REMOTE SUBSTITUENT EFFECTS ON POLAR AND NONPOLAR COVALENT BONDS

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 calcu lated by means of AM1 and PM3 semiempirical 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 go od concerning the BDEs of the unsubstituted compounds calculated at th e UHF level. At the RHF level, employing half-electron energies for th e radicals, the absolute accuracy is also acceptable. The relative acc uracy for the remote substituent effects is found to be rather poor. B oth the substituent effects and the absolute BDEs are markedly weaker at the UHF level than those published in previous work. At the RHF lev el the absolute BD Es are close to the published data, but the substit uent effect is still weaker. The final part of this work deals, with t he experimentally established fact that remote substituent effects on Z-X BDEs change with the polarity of the Z-X bond. Relationships deriv ed for simple molecules based on electronegativity have been used to e xplore this phenomenon qualitatively. This study shows that the change in remote substituent effect-with bond polarity can very well be acco unted for by relationships of this kind.