Theoretical studies of hydrogen abstraction from 2-propanol by OH radical

Hydrogen abstraction from 2-propanol by hydroxyl radical was investigated with ab initio quantum chemical methods at the level of MP2/6-31G*, with scaling of correlation energy. Both the geometries and the energetics of reactants, products, and transition state structures change significantly when electron correlation is included in the optimization process. An exhaustive search produced 16 transition state structures for the abstraction of the three distinct hydrogens in 2-propanol. Abstraction of the α-hydrogen has two distinct transition structures with very low barriers. The calculated rate constant for H_α-abstraction is close to that predicted by collision theory. Abstraction of the β-hydrogen has 11 different transition structures that can be classified into three groups on the basis of the presence or absence of hydrogen bonding between the OH radical and the hydroxy group of 2-propanol. The calculated rate constants for the individual pathways show that the non-hydrogen-bonded pathways contribute most of the flux for this process. There are three nearly degenerate transition structures in the abstraction of the hydroxyl hydrogen. The calculated rate constants for the combined (H_α + H_o) and H_β-abstractions, respectively, are in good agreement with available experimental data. The kinetic isotope effect (KIE) for H_β-abstraction agrees very well with experimental data. The calculated KIE for (H_α + H_o) abstraction shows a stronger temperature dependence than the experimental KIE. However, the weak temperature dependence supports the notion that H_α-abstraction may be collision controlled.