Conformational analysis and interpretation of ν(OH) bands in the IR spectra of 1′-hydroxyethyl derivatives of 1,4-benzo- and 5,8-dihydroxy-1,4-naphthoquinones: A DFT study

Article abstract of DOI:10.1007/s11172-009-0077-4

Detailed conformational analysis of the molecule of 1′-hydroxyethyl- 1,4-benzoquinone (3) by the B3LYP/cc-pVTZ method revealed predominance of rotamers with the free 1′-OH group in the gas phase. B3LYP/cc-pVTZ calculations with inclusion of solvent (cyclohexane) effect in the framework of the polarizable continuum model predict an increase in the percentage of such rotamers compared to the corresponding gas-phase values. The results obtained are in qualitative agreement with the experimentally observed pattern of ν(OH) bands in the IR spectrum of compound 3 in cyclohexane (hexane) solution. Conformational analysis, in cluding tautomerism and rotamerism, of 2-ethyl-1′,5,8-trihydroxy-1,4-naphthoquinone (2) was performed by the B3LYP method with the 6-31G(d), 6-311G(d), 6-311G(d,p), and cc-pVDZ basis sets. The most abundant tautomeric form of compound 2 is form A in which the substituent bearing 1 '-OH group is in the quinonoid nucleus. In the gas phase, the percentage of all rotamers in form A is about 86% (among them, the proportion of rotamers with the free 1'-OH group is more than 60%). The main reason for splitting of the v(OH) bands in the IR spectra of compounds 2 and 3 in solutions in nonpolar solvents is the equilibrium between rotamers with a relatively weak intramolecular hydrogen bond between the 1′-OH group and the carbonyl group and those having no this bond.

Full text of DOI:10.1007/s11172-009-0077-4

Russian Chemical Bulletin, International Edition, Vol. 58, No. 4, pp. 663—674, April, 2009
663
Conformational analysis and interpretation of ν(ОН) bands
in the IR spectra of 1´ꢀhydroxyethyl derivatives
of 1,4ꢀbenzoꢀ and 5,8ꢀdihydroxyꢀ1,4ꢀnaphthoquinones: a DFT study
V. P. Glazunov, D. V. Berdyshev, N. D. Pokhilo, and V. F. Anufriev
Pacific Institute of Bioorganic Chemistry, FarꢀEastern Branch of the Russian Academy of Sciences,
1
59 prosp. 100 let Vladivostoku, 690022 Vladivostok, Russian Federation.
Fax: +7 (423 2) 31 4050. Eꢀmail: glazunov@piboc.dvo.ru
Detailed conformational analysis of the molecule of 1´ꢀhydroxyethylꢀ1,4ꢀbenzoquinone
3) by the B3LYP/ccꢀpVTZ method revealed predominance of rotamers with the free 1´ꢀOH
(
group in the gas phase. B3LYP/ccꢀpVTZ calculations with inclusion of solvent (cyclohexane)
effect in the framework of the polarizable continuum model predict an increase in the percentꢀ
age of such rotamers compared to the corresponding gasꢀphase values. The results obtained are
in qualitative agreement with the experimentally observed pattern of ν(ОН) bands in the
IR spectrum of compound 3 in cyclohexane (hexane) solution. Conformational analysis, inꢀ
cluding tautomerism and rotamerism, of 2ꢀethylꢀ1´,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone (2)
was performed by the B3LYP method with the 6ꢀ31G(d), 6ꢀ311G(d), 6ꢀ311G(d,p), and
ccꢀpVDZ basis sets. The most abundant tautomeric form of compound 2 is form А in which the
substituent bearing 1´ꢀOH group is in the quinonoid nucleus. In the gas phase, the percentage
of all rotamers in form A is about 86% (among them, the proportion of rotamers with the free
1
´ꢀOH group is more than 60%). The main reason for splitting of the ν(ОН) bands in the
IR spectra of compounds 2 and 3 in solutions in nonpolar solvents is the equilibrium between
rotamers with a relatively weak intramolecular hydrogen bond between the 1´ꢀOH group and
the carbonyl group and those having no this bond.
Key words: 1´ꢀhydroxyethylꢀ1,4ꢀbenzoquinone; 2ꢀethylꢀ1´,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoꢀ
quinone, 2ꢀethylꢀ1´ꢀhydroxynaphthazarin, quantum chemical calculations, density functional
theory, potential energy surface; conformational analysis; rotamerism, tautomerism; IR specꢀ
troscopy.
5
6
7,8
9
In ancient times, extracts from roots of Alkanna tinctoꢀ
ria in Europe and from Lithospermum erithrorhizon in Eastꢀ
ern countries were used as natural darkꢀred dyes. In the
early 20th century, enantiomeric naphthoquinones shikoꢀ
nin (1a) and alkannin (1b) (racemic mixture of 1a and 1b
terial, antifungal, antiꢀinflammatory, anticancer, anꢀ
7
,10
11
algesic,
stimulating activity.
Tautomerism in 1´ꢀhydroxyalkylnaphthazarines is still
a moot question. Numerous attempts to reveal prototroꢀ
pic tautomerism in, e.g., shikalkin in solutions by Н and
С NMR spectroscopy have been made earlier.
and antipyretic ones, as well as immunoꢀ
1
2
1
1
is called shikalkin) were isolated from extracts of roots of
these plants and identified.2
13
13—15
Howꢀ
ever, the determination of the tautomeric composition of
shikalkin in solutions failed owing to high rate of tautoꢀ
meric transitions in naphthazarines (on the time scale of
NMR spectroscopy). Moreover, three different conꢀ
clusions have been made about the tautomeric form
(
Scheme 1) in which this compound exists in solution,
viz., only forms С and D (see Ref. 13), all forms А—D
see Ref. 14), or mainly form А (see Ref. 15).
(
1
6
Recently, we have studied prototropic tautomerism
More recently,3,4 these natural substances have been
found in many plants of the Boraginaecae family in the
form of free alcohols and their esters. They exhibit a variꢀ
ety of interesting biological properties including antibacꢀ
in 1´ꢀhydroxyalkylnaphthazarines in solutions in nonpoꢀ
lar solvents (CCl , nꢀhexane or cyclohexane) by IR specꢀ
4
troscopy taking ethylꢀ1´,5,8ꢀtrihydroxyꢀ1,4ꢀnaphthoquinone
(1´ꢀhydroxyethylnaphthazarine (2)) as an example. It was
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 648—659, April, 2009.
066ꢀ5285/09/5804ꢀ0663 © 2009 Springer Science+Business Media, Inc.
1

664
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
Scheme 1
A (rel. units)
3
635
3
637
0
.05
3
626
605
3
589
3
570
3
0
3
660 3640 3620 3600 3580 3560 ν/cm–1
Fig. 1. IR spectra of a solution of compound 2 in hexane.
Analysis of the contour of the ν(ОН) absorption band
in the spectrum of compound 2 in CCl solution showed
4
that, formally (due to broadening of spectral components),
the observed splitting into two components is about
–1
5
0 cm . This indicates that the 1´ꢀOH group participates
in the formation of very weak IMHB. The maximum of
the highꢀfrequency component of the ν(ОН) band in the
IR spectrum of a solution of 2 in hexane (cyclohexane) is
–
1
at 3635 cm , which is close to the ν(ОН) frequency in
aliphatic alcohols. Based on the splitting Δν, the IMHB in
compound 2 is comparable with IMHB in the оꢀvinylpheꢀ
1
7,18
assumed that intramolecular hydrogen bonds (IMHB)
nol molecule;
in the spectrum of the latter the band
О(1´)—Н…О(1)* in the "quinonoid" (2А) and "benzenoid"
corresponding to the free ОН group is much stronger than
the band corresponding to the bound OH group. An exꢀ
planation for this phenomenon is as follows : the proporꢀ
tion of the rotamers with IMHB determined with incluꢀ
(
2В) tautomers have different stability. Because of this,
1
8
the frequencies of the ν(ОН) absorption bands of tauꢀ
tomers 2А and 2В in the IR spectra of compound 2 should
be significantly different. This would help to follow the
changes in the tautomeric composition of compound 2 in
solutions. Indeed, the IR spectrum of a solution of comꢀ
sion of the solvent (CCl ) effect is at most 24%.
4
One can assume that, in addition to prototropic tautomeꢀ
rism, the following equilibria are also possible for compound
←
pound 2 in CCl exhibits two highꢀfrequency ν(ОН) bands
2 in solutions in nonpolar solvents: 2Аꢀa(b) → 2Аꢀf(g),
4
–
1
←
←
←
whose frequencies differ by 50 cm . This can be explained
by the assumed tautomeric equilibrium 2А → 2В. Howevꢀ
2Аꢀa(b) → 2Аꢀс(d,e),
2Вꢀa(b) → 2Вꢀf(g),
and
←
2Вꢀa(b) → 2Вꢀ(d,e). These equilibria are accompanied by
cleavage of the IMHB involving the 1´ꢀOH group (Scheꢀ
me 2 and Figs 2 and 3).
er, in the spectrum of compound 2 in hexane (cyclohexꢀ
ane) solution the lowꢀfrequency band, which is tentatively
assigned to the quinonoid tautomer 2А, is split into three
strongly overlapped components (Fig. 1). Emphasize that
hexane (cyclohexane) is a solution of lower polarity than
To establish the reasons for the complex pattern of the
ν(ОН) bands in the spectra of compound 2, we syntheꢀ
sized 1´ꢀhydroxyethylꢀ1,4ꢀbenzoquinone (3) (Scheme 3),
which possesses no prototropic tautomerism. Molecule 3
represents a model for the quinonoid fragment of moleꢀ
cule 2 in the tautomeric form А. To study the effect of tauꢀ
tomerism and rotamerism in the 1´ꢀhydroxyalkyl substituent
on the spectral characteristics of compounds 2 and 3, we
performed conformational analysis of these molecules with
the B3LYP density functional in different basis sets.
CCl . Decomposition of the contour of the lowꢀfrequency
4
ν(ОН) band gave three frequencies, namely, 3570, 3589,
–
1
and 3605 cm . Thus, the peakꢀtoꢀpeak separations are 19
and 16 cm–1. For the components at 3589 and 3570 cm–1
the areasꢀunderꢀcurve are nearly the same, whereas the
–
1
areaꢀunderꢀcurve for the component at 3605 cm is
.3 times smaller. In the spectrum of a solution of comꢀ
pound 2 in hexane, the highꢀfrequency ν(ОН) band at
635 cm–1 has an asymmetric contour. Its approximation
2
Results and Discussion
3
by Lorentzian and Gaussian functions revealed contribuꢀ
tions from at least two components. A possible reason
is that not only tautomers 2А and 2В, but also the
The IR spectra of solutions of compounds 2 and 3 in
CCl exhibit highꢀfrequency ν(ОН) doublets with a splitꢀ
4
–
1
ting of about 50 cm . Owing to the narrowing of spectral
bands, the spectrum of a solution of compound 3 in hexꢀ
ane (cyclohexane) clearly exhibits a splitting of the lowꢀ
frequency ν(ОН) band into two components with a peakꢀ
"
1,5ꢀquinonoid" tautomer 2С and the "4,8ꢀquinonoid" tauꢀ
1
6
tomer 2D are present in solution.
*
Figures in parentheses denote the number of the carbon atom
–
1
bearing the oxygen atom.
toꢀpeak separation of nearly 16 cm . The highꢀfrequency

DFT study of 1´ꢀhydroxyethylbenzoquinone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
665
Scheme 2
band at 3635 cm–1 has a slightly asymmetric contour
ponents. The IR spectra of solutions of compounds 2 and
3 in hexane match each other, except for a weak band at
3605 cm , which is observed for 2 only.
(
Fig. 4); its decomposition using Lorentzian and Gaussꢀ
–
1
ian functions reveals contributions from at least two comꢀ
Fig. 2. Structures of rotamers of the quinonoid tautomers 2А.

666
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
Fig. 3. Structures of rotamers of the benzenoid tautomers 2В.
Scheme 3
a superposition of at least four components. The larger
–
1
splitting of about 54—55 cm between the maximum of
–
1
the band at 3635 cm and the center of gravity of the two
–
1
components at 3586 and 3570 cm can be attributed to
rotamers 3a and 3c present in solution (Scheme 4).
–
1
The nature of the smaller splitting of about 16 cm
–
1
(
3586—3570 cm ) is still unclear.
Scheme 4
Analysis of the spectrum of a solution of the strucꢀ
turally simpler compound 3 in hexane shows that the conꢀ
tour of the ν(ОН) absorption band of the 1´ꢀOH group is
To locate all possible isomers of compound 3, we used
the B3LYP/6ꢀ31G method to construct a twoꢀdimension→al
(2D) section of the potential energy surface (PES, E (R )
0

DFT study of 1´ꢀhydroxyethylbenzoquinone
A (rel. units)
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
667
θO(9)/deg
3
634
g 4.5 6.1
.7
5.3
1.4
2.2
2.9
2.9
g 4.5
0
.05
6.1
5
.3
5.3
3.7
–
250
3
5.3
3
570
2.2
3
586
–200 1.4
1.4 0.58
e
1.4
d
d
6.1
c
0.58
2.3
3
624
6
.1
–
–
150
100
7.7
7
.7
8.5
9.3
6
.9
9.3
6.9
6.1
0
9.3
.58 3.7
7
.7
6
.9
ν/cm–1
3.7
3
660 3640 3620 3600 3580 3560
0
f 5.3 6.9
5.3
4.5
0.58 2.9
6
.1
2
2.2
–
50
0
2.9
a
a
2
.2
1.4
Fig. 4. IR spectrum of a solution of compound 3 in hexane.
11
1
0
6.1 8.5
12
9.3
.5
.7 4.5
6.1
10
12
1
2
7
3.7
7.7
.2
8
5
0
6.9
0.58b1.4
4
.5 5.3
g
5.3
4.5
of system 3 in the plane of the torsion angles θО(9)
O(9)—C(1´)—C(2)—C(1)) and θОН (Н(10)—O(9)—
C(1´)—C(2)) (from this point on, "the 2D cyclic poꢀ
g
1
1
00
(
2.9
2
.9
50 2.2
—
1.4 0.58
1.4
2.2
tential" v(θO(9), θOH)). When using highꢀlevel quantum
chemical methods and extended basis sets, this procedure
permits correct identification of the regions in the configꢀ
uration space corresponding to the potential wells and
saddle points on the PES. In the case of compound 3 it is
sufficient to use the density functional theory (DFT) with
hybrid exchangeꢀcorrelation functionals (e.g., B3LYP)
and the splitꢀvalence АО basis sets of 6ꢀ31G quality (and
more extended basis sets). The applicability of the B3LYP
method to solving such problems has been demonstratꢀ
ed earlier.1
100
0
–100
–200
θOH/deg
Fig. 5. 2D cyclic PES of compound 3.
6ꢀ311G(d) basis set, because their percentage is negligible
and they were left out of consideration (see below). Only
in two rotamers (3a,b) the 1´ꢀOH group is involved in the
formation of IMHB with the O atom of the С(1)O carboꢀ
nyl group (Scheme 5). The global minimum on the PES
→
(E (R )) corresponds to rotamer 3a (Table 1). The relaꢀ
0
8—23
tive energy of rotamer 3b calculated in the ccꢀpVTZ basis
–
1
Assuming that the first step of conformational analysis
involves only the establishment of possible conformations
of isomers of the compound under study and no exact
determination of their relative energies and geometric
parameters, the requirements for the quantum chemiꢀ
cal method to be used can be not too stringent. For
instance, in a study of the conformational mobility
set is 0.11 kcal mol .* The geometric parameters of rotaꢀ
mers 3a and 3b are as follows: R(O(7)—O(9)) = 2.826 and
2.820 Å, R(H(10)—O(7)) = 2.118 and 2.820 Å, the angle
O(7)—H(10)—O(9) = 128.9° and 128.4°, θОН = 62.24°
and –64.18°. Among these parameters, the θОН angles
differ to the greatest extent.
1
8,24
of oꢀvinylphenol we have shown
that the 2D sections
Scheme 5
of the PES obtained from the АМ1, MP2/6ꢀ31G, and
B3LYP/6ꢀ311G(d) calculations are qualitatively similar
and correctly reproduce details of the dynamic behavior of
the molecular system.
Figure 5 presents the 2D cyclic potential V(θO(9), θOH
obtained from B3LYP/6ꢀ31G calculations of compound
. Scanning was performed on a uniform grid with an
)
3
increment of –5.0° for each coordinate. For clarity, possiꢀ
ble pathways of transitions between rotamers are shown in
Fig. 5 where the coordinates were varied from 165° to
–295° (θО(9)) and from 80° to –355° (θОН).
Analysis of the PES thus constructed showed that comꢀ
pound 3 has seven rotamers 3a—g. The PES shown in Fig. 5
demonstrates only six out of seven minima because the
potential well for rotamer 3g (about 0.36 kcal mol–1) is
very shallow at this scale (neighboring isolines are sepaꢀ
–
1
rated by 0.8 kcal mol ). The minimum corresponding to
rotamer 3g is shown by the dashed line.
Full geometry optimization of rotamers 3a—e was
carried out by the B3LYP method in the 6ꢀ311G(d),
6
ꢀ311G(d,p), ccꢀpVDZ, and ccꢀpVTZ basis sets. The
* From this point on, by default we will discuss the energy and
geometric characteristics calculated in the ccꢀpVTZ basis set.
geometry of rotamers 3f,g was optimized only in the

668
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
Table 1. Electronic energies (E /au), total energies ((E = E + ZPE)/au), the Gibbs free energies (G/au), enthalpies (ΔH/kcal mol–1),
0
0
–
1
%
–1
relative Gibbs free energies (ΔG/kcal mol ), percentages (g (%)), ν(ОН) frequencies/cm , and intensities А (rel. units) calculated
for rotamers of 1´ꢀhydroxyethylbenzoquinone 3
Rotaꢀ
mer
Computational
method
–E0
–E
–G
–H
ΔG
g%
ν/A
3
3
3
3
3
а
b
с
d
е
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d)a
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
B3LYP/ccꢀpVTZ
535.428663
535.441124
535.445472
535.334981
535.500178
535.500178
535.513188
535.282242
535.294834
535.299351
535.188997
535.353975
535.361286
535.366898
535.317671
535.330374
535.334807
535.224388
535.389470
535.397157
535.402438
35.271317
535.283877
535.288410
535.178096
535.343029
535.349925
535.310953
0.23
0.38
0.25
0.13
0.20
0.19
0.41
22.99
19.86
22.86
26.62
21.67
21.79
18.13
3736.8/1.00
3733.3/1.00
3767.3/1.00
3701.7/1.00
3751.5/1.00
3564
b
B3LYP/ccꢀpVTZ
B3LYP/ccꢀpVTZa
—
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d)a
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
B3LYP/ccꢀpVTZ
535.428506
535.440862
535.444950
535.334264
535.500003
535.500003
535.512956
535.281942
535.294497
535.298747
535.188179
535.353686
535.361002
535.366392
535.317316
535.329887
535.334175
535.223510
535.389161
535.396851
535.401981
535.271098
535.283628
535.287872
535.177356
535.342802
535.349709
535.355548
0.46
0.68
0.65
0.68
0.39
0.38
0.69
15.78
11.86
11.71
10.52
15.62
15.76
11.18
3748.4/1.19
3741.4/1.13
3777.2/1.19
3709.4/1.21
3765.0/1.10
3577
b
B3LYP/ccꢀpVTZ
B3LYP/ccꢀpVTZa
—
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d)a
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
B3LYP/ccꢀpVTZ
535.428277
535.441027
535.445149
535.334578
535.499843
535.499843
535.515135
535.282298
535.295187
535.299453
535.188952
535.354010
535.361302
535.369156
535.318043
535.330976
535.335206
535.224600
535.389781
535.397462
535.403084
535.271168
535.284038
535.288308
535.177859
535.342860
535.349738
535.358026
0.00
0.00
0.00
0.00
0.00
0.00
0.00
34.09
37.58
34.89
33.15
30.12
30.09
35.94
3775.0/0.35
3773.7/0.46
3821.3/0.47
3767.5/0.35
3811.2/0.51
3621
b
B3LYP/ccꢀpVTZ
B3LYP/ccꢀpVTZa
—
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d)a
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
B3LYP/ccꢀpVTZ
535.426865
535.439773
535.444061
535.333649
535.498773
535.498773
535.512136
535.280928
–535.293741
535.298333
535.188047
535.353047
535.360333
535.366199
535.316631
535.329403
535.334009
535.223665
535.388801
535.396477
535.401763
535.269794
535.282666
535.237216
535.176966
535.341871
535.348742
535.157071
0.89
0.99
0.75
0.59
0.61
0.62
0.83
7.64
7.10
9.82
12.25
10.67
10.60
8.90
3766.4/0.38
3766.4/0.61
3813.9/0.45
3756.8/0.34
3805.1/0.48
3615
b
B3LYP/ccꢀpVTZ
B3LYP/ccꢀpVTZa
—
3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d)a
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
B3LYP/ccꢀpVTZ
535.427703
535.440330
535.444566
535.333873
535.499363
535.499363
535.512566
535.281841
535.294678
535.298997
535.188376
535.353695
535.360978
535.366704
535.317516
535.330537
535.334714
535.223989
535.389481
535.397156
535.402773
535.270689
535.283458
535.237813
535.177242
535.342486
535.349356
535.355552
0.33
0.28
0.31
0.38
0.19
0.19
0.20
19.51
23.60
20.72
17.46
21.92
21.76
25.85
3800.9/0.48
3798.9/0.63
3844.8/0.61
3790.6/0.46
3830.3/0.62
3639
b
B3LYP/ccꢀpVTZ
B3LYP/ccꢀpVTZa
—
3
3
f
B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
535.289596
535.422032
535.143122
535.276177
535.178953
535.312032
535.131943
535.264984
3.92
3.77
0.06
0.06
3735.8/0.22
3774.0/0.30
g
B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
535.290340
535.422654
535.143688
535.276627
535.179702
535.312672
535.132541
535.265456
3.45
3.37
0.13
0.12
3717.3/0.17
3755.3/0.23
a
b
Calculated with inclusion of solvent (cyclohexane) effect in the framework of the РСМ model.
Frequency calculations were carried out with a scale factor of 0.95.
Out of five rotamers with free 1´ꢀОН group, two (3f
and 3g) are energetically most unfavorable; their relative
substituent near the C(1)O carbonyl group lying in the
plane of the benzoquinone ring) and to the ability of the
1´ꢀOH group to participate in the formation of the IMHB
with the carbonyl group. Owing to the orientation effect of
energies (ΔЕ ) calculated in the 6ꢀ311G(d) basis set are
0
–
1
4
(
=
.16 and 3.77 kcal mol , respectively. Rotamers 3c—e
←
Scheme 6) have the following relative energies: ΔE (3c) =
the latter, oneꢀstep interconversion a → b occurs as conꢀ
0
–
1
0.21, ΔE (3d) = 0.88, and ΔE (3e) = 0.51 kcal mol
.
certed rotation of the 1´ꢀОН group and the whole substitꢀ
uent about the С(1´)—О(9) and С(1´)—С(2) bonds. This
basically distinguishes these transitions from the transiꢀ
tions between rotamers with the free 1´ꢀОН group. For
0
0
In the region –310° ≤ θO(9) ≤ –235° of the configuraꢀ
tion space, the potential V(θO(9), θOH) has a complex
shape. This is due to steric factor (passage of bulky methyl

DFT study of 1´ꢀhydroxyethylbenzoquinone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
669
Scheme 6
ccꢀpVTZ basis sets in the framework of the polarizable
continuum model (РСМ). The thermal corrections, GT,
2
5
to the Gibbs free energy were calculated in the 6ꢀ311G(d)
1
8
basis set. Rotamers 3f and 3g having percentages of at
most 0.18 were left out of consideration. The stretching
vibration frequencies calculated with inclusion of the solꢀ
–
1
vent effect decrease by 4—8 cm for rotamers 3a,b and by
—
1
0
—4 cm for rotamers 3c—e (see Table 1). In the gas
phase, the ratio (Y) of the percentage of the rotamers
with free ОН groups to that of the rotamers with
bound ОН groups is 1.58 (B3LYP/6ꢀ311G(d)) and 1.89
(
2
B3LYP/6ꢀ311G(d,p)). In cyclohexane, Y increases to
.15 (B3LYP/6ꢀ311G(d)). More exact calculations of Y
in the ccꢀpVTZ basis set give a value of 1.68 (gas phase)
and 2.41 (cyclohexane). The ratio of the areasꢀunderꢀ
curve, S3635/(S3586 + S3570), calculated from the experiꢀ
mental spectrum of compound 3 in hexane (cyclohexane)
solution is 1.77. Assuming that the absolute intensities of
the ν(ОН) bands of the free and bound ОН groups differ
insignificantly, good qualitative agreement with the "expeꢀ
rimental" Y value is provided by the Y values estimated
from the results of B3LYP/6ꢀ311G(d,p) and B3LYP/ccꢀ
pVTZ (gas phase) calculations. B3LYP/ccꢀpVTZ calcuꢀ
lations with inclusion of the solvent effect in the frameꢀ
work of the РСМ model overestimate the Y value. This is
probably due to the assumption made above.
The estimates of the Y ratio were obtained using the
results of calculations with no frequency scaling. When
the frequencies are scaled with a factor of 0.95, the Y
values change insignificantly, to 1.56 and 1.66 (cf. 1.58
and 1.68, respectively, with no scaling; B3LYP/6ꢀ311G(d)
and B3LYP/ccꢀpVTZ calculations). The scale factor was
chosen to be equal to the ratio of the frequency observed
in the spectrum of compound 3 for free 1´ꢀОН group
←
←
instance, transitions c → e and e → d can occur as a result
of rotation of the 1´ꢀОН group only, while transitions
←
c → f can occur as a result of rotation of the substituent
≈ const). An exception is the transition
d → g, which causes the θ angle to change by about 20°.
(
in this case, θ
ОН
←
ОН
Thus, molecule 3 can exist in the gas phase as a mixꢀ
ture of seven nonplanar rotamers 3a—g. In two of them
3a,b) the 1´ꢀОН group is involved in the formation of
IMHB.
The Gibbs free energies (G = Е + G ) were calculated
using the default scheme implemented in the GAUSSIANꢀ03
program. B3LYP calculations in the 6ꢀ311G(d),
ꢀ311G(d,р), ccꢀpVDZ, and ccꢀpVTZ basis sets predict
the lowest G value for rotamer 3c (see Table 1) with
R(O(7)—O(9)) = 4.213 Å. The IMHB between the H atom
of the 1´ꢀOH group and the carbonyl O atom of benzoꢀ
quinone is very weak; the inclusion of the entropy factor
causes a shift of the equilibrium toward the rotamers with
the free 1´ꢀОН group. The strength of the IMHB was
estimated as the enthalpy difference between rotamers 3c
(
0
Т
6
–
1
(
3635 cm ) to the average theoretically calculated freꢀ
–
1
quency ((3811.2 + 3805.1 + 3830.3)/3 = 3815.5 cm ).
Detailed conformational analysis of molecule 3 (see
above) allows one to interpret the contour of the ν(ОН)
band in the IR spectra of solutions of 3 in inert solvents.
–
1
and 3a (Scheme 7): ΔН = Н3a – Н = –0.15 kcal mol
cf. ΔG = G3a – G3c = 0.20 kcal mol–1).
3c
(
–1
The larger splitting of this band (∼ 50 cm ) is due to the
equilibrium between the rotamers 3a,b with the bound
ОН group and the rotamers 3c—g with the free ОН group.
Each of the two components of the ν(ОН) band originates
from the contributions of several rotamers. For instance,
Scheme 7
–
1
the smaller splitting (16 cm ) of the lowꢀfrequency comꢀ
ponent of the ν(ОН) band is due to the equilibrium
←
–1
3
a → 3b. The difference ν(ОН) – ν(ОН)3a = 13.5 cm
3b
calculated in the ccꢀpVTZ basis set for the gas phase is in
–1
reasonable agreement with the observed splitting (16 cm )
in the spectrum of a solution of compound 3 in hexane
(
see Fig. 4). Broadening of spectral bands in CCl and,
4
especially, chloroform makes the lowꢀfrequency compoꢀ
nent unresolvable, but, mathematically, the contour is deꢀ
scribed in this case by a doublet. According to gasꢀphase
calculations in the ccꢀpVTZ basis set, the ν(ОН) frequenꢀ
The effect of solvent (cyclohexane) on the geometric
parameters and energy characteristics of molecule 3 was
modeled by the B3LYP method with the 6ꢀ311G(d) and

670
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
cies of rotamers 3c and 3d differ by 5 cm–1 only and thus
are expected to appear in the spectra as a single band. The
calculated ν(ОН) frequency of rotamer 3e differs from
Scheme 8
–
1
those of rotamers 3c and 3d by about 17 cm and would
appear as a particular band with allowance for the perꢀ
centage of 3e (21.9%). However, in the spectrum of a
solution of compound 3 in hexane (see Fig. 4) the band at
3
635 cm–1 only has an asymmetric contour. This may
indicate than the B3LYP/ccꢀpVTZ method overestimates
the frequency of rotamer 3e. Owing to very low content of
the rotamers 3f and 3g, the contributions of their ν(ОН)
to the bandshape can be neglected. Thus, calculations preꢀ
dict a high percentage of rotamers with the free ОН group
in both the gas phase and solution, which corresponds to
the more intense highꢀfrequency component band ν(ОН)
in the spectrum of compound 3.
model benzoquinone 3 and the quinonoid tautomers of
compound 2, the enthalpy difference ΔН = Н – Н2Вꢀс
is 0.39 kcal mol . Thus, the formation of IMHB in the
benzenoid tautomers is an endothermic process.
2
Вꢀа
–1
Next, we studied the conformational mobility of comꢀ
pound 2. For all rotamers of 2, the initial values of geoꢀ
metric parameters of the 1´ꢀOH group in all four tautoꢀ
meric forms were equal to those obtained upon geometry
optimization of the model benzoquinone 3. Prescreening
of tautomericꢀrotameric states of molecule 2 was perꢀ
formed in the 6ꢀ31G(d) basis set. Having estimated the
percentage of all 28 tautomericꢀrotameric states (seven
rotamers for each out of four tautomers), we refined the
geometric and energy characteristics of the isomers havꢀ
ing percentages higher than 1% in more extended basis
sets 6ꢀ311G(d), 6ꢀ311G(d,p), and ccꢀpVDZ. The results
of calculations of the energy characteristics of all isomers
of compound 2 are listed in Table 2. The structures of all
rotamers for the tautomeric forms 2А and 2В are shown in
Figs 2 and 3.
Scheme 9
As mentioned above, isomers of only two tautomeric
forms, 2А and 2В, dominate in the gas phase. For 2B, the
percentage of all rotamers is at most 16% (among them,
only 4.4% of rotamers with IMHB, see Table 2). Thereꢀ
fore, the shape of the contour of the ν(ОН) bands in the
spectrum of compound 2 is mainly determined by the rotꢀ
amers of the quinonoid tautomer 2А. As in the spectrum
of benzoquinone 3, the highꢀfrequency band at 3635 cm–
in the spectrum of a solution of compound 2 in hexane is
stronger than three lowꢀfrequency bands at 3605, 3589,
and 3570 cm (see Fig. 1). It corresponds to isomers of
the tautomers 2А and 2В with no IMHB, while the lowꢀ
frequency band corresponds to the isomers with the 1´ꢀОН
group involved in the formation of IMHB with the carboꢀ
nyl O(1) atom. The ratio of the areaꢀunderꢀcurve for the
highꢀfrequency component of the ν(ОН) band to the sum
of the areasꢀunderꢀcurve for the lowꢀfrequency compoꢀ
nents is Y = 1.62, which is close to theoretical estimate of
the ratio of the percentage of all rotamers with the free
1´ꢀOH group to the percentage of rotamers with IMHB
equal to 1.81 (calculations in the ccꢀpVDZ basis set for
the gas phase). As was shown taking the model compound
3 as an example, the inclusion of the solvent effect on the
equilibrium between rotamers causes the ratio Y to inꢀ
crease; this also seems to be valid for compound 2.
Conformational analysis of molecule 2 showed that
the content of all rotamers in the tautomeric forms А and
B is 100% (form А — 85.8%, form B — 14.2%; B3LYP/6ꢀ
1
3
11(d) calculations); among them, the content of the rotꢀ
amers with IMHB is only 35.6% (see Table 2). Therefore,
the energy and geometric characteristics of isomers of
compound 2 were then refined in the Gꢀ311G(d,p) and
ccꢀpVDZ basis sets only for the tautomeric forms А and В.
Among all 28 isomers, the quinonoid isomer 2Аꢀа has
–
1
the lowest energy Е . As for compound 3, the inclusion of
0
the entropy factor causes the equilibrium between the isoꢀ
mers (Scheme 8) to shift toward 2Аꢀс (ΔG = G
– G2Аꢀс =
2
Аꢀа
1
=
0.13 kcal mol– , see Table 2). The enthalpy difference
–
1
ΔН = Н2Аꢀа – Н2Аꢀс = –0.10 kcal mol (B3LYP/ccꢀ
pVDZ calculations).*
Among the benzenoid tautomers, isomer 2Вꢀс (Scheꢀ
me 9) has the lowest energy Е . The inclusion of the entroꢀ
0
py factor causes the equilibrium to shift toward rotamers
with no IMHB to even greater extent, viz., one gets ΔG =
–
1
=
G2Вꢀа – G2Вꢀс = 0.63 kcal mol . In contrast to the
The highꢀfrequency ν(ОН) band appears in the specꢀ
trum as a single band owing to close frequencies of the
rotamers 2Аꢀс, 2Аꢀd, and 2Aꢀe. More "diffuse" contour of
*
From this point on, by default we will discuss the energy charꢀ
acteristics calculated in the ccꢀpVDZ basis set.

DFT study of 1´ꢀhydroxyethylbenzoquinone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
671
Table 2. Electronic energies (E /au), total energies ((E = E + ZPE)/au), the Gibbs free energies (G/au), enthalpies (ΔH/kcal mol–1),
0
0
–
1
%
–1
relative Gibbs free energies (ΔG/kcal mol ), percentages (g (%)), ν(ОН) frequencies/cm , and intensities А (rel. units) calculated
for rotamers of 1´ꢀhydroxyethylꢀ1,4ꢀnaphthazarine 2
Rotaꢀ
mer
Computational
method
–E0
–E
–G
–H
ΔG
g%
ν/A
2
2
2
2
2
Aꢀa B3LYP/6ꢀ31G(d)
839.420320
839.623313
839.653552
839.489155
839.216262
839.419706
839.450465
839.286194
839.256446
839.459991
839.490772
839.326344
839.201427
839.459991
839.435544
839.271391
0.00 36.32
0.24 20.16
0.27 19.81
0.13 23.35
3687.8/1.00
3734.5/1.00
3765.2/1.00
3700.4/1.00
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
Aꢀb B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.419496
839.623123
839.652992
839.488355
839.215299
839.419330
839.449772
839.285246
839.255404
839.459512
839.490003
839.325307
839.200539
839.404526
839.434923
839.270552
0.65 12.13
0.54 12.15
3697.6/1.04
3746.4/1.10
3776.6/1.09
3712.3/1.10
0.73
0.78
9.11
7.79
Aꢀс B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.419095
839.622935
839.653305
839.488848
839.215516
839.419773
839.450620
839.286193
839.255985
839.460372
839.491197
839.326547
839.200473
839.404678
839.435506
839.271224
0.29 22.26
0.00 30.23
0.00 31.25
0.00 29.08
3734.3/0.23
3773.8/0.31
3820.6/0.39
3767.3/0.22
Aꢀd B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.417926
839.621411
839.652097
839.487774
839.214354
839.418322
839.449408 –839.489891
839.285178 –839.325510
839.254742
839.458865
839.199314
839.403194
839.434298
839.270200
1.07
0.95
0.82
0.65
5.97
6.08
7.83
9.71
3729.1/0.26
3768.2/0.36
3815.3/0.40
3757.9/0.28
Aꢀе B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.418266
839.622332
839.652679
839.488079
839.214835
839.419299
839.450147
839.285608
839.255325
839.459832
839.490707
839.325978
839.199731
839.404168
839.434978
839.270572
0.70 11.14
0.34 17.03
0.31 18.52
0.36 15.84
3759.4/0.31
3801.7/0.40
3845.5/0.53
3791.7/0.38
2
2
2
Aꢀf B3LYP/6ꢀ31G(d)
839.412663
839.616144
839.209231
839.413091
839.249863
839.453786
839.194105
839.397938
4.13
4.13
0.03
0.03
3736.0/0.18
3775.7/0.25
B3LYP/6ꢀ311G(d)
Aꢀg B3LYP/6ꢀ31G(d)
839.413526
839.616945
839.209901
839.413730
839.250814
839.454735
839.194788
839.398570
3.53
3.54
0.09
0.08
3717.7/0.14
3755.0/0.19
B3LYP/6ꢀ311G(d)
Bꢀa B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.417847
839.621145
839.651228
839.486793
839.214039
839.417714
839.448374
839.284120
839.254138
839.457896
839.488578
839.324182
839.199157
839.402782
839.433413
839.269278
1.45
1.55
1.64
1.48
3.14
2.21
1.96
3.50
3716.1/0.57
3754.8/0.67
3790.5/0.65
3734.0/0.57
2
2
2
2
Bꢀb B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.416816
839.620718
839.650398
839.485705
839.212847
839.417086
839.447406
839.282889
839.252871
839.457182
839.487558
839.322886
839.198052
839.402153
839.432521
839.268122
2.24
2.00
2.28
2.30
0.83
1.03
0.67
0.90
3721.2/0.60
3763.4/0.71
3798.8/0.70
3742.0/0.64
Bꢀс B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpvDz
839.417680
839.621616
839.651828
839.487424
839.214173
839.418478
839.449221
839.284879
839.254599
839.458990
839.489740
839.325186
839.199124
839.403385
839.434104
839.269907
1.16
0.87
0.91
0.85
5.13
6.96
6.73
6.93
3733.9/0.22
3773.7/0.30
3820.3/0.38
3767.0/0.27
Bꢀd B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.416180
839.619772
839.650272
839.486032
839.212668
839.416671
839.447667
839.283548
839.252950
839.457092
839.488032
839.323771
839.197637
839.401549
839.432564
839.268577
2.19
2.06
1.99
1.74
0.90
0.93
1.09
1.54
3730.1/0.26
3769.3/0.37
3816.5/0.41
3759.1/0.29
Bꢀе B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
B3LYP/6ꢀ311G(d,p)
B3LYP/ccꢀpVDZ
839.416543
839.620662
839.650891
839.486366
839.213224
839.417722
839.448486
839.284032
839.253672
839.458200
839.488991
839.324335
839.198106
839.402577
839.433304
839.268988
1.74
1.36
1.38
1.39
1.93
3.04
3.04
2.78
3759.1/0.30
3801.7/0.38
3845.6/0.51
3791.9/0.37
2
2
2
Bꢀf B3LYP/6ꢀ31G(d)
839.411744
839.616078
839.208421
839.412153
839.249004
839.452842
839.193269
839.396961
4.67
4.73
0.01
0.01
3737.0/0.17
3776.4/0.24
B3LYP/6ꢀ311G(d)
Bꢀg B3LYP/6ꢀ31G(d)
839.411943
839.615334
839.208440
839.412223
839.249299
839.453208
839.193311
839.397043
4.48
4.50
0.02
0.02
3719.6/0.14
3757.3/0.18
B3LYP/6ꢀ311G(d)
Cꢀa B3LYP/6ꢀ31G(d)
839.413445
839.615778
839.210063
839.412660
839.250037
839.452709
839.195333
839.397886
4.02
4.81
0.04
9.0•10–
3667.1/1.19
3715.2/1.32
3
B3LYP/6ꢀ311G(d)
(
to be continued)

672
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
Table 2 (continued)
Rotaꢀ
mer
Computational
method
–E0
–E
–G
–H
ΔG
g%
ν/A
2
2
2
2
2
2
2
2
2
2
2
2
2
Cꢀb B3LYP/6ꢀ31G(d)
839.412489
839.615481
839.209011
839.412189
839.248930
839.452141
839.194347
839.397502
4.72
5.17
0.01
3676.3/1.23
3727.1/1.30
5.0•10–
3
B3LYP/6ꢀ311G(d)
Cꢀс
B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
839.411773
839.614979
839.208983
839.412377
839.249306
839.452800
839.194014
839.452800
4.48
4.75
0.02
0.01
3734.0/0.23
3773.5/0.31
Cꢀd B3LYP/6ꢀ31G(d)
839.410542
839.613396
839.207806
839.410911
839.248071
839.451306
839.192827
839.395860
5.26
5.69
0.01
2.0•10–
3730.1/0.28
3769.8/0.38
3
B3LYP/6ꢀ311G(d)
Cꢀe B3LYP/6ꢀ31G(d)
839.411069
839.614526
839.208412
839.412035
839.248730
839.452363
839.193388
839.397002
4.84
5.03
0.01
6.0•10–
2.3•10–
6.9•10–6 3775.1/0.25
6.0•10–
1.8•10–
2.0•10–
4.5•10–
6.1•10–
2.4•10–
3.0•10–
1.0•10–
6.9•10–
2.2•10–
1.0•10–
6.1•10–
1.3•10–
3.1•10–6 3776.0/0.27
3759.9/0.32
3802.0/0.40
3
5
B3LYP/6ꢀ311G(d)
Cꢀf
B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
839.405094
839.607879
839.202516
839.405429
839.242997
839.445934
839.187463
839.390389
8.44
9.06
3735.6/0.17
5
Cꢀg B3LYP/6ꢀ31G(d)
839.405689
839.608422
839.202943
839.405856
839.243877
839.446852
839.187876
839.390753
7.89
8.48
3718.8/0.14
3756.7/0.19
5
B3LYP/6ꢀ311G(d)
3
4
Dꢀa B3LYP/6ꢀ31G(d)
839.409988
839.612562
839.207240
839.409819
839.247264
839.449884
839.192425
839.394984
5.76
6.58
3721.5/0.59
3761.1/0.67
B3LYP/6ꢀ311G(d)
4
4
Dꢀb B3LYP/6ꢀ31G(d)
839.409086
839.612251
839.206131
839.409289
839.246076
839.449278
839.191405
839.394550
6.51
6.96
3727.5/0.64
3770.5/0.76
B3LYP/6ꢀ311G(d)
3
3
Dꢀс B3LYP/6ꢀ31G(d)
839.409952
839.613160
839.207353
839.410622
839.247641
839.450976
839.192401
839.395653
5.53
5.90
3733.8/0.24
3772.9/0.33
B3LYP/6ꢀ311G(d)
4
4
Dꢀd B3LYP/6ꢀ31G(d)
839.408614
839.611435
839.206010
839.408954
839.246178
839.449213
839.191673
839.393964
6.44
7.00
3729.6/0.28
3768.1/0.38
B3LYP/6ꢀ311G(d)
3
4
Dꢀe B3LYP/6ꢀ31G(d)
839.408805
839.612183
839.206427
839.409842
839.246748
839.450159
839.191405
839.394823
6.09
6.41
3759.6/0.33
3801.3/0.41
B3LYP/6ꢀ311G(d)
5
Dꢀf
B3LYP/6ꢀ31G(d)
B3LYP/6ꢀ311G(d)
839.404403
839.607082
839.201949
839.404696
839.242400
839.445187
839.186900
839.389647
8.81
9.53
3736.5/0.19
1.5•10–
4.0•10–6 3756.0/0.20
5
3718.2/0.15
Dꢀg B3LYP/6ꢀ31G(d)
839.404701
839.607413
839.201978
839.404796
839.242557
839.445413
839.186978
839.389774
8.72
9.39
B3LYP/6ꢀ311G(d)
this band is also favored by the overlap of the ν(ОН) bands
of the rotamers 2Вꢀс, 2Вꢀd, and 2Вꢀe with close frequenꢀ
cies (their contribution is at most 12%). The splitting of the
obtained from B3LYP/6ꢀ31G calculations. Analysis of
their topography revealed seven rotamers. In two of them,
the 1´ꢀOH group is involved in the IMHB with the carboꢀ
nyl O(1) atom.
–
1
lowꢀfrequency component of the ν(ОН) band (∼ 19 cm
)
is in reasonable agreement with the frequency difference
T→he global minimum on the Gibbs free energy surface
G0(R )) corresponds to rotamer 3с with the free 1´ꢀOH
group. In the gas phase, the percentage of rotamers with
the free 1´ꢀOH group is about 63% (estimated from the
Gibbs free energy values), which is in qualitative agreeꢀ
ment with experimental data. With allowance for the solꢀ
vent (cyclohexane) effect in the framework of the РСМ
model, this estimate increases to nearly 71%.
Thus, we carried out the conformational analysis
of 1´ꢀhydroxyethylnaphthazarine 2 in the 6ꢀ31G(d),
6ꢀ311G(d), 6ꢀ311G(d,p), and ccꢀpVDZ basis sets. As for
compound 3, four tautomeric forms are possible for naphꢀ
thazarine 2 and seven rotamers are possible for each tauꢀ
tomer. The most abundant is tautomeric form А (percentꢀ
age ∼ 86%) in which the substituent bearing the 1´ꢀOH
group is in the quinonoid nucleus. Rotamer 2Аꢀс with free
1´ꢀOH group has the lowest Gibbs free energy. In the gas
phase, the percentage of all rotamers with free 1´ꢀOH
between the rotamers 2Aꢀa and 2Aꢀb calculated in the
–
1
ccꢀpVDZ basis set (12 cm ).
–
1
A weak ν(ОН) band at 3605 cm in the spectrum of
a solution of compound 2 in hexane probably originates
from the "benzenoid" tautomers 2Вꢀa and 2Вꢀb whose
frequencies differ by only 8 cm (see Table 2), as preꢀ
dicted by calculations in the 6ꢀ311G(d,p) and ccꢀpVDZ
basis sets. Therefore, it is most likely that they will apꢀ
pear in the spectra as a single band. The frequency difꢀ
ference estimated from the results of calculations in the
–
1
ccꢀpVDZ basis set, νOH(2Aꢀa) – ν (2Bꢀa, 2Bꢀb) =
OH
–
1
=
(3734 + 3742)/2 – 3700 = 38 cm , is in good agreeꢀ
ment with the frequency difference between maxima
of the ν(ОН) bands in the spectrum of compound 2:
–
1
3
605 – 3570 = 35 cm (see Fig. 1).
Twoꢀdimensional sections of the PES of benzoquinoꢀ
ne 3 in the plane of the θО(9) and θОН torsion angles were

DFT study of 1´ꢀhydroxyethylbenzoquinone
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
673
group exceeds 60%, which is in qualitative agreement with
the pattern of the ν(ОН) bands in the IR spectra of
a solution of 2 in hexane. The larger splitting of the ν(ОН)
band in the IR spectra of solutions of 2 in nonpolar solꢀ
vents is due to different frequencies of the rotamers resultꢀ
ing from cleavage of the relatively weak IMHB between
the 1´ꢀOH group and the carbonyl group. The energy of
to 2.50 mm). The frequency and areaꢀunderꢀcurve measureꢀ
ments and the decomposition of contours of the ν(OH) stretchꢀ
ing absorption bands were performed using the OPUS/IR 02
Version 3.0.2 program package. The reproducibility in frequenꢀ
cy measurements was at least 0.5 cm– . The concentrations of
1
–
1
solutions of the compounds under study were 5 to 20 mmol L
.
The course of reactions was monitored, and individual charꢀ
acter of the compounds synthesized was confirmed, by TLC
(Merck 60Fꢀ254 plates, hexane—acetone 3 : 1). Individual comꢀ
pounds were isolated from mixtures of reaction products by
preparative TLC on 20×20ꢀcm plates with fixed silica gel layer
the IMHB was estimated at 0.10 kcal mol–1
.
Experimental
5
—40 μm thick. The yields of the compounds were not optiꢀ
mized. Elemental analysis was done on a Flash EA1112 С,Н,N
analyzer.
Quantum chemical calculations were carried out in the frameꢀ
work of the density functional theory (exchangeꢀcorrelation funcꢀ
Acetylhydroquinone (6). To a melt of anhydrous AlCl (145 g,
3
2
6
tional B3LYP ) with the 6ꢀ31G(d), 6ꢀ311G(d), 6ꢀ311G(d,p),
ccꢀpVDZ, and ccꢀpVTZ basis sets and the GAUSSIAN 03 proꢀ
gram27 (byꢀdefault algorithms were used). Geometric parameꢀ
ters were optimized in internal coordinates using the Berni algoꢀ
rithm ("Tight" mode). Integrals were calculated by numerical
integration on a grid with byꢀdefault parameters.
1.1 mol) and NaCl (27 g, 0.5 mol), hydroquinone diacetate (4)
(31 g, 0.16 mol) was added portionwise with vigorous stirring at
140 °C. The temperature of the mixture was raised to 195 °С and
the melt was stirred for 9 min. The reaction mixture was cooled
and hydrolyzed with a solution of conc. HCl (150 mL) in water
(2.0 L). After 12 h, the precipitate was filtered off, washed with
hot water (0.5 L), and dried. A mixture of acetylhydroquinone
Oꢀacetate (5) and acetylhydroquinone (6) was obtained (26 g).
In constructing the 2D section of the PES, the energies (Е0)
of the ground electronic state of 1´ꢀhydroxyethylbenzoquinone
1
3
were calculated by optimizing all geometric parameters of the
Н NMR for 6 (δ, J/Hz): 2.60 (s, 3 Н, Ме); 4.62 (s, 1 Н,
molecule except for θOH and θO(9) (these angles were chosen as
the PES scan variables). Stationary points on the PES were loꢀ
cated upon full optimization of the molecular geometry.
Geometry optimization was conducted until meeting the followꢀ
С(4)ОН); 6.89 (d, 1 Н, Н(6), J = 8.5); 7.03 (dd, 1 Н, Н(5),
J = 8.5, J = 2.9); 7.19 (d, 1 Н, Н(3), J = 2.9); 11.81 (s, 1 Н,
С(1)ОН). Н NMR for 5 (δ, J/Hz): 2.31, 2.62 (both s, 3 Н, Ме);
1
2
1
6.99 (d, 1 Н, Н(6), J = 8.5); 7.21 (dd, 1 Н, Н(5), J = 8.5,
1
–
6
–1
ing condition for the norm of the gradient: |grad| ≤ 10 au Å
.
J2 = 2.9); 7.45 (d, 1 Н, Н(3), J = 2.9); 12.13 (s, 1 Н, С(1)ОН).
A mixture of 5 and 6 was boiled for 0.5 h in HCl (100 mL),
filtered, the residue was washed with water and dried to give 20.5 g
(85%) of acetylhydroquinone (6), m.p. 198—200 °C (cf. Ref. 29:
If normalꢀmode vibrational frequency calculations at certain staꢀ
tionary points on the PES gave no imaginary frequencies, these
stationary points were treated as energy minima.
–
1
The Gibbs free energies, G, were calculated with inclusion of
all electronic, translational, rotational, and vibrational degrees
of freedom for Т = 298.15 К. The expression used for a molecule
in the gas phase is as follows: G = E + GT,i, where i is the
m.p. 202—203 °C). IR spectrum (СНСl ), ν/cm : 3601, 3100
3
(О—Н); 1647 (С=О); 1629, ∼ 1615, ∼ 1600, 1590 (С=С). Mass
+
spectrum, m/z (Irel (%)): 153 [М + 1] (9.1), 152 [М] (100), 43 (2.9).
1´ꢀHydroxyethylbenzoquinone (3). Acetylhydroquinone 6
i
0,i
i
number of the rotamer and GT,i = GT,i,r + GT,i,v + GT,i,t is the sum
of the rotational, vibrational, and translational contributions,
respectively, calculated in the "harmonic oscillator—rigid rotaꢀ
tor" approximation. The Gibbs free energy of the ith rotamer of
the molecule in solution (s) was calculated in the framework of
the РСМ model2 as Gi,s = (E0,i)s + (GT,i)s, where (E0,i)s is the
electronic contribution and (GT,i)s is the sum of the translational,
vibrational, and rotational contributions calculated using the PC
GAMESS program28 with inclusion of electrostatic interaction
between the molecule and the solvent. All calculations were
performed for the optimized structures using the byꢀdefault alꢀ
gorithms. The molecular geometry was optimized with the quaꢀ
dratic approximation (QA) algorithm. Integrals were calculated
with an accuracy of 11 decimal places, the norm of the gradient
(150 mg, 1 mmol) was dissolved in Pr OH (10 mL) and sodium
borohydride (40 mg, 1 mmol) was added with stirring. The course
of the reaction was monitored by TLC (hexane—acetone, 3 : 1).
A few minutes after completion of the reaction, the reaction
mixture was diluted with water, carefully acidified with НCl
until a weak acid reaction, and extracted with ethyl acetate. The
extract was dried with Na SO , concentrated under reduced presꢀ
5
2
4
sure, and the dark oily residue was dissolved in MeCN (3 mL).
To this solution, a solution of 1 g (NH ) Ce(NO ) in a mixture
4
2
3 6
of MeCN (1 mL) and water (3 mL) was added dropwise with
stirring and iceꢀcooling. The course of the reaction was moniꢀ
tored by TLC. After 30 min, the reaction mixture was poured
onto ice and after an additional 1 h extracted with Et O. The
2
ethereal layer was washed with water, dried with Na SO , and
2
4
–
6
–1
was minimized until a value of 10 Hartree Bohr . The perꢀ
concentrated. The yield of 1´ꢀhydroxyethylbenzoquinone (3) was
98 mg (65%), lightꢀyellow oil; after additional purification by
preparative TLC the oil crystallizes in a refrigerator, m.p.
55—57 °C. Found (%): С, 63.23; Н, 5.28. С Н О . Calculatꢀ
%
centage of the ith rotamer was calculated as gi = g •100%.
i
The melting points were determined on a Boetius apparatus
1
13
and are uncorrected. Н and С NMR spectra were recorded
8
8
3
1
13
1
on a Bruker Avance DRXꢀ500 spectrometer ( Н, 500 MHz; С,
25 MHz) in CDCl with Me Si as the internal reference. Mass
ed (%): С, 63.15; Н, 5.30. Н NMR (δ, J/Hz): 1.38 (d, 3 Н, Ме,
J = 6.5 Hz); 4.82 (qd, 1 Н, СН, J = 6.5, J = 1.4 Hz); 4.09 (br. s,
1 Н, ОН); 6.71 (m, 2 Н, Н(5), Н(6)); 6.77 (q, 1 Н, Н(3), J = 1.4 Hz).
1
3
4
spectra (EI) were recorded on an LKBꢀ9000S instrument with
direct inlet at electron ionizing energies of 18 and 70 eV.
IR spectra were recorded on a Bruker Vector 22 Fourier spectroꢀ
photometer with a resolution of 2 cm–1 in solutions in nꢀhexane,
cyclohexane, CCl , and CDCl (CaF cells, layer thickness 1.00
13
С NMR (δ): 188.0, 187.6 (С=О), 151.1 (С(2)), 136.9, 136.4
(С(5), С(6)), 130.5 (С(3)), 64.5 (С(1´)), 22.6 (Ме). Mass specꢀ
+
trum, m/z (Irel (%)): 152 [М] (100), 136 (53), 135 (31), 123
(46), 109 (28), 95 (8), 76 (15).
4
3
2

674
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 4, April, 2009
Glazunov et al.
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