Sulfur trifluoride cation (SF3+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the polyatomic cation SF3+. The dimeric ions Py1SF3+Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3+)]/[Py(SF3+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indication of agostic binding. The most stable form of the 2-ethylpyridine/SF3+ adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.
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