Psh. Wong et al., SULFUR TRIFLUORIDE CATION (SF3- STEREOELECTRONIC EFFECTS IN THE GAS-PHASE() AFFINITIES OF PYRIDINES DETERMINED BY THE KINETIC METHOD ), Journal of the American Society for Mass Spectrometry, 8(1), 1997, pp. 68-75
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