Collision-induced dissociation of fluoropyridinide anions

Collision-induced dissociation of ortho-fluoro, meta-fluoro, and 2,6-difluoropyridinide anions are studied using the selected ion flow tube technique. Structures and energetics of the reactants, transition states, and products are calculated at the MP4(SDQ)/6-31 + G(d) level of theory based on the B3LYP/6-311++G(d,p) and/or MP2/6-31 + G(d) optimized geometries. The monofluoropyridinide anions (C5NH3F−) dissociate almost exclusively via loss of an HF molecule, i.e., C5NH2− + HF at low collision energies, in addition to loss of F− at higher energies. 2,6-Difluoropyridinide anions (C5NH2F2−) dissociate via successive loss of HF molecules to form C5NHF− then C5N− depending on the collision energy. The CID results strongly suggest formation of ring-intact pyridynide structures (C5NH2−, C5NHF−) with a bent triple bond embedded in the azine ring systems. Calculated reaction energy diagrams are totally consistent with the experimental observations. Didehydropyridynides C5NH2− and C5NHF− have substantial barriers to decomposition. Tetradehydropyridynide C5N− is a highly strained ring system and metastable with a predicted barrier of about 5 kcal mol−1 (20 kJ mol−1) toward ring-opening to a linear NCCCCC− structure. The observed C5N− species is most likely the linear anion under experimental conditions; however, the ring-intact C5N− pyridynide is a highly energetic species releasing about 80 kcal mol−1 (340 kJ mol−1) of energy upon the ring-opening.