uncorrected, with a pulse width of 5 ms) through the ICR cell.
Conclusions
Methyl nitrite of ca. 1 × 10Ϫ7 Torr was used for the initiation of
the chemical ionization of the acids studied, eqns. (5)–(7). Each
The substituent effect on the gas-phase acidity of dimethyl-
phenylsilane is significantly smaller than that for carbon acids.
Resonance stabilization does not play an important role for the
stability of silyl anions. This is supported by geometrical
features of silyl anions obtained by ab initio calculations,
though the stabilization energies are not reproduced quanti-
tatively by these calculations.
MeONO ϩ eϪ
MeOϪ ϩ AoH
MeOϪ ϩ AH
AϪ ϩ AoH
MeOϪ ϩ NO
AoϪ ϩ MeOH
AϪ ϩ MeOH
AoϪ ϩ AH
(5)
(6)
(7)
(8)
Experimental
Chemicals and syntheses
neutral sample was separately introduced into the ICR cell
through a variable leak valve (ANELVA). The mass spectra and
timeplots were acquired and processed in a FT mode. After a
reaction period of 0.5 to 1 s, depending upon the pressure of
the neutrals, equilibrium was attained and relative abundances
The silanes used in this study were available from our previous
study24 except for the p-nitro derivative. p-Nitrophenyl-
dimethylsilane was prepared from p-nitrobenzoyl chloride and
1,1,2,2-tetramethyl-1,2-dichlorodisilane according to Rich’s
procedure.25
Ϫ
of AϪ and Ao were measured by signal intensities of ICR
spectra. Each measurement was performed at several ratios
of partial pressures and at different overall pressures. The
pressures of the neutral reactants were measured by means of
1,1,2,2-Tetramethyl-1,2-dichlorodisilane.26
A
mixture of
hexamethyldisilane (15 g, 100 mmol) and concentrated sulfuric
acid (100 g) was vigorously stirred at 23 ЊC for 2 days. The
mixture was then cooled to 0 ЊC and powdered ammonium
chloride (16 g, 300 mmol) was added over 30 min with stirring.
The oil bath was then heated up to 150 ЊC and the distilled
product was collected. The crude dichloride was redistilled
at 138–142 ЊC to give 1,1,2,2-tetramethyl-1,2-dichlorodisilane
(4.37 g, 23%).
a
Bayard–Alpert-type ionization gauge with appropriate
correction factors being applied to correct the gauge readings
for the different ionization cross sections of various com-
pounds.28 Arithmetic mean values of K from eqn. (9) were used
for the calculation of ∆GЊ at 343 K [eqn. (10)] with an average
uncertainty of 0.2 kcal molϪ1 in most cases.
K = [AoH/AH][AϪ/Ao
∆GЊ = RT ln K
]
(9)
Ϫ
p-Nitrophenyldimethylchlorosilane. A mixture of p-nitro-
benzoyl chloride (3.1 g, 17 mmol) and 1,1,2,2-tetramethyl-1,2-
dichlorodisilane (4.0 g, 21 mmol) was heated at 140 ЊC under
nitrogen. To the solution bis(benzonitrile)palladium chloride
(6.4 mg, 1.7 × 10Ϫ2 mmol) and triphenylphosphine (8.8 mg,
3.4 × 10Ϫ2 mmol) were added, resulting in the evolution of
carbon monoxide. The mixture was heated at 140 ЊC for 20 h.
Distillation (117–120 ЊC/5 mmHg) of the mixture afforded 1.5 g
(41%) of p-nitrophenyldimethylchlorosilane. δH(500 MHz;
CDCl3; Me4Si) 0.74 (6H, s, SiMe2), 7.81 (2H, d, J = 8.5 Hz, Ph),
8.24 (2H, d, J = 8.5 Hz, Ph).
(10)
The proton-transfer reactions were examined by ion-eject
experiments using the SWIFT technique.29 Each sample was
subjected to several freeze–pump–thaw cycles on the ICR inlet
vacuum system to remove entrapped impurities. The gas-phase
acidity values for the reference compounds were taken from the
literature.4
Calculations
Ab initio calculations were carried out using the Gaussian 98
program30 suite. The geometries were fully optimized at the
RHF/6-31ϩG* and RHF/6-311ϩϩG** levels of theory with
normal convergence. Vibrational normal mode analyses were
performed at the same level to ensure that each optimized
structure was a true minimum on the potential energy surface
and to calculate the thermal correction needed to obtain
the Gibbs free energies. The zero point energies used for the
thermal correction were unscaled. To improve the calculated
energies, single point MP2 calculations were also carried out at
the 6-31ϩG* and 6-311ϩϩG** basis sets using the frozen-core
approximation. Thermal corrections to the Gibbs free energy
evaluated at HF/6-31ϩG* and RHF/6-311ϩϩG** levels were
also applied to total energy of single point MP2 calculations.
For unsubstituted, m-fluoro and p-nitro derivatives, the geom-
etries were optimized at the MP2/6-31ϩG* level to examine the
dependency of calculated structures on the level of theory used.
p-Nitrophenyldimethylsilane. LiAlH4 (0.27 g, 7.0 mmol in
dry ether 100 mL) was added slowly to an ethereal solution of
p-nitrophenyldimethylchlorosilane (1.5 g, 7.0 mmol) under a
nitrogen atmosphere, then aq. HCl (10%, 30 mL) was added.
The mixture was extracted with ether, washed with saturated
aqueous NaCl solution, and dried over anhydrous magnesium
sulfate. Removal of solvent afforded the crude product (0.9 g,
28%) and purification by silica gel column chromatography
gave pure p-nitrophenyldimethylsilane (0.1 g), which was
1
characterized by H NMR and elemental analysis as follows.
δH(500 MHz; CDCl3; Me4Si) 0.74 (6H, s, SiMe2), 4.58 (1H, m,
SiH), 7.71 (2H, d, J = 8.5 Hz, Ph), 8.18 (2H, d, J = 8.5 Hz, Ph).
Anal. Calcd. for C8H11O2NSi: H, 6.21; C, 53.30; N, 7.53.
Found: H, 6.12; C, 53.01; N, 7.73%.
Gas-phase acidity measurements
The gas-phase acidity measurements were performed on an
Extrel FTMS 2001 Fourier transform mass spectrometer. Most
of the experimental techniques used for the measurements of
the equilibrium constants of the reversible proton-transfer
reaction (8) are the same as the general method described in the
literature.27 Only significant changes and additional procedures
will be given here.
References
1 E. W. Colvin, Silicon in Organic Synthesis, Butterworths, London,
1981, ch. 11.
2 D. M. Wetzel, K. E. Salomon, S. Berger and J. I. Brauman, J. Am.
Chem. Soc., 1989, 111, 3835.
3 R. Walsh, in The Chemistry of Organic Silicon Compounds, ed.
S. Patai and Z. Rappoport, John Wiley and Sons Ltd., New York,
1989, vol. I, pp. 371.
4 J. E. Bartmess, Negative Ion Energetics Data, ed. W. G. Mallard
and P. J. Linstrom, NIST Chemistry WebBook, NIST Standard
Reference Database Number 69, August 1997, National Institute of
All equilibrium measurements were performed at 50 ЊC at a
3.0 T magnetic field strength using a cubic (2 × 2 × 2 in.)
trapped analyzer cell. Typical operating pressures were 10Ϫ6
–
10Ϫ7 Torr. The proton-transfer reactions were initiated by
a pulsed electron beam (electron energy of 0.3–1.0 eV,
J. Chem. Soc., Perkin Trans. 2, 2001, 923–928
927