AM1 calculations on reactive oxygen species .1. Analysis of hydroxyl radical reactions

A variety of hydroxyl-radical reactions (hydrogen abstraction, (OH)-O-. addition to unsaturated molecules, and decomposition of the radical adducts) has been investigated by means of the AM1 semiempirical molecular-orbital method. A limited CI treatment was employed to explore the doubler ground-state potential surface of each reaction. The analysis focuses on energy barriers of the reactions and the temperature dependence of the pre-exponential term in the expression for the rate constant. Experimental values of the Arrhenius activation energy E(a) and the heat of reaction Delta H-f300(o) are compared with the results of the calculations. Although the AMI method generally exaggerates the height of energy barriers and the stability of radical reaction products, trends in E(a) and Delta H-f300(o) are qualitatively followed by the calculations. To quantify the relationship between E(a) and the calculated AM1 barrier Delta H-f(double dagger), the data obtained from 19 reactions were subjected to linear regression analysis. The resultant expression yields useful estimates of E(a) from computed values of Delta H-f(double dagger).