SICL3-SPECTROMETRY - AGOSTIC EFFECTS IN THE GAS-PHASE( AND SICL+ AFFINITIES FOR PYRIDINES DETERMINED BY USING THE KINETIC METHOD WITH MULTIPLE STAGE MASS)

Cluster ions, Py(1)SiCl(3)(+)Py(2) and Py(1)SiCl(+)Py(2), where Py(1) and Sy(2) represent substituted pyridines, formed upon reactive collis ions of mass-selected SiCl3+ or SiCl+ cations with a mixture of pyridi nes, are shown to have loosely bound structures by multiple stage mass spectrometry experiments in a pentaquadrupole mass spectrometer. The fragment ion abundance ratio, In([Py(1)SiCl(n)(+)]/[Py(2) SiCln+]) (n = 1 or 3) is used to estimate the relative SiCln+ or SiCl+ affinities of the constituent pyridines by the kinetic method. In the case of clu sters comprised of meta- and/or para-substituted pyridines (unhindered pyridines), the SiCl3+ and SiCl+ affinities are shown to display exce llent linear correlations with the proton affinities (PAs). On the ass umption that the effective temperatures of the SiCl3+- and SiCl+-bound dimers are 555 K (i.e., the same as those of the corresponding Cl+-bo und dimers), SiCl3+ and SiCl+ affinities of the substituted pyridines, relative to pyridine, are estimated to be 3-MePy (2.1 kcal/mol), 4-Me Py (3.2 kcal/mol), 3-EtPy (3.7 kcal/mol), 4-EtPy (4.2 kcal/mol), 3,5-d iMePy (4.8 kcal/mol), and 3,4-diMePy (5.4 kcal/mol). The SiCl3+ and Si Cl+ cation affinities are related to the proton affinities by the expr essions: relative (SiCl3+) affinity = 0.95 Delta PA and relative (SiCl +) affinity = 0.60 Delta PA. The smaller constant in the relationship between the relative SiCl+ affinity and the relative proton affinity i s the result of weaker bonding. Steric effects between the ortho-subst ituted alkyl group and the central SiCl3+ cation reduce the SiCl3+ aff inities of dimers that contain ortho-substituted pyridines. The magnit ude of the steric acceleration of fragmentation is used to measure a s et of gas-phase steric parameters (S-k). The steric effects in the SiC l3+ dimers are similar in magnitude to those in the corresponding Cl+- bound dimers but weaker than those produced by the bulky [OCNCO](+) gr oup. An inverted steric effect is observed in those SiCl+-bound dimers that incorporate ortho-substituted pyridines and is ascribed to auxil iary Si-H-C bonding, which stabilizes the ortho-substituted pyridine-S iCl+ bond. This auxiliary bonding appears to correspond to agostic bon ding, which is well characterized in solution and occurs in competitio n with steric effects that weaken the pyridine-SiCl- interaction. Ion- molecule reactions of pyridines with halosilicon radical cations SiCl2 (+.) and SiCl4+. as well as alkylated halosilicon cations Si(CH3Cl+ an d Si(CH3)Cl-2(+) also are investigated. In these cases, charge exchang e and associated reactions are the main reaction channels, and cluster ing is not observed.