Issue 12, 1996

Understanding the rates of hydrogen-atom abstraction reactions: empirical, semi-empirical and ab initio approaches

Abstract

The performance of the empirical algorithm proposed by Roberts and Steel (J. Chem. Soc., Perkin Trans. 2, 1994, 2155), which relates the activation energy for the H-atom transfer reaction (A) to properties of the A˙+ HB → AH + B˙(A) four molecules involved, is assessed. This approach has been strongly criticised by Zavitsas in two recent papers (J. Chem. Soc., Perkin Trans. 2, 1996, 391 and J. Am. Chem. Soc., 1995, 117, 10 645), mainly on the grounds that it performs badly in comparison with his semi-empirical method for estimating Ea. These criticisms are examined and, in the main, are found to be without foundation. The identity reaction H3Si˙+ H4Si is viewed as a test case and it is shown that our empirical approach predicts an activation energy in accord with that estimated here by ab initio methods and with published experimental data on related reactions. The value of Ea predicted by Zavitsas' published method (71.5 kJ mol–1) is very much higher than the value deduced from our empirical analysis (37.6 kJ mol–1) or that obtained by the ab initio procedure (46.2 kJ mol–1, which itself is probably too high). It is argued that polar effects are important in H-atom abstraction from thiols by alkyl radicals and that hydrogen-bonding interactions between A˙ and HB can be important, in particular when the leading atoms of A and B are oxygen or nitrogen. It is concluded that empirical models have a role to play in aiding the understanding of radical reactivity and that the approach of Roberts and Steel is relatively successful, given the limitations implicit in the model.

Article information

Article type
Paper

J. Chem. Soc., Perkin Trans. 2, 1996, 2719-2725

Understanding the rates of hydrogen-atom abstraction reactions: empirical, semi-empirical and ab initio approaches

B. P. Roberts, J. Chem. Soc., Perkin Trans. 2, 1996, 2719 DOI: 10.1039/P29960002719

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Spotlight

Advertisements