The electron affinities for a series of a-silyl-substituted silyl and carbon radicals have been measured. These electron affinities (kcal/mol) include the following: EA((Me3Si)zCH) = 36.0 f 0.2, EA(MezHSiSiMe2) = 32.3 f 0.8, EA((Me3Si)zSiH) = 44.7 f 1.9, and EA((MesSi)3Si) = 46.8 f 2.0. The electron affinity of the dimethylsilyl radical was also determined to be 24.7 f 0.5 kcal/mol. From these electron affinities and the bond dissociation energies, we derive the gas-phase acidities for these compounds. These quantities allow us to compare the stabilization in carbon and silyl anions by a-silyl groups. Ab initio calculations aid in understanding the stabilization mechanism.
Electron photodetachment spectra have been measured in an ICR spectrometer for the enolate ions of acetone, cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, methyl vinyl ketone, pinacolone, propionaldehyde, and 1,1,1-trifluoroacetone enolates. Electron affinities have been determined for acetone enolate radical 1.758±0.019, cyclobutanone enolate radical 1.801±0.008, cyclopentanone enolate radical 1.598±0.007, cyclohexanone enolate radical 1.526±0.010, cycloheptanone enolate radical 1.444+0.02/−0.002, tert-butyl methyl ketone (pinacolone) enolate radical 1.755+0.05/0.005, propionaldehyde enolate radical 1.621±0.006, and 1,1,1-trifluoroacetone enolate radical 2.625±0.010 eV. Autodetaching dipole-bound states are observed in some but not all of these spectra. The mechanism for autodetachment of these states is discussed and it is seen that the binding of an electron by a dipole is very sensitive to the motions of the dipole. The motions of the dipole can be predicted from the rotational motions of the molecule, allowing us to correlate the observation of dipole-bound states with the rotational motions of the dipole moment.
3835 f 1. Since it is much larger than the experimental value (log A = 6.4) for the high-temperature reaction, we conclude that it is unlikely that this reaction occurs by the unimolecular mechanism A (Scheme HI). many more assumptions than the first (e.g., rotational barriers in the starting species and frequencies of the vibrational modes in 3), is rather long and involved, and is not reported in detail here.This second calculation is in general agreement with the first and gives log A = IO.Thus, according to both calculations, the preexponential for the reaction outlined in Scheme VI is approximately log A = 11 Registry No. Ag( 1 lo), 7440-22-4; fert-butyl alcohol, 75-65-0; isobutylene oxide, 558-30-5; deuterium, 7782-39-0. Abstract:We have measured the equilibrium gas-phase acidities for a series of alkyl-and aryl-substituted organosilanes, and the cross sections for electron photodetachment of the corresponding conjugate bases, using ion cyclotron resonance spectrometry. Gas-phase acidities for the conjugate acids of these compounds are AHoa,id(SiH4) = 372.8 f 2 kcal/mol, AHoaCid(C6H5SiH,) = 370.7 f 2 kcal/mol, AHoacid(C6H5(CHJ)SiH2) = 374.2 f 2 kcal/mol, AH0,,,(CH3SiH3) = 378.3 f 2 kcal/mol, and AHoa,id((CH3)3SiH) L 382.8 f 2 kcal/mol. The electron affinities are EA(SiH3') = 32.4 f 0.6 kcal/mol, EA(C6H5SiH2:) = 33.1 f 0.1 kcal/mol, EA(C6H5(CH3)SiH') = 30.7 f 0.9 kcal/mol, EA(CH3SiH2') = 27.5 f 0.8 kcal/mol, and EA((CH,),Si ) = 22.4 f 0.6 kcal/mol. These values were used to determine the Si-H bond dissociation energies in the organosilicon hydrides: Do[SiH3-H] = 91.6 f 2 kcal/mol, Do[C6H5SiH2-H] = 90.2 f 2 kcal/mol, Do[C6H5(CH3)SiH-H] = 91.3 f 3 kcal/mol, Do[CH3SiH2-H] = 92.2 f 3 kcal/mol, and D0[(CH3),Si-H] Z 91 .O f 2 kcal/mol. The electronic structure of the organosilicon compounds studied is discussed and compared with that in the corresponding carbon compounds.Thermochemical properties are indispensable tools for understanding chemical reactivity and making predictions about intermediates, products, equilibria, and reaction mechanisms. It is surprising, therefore, that despite the prolific investigation and utilization of organosilicon chemistry in the past 20 years, thermochemical data for organosilicon compounds remain scarce. For example, the data base of ionic, gas-phase thermochemical measurements' which has grown impressively in recent years has only minimal coverage in other classes such as organometallics. Especially important, yet experimentally challenging, are thermochemical measurements involving reactive silicon intermediates* such as divalent silicon compounds, silyl radicals, cations, and anions. In this paper, we report our measurements of the gas-phase acidities of selected organosilanes and the electron affinities of the corresponding organosilyl radicals.In addition to contributing thermochemical data, gas-phase investigations involving equilibrium basicities and electron photodetachment spectroscopy of silyl and substituted silyl anions can provide information about substituent effec...
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