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|Type:||Artigo de periódico|
|Title:||Gas phase agostic bonding in pyridine SiFn+ (n = 1, 3) cluster ions investigated by the kinetic method|
|Abstract:||Loosely bonded cluster ions, Py1SiF3+Py2 and Py1SiF+Py2, where Py-1 and Py-2 represent substituted pyridines, are formed by ion/molecule reactions between mass-selected SiF3+ or SiF+ and a mixture of pyridines. The clusters are hown to have loosely bound symmetric structures by MS3 experiments and ab initio calculations. The SiF3+/pyridine dimer is shown to have a trigonal bipyrimidal structure. Relative SiF3+ and SiF+ affinities of the constituent pyridines are measured by the kinetic method, and excellent linear correlations with the proton affinity of meta- and para-substituted pyridines are observed. Gas-phase stereoelectronic parameters (S-k) for SiF3+ SiF+ are also experimentally measured and show that the binding of the ortho-substituted pyridines is governed by two opposing effects, steric hindrance and agostic bonding. Agostic bonding of the form C-H --- Si+, is evident in the SiF+ system, just as it is in the corresponding SiCl+/pyridine dimers. On the other hand, steric hindrance plays a key role in weakening the strength of the interaction of the central SiF3+ ion and the ortho-substituted pyridines compared with that in SiF+-bound cluster ions. The relatively larger Lewis acidity of fluorinated siliconium ions compared with the corresponding chlorinated species shortens the Si-N bond and makes overall steric effects larger in the SiFn+ (n = 1, 3) systems than in the SiCln+ (n = 1, 3) systems. The potential application of the kinetic method in recognizing agostic bonding in transition metal systems in the gas phase is also demonstrated in this study. (Int J Mass Spectrom 179/180 (1998) 195-205). (C) 1998 Elsevier Science B.V.|
|Editor:||Elsevier Science Bv|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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