THEORETICAL-STUDIES OF HYDROGEN ABSTRACTION FROM 2-PROPANOL BY OH RADICAL

Hydrogen abstraction from 2-propanol by hydroxyl radical was investiga ted with ab initio quantum chemical methods at the level of MP2/6-31G , with scaling of correlation energy. Both the geometries and the ener getics of reactants, products, and transition state structures change significantly when electron correlation is included in the optimizatio n process. An exhaustive search produced 16 transition state structure s for the abstraction of the three distinct hydrogens in 2-propanol, A bstraction of the alpha-hydrogen has two distinct transition structure s with very low barriers. The calculated rate constant for H-alpha-abs traction is close to that predicted by collision theory. Abstraction o f the beta-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 sh ow that the non-hydrogen-bonded pathways contribute most of the flux f or this process. There are three nearly degenerate transition structur es in the abstraction of the hydroxyl hydrogen. The calculated rate co nstants for the combined (H-alpha + H-0) and H-beta-abstractions, resp ectively, are in good agreement with available experimental data. The kinetic isotope effect (KIE) for H-beta-abstraction agrees very well w ith experimental data. The calculated KIE for (H-alpha + H-0) abstract ion shows a stronger temperature dependence than the experimental KIE. However, the weak temperature dependence supports the notion that H-a lpha-abstraction may be collision controlled.