GAS-PHASE ACIDITIES DERIVED FROM THRESHOLD ENERGIES FOR ACTIVATED REACTIONS

Appearance energies have been measured for the ionic products from activated unimolecular dissociations of organiccarboxylate ions, alkoxide ions, and ketone enolates as well as for products from endothermic bimolecular proton-transfer and nucleophilic displacement reactions. A correlation between the experimental energies of the reaction onsets and the reactionendothermicity permits the derivation of gas-phase acidities for the conjugate acids of the product carbanions by means of simple thermochemical cycles. The acidities so derived generally show good agreement with literature values or with predictionsbased on analogous systems, although certain of the reactions display threshold energies higher than what would be predicted from the reaction enthalpy, due to activation barriers or kinetic shifts, whereas others have threshold energies lower than expected. These latter systems have the common characteristic of acidic hydrogens in the side chain of the reactant ions, leading to the possibility of isomerization. General features of the collisional activation experiments and the data analysis are described. Gas-phase acidities are derived for methane, fluoromethane, ethylene, cyclopropane, and neopentane, as well as several otherspecies. © 1990, American Chemical Society. All rights reserved.