Collision-induced dissociation of fluoropyridinide anions

Abstract

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 (C 5NH 3F −) dissociate almost exclusively via loss of an HF molecule, i.e., C 5NH 2 − + HF at low collision energies, in addition to loss of F − at higher energies. 2,6-Difluoropyridinide anions (C 5NH 2F 2 −) dissociate via successive loss of HF molecules to form C 5NHF − then C 5N − depending on the collision energy. The CID results strongly suggest formation of ring-intact pyridynide structures (C 5NH 2 −, C 5NHF −) with a bent triple bond embedded in the azine ring systems. Calculated reaction energy diagrams are totally consistent with the experimental observations. Didehydropyridynides C 5NH 2 − and C 5NHF − have substantial barriers to decomposition. Tetradehydropyridynide C 5N − 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 C 5N − species is most likely the linear anion under experimental conditions; however, the ring-intact C 5N − pyridynide is a highly energetic species releasing about 80 kcal mol −1 (340 kJ mol −1) of energy upon the ring-opening.