Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State.

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH3CN and H2O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.