OBSERVATION OF THE HAMMICK INTERMEDIATE - REDUCTION OF THE PYRIDINE-2-YLID ION IN THE GAS-PHASE

Azacyclohexatriene-2-ylidene (1), the 2-isomer of pyridine (2), has be en generated by one-electron reduction of the corresponding radical ca tion in neutralization-reionization mass spectrometric experiments. Th e experimental finding that this molecule is stable on the microsecond time scale is in agreement with results of quantum chemical calculati ons that indicate both 1 and its radical cation, 1(.+), correspond to minima on the C5H5N and C5H5N.+ potential energy surfaces. The calcula tions predict that 1 is less stable than pyridine, 2, by 50 and 49 kca l/mol (MP2/6-31G* and CASSCF-MP2/6-31G**, respectively) or 47 kcal/mo l (B3LYP/6-31G*), whereas the radical cations 1(.+) and 2(.+) are muc h closer in energy. The ylid ion 1(.+) is predicted to be 6 and 7 kcal /mol lower in energy than 2(.+) at the MP2 and CASSCF-MP2/6-31G* leve ls, respectively, and 1 kcal/mol higher according to the hybrid densit y functional theory. Calculations also suggest that facile isomerizati on of the ions is prohibited by an energy barrier, amounting to 62 and 57 kcal/mol at MP2/6-31G* and B3LYP/6-31G**, respectively, relative to 1(.+), which is even larger than the 38 kcal/mol obtained at both l evels of theory required for the neutral transformation. Despite the s ubstantial impediments, isomerization of excited species is possible s ince the lowest dissociation channels lie even higher in energy but th e experimental observations confirm that neither the ions or neutrals undergo particularly facile isomerization. Using known thermochemical data a value for Delta H-f (1(.+)) = 237 +/- 5 kcal/mol was obtained f rom the measured appearance energy, 10.14 eV, of the C5H5N.+ ion gener ated from methyl picolinate, which is completely consistent with the t heoretical predictions of 237-242 kcal/mol; derived from the calculate d energy differences between the various species and the known heat of formation of 2.