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

Abstract

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 (CH 3 CN and H 2 O) 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.

Mechanism and Kinetics of the Reaction of Nitrate Radicals with Carboxylic Acids

“…For alkanes (whose ionization potentials are similar and around 10.2 eV), the reactivity order (per hydrogen) was determined: 3°(24) > 2°(4.0) > 1°(1.0). [16] For the carboxylic acids used in this study, qualitatively, the reactivity order is similar but there is not enough data and sufficient structural diversity to provide meaningful values. The ionization potential of formic and acetic acid are 11.3 and 10.7 eV, respectively-considerably greater than that of an alkane, and undoubtedly, this is also a factor contributing to their inertness to hydrogen abstraction by NO 3 * .…”

“…As we previously noted, the rate constants for H‐abstraction by NO 3 ⋅ depend both on the nature and strength of the C−H bond and the ionization potential of the substrate (with rate constants higher for more easily ionized substrates). For alkanes (whose ionization potentials are similar and around 10.2 eV), the reactivity order (per hydrogen) was determined: 3° (24)>2° (4.0)>1° (1.0) [16] . For the carboxylic acids used in this study, qualitatively, the reactivity order is similar but there is not enough data and sufficient structural diversity to provide meaningful values.…”

“…Laser flash photolysis was used to monitor the kinetics of NO 3 ⋅ reactions with carboxylic acids. The experimental setup was recently described in detail [16] . NO 3 ⋅ was generated at 25 °C from 3×10 −4 mol L −1 ceric ammonium nitrate (CAN) through excitation at 355 nm, and monitored at 635 nm in acetonitrile.…”

“…As depicted in Figure 1, as a result of this partial positive charge, electron withdrawing groups on carbon destabilize the transition state, and electron donating groups do the opposite. [14] Previous studies from our group have shown that hydrogen abstractions by electron-deficient radicals such as HO * [15] and NO 3 * [16] are influenced by the polar effect, and that the transition state may be stabilized or destabilized as a result of substituents that provide or compete for electron density in the transition state. Furthermore, solvent may also play a role in stabilizing the polarized transition state, as was shown for in the case of HO * .…”

Oxidative Damage of Aliphatic Amino Acid Residues by the Environmental Pollutant NO3·: Impact of Water on the Reactivity

“…The time-dependent consumption of NO 3 · was monitored at 630 nm. The kinetic measurements were performed in 6 M HNO 3 to prevent exchange of the nitrate ligands in CAN with water, which inhibits formation of NO 3 · . ,− To our knowledge, no method has been reported for the generation of NO 3 · in neutral aqueous solution for kinetic studies. The experiments were therefore augmented with DFT calculations to evaluate the impact of both H 2 O and H 3 O + on the energies of the reaction pathways, which will be discussed in Computational Studies.…”