GAS-PHASE REACTIVITY OF THE 2,6-DIMETHYLENEPYRIDINIUM AND 3,5-DIMETHYLENEPYRIDINIUM BIRADICALS

The 2,6- and 3,5-dimethylenepyridinium biradical ions were generated a nd structurally characterized in the gas phase by using Fourier transf orm ion cyclotron resonance mass spectrometry. Their reactivity was ex amined toward several commonly used spin traps. Reaction rates and pro duct distributions were determined. The isomeric biradicals were found to possess remarkably different chemical properties. Most importantly , the 2,6-dimethylenepyridinium biradical is highly reactive toward al l the reagents studied, while the 3,5-dimethylenepyridinium biradical reacts slowly or not at all. The reaction of the 2,6-dimethylenepyridi nium biradical with tert-butyl isocyanide occurs via HCN abstraction a nd addition, the same pathways observed for isomeric closed-shell pyri dylmethyl cations. In contrast, the 3,5-dimethylenepyridinium biradica l reacts exclusively by slow addition. The same reaction was observed for the analogous 3-methylenepyridinium monoradical ion. The 3,5-dimet hylenepyridinium biradical and the 3-methylenepyridinium monoradical a lso display similar reactivity toward the other reagents studied, whil e this is not the case for the 2,6-dimethylenepyridinium biradical and the pyridylmethyl cations. Some of the differences observed in the re activity of the isomeric biradicals are likely explained by their diff erent ground-state multiplicities. Earlier ab initio molecular orbital calculations predict a triplet ground state for the 3,5-dimethylenepy ridinium biradical (by about 6 kcal/mol) and a closed-shell singlet gr ound state for the 2,6-dimethylenepyridinium biradical (by about 2 kca l/mol). The different location of the protonated nitrogen relative to the methylene groups probably has a minor influence over the types of reaction products obtained since this is the case for the analogous mo noradicals.