Experimental investigation of the absolute enthalpies of formation of 2,3-, 2,4-, and 3,4-pyridynes

The absolute enthalpies of formation of 3,4-, 2,3-, and/or 2,4-didehydropyridines (3,4-, 2,3- and 2,4-pyridynes) have been determined by using energy-resolved collision-induced dissociation of deprotonated 2- and 3-chloropyridines. Bracketing experiments find the gas-phase acidities of 2- and 3-chloropyridines to be 383 ± 2 and 378 ± 2 kcal/mol, respectively. Whereas deprotonation of 3-chloropyridine leads to formation of a single ion isomer, deprotonation of the 2-chloro isomer results in a nearly 60:40 mixture of regioisomers. The enthalpy of formation of 3,4-pyridyne is measured to be 121 ± 3 kcal/mol by using the chloride dissociation energy for deprotonated 3-chloropyridine. The structure of the product formed upon dissociation of the ion from 2-chloropyridine cannot be unequivocally assigned because of the isomeric mixture of reactant ions and the fact that the potential neutral products (2,3-pyridyne and 2,4-pyridyne) are predicted by high level spin-flip coupled-cluster calculations to be nearly the same in energy. Consequently, the enthalpies of formation for both neutral products are assigned to be 130 ± 3 kcal/mol. Comparison of the enthalpies of dehydrogenation of benzene and pyridine indicates that the nitrogen in the pyridine ring does not have any effect on the stability of the aryne triple bond in 3,4-pyridyne, destabilizes the aryne triple bond in 2,3-pyridyne, and stabilizes the 1,3-interaction in 2,4-pyridyne compared to that in m-benzyne. Natural bond order calculations show that the effects on the 2,3- and 2,4-pyridynes result from polarization of the electrons caused by interaction with the lone pair. The polarization in 2,4-pyridyne is stabilizing because it creates a 1,2-interaction between the nitrogen and dehydrocarbons that is stronger than the 1,3-interaction between the dehydrocarbons.