SULFUR TRIFLUORIDE CATION (SF3- STEREOELECTRONIC EFFECTS IN THE GAS-PHASE() AFFINITIES OF PYRIDINES DETERMINED BY THE KINETIC METHOD )

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 Py(1)SF(3)(+)Py(2), w here Py(1) and Py(2) represent substituted pyridines, are shown to hav e loosely bound structures by collision-induced dissociation (MS(3)) e xperiments and by semiempirical AM1 and ab initio RHF/G-S1G(d, p) mole cular orbital calculations. In the case of dimers comprised of meta- a nd/or para-substituted pyridines (unhindered pyridines), there is an e xcellent Linear correlation between the logarithm of the fragment ion abundance ratio In{[Py(1)(SF3+)]/[Py(SF3+)]} and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are use d to estimate the SF3+ affinities of pyridines which are found to be i n the range of 25-31 kcal/mol. The SF3+ affinities show an excellent l inear correlation with the proton affinities of the pyridines, and the relationship SF3+ affinity (kcal/mol) = 0.73PA - 135.8 between the tw o affinities is derived. The effective temperature of the dimeric ions is determined to be 595 +/- 69 K, which is in good agreement with val ues of around 600 K obtained experimentally in studies on many other s ystems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects t hat decrease the cation affinities. Gas-phase stereoelectronic paramet ers (S-k) 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+-bou nd dimers but smaller than those in the bulky [OCNCO+] system. Calcula tions show evidence for agostic bonding that offsets the steric effect s in some eases. The eclipsed conformation of 2-methylpyridine/SF3+ ad duct is found to be more stable than the staggered form by 0.8 kcal/mo l, 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 a tom (+1.99) and the charge on the bonding hydrogen S-H-C (+0.07) is co nsiderably lower than that on the other two methyl hydrogens (+0.14), which appears to be a good indication of agostic binding. The most sta ble form of the 2-ethylpyridine/SF3+ adduct is found when the N-C-1-C- alpha-C-beta dihedral angle is approximately 60 degrees, where the eth yl hydrogen is directed toward the SF3 group via an interesting six-me mbered 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 atom s further stabilize this form. (C) 1997 American Society for Mass Spec trometry