RELATIONSHIP BETWEEN SUBSTITUENT ELECTRONEGATIVITY AND FIELD PARAMETER IN THE ALIPHATIC SERIES - INTRODUCTION OF CHARGE SEPARATION PARAMETER (LAMBDA)

The problem of non‐proportionality between the substituent field parameter (σF) and substituent electronegativity (Δι or σχ) was clarified by the introduction of the ‘charge‐separation parameter’ (λ), which is the component for the direction of the HX axis of the distance between positive and negative poles of the dipole in the HX molecule. Thus σF was correlated with Δι or σχ by the equation σF = aΔιλ (or σF = a′σχλ). 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 XH…HCO 2−) at shorter distances (2·0–4·5 Å). It was further shown that field parameters such as σi, F and σF(theor), and the pKa values in the series of aliphatic acids, and spectral data (UPS and 13C‐SCS) were also well correlated with Διλ or σχλ. It is concluded that λ and Δι or σχ 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. Copyright © 1990 John Wiley & Sons Ltd.