12362 J. Am. Chem. Soc., Vol. 123, No. 49, 2001
Halter et al.
approximation limited to single and double excitations (CCSD)77 and
CCSD(T) in which CCSD is augmented by a correction for triple-
excitation effects.78 Analytical gradient methods79,80 were used to
determine the equilibrium geometries and corresponding dipole mo-
ments. Basis sets used in the geometry optimizations were a triple-ú
plus double-polarization (TZ2P) basis81 and the correlation consistent
cc-pVTZ basis of Dunning.82
contained unknown impurities, but these impurities did not interfere
with the microwave experiments.
Propiolamide (14). In a slight modification of the literature
procedure,37 10 mL of NH4OH was cooled to -30 °C and methyl
propiolate (5.03 g, 59.8 mmol) added dropwise. The reaction was stirred
for 20 min at -30 °C. Solvent was removed under vacuum (0.1 mmHg)
overnight to yield 3.86 g (94%) of amide 14 as a yellow solid: 1H
NMR δ 6.2 (br s, 1H), 5.9 (br s, 1H), 2.85 (s, 1H); 13C NMR δ 153.8,
117.6, 74.5.
In addition, the quadratic and cubic force fields were determined at
the self-consistent field level of theory by a procedure based on
numerical differentiation of analytically computed second deriva-
tives.50,83,84 These force fields were then used to calculate vibrational
corrections to the rotational constants according to the formulas
described by Mills.85 A (nearly negligible) correction for centrifugal
distortion effects needed to convert rotational constants belonging to
the set of Watson’s determinable parameters to equilibrium rotational
constants was calculated on the basis of the equations given by
Kirchhoff.86
Propiolonitrile (15). In a slight modification of the published
procedure,38 P2O5 (40 g, 280 mmol) and 14 (3.86 g, 55.9 mmol) were
well mixed, as solids, under N2 in the distillation pot of a dry, short-
path distillation apparatus. The distillation pot was heated to 125 °C,
and 1.74 g (61%) of nitrile 15 was collected as a white solid at -78
°C. Caution: This product is a seVere Vesicant and great care should
be taken when using it. It will penetrate gloVes with ease and causes
1
painful burns and blistering: bp 44 °C; H NMR δ 2.50.
General Synthetic Methods. NEt3 was stirred over CaH2 overnight
and then distilled under N2. LiBr was oven dried for 24 h and stored
in a desiccator. Iodine and fumaronitrile were sublimed under vacuum
before use. Isotopically labeled 15NH4OH was purchased from Cam-
bridge Isotope Laboratories. Other compounds from commercial sources
were used without further purification. Unless otherwise noted, 1H and
13C NMR spectra (Bruker AC-300 spectrometer, Varian Unity-500)
and 2H NMR spectra (Bruker AC-250 spectrometer) were obtained in
CDCl3 or CHCl3, respectively. 1H and 13C chemical shifts were
referenced to residual solvent, while 2H chemical shifts were referenced
to CD2Cl2. All air- or moisture-sensitive reactions were carried out under
an argon atmosphere under anhydrous conditions. Flash column
chromatography was conducted on silica gel (E. Merck 60, 230-400
mesh).
(Z)-3-Bromoacrylonitrile (16). Synthesized according to the lit-
erature procedure39 and purified by vacuum distillation (20 mmHg, 80
°C). Caution: This product is a seVere Vesicant and great care should
be taken when using it. It will penetrate gloVes with ease and causes
painful burns and blistering as well as redness of the eyes. It should
be worked with only in a fume hood: 1H NMR δ 7.23 (d, J ) 8.1 Hz,
1H), 6.28 (d, J ) 8.1 Hz, 1H); 13C NMR δ 129.4, 114.8, 106.9.
(Z)-5-Trimethylsilylpent-2-en-4-ynenitrile (17). A dry flask charged
with Pd(PPh3)4 (1.0 g, 0.87 mmol) and CuI (0.34 g, 1.8 mmol) was
subjected to three pump/Ar flush cycles. NEt3 (25 mL) was added and
the solution turned black. 16 (2.27 g, 17.2 mmol) and trimethylsilyl-
acetylene (5.5 mL, 39 mmol) were added. After stirring for 70 min,
the solution was quenched by pouring into a mixture of H2O (25 mL)
and saturated aqueous NH4Cl (25 mL). The aqueous solution was
extracted with Et2O (3 × 25 mL), and the organic layers were combined,
washed with H2O (25 mL) and brine (25 mL), dried over MgSO4, and
filtered, and solvent was removed by rotary evaporation. The resulting
black oil was purified by flash column chromatography (SiO2, 5%
EtOAc/hexanes) to yield 1.62 g (63%) of the protected enynenitrile 17
Sample Preparation and Handling. Samples of enynenitrile 8 were
prepared by KOH-mediated deprotection of a trimethylsilyl-substituted
precursor 17. The deprotected enynenitrile was collected in a liquid
nitrogen-cooled flask, thawed, transferred to a dry glass ampule via
pipet, cooled in liquid nitrogen, and sealed under vacuum. In the case
of the deuterated samples, the ampules were prewashed with D2O in
order to minimize loss of deuterium via isotopic exchange with the
glass surface. Ampules were packaged in a Styrofoam container in dry
ice and shipped to the University of Michigan for characterization by
1
as a clear oil: Rf ) 0.20; H NMR δ 6.28 (d, J ) 11.1 Hz, 1H), 5.61
(d, J ) 11.1 Hz, 1H), 0.23 (s, 9H); 13C NMR δ 129.7, 115.6. 109.8,
108.5, 99.0, -0.53.
(Z)-Pent-2-en-4-ynenitrile (8). 17 (3.44 g, 23.0 mmol) was added
to 40 mL of a 0.13 M KOH solution in MeOH and and the resultant
mixture stirred for 30 min at -41 °C. After warming to room
temperature, H2O (40 mL) and CH2Cl2 (20 mL) were added and the
layers separated. The aqueous layer was extracted with CH2Cl2 (2 ×
20 mL), and the organic layers were combined, washed with H2O (40
mL) and brine (40 mL), dried over MgSO4, and filtered, and solvent
was removed under vacuum (0.02 mmHg) at -41 °C. The nitrile 8
was collected at 77 K upon warming to room temperature: 1H NMR
δ 6.29 (dd, J ) 10.8, 2.1 Hz, 1H), 5.73 (dd, J ) 11.1, 0.9 Hz, 1H),
3.65 (dd, J ) 2.4, 0.6 Hz, 1H); 13C NMR δ 128.9, 115.3, 110.2, 90.0,
78.1.
[15N]-Propiolamide ([15N]-14). A 6 N 15NH4OH solution (5 mL)
was cooled to -20 °C and methyl propiolate (2.18 g, 25.9 mmol) added
dropwise. The reaction was stirred for 20 min at -20 °C. Solvent was
removed under vacuum (0.1 mmHg) overnight to yield 1.20 g (66.7%)
of the amide as a yellow solid: 1H NMR δ 6.24 (dd, J[15N-1H] )
89.9 Hz, 2.7 Hz, 1H), 5.90 (dd, J[15N-1H] ) 90.6 Hz, 2.7 Hz, 1H),
2.85 (s, 1H).
[15N]-(Z)-3-Bromoacrylonitrile ([15N]-16). [15N]-Propiolamide (1.20
g, 17.1 mmol) and [14N]-propiolamide (3.78 g, 54.7 mmol) were mixed
to provide 24% 15N enrichment. 15N-enriched (Z)-3-bromoacrylonitrile
was then synthesized as described above: 1H NMR δ 7.23 (d, J ) 8.1
Hz, 1H), 6.28 (d, J ) 8.1 Hz, 1H); 13C NMR δ 129.3, 114.7 (d, J[15N-
13C] ) 17.8 Hz), 106.9, (d, J[15N-13C] ) 3.8 Hz).
[15N]-(Z)-5-Trimethylsilylpent-2-en-4-ynenitrile ([15N]-17). The
reaction was performed as described for the normal isotopomer using
15N-enriched (Z)-3-bromoacrylonitrile [15N]-16: 1H NMR δ 6.28 (d, J
) 11.1 Hz, 1H), 5.61 (d, J ) 11.1 Hz, 1H), 0.23 (s, 9H); 13C NMR δ
129.7, 115.6 (d, J[15N-13C] ) 18.4 Hz), 109.8, 108.5 (d, J[15N-13C]
) 3.9 Hz), 99.0, -0.53.
1
Fourier transform microwave spectroscopy. H NMR spectroscopy of
these samples often revealed Me3SiF as an impurity, but this impurity
did not interfere with accurate measurement of rotational spectra. The
short warming to room temperature necessary for ampule preparation
resulted in discoloration of the sample. The products causing this
1
discoloration were not visible in the H NMR spectra, nor did they
interfere with measurement of rotational spectra. Samples of 9 were
relatively stable solids and were shipped to Michigan in glass vials
packed in dry ice. 1H NMR spectra indicated that the dinitrile samples
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