Enthalpies of Formation of o-, m-, and p-Benzoquinone
A R T I C L E S
using the Becke three-parameter hybrid exchange16 and Lee-Yang-
Parr17 correlation density functional (B3LYP) and the 6-31G(d) and
6-31+G(d) basis sets. B3LYP single-point energies were obtained with
the 6-311G(d,p) and 6-311+G(2df,2pd)18 basis sets, and all of the
resulting energetic quantities include zero-point energies and have been
adjusted to 298.15 K, except for electron affinities which are at 0 K.
Unscaled vibrational frequencies were used for the density functional
theory (DFT) calculations, whereas the Hartree-Fock frequencies were
scaled by 0.8929 as called for in the G3 procedure. However, HF
frequencies can lead to significant errors in zero-point energies and
temperature corrections; thus, G3 results with DFT frequencies also
are given, and they are provided in parentheses.
Chart 1
Experimental Section
o-Benzoquinone (1o). This compound was prepared by a modifica-
tion of Fischer and Henderson’s previously reported procedure.8 That
is, a well-stirred solution of 0.11 g (1.0 mmol) of catechol (1,2-
dihydroxybenzene) and 3 g of anhydrous magnesium sulfate in 10 mL
of dry ether was cooled to 0 °C. Cerium (IV) on silica gel (6.94 g, 2.1
mmol)8 was added, and the mixture was shaken for exactly 3 min. It
was then rapidly suction-filtered through a jacketed funnel and cooled
to -78 °C. The residue was washed with 20 mL of cold ether and
combined with the filtrate. Concentration of the solution at -25 °C
under reduced pressure to a volume of ∼3 mL was followed by
crystallization at -78 °C to give 90 mg (83%) of the red product. This
quinone can be kept unchanged for two months by storing it in the
dark at -78 °C. 1H NMR (300 MHz, CDCl3) δ 6.43 (m, 2H), 7.06 (m,
2H).
Ion Energetics. Gas-phase experiments were carried out with a dual
cell model 2001 Finnigan Fourier transform mass spectrometer (FTMS)
equipped with a 3 T superconducting magnet and controlled by a Sun
workstation running the Odyssey software 4.2 package or a similar
instrument which is controlled by an Ion Spec data system running
IonSpec99 ver. 7.0. ortho-Benzoquinone and para-benzoquinone radical
anions were generated by electron attachment using low energy elec-
trons (3.5 eV), and the meta isomer was prepared from m-nitrophen-
oxide (which was made by reacting the trimethylsilyl ether of m-nitro-
phenol with fluoride ion) by collision-induced dissociation (CID) as
previously reported.9 The desired m/z 108 ions were isolated using
stored-waveform inverse Fourier transform (SWIFT)10 excitations and
then were transferred to the second (source) cell where they were trans-
lationally and vibrationally cooled with a pulse of argon up to a pressure
of ∼1 × 10-5 Torr. Neutral reagents were added to the source cell via
slow leak valves or a solid inlet probe, and all of the resulting reactions
were monitored as a function of time. For the electron affinity bracketing
experiments it was particularly important to use low pressures (∼1 ×
10-8 Torr) of the reference compounds to minimize adventitious electron
capture from stray electrons. This process was monitored by double
resonance (i.e., by continually ejecting the m/z 108 ion and observing
the effect on electron transfer) as well as by running the reactions
without transferring the ion of interest to the source cell.
Combustion Calorimetry
Materials and Purity Control. 3,5-Di-tert-butyl-o-benzo-
quinone and benzil (PhCOCOPh) were obtained commercially
from Aldrich with mass fraction purities of 0.993 and 0.9980,
respectively, as determined by gas-liquid chromatography.
These compounds were further purified by repeated vacuum
sublimations before the calorimetric measurements. Their purity
was assessed by differential scanning calorimetry (Setaram DSC
141) via a fractional fusion technique.19 The samples, hermeti-
cally sealed in stainless steel crucibles, were heated using a
heating rate of 1.67 × 10-2 K s-1, and the resulting thermograms
did not show any phase transitions between T ) 298 K and the
melting temperature of the studied compounds. Three high-
purity reference materials (naphthalene, benzoic acid, and
indium)20 were used to calibrate the temperature scale of the
calorimeter, and its power scale was calibrated with high purity
indium (mass fraction > 0.99999). Enthalpies of fusion of the
crystalline compounds and their purity were derived from the
DSC experiments and are presented in Table 1, where the
uncertainties are twice the standard deviations of the mean of
six independent runs.
Sample purities also were confirmed via carbon dioxide
recovery ratios. In particular, the average ratios of the mass of
(12) GAMESS-UK is a package of ab initio programs written by Guest, M. F.;
van Lenthe, J. H.; Kendrick, J.; Schoffel, K.; Sherwood, P., with
contributions from Amos, R. D.; Buenker, R. J.; van Dam, H. J. J.; Dupuis,
M.; Handy, N. C.; Hillier, I. H.; Knowles, P. J.; Bonacic-Koutecky, V.;
von Niessen, W.; Harrison, R. J.; Rendell, A. P.; Saunders, V. R.; Stone,
A. J.; de Vries, A. H. The package is derived from the original GAMESS
code due to Dupuis, M.; Spangler, D.; Wendoloski, J. NRCC Software
Catalog, Vol. 1, Program No. QG01 (GAMESS), 1980.
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under the auspices of EPSRC’s Collaborative Computational Project No.
1 (CCP1) (1995-1997).
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A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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To confirm the structure of the o-benzoquinone radical anion, it was
converted to the conjugate base of catechol by reaction with tert-butyl
mercaptan using sustained off-resonance irradiation (SORI)11 to drive
the process. The resulting hydrogen-atom transfer product was bracketed
with standard reference acids and further characterized by CID. These
results were compared to the authentic conjugate base of catechol, which
was generated by deprotonating 1,2-dihydroxybenzene with OH-;
hydroxide ion was made by electron ionization of H2O at 9 eV. We
also converted the meta isomer to its nitrite (C6H4(ONO)O-) by addition
of nitric oxide to 2m, and the resulting species was characterized by
CID and bracketing its proton affinity.
Computations. Calculations were carried out using the U.K. version
of GAMESS12,13 and Gaussian 200314 on IBM and SGI workstations.
G3 theory was employed for the parent quinones (1o and 1p) and related
species.15 The larger tert-butyl substituted compounds were optimized
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Torres, L. A. Thermochim. Acta 1999, 331, 93-204.
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