Gas-Phase C1+ Affinities of Pyridines Determined by the Kinetic Method Using Multiple-Stage ( MS3) Mass Spectrometry

The relative gas-phase halogen cation affinities of a group of substituted pyridines have been ordered, and absolute C1+ affinity values have been estimated. The C1+-bound dimer of two pyridines is generated in an ion/molecule reaction using mass-selected Cl-C=O+ as the chlorinating agent, and its competitive fragmentations to yield the C1+-pyridine monomers are monitored by multiple-stage (MS3) experiments. These data yield approximate C1+ affinities which show an excellent linear correlation with literature proton affinity (PA) values. The relationship C1+ affinity (kcal/mol) = 0.83PA 42.5 between the two affinities is derived, and both slope and intercept are rationalized in terms of the greater polarizability of C1+ ion. While proton affinities are unaffected by hindrance near the bonding site in the corresponding proton-bound dimers, the affinities for the larger C1+ ion are significantly decreased by intramolecular steric effects in those C1+-bound dimers which involve ortho-substituted pyridines. .Electronic effects are separated from steric effects by comparing the fragmentation of the C1+and H+-bound dimers composed of a hindered and an unhindered pyridine. In this way, ortho substituents are ordered in terms of the magnitudes of their steric effects. The intramolecular steric effects of ortho substituents, defined here as a gas-phase steric parameter 9, are found to increase, not only with the size of the substituent but also as the C1+ affinity of the pyridine increases, due to shortening of the N C P bond. The 9 values are found also to fall in the same order as the corresponding SO steric parameters obtained by solution kinetic measurements. Exceptions occur for 2-methoxypyridine and quinoline, where an additional, through-space electronic interaction between the electron-rich substituent and C1+ is proposed. The methodology used to order Cl+ affinities can be extended to Br+ and I+ affinities, and, in the cases examined, the magnitude of the steric effect falls in the order Br+ > I+ C1+ >> H+. The intramolecular steric effect in the I+-bound dimers is reduced because of the long N-I bond. The quality of the data obtained is such that it is possible to predict with an estimated uncertainty of 2 kcal/mol C1+ affinities for compounds which were not examined. To check further on the experimental data and predictions, semiempirical AM 1 molecular orbital calculations are used to estimate absolute values of C1+ affinities. An excellent correlation is obtained between the experimental values and the AM1 C1+ affinities of unhindered pyridines. The calculation overestimates the C1+ affinities of the hindered pyridines, and this confirms that steric, not electronic, effects are responsible for the decreases observed in the C1+ affinities of ortho-substituted pyridines. Ab initio MP2/6-3 lG(d,p)//6-31G(d,p) molecular orbital calculations are used to confirm that C1+ addition to pyridine occurs at the nitrogen and that the lowest energy structure of the C1+-bound dimer is the N-Cl+-N-bound species.