Methyl and fluorine substituent effects on the gas-phase Lewis acidities of silanes by ion cyclotron resonance spectroscopy
Formation of gas-phase Lewis acid-base adducts (CH/sub 3/)/sub n/F/sub 4-n/SiF/sup -/ (n = 0-2) in reactions of SF/sub 6//sup -/ and SF/sub 5//sup -/ with neutral fluoromethylsilanes is examined using trapped-anion ion cyclotron resonance spectroscopy. Fluoride transfer reactions observed in binary mixtures of the silanes and of silanes with borane Lewis acids R/sub 3/B(R = CH/sub 3/, C/sub 2/H/sub 5/, i-C/sub 3/H/sub 7/, F), in the presence of trace amounts of SF/sub 6/ as the fluoride source, establish the Lewis acidity order BF/sub 3/ is greater than SiF/sub 4/ is greater than (i-C/sub 3/H/sub 7/)/sub 2/FB is greater than (i-C/sub 3/H/sub 7/)/sub 3/B is greater than (C/sub 2/H/sub 5/)/sub 2/FB is greater than (C/sub 2/H/sub 5/)/sub 3/B is greater than (CH/sub 3/)/sub 2/FB is greater than CH/sub 3/SiF/sub 3/ is greater than (CH/sub 3/)/sub 3/B is greater than (CH/sub 3/)/sub 2/SiF/sub 2/ is greater than SF/sub 4/ is greater than (CH/sub 3/)/sub 3/SiF in the gas phase for F/sup -/ as reference base. Quantitative estimates of adduct bond dissociation energies D((CH/sub 3/)/sub n/F/sub 4-n/Si-F/sup -/) (n = 0-2) and heats of formation of adducts (CH/sub 3/)/sub n/F/sub 4-n/SiF/sup -/ are derived. Variations in adduct bond strengths are discussed in terms of the effects of methyl and fluorine substitution on silicon. Analogies are drawn to similar substitutent effects observed in the isoelectronic neutral fluoromethylphosphoranes.
- Research Organization:
- California Inst. of Tech., Pasadena
- OSTI ID:
- 7087584
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 99:15
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
SILANES
BINDING ENERGY
ADDUCTS
FLUORINATION
ION CYCLOTRON RESONANCE SPECTROSCOPY
LEWIS ACIDS
METHYLATION
CHEMICAL REACTIONS
ENERGY
HALOGENATION
HYDRIDES
HYDROGEN COMPOUNDS
INORGANIC ACIDS
ION SPECTROSCOPY
SILICON COMPOUNDS
SPECTROSCOPY
400201* - Chemical & Physicochemical Properties