Relative Cyanide Cation (+CN) Affinities of Pyridines Determined by the Kinetic Method Using Multiple-stage (MS3) Mass Spectrometry

Ion-molecule reactions occurring in a pentaquadrupole mass spectrometer are used to generate and characterize ions in which one or two pyridine molecules are bound by a +CN cation. Cyanide cation binds strongly to the nitrogen atom of pyridine to generate a mono-adduct, which undergoes pyridine exchange reactions and from which one can generate the dipyridine adduct in low abundance. The dimeric ions have two structures, loosely bound and covalently bound, and both fragment to yield the constituent cyanide-bound monomers. In the case of dimers comprised of meta-substituted alkylpyridines, there is a quantitative correlation between relative cyanide cation affinity, as measured using the kinetic method, and literature values of relative proton affinities. These dimers fragment analogously to the corresponding H+- and CI+- bound dimers, and on this basis are assigned analogous structures, viz. the loosely bound form Py ,-' CN-Py,. Semi-empirical molecular orbital calculations show that both pyridines are bound to the carbon atom of the cyanide cation. Making the assumption that the effective temperatures of the activated cyanide-bound dimers are similar to those of the corresponding CI+- and H+-bound dimers, relative *CN cation affinities are estimated to be 1.5 kcal mol-' (3-MePy), 1.7 kcal mol-' (CMePy), 2.6 kcal mol-' (3-EtPy), 3.5 kcal mol-' (3-n-BuPy) and 3.6 kcal mol-' (3,ldiMePy), all expressed relative to pyridine (1 kcal = 4.184 kJ). A linear relationship between the relative +CN affinity and relative proton affinity (PA) is derived as A +CN affinity (kcal mol-I) = 0.78 (APA), with the assumption that the 'CN dimer effective temperature is 600 K. The estimated uncertainty is 0.5 kcal mol-'. Relative +CN affinities of pairs of pyridines are smaller by ca. 1 kcal mol-' than the corresponding CI+ affinities. Dimers in which one of the pyridines is meta-chlorine- or para-alkyl-substituted have the covalently bound, ring-carbon-substituted structure, in which the 'CN group is attached to the pyridine nitrogen and the second pyridine molecule is bound to a ring carbon. The fragmentation of these isomeric dimers yields the corresponding monomers, in addition to other minor ions, but the distribution of the cyanide cation between the two pyridines does not correlate with CI' affinity or proton affinity. In the special cases of the 3-methylpyridine3-n-butylpyridine and the 4-methylpyridine-pyridine cyanide cation adducts, both the loosely bound dimer and the covalently bound adduct are generated and distinguished by their fragmentation behavior. Evidence for the formation of the covalently bound, ring-carbon-substituted structure was also obtained in semi empirical AM1 calculations.