Remote substituent effects on polar and non-polar covalent bonds

Abstract

In this work the homolytic Z–X bond dissociation energies (BDE, E_d) for 48 molecules with the general formula 4-Y–C₆H₄–Z–X have been calculated by means of AM1 and PM3 semi-empirical quantum chemical methods. The BDEs and the remote substituent effects of Y on the Z–X BDEs have been compared with previously published data in terms of both relative and absolute accuracy. The relative accuracy is found to be rather good concerning the BDEs of the unsubstituted compounds calculated at the UHF level. At the RHF level, employing half-electron energies for the radicals, the absolute accuracy is also acceptable. The relative accuracy for the remote substituent effects is found to be rather poor. Both the substituent effects and the absolute BDEs are markedly weaker at the UHF level than those published in previous work. At the RHF level the absolute BDEs are close to the published data, but the substituent effect is still weaker.

The final part of this work deals with the experimentally established fact that remote substituent effects on Z–X BDEs change with the polarity of the Z–X bond. Relationships derived for simple molecules based on electronegativity have been used to explore this phenomenon qualitatively. This study shows that the change in remote substituent effect with bond polarity can very well be accounted for by relationships of this kind.