The Journal of Physical Chemistry A
Article
improve the confidence in the experimental values obtained by
least-squares fitting transitions of coupled dyads and polyads.
Independent of the ambiguities with the higher-order
centrifugal distortion constants and Coriolis-coupling terms,
this work provides a highly accurate and precise energy
difference between these two vibrationally excited states, ν17
and ν27. Pure rotational spectroscopy, however, cannot directly
determine the individual energy values of these states. Due to
their isolation from the ground vibrational state, high-
resolution IR spectroscopy would be necessary for accurate
measurement of the energy of these fundamentals. The
experimental measurement of ΔE17,27 serves as a benchmark
for future computational estimations and, combined with the
spectroscopic constants determined for (cyanomethylene)-
cyclopropane (5) in this work, makes the fitting of the high-
resolution IR spectra of ν17 and ν27 a one-variable problem.
General Experimental Methods. All commercial re-
agents were purchased from Sigma-Aldrich or Oakwood
91.8 Hz). Chemical shifts are consistent with previous
reports.38
(Cyanomethylene)cyclopropane (5). To a flame-dried 250
mL single-neck round-bottom flask (cooled under a stream of
N2) was added 1-ethoxycyclopropanol (8, 2.07 g, 20.3 mmol),
(cyanomethylene)triphenylphosphorane (9, 6.01 g, 20 mmol),
benzoic acid (243 mg, 2 mmol, 10 mol %), and 115 mL of
benzene. The reaction vessel was fitted with a water-cooled
reflux condenser (with N2 vent) and heated to reflux for 24 h.
After cooling to room temperature, the flask was fitted with a
fractionating column (23 cm, glass bead-packed) and a short-
path distillation apparatus. Most of the benzene was removed
by distillation at atmospheric pressure. To the remaining liquid
(∼40 mL) in the boiling flask, 100 mL of pentane was added,
and the solution was cooled on ice to precipitate
triphenylphosphine oxide. The resulting mixture was filtered
through a fine glass frit into a separate boiling flask, and
pentane was removed by short-path distillation with a
fractionating column at atmospheric pressure, leaving the
crude product as a bright yellow liquid. The crude product was
purified via short-path vacuum distillation to yield
(cyanomethylene)cyclopropane (5) (bp = 88 °C at 100
Torr) as a colorless oil (820 mg, 52%). TLC: Rf = 0.5
1
Chemical and used as received, unless otherwise noted. H
NMR spectra (400 or 500 MHz) and 13C{1H}-NMR spectra
(100 or 125 MHz) were obtained in CDCl3 on a Bruker 400
MHz AVANCE III or Bruker 500 MHz DCH AVANCE III
spectrometer; chemical shifts (δ) are reported as ppm
downfield from internal standard SiMe4 or referenced to
residual solvent signals. Mass spectra were acquired using
electrospray ionization (ESI) or the atmospheric solids analysis
probe (ASAP) on a Thermo Scientific Q-Exactive Plus mass
spectrometer. IR spectra were obtained on a Bruker TENSOR
Fourier transform IR instrument as neat samples using an
attenuated total reflectance accessory (Bruker PLATINUM
ATR).
1
(pentane/diethyl ether, 6:1; KMnO4). H NMR (CDCl3, 400
MHz): δ (ppm) 5.76 (p, J = 2 Hz, 1H), 1.40 (d, J = 2 Hz, 4H).
13C{1H}-NMR (CDCl3, 100 MHz): δ (ppm) 151.9, 116.8,
90.4, 5.2, 4.1 IR (neat): (cm−1) 3072 (w), 3020 (w), 2987 (w),
2221 (m), 1744 (w), 1403 (w), 1230 (w), 1058 (m), 1003
(m), 939 (m), 913 (m), 760 (s), 732 (m), 682 (m). HRMS
(ESI) m/z: [M + H]+ calcd for C5H6N, 80.0495; found,
80.0494.
1-Ethoxycyclopropanol (8).28 A solution of (1-
ethoxycyclopropoxy)trimethylsilane (10, 9.98 g, 57 mmol) in
methanol (25 mL) was stirred at room temperature for 24 h.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
■
sı
1
The reaction was monitored via H NMR and TLC, which
confirmed complete deprotection of the trimethylsilane group
at 24 h. The methanol solvent was subsequently removed in
vacuo to afford a clear, colorless oil. The crude oil was purified
via short-path distillation to yield 1-ethoxycyclopropanol (8,
bp = 88−90 °C at 100 Torr) as a colorless oil (5.0 g, 48.5
mmol, 85%). TLC: Rf = 0.3 (hexane/EtOAc, 4:1; KMnO4). 1H
NMR (CDCl3, 500 MHz): δ (ppm) 4.37 (br s, 1H), 3.77 (q, J
= 7.2 Hz, 2H), 1.21 (t, J = 7.1 Hz, 3H), 0.98−0.87 (m, 4H).
13C{1H}-NMR (CDCl3, 125 MHz): δ (ppm) 85.5, 62.0, 15.4,
14.2. IR (neat): (cm−1) 3386 (m), 2978 (w), 1734 (m), 1460
(w), 1298 (m), 1225 (m), 1052 (m), 943 (m), 487 (w).
HRMS (ESI) m/z: [M + H]+ calcd for C5H11O2, 103.0754;
found, 103.0753.
Experimental IR, 1H NMR, 13C NMR, and mass spectra
Computational chemistry output files, least-squares
fitting output files from ASFIT and SPFIT (ZIP)
AUTHOR INFORMATION
Corresponding Authors
■
Robert J. McMahon − Department of Chemistry, University of
Wisconsin−Madison, Madison, Wisconsin 53706-1322,
R. Claude Woods − Department of Chemistry, University of
Wisconsin−Madison, Madison, Wisconsin 53706-1322,
(Cyanomethylene)triphenylphosphorane (9).27 To a
stirred solution of (cyanomethyl)triphenylphosphonium chlor-
ide (11, 120.4 g, 60.5 mmol) in 200 mL of deionized water was
added, dropwise, an aqueous solution of sodium hydroxide
(50% w/w, 10.0 mL) over 5 min at room temperature. The
addition resulted in the formation of a white precipitate, and
the mixture was stirred for an additional 30 min. The resulting
white solid was isolated by filtration via a coarse glass fritted
filter and dried in vacuo for 24 h to yield (cyanomethylene)-
Authors
Brian J. Esselman − Department of Chemistry, University of
Wisconsin−Madison, Madison, Wisconsin 53706-1322,
Samuel M. Kougias − Department of Chemistry, University of
Wisconsin−Madison, Madison, Wisconsin 53706-1322,
Maria A. Zdanovskaia − Department of Chemistry, University
of Wisconsin−Madison, Madison, Wisconsin 53706-1322,
1
triphenylphosphorane (9, 17.0 g, 56.5 mmol, 93%). H NMR
(CDCl3, 500 MHz): δ (ppm) 7.43−7.71 (m, 15H), 1.60 (s,
1H). 13C{1H}-NMR (125 MHz, CDCl3): δ (ppm) 132.80 (d, J
= 10.1 Hz), 132.57 (d, J = 3.0 Hz), 132.11 (d, J = 9.8 Hz),
129.08 (d, J = 12.4 Hz), 128.50 (d, J = 12.1 Hz), 127.43 (d, J =
Complete contact information is available at:
5612
J. Phys. Chem. A 2021, 125, 5601−5614