Relationship between substituent electronegativity and field parameter in the aliphatic series. Introduction of charge separation parameter (λ)

The problem of non‐proportionality between the substituent field parameter (σ F ) and substituent electronegativity (Δι or σ χ ) was clarified by the introduction of the ‘charge‐separation parameter’ (λ), which is the component for the direction of the HX axis of the distance between positive and negative poles of the dipole in the HX molecule. Thus σ F was correlated with Δι or σ χ by the equation σ F = a Διλ (or σ F = a ′σ χ λ). The above proposal was further supported by performing an energy decomposition analysis for the isodesmic reaction in the isolated molecule system ( \documentclass{article}\pagestyle{empty}\begin{document}${\rm X}\raise1pt\hbox{---} {\rm H} \cdots {\rm H}\mathop {\rm N}\limits^ + {\rm H}_3 $\end{document} or XH…HCO   2 − ) at shorter distances (2·0–4·5 Å). It was further shown that field parameters such as σ i , F and σ F(theor) , and the p K a values in the series of aliphatic acids, and spectral data (UPS and 13 C‐SCS) were also well correlated with Διλ or σ χ λ. It is concluded that λ and Δι or σ χ are essential parameters for interpreting the electrostatic field effect in aliphatic substituent effects, and that the transmission due to the σ‐inductive effect is considered to be not as significant as the field effect after a few bonds away from the substituent.