A bracketing method for proton affinity measurements of dehydro- and didehydropyridines

Proton affinity (PA) is a fundamental property that is related to the structure and reactivity of a molecule. Currently, very few experimental PA values are available for organic radicals and none for biradicals. Equilibrium methods cannot be used for these measurements. The traditional bracketing method is based on monitoring reactions of different reference bases with known proton affinities with the protonated analyte for the occurrence of exothermic proton transfer to determine the upper and lower limits of proton affinity. However, the energy deposited into the precursor ions upon CAD when forming protonated radicals may cause endothermic proton transfer reactions occur albeit slowly. Their occurrence limits the accuracy of the measurement. The bracketing method was modified to allow distinction between exothermic and slightly endothermic proton transfer reactions. This method was used to measure the PA values for three dehydropyridine radicals and six didehydropyridine biradicals. The values were compared to those obtained using quantum chemical calculations