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Struct Chem (2011) 22:615–625
DOI 10.1007/s11224-011-9735-x

ORIGINAL RESEARCH

Theoretical study of substituent and solvent effects
on the thermodynamics for cis/trans isomerization
and intramolecular rearrangements of 2,20 -diphenoquinones
Sierra Rayne • Kaya Forest

Received: 12 November 2010 / Accepted: 6 January 2011 / Published online: 23 January 2011
Ó Springer Science+Business Media, LLC 2011

Abstract Enthalpies (DisomHo(g)), Gibbs free energies
(DisomGo(g)), and equilibrium constants (log Kisom) for the
trans ? cis isomerization of various 3,30 -, 4,40 -, and 5,50 disubstituted 2,20 -diphenoquinones with a range of electron
withdrawing and releasing moieties (methyl, fluoro, chloro,
bromo, trifluoromethyl, and amino) were calculated in the
gas phase and in the solvent phase (n-hexane, benzene,
n-octanol, acetonitrile, and water). In the gas phase, the
trans isomer of the parent and all substituted 2,20 -diphenoquinones is predicted to be more thermodynamically
stable than the cis configuration, with log Kisom ranging
from -2.8 to -7.0. For all compounds, increasing solvent
polarity/proticity progressively favors shifting the cis/trans
equilibrium towards greater contributions of the cis configuration and substantially increases the log Kisom by up to
5.1 units relative to the gas phase. In polar protic and polar
aprotic solvents, the estimated log Kisom ranges as low as
-0.4, indicating significant populations of the cis isomers
should be present. The findings support the polar solvent
phase mechanistic predictions for a cis configuration
of 2,20 -diphenoquinones participating in the thermal
transformation of trans-2,20 -diphenoquinones to oxepino
[2,3-b]benzofurans. With limited exceptions for some
amino derivatives, the cis-2,20 -diphenoquinone to oxepino
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-011-9735-x) contains supplementary
material, which is available to authorized users.
S. Rayne (&)
Ecologica Research, 301-1965 Pandosy Street, Kelowna,
BC V1Y 1R9, Canada
e-mail: rayne.sierra@gmail.com
K. Forest
Department of Chemistry, Okanagan College, 583 Duncan
Avenue West, Penticton, BC V2A 8E1, Canada

[2,3-b]benzofuran isomerization is expected to be thermodynamically favorable for all substituents/phases under
consideration. The cis-2,20 -diphenoquinone to oxepino
[2,3-b]benzofuran rearrangement is predicted to become
less thermodynamically favored with increasing solvent
polarity/proticity.
Keywords 2,20 -Diphenoquinones cis/trans
isomerization Intramolecular rearrangements
Equilibrium constants Substituent and solvent effects
Theoretical study

Introduction
In contrast to their 4,40 -diphenoquinone analogs, the 2,20 diphenoquinones remain a synthetic challenge to generate
and isolate. Despite over a century of study, only a select few
2,20 -diphenoquinones have been spectroscopically characterized and reported in the literature. In research efforts
during the 1960s through 1980s, the groups of Hewgill [1–7]
and Becker [8, 9] employed oxidative approaches to generate—and in some cases isolate—these elusive compounds.
More recently, during the 1990s and 2000s, it was shown that
2,20 -diphenoquinones may be photochemically generated
from a range of dibenzo[1,4]dioxins (including the parent
system) having electron withdrawing and donating groups
[10–13], as well as via traditional thermal oxidative methods with 3,30 ,5,50 -tetraaryl substituents (e.g., phenyl and
4-methoxyphenyl) [14].
The 2,20 -diphenoquinones can exist in the trans and cis
configurations (Fig. 1). The relative energies of these isomers are of interest, as mechanisms for the thermal intramolecular rearrangement of 2,20 -diphenoquinones into
oxepino[2,3-b]benzofurans have been proposed that require

123

616

involvement of the cis configuration [7, 9, 13, 14]. In
addition, the expected different spectral properties of the
cis/trans isomers, and their potential thermal/excited state
interconversions, offer possible utility as molecular switch
motifs. The ability to generate 2,20 -diphenoquinones photochemically from the well-known chlorinated dibenzo[1,4]dioxin environmental contaminants, as well as via
thermal oxidation of polychlorinated biphenyl (PCB)
metabolites (i.e., chlorinated-2,20 -dihydroxybiphenyls),
also warrants further mechanistic investigation regarding
their role in the toxicological effects of the parent compounds [12, 15]. Consequently, in this study we employ
various levels of theoretical methods to examine substituent and solvent effects on the cis/trans isomerization
equilibrium of 2,20 -diphenoquinones, as well as the thermodynamics of their intramolecular rearrangements.

Computational methods
Calculations were performed using Gaussian 09 [16], the
Hartree–Fock model chemistry, the M062X [17], wB97XD
[18], LC-wPBE [19–22], B97D [23], BLYP [24–26], BP86
[24, 27], CAM-B3LYP [28], PBE0 [29–31], B3LYP [25,
26, 32], and BMK [33] density functionals with the
6-311??G(d,p) [34, 35] basis set, and the CBS-Q//B3 [36,
37], G4MP2 [38], and G4 [39] composite method levels of
theory. Geometry optimizations and subsequent frequency
analyses were conducted at the same level of theory; no
imaginary frequencies were present in final geometries. All
enthalpies and free energies include thermal and zero point
energy corrections. Solvent phase calculations employed
the SMD [40], IEFPCM-UFF [41, 42], and CPCM [43, 44]
implicit solvation models. Structures were visualized using
Gabedit 2.2.12 [45].

Results and discussion
Gas phase (298.15 K, 1 atm) enthalpies (DisomHo(g)), Gibbs
free energies (DisomGo(g)), and equilibrium constants (log
Kisom) for the trans ? cis isomerization of the parent 2,20 diphenoquinone were calculated using the Hartree–Fock

Struct Chem (2011) 22:615–625

method and various density functionals—all with the
6-311??G(d,p) basis set—as well as via the CBS-Q//B3
composite method (Table 1). All levels of theory predict
the trans configuration is more thermodynamically stable
(by 5.3–7.4 kcal/mol) than the cis, yielding log Kisom
ranging from -3.9 to -5.4 strongly favoring the trans
isomer. Based on prior isomerization energy benchmarking
efforts [46, 47], CBS-Q//B3 values are taken as reference
estimates (DisomHo(g) = 4.6 kcal/mol; DisomGo(g) = 5.4 kcal/
mol; and log Kisom = -4.0). The M062X functional with
the 6-311??G(d,p) basis set provided near CBS-Q//B3
quality estimates (DisomHo(g) = 5.5 kcal/mol; DisomGo(g) =
5.7 kcal/mol; and log Kisom = -4.1), in agreement with
previous investigations [46–50], as did the long-range
corrected wB97XD (which also includes an empirical
dispersion correction) and LC-wPBE functionals. Good
performance was also obtained on this system from the
B97D dispersion corrected functional, although this
method has been shown to yield inconsistent isomerization
energy accuracy on small and larger organic compounds
[47, 49, 51]. Given the consistent isomerization energy
performance to date in the literature of the M062X functional for a broad range of small and large organic molecules with varying levels of strain, it was chosen for further
calculations.
Analogous calculations were conducted on the isomerizations of cis-2,20 -diphenoquinone to oxepino[2,3-b]benzofuran, trans-2,20 -diphenoquinone to 4,40 -diphenoquinone,
and trans-2,20 -diphenoquinone to 2,40 -diphenoquinone. The
isomerization of cis-2,20 -diphenoquinone to oxepino[2,
3-b]benzofuran is strongly thermodynamically favored
(CBS-Q//B3 DisomHo(g) = -16.0 kcal/mol, DisomGo(g) =
-14.2 kcal/mol, and log Kisom = 10.4; M062X/6-311?
?G(d,p)
DisomHo(g) = -18.1 kcal/mol,
DisomGo(g) =
-16.1 kcal/mol, and log Kisom = 11.8), supporting prior
experimental studies showing these compounds can be
formed via the thermal intramolecular rearrangement of
various 2,20 -diphenoquinones [7, 9, 13, 14]. In contrast to the
trans ? cis isomerization calculations for 2,20 -diphenoquinone, there are wide differences in the isomerization
enthalpies and free energies predicted by the various levels
of theory, ranging from a DisomGo(g) of ?0.3 kcal/mol (log
Kisom = -0.2) at the BLYP/6-311??G(d,p) level slightly

Fig. 1 General structure and numbering system for the trans and cis configurations of 2,20 -diphenoquinones and the proposed general
mechanism for the formation of oxepino[2,3-b]benzofurans via the cis configuration of 2,20 -diphenoquinones (adapted from [7, 9])

123

7.4 [2.0]

-5.4 [1.4]

-13.8 [2.2]

-12.0 [2.2]
6.7 [2.1]

Absolute deviations from the CBS-Q//B3 estimates are given in brackets

2.7 [0.3]

2.3 [0.1]
-3.2 [0.1]
-4.3 [0.7]
6.5 [0.0]
-8.9 [0.0]
-8.3 [1.1]

2.7 [0.3]

BMK/6-311??G(d,p)

6.8 [1.4]

-5.0 [1.0]

-8.3 [7.7]

-6.4 [7.8]
6.1 [1.5]

8.8 [1.6]

-3.7 [0.4]
-5.0 [0.0]
6.7 [0.2]
-9.2 [0.3]
-9.3 [0.1]

2.6 [0.2]

B3LYP/6-311??G(d,p)

6.7 [1.3]

-4.9 [0.9]

-15.5 [0.5]

-13.6 [0.6]
6.2 [1.6]

4.7 [5.7]

-3.6 [0.3]
-4.8 [0.2]
6.7 [0.2]
-9.1 [0.2]
-9.0 [0.4]

2.4 [0.0]

PBE0/6-311??G(d,p)

9.9 [0.5]

-3.5 [0.2]

-3.2 [0.1]
-4.2 [0.8]

-4.5 [0.5]
6.2 [0.3]

6.2 [0.3]
-8.5 [0.4]

-8.5 [0.4]
-8.2 [1.2]
9.3 [1.1]

-8.3 [1.1]
-7.2 [8.8]

-14.7 [1.3]
-4.8 [0.8]

-4.8 [0.8]
6.5 [1.1]

6.5 [1.1]

-5.3 [8.9]

6.2 [1.6]
CAM-B3LYP/6-311??G(d,p)

-12.7 [1.5]

6.0 [1.4]
BP86/6-311??G(d,p)

-1.5 [14.5]
6.4 [1.0]

-4.7 [0.7]

0.3 [14.5]
5.7 [1.1]

3.9 [6.5]

3.9 [1.5]

2.4 [0.0]
-3.2 [0.1]
-4.6 [0.4]
6.4 [0.1]
-8.7 [0.2]
-8.8 [0.6]

1.5 [0.9]

BLYP/6-311??G(d,p)

-10.3 [5.7]
6.0 [0.6]

-4.4 [0.4]

-8.2 [6.0]
5.9 [1.3]

-0.2 [10.6]

-5.4 [2.1]
-6.2 [1.2]
8.7 [2.2]
-11.8 [2.9]
-11.4 [2.0]

2.5 [0.1]

HF/6-311??G(d,p)

6.0 [4.4]

-2.0 [1.3]
-5.4 [0.4]
5.8 [0.7]
-7.9 [1.0]
-10.0 [0.6]
5.7 [0.3]

-4.2 [0.2]

-1.7 [14.3]

0.0 [14.2]
4.6 [1.0]
B97D/6-311??G(d,p)

0.0 [10.4]

2.6 [0.2]
-3.5 [0.2]

-3.3 [0.0]
-4.0 [1.0]

-4.1 [0.9]
6.1 [0.4]

6.1 [0.4]
-8.3 [0.6]

-8.3 [0.6]
-7.6 [1.8]
8.8 [1.6]

-7.7 [1.7]
-20.3 [4.3]

-14.2 [1.8]
-3.9 [0.1]

-3.9 [0.1]
5.4 [0.0]

5.3 [0.1]

-18.2 [4.0]
5.3 [0.7]
LC-wPBE/6-311??G(d,p)

-12.0 [2.2]
5.5 [0.9]
wB97XD/6-311??G(d,p)

13.3 [2.9]

1.7 [0.7]

2.4
-3.3

-2.4 [0.9]
-4.6 [0.4]

-5.0
6.5

6.5 [0.0]
-8.9 [0.0]

-8.9
-9.4

-8.0 [1.4]
11.8 [1.4]
-18.1 [2.1]

-16.0
-4.0

-4.1 [0.1]
5.7 [0.3]

5.4

-16.1 [1.9]
5.5 [0.9]
M062X/6-311??G(d,p)

-14.2
4.6
CBS-Q//B3

10.4

log
Kisom
DisomGo(g)
(kcal/mol)
DisomHo(g)
(kcal/mol)
DisomGo(g)
(kcal/mol)

log Kisom

DisomGo(g)
(kcal/mol)
DisomHo(g)
(kcal/mol)
DisomHo(g)
(kcal/mol)
Level of theory

log Kisom

DisomHo(g)
(kcal/mol)

DisomGo(g)
(kcal/mol)

log Kisom

trans-2,20 -DPQ ? 2,40 -DPQ
cis-2,20 -DPQ ? oxepino[2,3-b]benzofuran

trans-2,20 -DPQ ? 4,40 -DPQ

617

trans-2,20 -DPQ ? cis-2,20 -DPQ

Table 1 Gas phase (298.15 K, 1 atm) enthalpies (DisomHo(g)), Gibbs free energies (DisomGo(g)), and equilibrium constants (log Kisom) for various isomerization reactions of trans- and cis-2,20 diphenoquinone (2,20 -DPQ), oxepino[2,3-b]benzofuran, 4,40 -diphenoquinone (4,40 -DPQ), and 2,40 -diphenoquinone (2,40 -DPQ) at various levels of theory

Struct Chem (2011) 22:615–625

favoring the cis-2,20 -diphenoquinone to a DisomGo(g) of
-18.2 kcal/mol (log Kisom = ?13.3) at the LC-wPBE/6-311?
?G(d,p) level that strongly favors the oxepino[2,3-b]benzofuran. The reasonable agreement between the CBS-Q//B3 results
and DFT calculations with the M062X, wB97XD, LC-wPBE,
CAM-B3LYP, PBE0, and BMK functionals suggests the gas
phase (298.15 K, 1 atm) cis-2,20 -diphenoquinone ? oxepino
[2,3-b]benzofuran intramolecular rearrangement is exothermic
by about 15 kcal/mol.
Good thermodynamic agreement was found between all
DFT and CBS-Q//B3 calculations on the isomerization of
trans-2,20 -diphenoquinone to 4,40 -diphenoquinone (Fig. 2),
with the HF results indicating slightly more exothermicity
than at the DFT and CBS-Q//B3 levels. The 4,40 -diphenoquinone is predicted to be more thermodynamically
stable than trans-2,20 -diphenoquinone by about 9 kcal/mol
(CBS-Q//B3 DisomHo(g) = -9.4 kcal/mol, DisomGo(g) =
-8.9 kcal/mol, and log Kisom = 6.5; DFT ranges of
DisomHo(g) = -7.6 to -10.0 kcal/mol, DisomGo(g) = -7.9 to
-9.2 kcal/mol, and log Kisom = 5.8 to 6.7). 2,40 -Diphenoquinone is approximately midpoint in thermodynamic
stability between the 4,40 -diphenoquinone and trans-2,20 diphenoquinone, resulting in the a priori expected gas
phase relative thermodynamic stability rank order of
oxepino[2,3-b]benzofuran [ 4,40 -diphenoquinone [ 2,40 diphenoquinone [ trans-2,20 -diphenoquinone [ cis-2,20 diphenoquinone. Excellent agreement was obtained at all
levels of theory on the isomerization of trans-2,20 -diphenoquinone to 2,40 -diphenoquinone. Again, the HF results
are modestly more exothermic than those at the DFT and
CBS-Q//B3 levels of theory. The 2,40 -diphenoquinone is
predicted to be more thermodynamically stable than trans2,20 -diphenoquinone by about 3 kcal/mol (CBS-Q//B3
DisomHo(g) = -5.0 kcal/mol, DisomGo(g) = -3.3 kcal/mol,
and log Kisom = 2.4; DFT ranges of DisomHo(g) = -4.0 to
-5.4 kcal/mol, DisomGo(g) = -2.0 to -3.7 kcal/mol, and
log Kisom = 1.5 to 2.7).
Temperature is predicted to have negligible influence on
DisomHo(g), DisomGo(g), and log Kisom for the parent 2,20 -diphenoquinone trans ? cis isomerization, with increases
of only 0.1 kcal/mol, 0.2 kcal/mol, and 0.2 units, respectively, between 0 and 500 K at the M062X/6311??G(d,p) level of theory. Similarly, the solvent model
employed results in little differentiation of predicted
DisomHo(g) and DisomGo(g) in nonpolar and polar aprotic and
polar protic organic solvents and water (Table 2). Using the

Fig. 2 Structures of 4,40 -diphenoquinone and 2,20 -diphenoquinone

123

618

Struct Chem (2011) 22:615–625

SMD, IEFPCM-UFF, and CPCM implicit solvation models,
DisomHo(g), DisomGo(g), and log Kisom variations of B1.2 kcal/
mol, B1.3 kcal/mol, and B0.9 units, respectively, were
observed. The thermodynamic differences between the
solvation models are negligible for nonpolar aprotic organic
solvents such as n-hexane and benzene, and progressively
increase with both solvent polarity and proticity. Given the
demonstrated success of the SMD model for tackling difficult solvation problems on a broad range of functional
groups [38, 52–54], it was used for further calculations.
Analogous temperature calculations on the cis-2,20 -diphenoquinone to oxepino[2,3-b]benzofuran isomerization
between 100 and 500 K indicate modest decreasing trends
in DisomHo(g) (100 K, -17.5; 200 K, -17.8; 300 K, -18.1;
400 K, -18.4; and 500 K, -18.5 kcal/mol) and log Kisom
(100 K, 12.6; 200 K, 12.2; 300 K, 11.8; 400 K, 11.2;
and 500 K, 10.7), and corresponding increasing trend in
DisomGo(g) (100 K, -17.2; 200 K, -16.7; 300 K, -16.1;
400 K, -15.3; and 500 K, -14.5 kcal/mol).
Gas and solvent phase (n-hexane, benzene, n-octanol,
acetonitrile, and water) calculations were subsequently
conducted at the SMD-M062X/6-311??G(d,p) level on
the 3,30 -, 4,40 -, and 5,50 -dimethyl, difluoro, dichloro,
dibromo, di(trifluoromethyl), and diamino substituted cisand trans-2,20 -diphenoquinones. Assuming errors on the
order of *±1 kcal/mol in each of the theoretical calculations, there are few clear general trends in DisomHo(g),
DisomGo(g), and log Kisom for the trans- to cis-2,20 -diphenoquinone isomerizations that are dependent on substitution pattern for a given substituent, or upon the electron
withdrawing or releasing ability of a substituent at a particular position (Table 3). The possible exceptions are the
4,40 - and 5,50 -diamino-2,20 -diphenoquinones, which have
estimated gas phase log Kisom (-7.0 and -6.1, respectively) significantly lower than the other substituents at
these positions (-3.9 to -5.0).
For the methyl and amino moieties, the 3,30 -disubstituted 2,20 -diphenoquinones have substantially higher log
Kisom (-3.0 and -2.8, respectively) compared to their 4,40 and 5,50 -analogs (-4.6/-4.7 and -7.0/-6.1, respectively).

Intramolecular hydrogen bonding between these substituents and the carbonyl oxygens is not the likely cause of
these differences, as internuclear distances between the
substituent hydrogen atoms and adjacent carbonyl oxygen
˚ ; cis: 2.681
on the 3,30 -dimethyl (trans: 2.698 and 2.801 A
0
˚
˚ ; cis:
and 2.970 A) and 3,3 -diamino (trans: 2.183 A
˚
2.255 A) are longer than typical O H hydrogen bonds
[55]. There is also little difference in the O O internuclear
distances for the cis configurations of the 3,30 -dimethyl
˚ ) and 3,30 -diamino (2.793 A
˚ ) 2,20 -diphenoqui(2.834 A
0
nones compared to the 3,3 -di(trifluoromethyl) analog
˚ ), suggesting potential attractive (methyl and
(2.831 A
amino) or repulsive (di(trifluoromethyl)) through-space
interactions between the 3,30 -diortho substituents and the
neighboring carbonyl moiety are not determinative on the
isomerization thermodynamics.
The difference between the log Kisom of the 3,30 -dimethyl and diamino 2,20 -diphenoquinones and their 4,40 -/5,50 counterparts also decreases progressively in moving from
the gas phase to nonpolar aprotic solvents (n-hexane/
benzene) to polar aprotic (acetonitrile) and polar protic
(n-octanol/water) solvents. For example, the predicted log
Kisom variation among the three dimethyl isomers under
study is 1.7 units in the gas phase, decreasing to 1.1 units in
both n-hexane and benzene, and further declining to 0.3,
0.5, and 0.4 units for n-octanol, acetonitrile, and water,
respectively. Similarly, the corresponding diamino log
Kisom variation declines from 4.2 units (gas phase) to 1.8
and 2.3 units (n-hexane and benzene, respectively) to 1.9,
2.4, and 1.0 units (n-octanol, acetonitrile, and water,
respectively). The difluoro, dichloro, dibromo, and di(trifluoromethyl) substituted 2,20 -diphenoquinones do not
exhibit this declining inter-isomer log Kisom variation with
increasing solvent polarity/proticity, suggesting a particularly strong solvent effect on the log Kisom for the methyl
and amino-substituted derivatives.
For all compounds, increasing solvent polarity/proticity
substantially increases the log Kisom, thereby progressively
shifting the equilibrium towards greater contributions of the
cis isomer. The log Kisom difference between the gas and

Table 2 Comparison of estimated enthalpies (DisomHo(g)), Gibbs free
energies (DisomGo(g)), and equilibrium constants (log Kisom) for the
trans ? cis isomerization of 2,20 -diphenoquinone at the M062X/6-

311??G(d,p) level of theory in various solvents with the SMD,
IEFPCM-UFF, and CPCM implicit solvation models

DisomHo(g) (kcal/mol)

n-Hexane
Benzene

DisomGo(g) (kcal/mol)

log Kisom

SMD

IEFPCM-UFF

CPCM

SMD

IEFPCM-UFF

CPCM

SMD

IEFPCM-UFF

CPCM

4.4
4.0

4.5
4.3

4.4
4.1

4.6
4.3

4.7
4.4

4.5
4.2

-3.3
-3.1

-3.4
-3.2

-3.3
-3.1

n-Octanol

2.1

2.9

2.8

2.2

3.0

2.9

-1.6

-2.2

-2.1

Acetonitrile

2.0

2.5

2.5

2.2

2.6

2.6

-1.6

-1.9

-1.9

Water

1.2

2.4

2.4

1.2

2.5

2.5

-0.9

-1.8

-1.8

123

6.3

5.7

6.0

6.8

5.7

5.7

6.3

5.3

5.7

6.3

5.2

6.8

5.0

5.5

4.7

9.4

7.3

4,40 -Methyl

5,50 -Methyl

3,30 -Fluoro

4,40 -Fluoro

5,50 -Fluoro

3,30 -Chloro

4,40 -Chloro

5,50 -Chloro

3,30 -Bromo

4,40 -Bromo

5,50 -Bromo

3,30 -Trifluoromethyl

4,40 -Trifluoromethyl

5,50 -Trifluoromethyl

3,30 -Amino

4,40 -Amino

5,50 -Amino

5.8

7.9

3.4

4.4

3.8

5.7

4.0

4.9

4.9

4.3

5.0

4.7

4.7

5.4

5.0

3.9

5.0

3.6

4.0

5.3

7.5

3.2

4.2

3.5

5.3

3.7

4.6

4.6

4.0

4.5

4.4

4.4

5.1

5.3

3.6

4.7

3.3

2.1

2.6

4.4

1.5

2.9

1.7

3.2

2.4

2.8

2.9

2.6

2.7

2.7

2.6

3.0

3.0

2.1

2.7

2.0

2.0

2.6

3.6

2.3

2.8

1.3

2.4

2.2

2.5

2.3

2.4

2.3

2.0

2.7

2.7

2.6

1.9

2.0

2.0

1.5

2.5

1.9

2.4

1.0

1.9

2.0

2.0

1.4

2.0

2.0

1.7

1.9

2.1

2.0

1.2

1.8

1.3

1.2

Water
5.7

8.3

9.6

3.8

5.6

5.3

6.9

5.6

6.3

5.9

6.0

6.1

6.2

6.6

6.8

6.6

6.4

6.2

4.1

6.4

7.6

5.2

5.1

4.3

6.0

4.3

5.3

5.0

5.1

4.8

5.2

5.7

5.8

6.4

4.4

4.6

3.0

4.6

n-Hexane

5.7

7.7

4.6

4.8

3.9

5.7

4.3

4.5

4.7

4.7

3.9

4.9

5.3

5.5

4.3

4.0

4.2

2.7

4.3

Benzene

4.4

5.5

4.5

Parent

3,30 -Methyl

Acetonitrile

Gas

n-Octanol

n-Hexane

Gas

Benzene

DisomGo(g) (kcal/mol)

DisomHo(g) (kcal/mol)

2.5

4.2

1.5

3.1

1.5

2.3

2.8

2.8

2.9

2.9

2.1

3.4

3.1

3.3

3.5

2.8

2.6

2.5

2.2

n-Octanol

2.3

4.5

1.2

3.0

1.0

2.6

2.4

2.5

2.5

3.1

1.8

1.8

3.2

2.9

2.9

2.6

1.9

2.5

2.2

Acetonitrile

1.4

2.8

1.5

2.5

0.5

1.8

2.1

1.7

1.4

2.1

2.2

2.6

2.3

2.4

2.5

2.2

2.0

1.6

1.2

Water

-6.1

-7.0

-2.8

-4.1

-3.9

-5.0

-4.1

-4.6

-4.3

-4.4

-4.4

-4.5

-4.8

-5.0

-4.8

-4.7

-4.6

-3.0

-4.1

Gas

-4.7

-5.6

-3.8

-3.7

-3.1

-4.4

-3.1

-3.9

-3.7

-3.7

-3.5

-3.8

-4.2

-4.2

-4.7

-3.2

-3.3

-2.2

-3.3

n-Hexane

log Kisom

-4.2

-5.7

-3.4

-3.6

-2.9

-4.2

-3.1

-3.3

-3.4

-3.4

-2.9

-3.6

-3.9

-4.0

-3.1

-2.9

-3.1

-2.0

-3.1

Benzene

-1.8

-3.0

-1.1

-2.3

-1.1

-1.7

-2.0

-2.1

-2.1

-2.1

-1.5

-2.5

-2.3

-2.4

-2.6

-2.1

-1.9

-1.8

-1.6

n-Octanol

-1.7

-3.3

-0.9

-2.2

-0.8

-1.9

-1.7

-1.8

-1.9

-2.3

-1.3

-1.3

-2.3

-2.1

-2.1

-1.9

-1.4

-1.8

-1.6

Acetonitrile

-1.0

-2.0

-1.1

-1.8

-0.4

-1.3

-1.6

-1.2

-1.0

-1.5

-1.6

-1.9

-1.7

-1.7

-1.9

-1.6

-1.4

-1.2

-0.9

Water

Table 3 Enthalpies (DisomHo(g)), Gibbs free energies (DisomGo(g)), and equilibrium constants (log Kisom) for the trans ? cis isomerization of 3,30 -, 4,40 -, and 5,5-disubstituted 2,20 -diphenoquinones at the M062X/6-311??G(d,p) level of theory in the gas phase and various solvents using the SMD implicit solvation model

Struct Chem (2011) 22:615–625
619

123

620

aqueous phases ranges up to 5.1 units (5,50 -diamino-2,20 diphenoquinone), with a corresponding difference of up to
3.7 units between the nonpolar aprotic solvents (n-hexane
and benzene) and water. In the polar protic (n-octanol
and water) and polar aprotic (acetonitrile) solvents, the
estimated log Kisom ranges as low as -0.4 (4,40 -di(trifluoromethyl)-2,20 -diphenoquinone), indicating significant
populations of the cis isomers should be present in these
more polar solvents. The findings support the polar solvent
phase mechanistic predictions (and requirements) for a cis
configuration of 2,20 -diphenoquinones participating in the
thermal transformation of trans-2,20 -diphenoquinones to
oxepino[2,3-b]benzofurans.
To further probe this question, the corresponding gas and
solvent phase (n-hexane, benzene, n-octanol, acetonitrile,
and water) calculations were also conducted at the SMDM062X/6-311??G(d,p) level on the 3,30 -, 4,40 -, and 5,50 dimethyl, difluoro, dichloro, dibromo, di(trifluoromethyl),
and diamino disubstituted oxepino[2,3-b]benzofurans,
facilitating estimation of the DisomHo(g), DisomGo(g), and log
Kisom for the cis-2,20 -diphenoquinone to oxepino[2,3-b]
benzofuran isomerizations (Table 4). For all compounds,
increasing solvent polarity/proticity strongly decreases the
thermodynamic favorability of the cis-2,20 -diphenoquinone
to oxepino[2,3-b]benzofuran isomerization. Differences in
log Kisom between the gas phase and water and n-hexane and
water solvent systems range between 2.5 (3,30 -diamino) to
12.2 (4,40 -diamino) units (gas ? water) and 1.9 (3,30 -diamino) to 8.7 (4,40 -diamino) units (n-hexane ? water). In the
gas phase and nonpolar aprotic solvents, the relative cis-2,20 diphenoquinone to oxepino[2,3-b]benzofuran log Kisom rank
order is 3,30 [ 5,50 [ 4,40 for all substituents except the
trifluoromethyl group (where the log Kisom rank order is
5,50 [ 4,40 & 3,30 ). The relative log Kisom rank orders
remain constant with increasing solvent polarity/proticity for
all substituents except the chlorinated and brominated
derivatives, whose rank orders shift from 3,30 [ 5,50 [ 4,40
in the gas phase and nonpolar aprotic solvents to 5,50 [
3,30 [ 4,40 in the polar aprotic and polar protic solvents.
In all solvents, the cis-2,20 -diphenoquinone to oxepino[2,3-b]benzofuran equilibrium is expected to favor the
oxepino[2,3-b]benzofuran for all substituents considered
except the diamino derivatives. Only the 3,30 -diaminooxepino[2,3-b]benzofuran is expected to be thermodynamically favored compared to the corresponding cis-2,20 diphenoquinone. In contrast, the 4,40 -diaminooxepino[2,3b]benzofuran is predicted to be less thermodynamically
stable than the 4,40 -diamino-cis-2,20 -diphenoquinone in all
solvents (including the nonpolar n-hexane and benzene
systems), while the 5,50 -diamino-cis-2,20 -diphenoquinone
to 5,50 -diaminooxepino[2,3-b]benzofuran isomerization is
expected to transition from favoring the 5,50 -diaminooxepino[2,3-b]benzofuran in the gas phase and nonpolar

123

Struct Chem (2011) 22:615–625

aprotic solvents to favoring the 5,50 -diamino-cis-2,20 -diphenoquinone in polar aprotic and polar protic solvents.
For most compounds, solvent proticity is a more important
determinant on the cis-2,20 -diphenoquinone to oxepino
[2,3-b]benzofuran log Kisom than is solvent polarity, as evident by the generally closer agreement in log Kisom values
between n-octanol and water versus acetonitrile and water
(acetonitrile [e/eo = 35.7] is substantially closer in polarity
to water [e/eo = 78.4] than is n-octanol [e/eo = 9.9]).
The relative gas phase enthalpies of formation (DfHo(g);
298.15 K, 1 atm) for the 2,20 -diphenoquinones are also of
interest, particularly when compared to the ortho- and
para-benzoquinones. Fattahi et al. [56] have recently
reported the following experimental and high-level composite method DfHo(g) for the ortho- and para-benzoquinones: o-benzoquinone, expt. DfHo(g) = -23.1 ± 4.1, G3
DfHo(g) = -19.7 kcal/mol; p-benzoquinone, expt. DfHo(g) =
-27.7 ± 3.0, G3 DfHo(g) = -27.6 kcal/mol. Rojas-Aguilar
et al. [57] obtained a similar experimental DfHo(g) for
p-benzoquinone of -28.8 ± 0.5 kcal/mol. At the B3LYP/
GTlarge level using isodesmic reactions, Altarawneh
et al. [58] have recently calculated DfHo(g) of -17.4 and
-25.2 kcal/mol for o- and p-benzoquinone, respectively.
Using the atomization enthalpy approach [59, 60] at
the G4MP2 and G4 levels of theory, we obtain equivalent DfHo(g) (o-benzoquinone: G4MP2 DfHo(g) = -19.8,
G4 DfHo(g) = -19.7 kcal/mol; p-benzoquinone: G4MP2
DfHo(g) = -27.5, G4 DfHo(g) = -27.3 kcal/mol) as the G3
results from Fattahi et al. [56], and in good agreement with
the B3LYP/GTlarge results of Altarawneh et al. [58].
Consequently, Fattahi et al. [56] found an experimental
DDfHo(g) of -4.6 ± 5.1 kcal/mol between the o- and
p-benzoquinones, similar to their G3 calculation
(-7.9 kcal/mol) and our G4MP2 (-7.7 kcal/mol) and G4
(-7.6 kcal/mol) results.
Using the isodesmic reaction o-benzoquinone ? 4,40 -diphenoquinone ? p-benzoquinone ? 2,20 -diphenoquinone,
we obtain enthalpies of reaction (DrxnHo(g); 298.15 K,
1 atm) at various levels of theory that are about 1 kcal/mol
(Table 5). Assuming this thermochemical magnitude is
within the experimental and theoretical limits of accuracy,
the relative DfHo(g) of the 2,20 - and 4,40 -diphenoquinones
appear to be equivalent to the relative DfHo(g) of the o- and
p-benzoquinones. Thus, the DfHo(g) of 2,20 -diphenoquinone
is expected to be about 8 kcal/mol higher than that of 4,40 diphenoquinone. G4MP2 and G4 calculations on the
diphenoquinones were cost prohibitive, and DFT-based
DfHo(g) atomization method estimates with moderate basis
sets are known to be in significant error for this size of
molecule [46, 61–63].
At the CBS-Q//B3 level of theory, atomization method
estimates yield DfHo(g) of -18.9 kcal/mol for o-benzoquinone, -26.9 kcal/mol for p-benzoquinone, 21.6 kcal/mol

-15.3
-15.6
-16.7
-18.0
-10.9

-15.4 -12.6

-22.6 -19.7

-12.3

-17.2 -14.9

-18.5 -16.0

-15.0 -12.6

-17.7 -15.5

-18.1 -15.8

-15.8 -13.4

-18.0 -15.7

5,50 -Methyl

3,30 -Fluoro

4,40 -Fluoro

5,50 -Fluoro

3,30 -Chloro

4,40 -Chloro

5,50 -Chloro

3,30 -Bromo

4,40 -Bromo

5,50 -Bromo

3,30 -Trifluoromethyl -19.4 -16.4

4,40 -Trifluoromethyl -19.7 -17.3

5,50 -Trifluoromethyl -20.9 -18.5

3,30 -Amino

-6.5

-11.0

4,40 -Amino

5,50 -Amino

-7.5

-2.2

-12.2 -11.1

-9.7

-14.2 -11.3

4,40 -Methyl

-14.9

-6.7

-1.2

-12.9

-15.2

-15.0

-11.9

-15.4

-14.4

-9.1

-19.0

-11.9

-10.6

-15.5

-18.1 -15.5

-17.6 -15.9

Parent

-11.8

-1.3

6.8

-10.2

-15.8

-14.8

-11.6

-13.1

-10.3

-12.3

-12.8

-9.2

-12.1

-12.1

-6.0

-15.5

-8.6

-6.9

-13.1

-12.5

-2.5

5.9

-11.0

-16.0

-14.7

-11.1

-13.6

-10.6

-11.8

-13.3

-9.5

-11.7

-12.6

-6.4

-15.3

-9.4

-7.6

-13.9

-10.2 -16.1 -13.5
-8.9

-7.9

1.6

10.8
-9.1

-4.2

-9.6 -10.5
-5.2

0.6

-9.6

-14.5 -18.2 -16.7

-13.5 -17.6 -15.4

-9.3 -17.6 -14.8

-12.0 -15.8 -13.4

-8.7 -13.7 -11.6

-10.3 -15.8 -13.6

-11.6 -15.5 -13.4

-7.8 -12.6 -10.3

-10.2 -16.6 -14.2

-10.7 -15.2 -13.2

-4.2 -10.3

-13.6 -20.5 -18.0

-6.8 -13.7 -11.1

-4.8 -12.3

-11.6 -15.2 -13.3

-12.9

-4.3

1.2

-9.4

-16.3

-14.8

-14.0

-13.1

-10.5

-13.0

-13.0

-9.2

-13.7

-12.7

-7.3

-17.3

-10.5

-8.2

-12.7

-9.9

1.3

9.1

-7.8

-13.9

-12.7

-9.0

-11.0

-8.2

-10.1

-10.9

-6.5

-10.5

-10.4

-4.3

-13.9

-7.2

-5.0

-11.5

-10.6

0.3

7.3

-9.3

-14.0

-12.5

-9.2

-11.5

-8.6

-9.9

-11.3

-7.1

-9.2

-10.9

-4.6

-13.5

-7.9

-5.4

-12.1

-8.2 11.8
9.0

7.5

9.2

3.4

12.5

-7.1

9.9

7.1

12.3

11.3

10.9

9.8

8.5

10.0

9.8

7.6

10.4

9.7

5.8

13.2

8.1

6.5

9.8

6.6

3.8

3.1 -0.4

7.7

-12.3 13.3

-11.1 12.9

-7.6 12.9

-9.7 11.5

-5.8 10.1

-8.2 11.6

-9.6 11.4

-5.8

-8.2 12.1

-8.9 11.2

-2.5

-11.9 15.0

-5.3 10.0

-3.2

-9.8 11.2

3.1

-0.9

6.9

12.0

10.9

10.3

9.6

7.7

9.6

9.5

6.7

10.0

9.3

5.3

12.7

7.7

6.0

9.3

9.5

-0.9

-6.7

5.7

10.2

9.3

6.6

8.1

6.0

7.4

8.0

4.8

7.7

7.6

3.2

10.2

5.3

3.7

8.4

7.3

-0.2

-5.3

6.8

10.2

9.1

6.8

8.4

6.3

7.3

8.3

5.2

6.8

8.0

3.4

9.9

5.8

4.0

8.9

7.8

-2.5

-9.1

5.2

9.0

8.2

5.5

7.1

4.3

6.0

7.0

4.3

6.0

6.5

1.8

8.7

3.9

2.3

7.2

6.0

n-Hexane Benzene n-Octanol Acetonitrile Water

log Kisom

n-Hexane Benzene n-Octanol Acetonitrile Water Gas

DisomGo(g) (kcal/mol)

n-Hexane Benzene n-Octanol Acetonitrile Water Gas

3,30 -Methyl

Gas

DisomHo(g) (kcal/mol)

Table 4 Enthalpies (DisomHo(g)), Gibbs free energies (DisomGo(g)), and equilibrium constants (log Kisom) for the isomerization of 3,30 -, 4,40 -, and 5,5-disubstituted cis-2,20 -diphenoquinones to the
corresponding oxepino[2,3-b]benzofurans at the M062X/6-311??G(d,p) level of theory in the gas phase and various solvents using the SMD implicit solvation model

Struct Chem (2011) 22:615–625
621

123

622

Struct Chem (2011) 22:615–625

Table 5 Gas phase (298.15 K, 1 atm) enthalpies of reaction
(DrxnHo(g)) for isodesmic reactions involving o-benzoquinone
(o-BQ), 4,40 -diphenoquinone (4,40 -DPQ), p-benzoquinone (p-BQ),
2,20 -diphenoquinone (2,20 -DPQ), biphenyl, and benzene at various

levels of theory, and the corresponding enthalpy of formation (DfHo(g))
for 2,20 -DPQ using the isodesmic reaction o-BQ ? biphenyl ?
benzene ? 2,20 -DPQ. Absolute deviations from the CBS-Q//B3
estimates are given in brackets

o-BQ ? 4,40 -DPQ ? p-BQ ? 2,20 -DPQ

o-BQ ? biphenyl ? benzene ? 2,20 -DPQ

DrxnHo(g)

DrxnHo(g)

Isodesmic DfHo(g) [2,20 -DPQ]

CBS-Q//B3

1.4

17.5

17.7

M062X/6-311??G(d,p)

0.9 [0.5]

18.0 [0.5]

18.2 [0.5]

Level of theory

wB97XD/6-311??G(d,p)

0.5 [0.9]

18.2 [0.7]

18.3 [0.6]

LC-wPBE/6-311??G(d,p)

0.4 [1.0]

20.0 [2.5]

20.1 [2.4]

B97D/6-311??G(d,p)

3.0 [1.6]

11.9 [5.6]

12.1 [5.6]

HF/6-311??G(d,p)

2.1 [0.7]

20.8 [3.3]

21.0 [3.3]

BLYP/6-311??G(d,p)

1.8 [0.4]

11.0 [6.5]

11.1 [6.6]

BP86/6-311??G(d,p)

1.3 [0.1]

10.6 [6.9]

10.8 [6.9]

CAM-B3LYP/6-311??G(d,p)

0.7 [0.7]

17.1 [0.4]

17.2 [0.5]

PBE0/6-311??G(d,p)

1.5 [0.1]

14.9 [2.6]

15.1 [2.6]

B3LYP/6-311??G(d,p)
BMK/6-311??G(d,p)

1.7 [0.3]
0.7 [0.7]

14.2 [3.3]
16.6 [0.9]

14.3 [3.4]
16.7 [1.0]

Values are in kcal/mol

for benzene, and 45.4 kcal/mol for biphenyl. The CBS-Q//
B3 DfHo(g) for o- and p-benzoquinone are in excellent
agreement with the B3LYP/GTlarge, G3, G4MP2, and G4
estimates discussed above, and within experimental errors
for both compounds. Benzene has an evaluated experimental DfHo(g) of 19.8 ± 0.2 kcal/mol [64], in good agreement with the CBS-Q//B3 atomization estimate (?1.8 kcal/
mol error). Biphenyl has an evaluated experimental
DfHo(g) of 43.1 ± 0.8 kcal/mol [64], yielding a CBS-Q//B3
atomization error of ?2.3 kcal/mol. Based on this molecular
size-atomization method DfHo(g) error relationship, and also
in the context of our previous work on a large set and wide
range of organic compounds [46], we expect CBS-Q//B3
atomization method DfHo(g) estimates for the parent diphenoquinones to be in positive error by about 3 kcal/mol using
the general molecular weight-error regression equation
presented in [46]. As a result, the directly calculated atomization approach CBS-Q//B3 DfHo(g) of 22.3 and 13.0 kcal/
mol for the 2,20 - and 4,40 -diphenoquinones, respectively,
need to be adjusted down to final theoretical DfHo(g) estimates of 19 and 10 kcal/mol, respectively.
An alternate route to estimate the DfHo(g) of 2,20 -diphenoquinone is via the following isodesmic reaction:
o-benzoquinone ? biphenyl ? benzene ? trans-2,20 -diphenoquinone. The DrxnHo(g) (298.15 K, 1 atm) for this
reaction at various levels of theory is given in Table 5.
With the exception of the B97D, BLYP, and BP86 functionals, all methods are in good agreement with a DrxnHo(g) of
about 17 to 18 kcal/mol. Using the CBS-Q//B3 results

123

with the experimental DfHo(g) quoted above for benzene,
biphenyl, and o-benzoquinone, an isodesmic DfHo(g) estimate of 17.7 kcal/mol is obtained for 2,20 -diphenoquinone,
in excellent agreement with the adjusted atomization
method estimate. Substituting the G3/G4 DfHo(g) for
o-benzoquinone yields a higher isodesmic DfHo(g) estimate
of 21.1 kcal/mol. When the M062X/6-311??G(d,p)
enthalpies are employed with the experimental DfHo(g) for
o-benzoquinone, an isodesmic DfHo(g) estimate of 18.2 kcal/
mol is obtained (compared with an isodesmic M062X/
6-311??G(d,p) DfHo(g) of 21.6 kcal/mol using the G3/G4
DfHo(g) for o-benzoquinone).
The excellent agreement between the isodesmic reaction
CBS-Q//B3 and M062X/6-311??G(d,p) DfHo(g) for the
parent trans-2,20 -diphenoquinone led us to use this functional and the following isodesmic reaction series to estimate
the DfHo(g) for the 3,30 -, 4,40 -, and 5,5-disubstituted-trans2,20 -diphenoquinones with methyl, fluoro, chloro, bromo,
trifluoromethyl, and amino substituents (Table 6): 2 9
substituted benzene ? 2,20 -diphenoquinone ? 2 9 benzene ? disubstituted trans-2,20 -diphenoquinone. All disubstituted trans-2,20 -diphenoquinones considered are expected
to have modestly exothermic formation enthalpies (-0.1
[3,30 -dibromo and 5,50 -difluoro] to -14.2 [4,40 -diamino]
kcal/mol) with the exception of the predicted endothermic
3,30 -dichloro (DfHo(g) = 1.8 kcal/mol) and 3,30 -difluoro
(DfHo(g) = 6.8 kcal/mol) derivatives. The thermodynamic
stability rank within each class of disubstituted derivatives
generally follows the trend 4,40 [ 5,50 or 3,30 except for the

Struct Chem (2011) 22:615–625

623

Table 6 Gas phase (298.15 K, 1 atm) enthalpies of reaction
(DrxnHo(g)) for the following isodesmic reactions at the M062X/6311??G(d,p) level of theory, 2 9 substituted benzene ? 2,20 Substituent

diphenoquinone ? 2 9 benzene ? disubstituted trans-2,20 -diphenoquinone, and the corresponding enthalpies of formation (DfHo(g)) for
the various disubstituted trans-2,20 -diphenoquinones

Substitution pattern
3,30
DrxnHo(g)

4,40
Isodesmic DfHo(g)

5,50

DrxnHo(g)

Isodesmic DfHo(g)

DrxnHo(g)

Isodesmic DfHo(g)

Methyl

-8.4

-6.4

-9.2

-7.2

-7.9

-5.9

Fluoro

?4.8

6.8

-3.9

-1.9

-2.2

-0.1

Chloro

-0.3

1.8

-4.4

-2.4

-3.4

-1.4

Bromo

-2.1

-0.1

-5.6

-3.6

-4.5

-2.5

-5.9

-3.9

-7.6

-5.6

-9.2

-7.2

-11.6

-9.6

-16.2

-14.2

-11.8

-9.8

Trifluoromethyl
Amino
Values are in kcal/mol

di(trifluoromethyl) substituted trans-2,20 -diphenoquinones
(5,50 [ 4,40 [ 3,30 ).

Conclusions
Gas and solvent phase theoretical calculations on various
3,30 -, 4,40 -, and 5,50 -disubstituted 2,20 -diphenoquinones with
a range of electron withdrawing and releasing moieties
consistently indicate that the trans configuration is lower in
energy than the cis isomer, regardless of substituent type or
location. However, increasing solvent polarity/proticity
progressively and substantially favors shifting the cis/trans
equilibrium towards greater contributions of the cis configuration. Consequently, in polar protic and polar aprotic solvents, significant populations of the cis 2,20 -diphenoquinone
isomers should be present, supporting proposed mechanisms
requiring an intermediate cis configuration of 2,20 -diphenoquinones during the thermal transformation of trans-2,20 diphenoquinones to oxepino[2,3-b]benzofurans. In both the
gas and solvent phase, the cis-2,20 -diphenoquinone to oxepino[2,3-b]benzofuran equilibrium is expected to favor the
oxepino[2,3-b]benzofuran for all electron withdrawing and
releasing substituents and phases considered, with the
exception of the 4,40 - (disfavored in all solvents) and 5,50 (disfavored in polar aprotic and polar protic solvents)
diamino derivatives, whose equilibrium favors the corresponding cis-2,20 -diphenoquinone. Increasing solvent
polarity/proticity disfavors the cis-2,20 -diphenoquinone to
oxepino[2,3-b]benzofuran rearrangement.
Acknowledgments This study was made possible by the facilities
of the Western Canada Research Grid (WestGrid: project 100185),
the Shared Hierarchical Academic Research Computing Network
(SHARCNET: project sn4612), and Compute/Calcul Canada. We
extend sincere thanks to an anonymous reviewer whose suggestions
to include calculations on the oxepino[2,3-b]benzofuran systems and
the various isodesmic reactions greatly improved the quality of our
study.

References
1. Hewgill FR, Hewitt DG (1967) Oxidation of alkoxyphenols. Part
IX. A dipheno-2,20 -quinone from 2,20 -dihydroxy-5,50 -dimethoxy4,40 -di-t-butylbiphenyl. J Chem Soc (C) 723–725
2. Adderley CJR, Hewgill FR (1968) Oxidation of alkoxyphenols.
Part XII. The oxidation products of some polymethoxyphenols,
and the mechanism of demethylation in neutral solution. J Chem
Soc (C) 1434–1438
3. Bowman DF, Hewgill FR (1971) Oxidation of alkoxyphenols.
Part XIX. Base-induced coupling of halogenophenols. J Chem
Soc (C) 1777–1788
4. Hewgill FR (1978) Oxidation of alkoxylphenols—XXIV. Tetrahedron 34:1595–1596
5. Byrne LT, Hewgill FR, Legge F, Skelton BW, White AH (1982)
Oxidation of alkoxyphenols. Part 26. The trimer of a 2, 20 -diphenoquinone (1, 10 -bicyclohexa-3, 5-dienylidene-2, 20 -dione).
J Chem Soc Perkin Trans 1:2855–2862
6. Hewgill FR, Legge F (1982) Oxidation of alkoxyphenols. Part 27.
The mechanism of formation of dibenzo[d, f]dioxepins. J Chem
Soc Perkin Trans 1:2863–2866
7. Hewgill FR, La Greca B, Legge F, Roga PE (1983) Oxidation of
alkoxyphenols. Part 28. On the configuration of 2, 20 -diphenoquinones. J Chem Soc Perkin Trans 1:131–134
8. Becker HD, Gustafsson K (1976) On the formation of spirosubstituted benzoxetes by phenol oxidation: preparation and
valence isomerization of 3,30 -di-tert-butyl-5,50 -ditrityl-2,20 -diphenoquinone. Tetrahedron Lett 52:4883–4886
9. Becker HD, Lingnert H (1982) Oxidative conversions of 2,20 diphenoquinone valence isomers with 2,3-dichloro-5,6-dicyanobenzoquinone. Synthesis and spectroscopic properties of
(E)-[3,30 ]bibenzofuranylidene-2,20 -diones (isoxindigos). J Org
Chem 47:1095–1101
10. Guan B, Wan P (1993) Photochemistry of dibenzo-1,4-dioxin:
formation of 2,20 -biphenylquinone as an observable intermediate.
J Chem Soc Chem Commun 409–410
11. Guan B, Wan P (1994) Photochemistry of dibenzo-1,4-dioxins:
intramolecular rearrangement-reduction through observable 2,20 biphenylquinones. J Photochem Photobiol A 80:199–210
12. Rayne S, Wan P, Ikonomou MG, Konstantinov AD (2002)
Photochemical mass balance of 2,3,7,8-TeCDD in aqueous
solution under UV light shows formation of chlorinated dihydroxybiphenyls, phenoxyphenols, and dechlorination products.
Environ Sci Technol 36:1995–2002
13. Rayne S, Sasaki R, Wan P (2005) Photochemical rearrangement of dibenzo[1,4]dioxins proceeds through reactive

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