Adducts and dimers of SFn+ (n = 1-5) with benzene, acetonitrile, and pyridine. Gas-phase generation and ab initio structures and thermochemistry

Pentaquadrupole (QqQqQ) mass spectrometry is used to explore the abilities of gaseous SFn+ (n = 1-5) ions to form adducts and dimers with three pi-ele ctron rich molecules-benzene, acetonitrile, and pyridine, whereas ab initio calculations estimate most feasible structures, bond dissociation energies (BDEs), and reaction enthalpies of the observed products. With benzene, SF + reacts by net H-by-SF+. replacement. As suggested by the calculations, th is novel benzene reaction forms ionized benzenesulfenyl fluoride, C6H5-SF+. , via a Wheland-type intermediate that spontaneously loses a H atom. SF3+ f orms a rare, loosely bonded n complex with benzene, [Bz ... SF3](+), which is stable toward both H and HF loss. No dimer. Bz(2)SF(3)(+), is formed. Ac cording to calculations, an unsymmetrically bonded, pi-coordinated Bz(2)SF( 3)(+) dimer exists, i.e. (Bz-SF(3 ...)Bz)(+) but its formation from [Bz ... SF3](+) is endothermic; hence, thermodynamically unfavorable. With acetoni trile, SF2+., SF3+, and SF5+ form both adducts and dimers. CH3-C-.=N-SF2+ ( a new distonic ion) and CH3CN-SF5+ are covalently bonded, but CH3CN ... SF3 + is loosely bonded. The binding natures of the acetonitrile adducts are re flected in the dimers; [CH3CN-SF2 NCCH3](+.) and [CH3CN-SF5... NCCH3](+) ar e unsymmetrically bonded, whereas [CH3CN ... SF3... NCCH3](+) is symmetrica lly and loosely bonded. Such dimers as [CH3CN ... SF3... NCCH3](+) are idea l for measurements of ion affinity via the Cooks' kinetic method. With pyri dine, only SF3+ forms adduct and dimer. Py-SF3+ is covalently bonded throug h nitrogen; [Py ... SF3... Py](+) is loosely but unsymmetrically bonded. Th e unsymmetric 2.28 and 2.44 Angstrom long N-S bonds in [Py ... SF3... Py](), which are expected to rapidly interconvert, result Likely from steric hi ndrance that forces orthogonal alignment of the two pyridine rings. Most ob served adducts and dimers display relatively high BDEs, i.e. they are forme d in thermodynamically favorable reactions. The extents of dissociation of the adducts and dimers observed in MS3 experiments reflect the structures a nd BDEs predicted by the calculations. (C) 1999 Elsevier Science B.V.