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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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