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

The relative gas-phase halogen cation affinities of a group of substit uted pyridines have been ordered, and absolute Cl+ affinity values hav e been estimated. The Cl+-bound dimer of two pyridines is generated in an ion/molecule reaction using mass-selected Cl-C=O+ as the chlorinat ing agent, and its competitive fragmentations to yield the Cl+-pyridin e monomers are monitored by multiple-stage (MS3) experiments. These da ta yield approximate Cl+ affinities which show an excellent linear cor relation 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 th e greater polarizability of Cl+ ion. While proton affinities are unaff ected by hindrance near the bonding site in the corresponding proton-b ound dimers, the affinities for the larger Cl+ ion are significantly d ecreased by intramolecular steric effects in those Cl+-bound dimers wh ich involve ortho-substituted pyridines. Electronic effects are separa ted 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 o f their steric effects. The intramolecular steric effects of ortho sub stituents, defined here as a gas-phase steric parameter S(k), are foun d to increase, not only with the size of the substituent but also as t he 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 me asurements. Exceptions occur for 2-methoxypyridine and quinoline, wher e an additional, through-space electronic interaction between the elec tron-rich substituent and Cl+ is proposed. The methodology used to ord er Cl+ affinities can be extended to Br+ and I+ affinities, and, in th e cases examined, the magnitude of the steric effect falls in the orde r 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 qual ity of the data obtained is such that it is possible to predict with a n estimated uncertainty of 2 kcal/mol Cl+ affinities for compounds whi ch were not examined. To check further on the experimental data and pr edictions, semiempirical AM1 molecular orbital calculations are used t o estimate absolute values of Cl+ affinities. An excellent correlation is obtained between the experimental values and the AM1 Cl+ affinitie s of unhindered pyridines. The calculation overestimates the Cl+ affin ities of the hindered pyridines, and this confirms that steric, not el ectronic, 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 ener gy structure of the Cl+-bound dimer is the N-Cl+-N-bound species.