Regarding the inherent dependence of resonance effects of strongly conjugated substituents on electron demand

A generalized treatment of the effects of molecular structure on gas-phase proton-transfer equilibria is reported. The treatment involves the summation of simple product terms for substituent field/inductive, polarizability, and a electron delocalization (resonance) effects. Independently determined substituent parameters are used that are shown to be non-colinear within each acid series analyzed. The treatment reproduces satisfactorily the observed substituent effects on gas-phase proton-transfer equilibria for a wide variety of both aliphatic and aromatic series acids. The reaction parameters follow distinctly different trends with the molecular structures of the acidity series, as expected from theoretical considerations. Well-defined limitations for the treatment have been established. For strongly conjugated substituents the simple product term for inherent substituent resonance effects is found to describe well the effects of widely varying changes in a electron demands from the reaction centers. Long-standing controversies' on the effects of a electron demand on substituent a electron delocalization (resonance) effects are based upon experimental observations in solution which are generally complicated by solvent effects2v3 We have completed substituent effect studies on the gas-phase proton-transfer acidities of 38 cationic and neutral aliphatic and aromatic series families having a critical selection of substituents! Seldom are there found to be precise general linear free energy relationships between the gas phase and corresponding solution results.24 Nevertheless, the gas-phase results for the most part support the principles of substituent resonance (R) and field/inductive (F) effects that are frequently presented at the level of introductory courses for substituent effects in solution. The major conceptional additions to substituent effect theory that are provided by the gas-phase results are the following: (a) substituent polarizability (P) ef- fect~,~,~ (b) substituent solvation effects," and (c) the attenuation in R, F, and P effects that are associated with various acid and solvent structure^.^^^ The gas-phase substituent acidity effects (expressed as -6AGO values) follow with useful quantitative approximation the sum of product functions for each of the three kinds (R, F, and P) of contribution^,^*^ although appropriate modifications (as given below) are required. Linear regression analysis' with eq 1 has been utilized, and the requirement of non-colinearity between the