102.
We present a preliminary work for a general method of computing the partition of σ and π electronic effects of a given atom A or substituent R on a given substrate. In this aim, the nuclear charge
Z* of a fictitious hydrogen atom H
* is fitted in order that the A–H
* (or R–H
*) bond be purely covalent, i.e. the Mulliken electron population be one electron on H
*. We obtain this way entities of the same electronegativity as A or R, thus having a comparable σ effect, without any π effect.
The values of Z* obtained for A–H* diatomic molecules (A=H–Br) exhibit a good linear correlation with the Allred–Rochow scale of electronegativity, as it could be expected on theoretical grounds. The method, applied to R–H* molecules, allows a determination of the electronegativity of a variety of polyatomic R substituents, and provides H*(R) having the same inductive effect as R. These results are discussed by comparison with some previous theoretical and experimental data.
As an example of application, the partition of σ and π contributions of R on the 13C chemical shifts in a series of monosubstituted benzenes RC6H5 has been computed. 相似文献