Application of the Seth-Paul-Van Duyse equation—I : A simple relation defining new X+(R) substituent constants on the basis of electrophilic σ+ constants

The Seth-Paul-Van Duyse equation (SPVDE) correlating the CO stretching frequencies of a great number of R₁COR₂ molecules with X (R) substituent constants has been reinvestigated. A simple empirical relation defining new X⁺(R) constants on the basis of electrophilic σ constants has been derived: X⁺ (R) = 0·238 σ⁺ + 1·077. The three earlier reported relationships between the X(R) constants and Hammett σ values have been replaced by this single relationships and a set of new X⁺(R) constants has been calculated. The new X⁺(R) constants applied to 287 R₁COR₂ compounds containing the para- or meta-substituted benzene rings fit very well the SPVDE. It has been possible to extend the SPVDE to various aromatic systems.