coupling constants. The BHandHLYP functional we employed contains 50% HF exchange.  We fully decontracted the well-tempered basis set to maximize its flexibility in describing the core electron density.  The BHandHLYP/WTBS calculation predicts that = 4.571 a₀^-3 = 3.084 x 10³¹ m^-3.  For a 5d electron,  = 0, so the Fermi contact term due to the unpaired 5d electron by itself should be b_F = 0.    However, the 5d electron will induce some spin polarization of the electron density in the Ta core s orbitals, leading to a non-zero Fermi contact term.  The BHandHLYP/WTBS calculation predicts that b_F = 102.7 MHz.  The contribution of a full unpaired 6s electron to the Fermi contact term b_F was estimated by a calculation on the electronic state in which the singly occupied molecular orbital was Ta(6s) in character.  The BHandHLYP/WTBS method was again used to optimize the TaO geometry and to predict its hyperfine coupling constants.  The calculation predicts that b_F = 4189 MHz.

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