GAS-PHASE CL+ AFFINITIES OF PYRIDINES DETERMINED BY THE KINETIC METHOD USING MULTIPLE-STAGE [MS(3)] MASS-SPECTROMETRY

The relative gas-phase halogen cation affinities of a group of substituted pyridines have been ordered, and absolute Cl+ affinity values have been estimated. The Cl+-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 Cl+-pyridine monomers are monitored by multiple-stage (MS3) experiments. These data yield approximate Cl+ affinities which show an excellent linear correlation with literature proton affinity (PA) values. The relationship Cl+ 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 Cl+ ion. While proton affinities are unaffected by hindrance near the bonding site in the corresponding proton-bound dimers, the affinities for the larger Cl+ ion are significantly decreased by intramolecular steric effects in those Cl+-bound dimers which involve ortho-substituted pyridines. Electronic effects are separated from steric effects by comparing the fragmentation of the Cl+- 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 S(k), are found to increase, not only with the size of the substituent but also as the Cl+ affinity of the pyridine increases, due to shortening of the N-Cl+ bond. The S(k) values are found also to fall in the same order as the corresponding S0 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 Cl+ 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+ congruent-to Cl+ >> H+. The intramolecular steric effect in the I+-bound dimers is reduced because of the long N-1 bond. The quality of the data obtained is such that it is possible to predict with an estimated uncertainty of 2 kcal/mol Cl+ affinities for compounds which were not examined. To check further on the experimental data and predictions, semiempirical AM1 molecular orbital calculations are used to estimate absolute values of Cl+ affinities. An excellent correlation is obtained between the experimental values and the AM1 Cl+ affinities of unhindered pyridines. The calculation overestimates the Cl+ affinities of the hindered pyridines, and this confirms that steric, not electronic, effects are responsible for the decreases observed in the Cl+ affinities of ortho-substituted pyridines. Ab initio MP2/6-31G(d,p)//6-31G(d,p) molecular orbital calculations are used to confirm that Cl+ addition to pyridine occurs at the nitrogen and that the lowest energy structure of the Cl+-bound dimer is the N-Cl+-N-bound species.