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Photoionization mass spectrometry of the fluoromethylsilanes (CH/sub 3/)/sub n/F/sub 4-n/Si(n=1-4)

Journal Article · · J. Am. Chem. Soc.; (United States)
DOI:https://doi.org/10.1021/ja00449a013· OSTI ID:7216757
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Photoionization efficiency curves for molecular ions P/sup +/ and lowest energy fragments (P--CH/sub 3/)/sup +/ for the series of methyl and fluoro substituted silanes (CH/sub 3/)/sub n/F/sub 4-n/Si (n = 1-4) are reported for photon energy ranges extending from 1 to 2 eV above the respective parent thresholds. Adiabatic ionization potentials of 9.80 +- 0.03, 10.31 +- 0.04, 11.03 +- 0.03, and 12.48 +- 0.04 eV are determined for (CH/sub 3/)/sub 4/Si, (CH/sub 3/)/sub 3/SiF, (CH/sub 3/)/sub 2/SiF/sub 2/, and CH/sub 3/SiF/sub 3/, respectively, with appearance thresholds of 10.03 +- 0.04, 10.70 +- 0.04, 11.70 +- 0.03, and 13.33 +- 0.04 eV for siliconium ion fragments (CH/sub 3/)/sub 3/Si/sup +/, (CH/sub 3/)/sub 2/FSi/sup +/, (CH/sub 3/)F/sub 2/Si/sup +/, and F/sub 3/Si/sup +/ arising from CH/sub 3/ loss. These data are interpreted in terms of the thermochemistry of the various ionic and neutral silicon species. Derived heats of formation afford accurate calculation of fluoride affinities (heterolytic bond dissociation energies, D(R/sub 3/Si/sup +/--F/sup -/)) of 219.5, 237.3, 258.4, and 302.0 kcal/mol for the siliconium ions (CH/sub 3/)/sub 3/Si/sup +/, (CH/sub 3/)/sub 2/FSi/sup +/, (CH/sub 3/)F/sub 2/Si/sup +/, and F/sub 3/Si/sup +/, respectively. These values are 30 to 50 kcal/mol higher than the analogous carbonium ions.

Research Organization:
California Inst. of Tech., Pasadena
OSTI ID:
7216757
Journal Information:
J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 99:7; ISSN JACSA
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400104 -- Spectral Procedures-- (-1987)
400500* -- Photochemistry
CHARGED PARTICLES
ENTHALPY
FORMATION HEAT
HYDRIDES
HYDROGEN COMPOUNDS
IONIZATION
IONIZATION POTENTIAL
IONS
MOLECULAR IONS
PHOTOIONIZATION
PHYSICAL PROPERTIES
REACTION HEAT
SILANES
SILICON COMPOUNDS
THERMODYNAMIC PROPERTIES