Structure, tautomerism, and features of 1-(5-acetyl-2,4-dihydroxyphenyl)-3- (furan-2-yl)prop-2-en-1-one (FC) and 1,1′-(4,6-dihydroxybenzene-1,3-diyl) bis[3-(furan-2-yl)prop-2-en-1-one] (FDC)

Article abstract of DOI:10.1007/s11224-013-0378-y

Quantum chemistry studies of various tautomeric/rotameric forms of the furanyl analogues of acetyl dihydroxychalcone (FC) and dihydroxydichalcone (FDC) have shown that the thermodynamically most stable molecules are planar. In the crystalline solid phase, both molecules are more (FDC) or less (FC) non-planar. Two relatively strong intramolecular H-bonds, whose existence is evidenced by X-ray, computational, and spectral investigations, stabilize the planar geometry and influence the features of the lowest energy tautomers/rotamers of both compounds. Extending to the visible region, the electronic absorption of FC and FDC is accompanied by an intramolecular electron density shift, a property that could have analytical implications.

Full text of DOI:10.1007/s11224-013-0378-y

Struct Chem (2014) 25:969–977
DOI 10.1007/s11224-013-0378-y
ORIGINAL RESEARCH
Structure, tautomerism, and features of 1-(5-acetyl-2,4-
dihydroxyphenyl)-3-(furan-2-yl)prop-2-en-1-one (FC) and 1,10-
(4,6-dihydroxybenzene-1,3-diyl)bis[3-(furan-2-yl)prop-2-en-1-one]
(FDC)
•
Michał Wera Andriy G. Chalyi Alexander D. Roshal
•
•
•
Beata Zadykowicz Jerzy Błazejowski
_
Received: 9 October 2013 / Accepted: 21 November 2013 / Published online: 6 December 2013
Ó The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract Quantum chemistry studies of various tauto-
meric/rotameric forms of the furanyl analogues of acetyl
dihydroxychalcone (FC) and dihydroxydichalcone (FDC)
have shown that the thermodynamically most stable mol-
ecules are planar. In the crystalline solid phase, both
molecules are more (FDC) or less (FC) non-planar. Two
relatively strong intramolecular H-bonds, whose existence
is evidenced by X-ray, computational, and spectral inves-
tigations, stabilize the planar geometry and influence the
features of the lowest energy tautomers/rotamers of both
compounds. Extending to the visible region, the electronic
absorption of FC and FDC is accompanied by an
intramolecular electron density shift, a property that could
have analytical implications.
Introduction
Derivatives of (2E)-1,3-diphenylprop-2-en-1-one (chal-
cones) are naturally occurring compounds which, due to
their biological activity, have been the subject of interest
to the food and health sciences [1, 2]. They have been
thoroughly investigated in the context of their role in
plant growth, human diet, and healthcare [1–7]. Our
interest in these compounds arises from the fact that they
are precursors in the synthesis of flavonols, which are
derivatives of 3-hydroxy-2-phenyl-4H-chromene-4-one
[8–10]. Following the synthesis of various chalcones
(C) and dichalcones (DC) [derived from (2E,20E)-1,10-
(1,3-phenylene)bis(3-phenylprop-2-en-1-one)] and related
derivatives, we obtained two compounds that were furanyl
(F) analogues of these entities, namely, 1-(5-acetyl-2,4-
dihydroxyphenyl)-3-(furan-2-yl)prop-2-en-1-one (FC) and
1,10-(4,6-dihydroxybenzene-1,3-diyl)bis[3-(furan-2-yl)prop-
2-en-1-one] (FDC). The molecules of these compounds
provide convenient models for studying the properties of
intramolecular H-bonds, as there are two sites present in
them where a hydroxyl group appears in the vicinity of
the carbonyl group and where such bonds can occur.
Another interesting property of FC and FDC is that both
can exist in several tautomeric and rotameric forms.
With these compounds in hand, we can investigate these
phenomena. Finally, we may infer from the canonical
structures of FC and FDC that both molecules should be
planar. Thus, it would be interesting to know how such
relatively large molecules are packed in the crystalline
solid phase. To shed more light on these issues and to
elucidate the properties of both these molecules, we
carried out X-ray, spectroscopic, and computational
investigations, the results of which are presented in this
Keywords Chalcone and dichalcone furanyl
analogues ꢀ Crystal and computed structures ꢀ
Tautomerism ꢀ Intra- and intermolecular interactions ꢀ
Spectral features
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-013-0378-y) contains supplementary
material, which is available to authorized users.
M. Wera ꢀ B. Zadykowicz ꢀ J. Błaz_ejowski (&)
´
Faculty of Chemistry, University of Gdansk, Wita Stwosza 63,
´
80-308 Gdansk, Poland
e-mail: jerzy.blazejowski@ug.edu.pl
A. G. Chalyi ꢀ A. D. Roshal
Institute of Chemistry, V. N. Karazin National University,
Svoboda 4, Kharkiv 61077, Ukraine
publication.
A
search of the Cambridge Structure
123

970
Struct Chem (2014) 25:969–977
Computations
Database [11] showed that the structures of several
chalcones have been determined by X-ray diffractome-
try, but that no structures of dichalcones have as yet
been established.
Geometry optimizations were carried out at the DFT level
of theory [13] with the B3LYP functional [14–17] and
6–31G** basis set [18, 19] and, in the case of selected
entities, at the MP2 level [20] using the cc-pVDZ basis set
[21, 22] and the Gaussian 03 program package [23].
Materials and methods
FC
FDC
Synthesis and spectral investigations
H
H
H
H
H
H
O
O
O
1-(5-Acetyl-2,4-dihydroxyphenyl)-3-(furan-2-yl)prop-2-
en-1-one (FC) and 1,10-(4,6-dihydroxybenzene-1,3-diyl)-
bis[3-(furan-2-yl)prop-2-en-1-one] (FDC) were synthe-
sized by condensation (with removal of H2O) of
commercially available 1,10-(4,6-dihydroxybenzene-1,3-
diyl)diethanone with furan-2-carbaldehyde dissolved in
methanol/50 % aqueous NaOH (1/1 v/v) [8, 9, 12]. The
reactant mixture was kept at room temperature for 48 h,
then diluted twice with distilled water and neutralized with
aqueous HCl to pH 5. The resulting precipitate was filtered
off, rinsed with hot (333–343 K) water and dried. The
products obtained were separated by gravitational column
chromatography (Silica Gel 60, ethyl acetate/hexane, 1/2
v/v). Yellow-orange and orange crystals of FC and FDC,
respectively, suitable for X-ray and other investigations
were grown from an ethanol/n-hexane (4/1 v/v) solution
(FC: m.p. 452–453 K, m/z = 273 (M–H?); FDC: m.p.
444–445 K, m/z = 351 (M–H?)).
H
H
H
H
H
H
H
H
H
CH
H
H
3
O
O
O
O
O
O
HO
OH
HO
OH
H
H
a1
a1
H
H
H
H
H
H
O
O
O
H
H
H
H
H
H
CH
H
H
H
H
H
2
HO
O
OH
HO
OH
O
O
H
H
H
a1(ii)
a
1(i)
The relevant spectral grade solvents were used
throughout in the spectroscopic investigations. UV spectra
were recorded on a Hitachi U3210 spectrophotometer. IR
spectra were recorded on a Bruker FTIR IFS 66 spectro-
photometer and NMR spectra on a Varian Unity 500 Plus
spectrometer operating at a proton frequency of 500 MHz
(carbon frequency = 125 MHz).
H
H
H
H
H
H
O
O
O
H
H
H
H
H
H
H
CH
O
H
H
3
H
H
O
OH
OH
OH
O
OH
Single crystal X-ray measurements
H
H
d1
c
1
X-ray data for the title compounds were collected on an
Oxford Diffraction Gemini R Ultra Ruby CCD diffractom-
eter (MoKa radiation, graphite monochromator). Data col-
lection and cell refinement were carried out with CrysAlis
CCD and data reduction with CrysAlis RED. Structure
solution and refinement was carried out with the SHELXS97
package. All C and O atoms were refined anisotropically. H
atoms of C–H bonds were positioned geometrically and
constrained to ride on their parent atoms, H atoms involved
in O–HꢀꢀꢀO hydrogen bonds were located on a difference
Fourier map and refined isotropically. The asymmetric unit
consists of two crystallographically independent molecules
of FC and one half of the molecule of FDC.
H
H
O
H
H
CH
H
H
3
HO
O
HO
O
H
e
1
Fig. 1 Canonical structures of selected tautomeric forms of FC and
FDC
123

Struct Chem (2014) 25:969–977
971
Table 1 Thermodynamic data for the selected FC and FDC tau-
tomers/rotamers in the gaseous phase
Tautomer/rotamer
DH
DG
DFT
MP2
DFT
MP2
FC
a1
0.0
0.0
7.8
0.0
0.0
4.0
a2
7.2
97.4
4.5
98.8
a1(i)
b1*
c1*
d1
88.8
89.5
148.1
153.4
154.8
162.2
148.4
156.1
144.8
153.3
146.2
142.5
148.1
155.0
147.1
154.5
138.9
146.7
e1
FDC
a1
0.0
6.6
0.0
1.4
0.0
9.9
0.0
4.7
a2
a1(ii)
b1*
c1
103.3
148.2
153.0
95.5
104.4
150.7
152.9
96.5
144.6
132.0
152.2
158.1
Values obtained at the DFT(B3LYP)/6–31G** (DFT) or MP2/cc-
pVDZ (MP2) levels of theory
DH represents the enthalpy of a given tautomer/rotamer (indicated in
Figs. 1 and 1S–3S in the Supplementary data) relative to the enthalpy
of form a1 (given in Table 1S in the Supplementary data); DG is the
Gibbs free energy of a given tautomer/rotamer (indicated in Figs. 1
and 1S–3S in the Supplementary data) relative to the Gibbs free
energy of form a1 (given in Table 1S in the Supplementary data);
DH and DG, in kJ/mol, correspond to 298.14 K and standard pressure
C and O, O and C, or two O atoms [24], which leads to a
substantial increase in the number of tautomers. To take
into account such a possibility we considered selected
structures relevant to the a1 forms (found to be the lowest
energy ones) of FC (fifteen structures) and FDC (four
structures) (Fig. 3S in the Supplementary data).
Fig. 2 MP2 optimized structures of selected tautomeric forms of FC
and FDC (for canonical structures, see Fig. 1)
Results and discussion
Optimization of the canonical structures a1, b1, and c1
of FC or a1 and b1 of FDC leads to the most stable
molecular structure a1 for each compound (Fig. 2). An
analogous result was obtained for the respective rotamers
of FC (a2) and FDC (a2 and a3); the relevant molecular
structures are shown in Figs. 4S–6S (Supplementary data),
also those in which only one H-bond is present (d and e of
FC, and c of FDC). The data in Table 1 (Tables 1S, 2S in
the Supplementary data) indicate that the thermodynami-
cally most stable tautomer of FC and FDC is predicted to
be a, which can occur as two rotamers (a1 and a2) in the
former compound and three rotamers (a1, a2, and a3) in
the latter. The enthalpy and Gibbs free energy differences
between the most and the least stable rotamers of a are no
greater than a dozen or so kJ/mol, which implies that
various entities of this kind can occur in equilibrium at
room temperature (the equilibrium constants expressing the
ratio of the number of molecules of a2 and a1 should be
According to the relevant canonical structures (Figs. 1 and
1S, 2S in the Supplementary data), we can expect FC and
FDC to have, respectively, five and three tautomeric forms
(resulting from the migration of the H atom between two O
atoms) with two or one intramolecular H-bond(s) [24]. In
the case of only one intramolecular H-bond, four tauto-
meric/rotameric canonical structures can be expected for
both FC and FDC. Rotation of furanyl fragment(s) around a
C–C bond, examined to ascertain whether the crystalline
phase consists of the lowest energy rotamers, makes it
possible for each FC tautomer to occur in two rotameric
forms, and each FDC tautomer in three or four such forms.
Investigation of the rotation of the O–H group around a
C–O bond in four tautomers/rotamers of both FC and FDC
enabled us to ascertain whether this would be an adequate
reflection of the thermodynamics of H-bonds. Tautomerism
may also result from the migration of the H atom between
123

972
Struct Chem (2014) 25:969–977
0.201 and 0.152 for FC and FDC, respectively; from MP2
data). Other tautomers/rotamers of both compounds,
including those resulting from the migration of the H atom
between C–O, O–C, or two O atoms are over 89 kJ/mol
thermodynamically less stable than tautomer/rotamer a1
and cannot be expected to be present in measurable
amounts under such conditions.
symmetry is present in the asymmetric unit of FDC (Figs. 4
and 10S in the Supplementary data). The molecular
structure of FC is stabilized by two S(6) type intramole-
cular O–HꢀꢀꢀO H-bonds (Table 6S in the Supplementary
data), and the crystal structure of the compound by a net-
work of intermolecular C–HꢀꢀꢀO interactions (Table 7S and
The stability of tautomers a [b, c, a1(i)–a1(vii)] of FC
and a (b) of FDC should be substantially enhanced by two
intramolecular H-bonds (Figs. 1 and 1S–3S in the Sup-
plementary data). In other tautomers of the compounds
considered only one such bond occurs. Therefore, the dif-
ferences between the enthalpy (DH) and Gibbs free energy
(DG) of the respective tautomeric/rotameric forms [d, e and
a (b, c) of FC and c and a (b) of FDC] could tentatively be
regarded as relevant thermodynamic characteristics of a
single H-bond (tautomeric forms resulting from the
migration of the H atom between C and O, O and C or two
O atoms cannot be used for such estimations). The DH and
DG values shown in Tables 1 and 2S (Supplementary data)
are, however, much higher than those we found earlier for a
single H-bond (DG for such a bond was estimated to be
78.3 kJ/mol [25]). Presumably, then, the transformation
from tautomer a to other tautomers of FC (d, e) and FDC
(c) is accompanied not only by the disappearance of one
H-bond, but also by substantial structural changes (Figs. 2
and 4S–6S in the Supplementary data), which lead to the
additional thermodynamic destabilization of the latter
molecules. If we compare tautomer a with tautomers d and
e of FC, and tautomer a with tautomer c of FDC, we find
that they occur as a result of H atom transfer between non-
vicinal sites of the molecules. On the other hand, the forms
b* and c* originating from b and c of FC and b* origi-
nating from b of FDC, which occur as a result of H atom
transfer between vicinal sites of the molecules, are ther-
modynamically much less stable. The DH and DG values of
the respective latter forms are comparable to the former
ones mentioned above, which means that they probably do
not reflect thermodynamic characteristics of the single
H-bond. The forms b* and c* of FC and b* of FDC can
exist since there is an enthalpy and Gibbs free energy
barrier to a H atom returning to the position it occupied in
tautomers b and c of FC and b of FDC, respectively (the
structures of molecules in transition states for such a pro-
cess are presented in Figs. 7S and 8S, and the activation
barriers are given in Table 3S in the Supplementary data).
Crystals of FC and FDC consist of tautomers/rotamers
a1, as demonstrated by the results of X-ray investigations
[the details of the refinement (Tables 4S, 5S) are given in
the Supplementary data]. Two structurally similar but
crystallographically independent molecules A and B are
present in the asymmetric unit of FC (Figs. 3 and 9S in the
Supplementary data), while one half of the molecule of C2
Fig. 3 ORTEP view of two structurally similar but crystallograph-
ically independent A and B molecules in the asymmetric unit of the
crystalline solid phase of FC (dashed lines H-bonds)
Fig. 4 ORTEP view of the FDC molecule in the crystalline solid
phase (dashed lines H-bonds)
123

Struct Chem (2014) 25:969–977
973
Table 2 Comparison of
Bonda
X-rayb
DFTc
MP2c
experimental (X-ray) and
predicted (DFT(B3LYP)/
6–31G** and MP2/cc-pVDZ
levels of theory) lengths of C–C,
˚
C–O and C=O bonds (in A) in
FC
A
B
dd
dd
C1–C2
1.417 (4)
1.386 (4)
1.457 (4)
1.372 (4)
1.344 (4)
1.363 (4)
1.416 (4)
1.344 (4)
1.387 (4)
1.444 (4)
1.241 (3)
1.499 (4)
1.246 (4)
1.454 (4)
1.332 (4)
1.423 (4)
1.361 (4)
1.353 (4)
1.355 (4)
1.330 (4)
1.401 (4)
1.414 (4)
1.385 (4)
1.463 (4)
1.373 (4)
1.350 (4)
1.373 (4)
1.403 (4)
1.347 (4)
1.386 (4)
1.458 (4)
1.243 (3)
1.497 (4)
1.251 (4)
1.448 (4)
1.336 (4)
1.409 (4)
1.367 (3)
1.353 (4)
1.361 (4)
1.334 (4)
1.391 (4)
1.442
1.398
1.471
1.395
1.329
1.393
1.439
1.331
1.398
1.464
1.243
1.516
1.255
1.470
1.353
1.430
1.376
1.377
1.357
1.365
1.424
1.764/1.980
0.866/0.939
0.961/0.547
1.676/1.602
1.116/1.556
2.201/1.457
1.624/2.566
0.967/1.113
0.793/0.865
1.385/0.412
0.161/0.0
1.437
1.406
1.481
1.402
1.339
1.400
1.434
1.341
1.404
1.479
1.243
1.516
1.251
1.482
1.362
1.440
1.370
1.392
1.362
1.381
1.428
1.411/1.627
1.443/1.516
1.647/1.230
2.187/2.112
0.372/0.815
2.715/1.966
1.271/2.210
0.223/0.445
1.226/1.299
2.424/1.440
0.161/0.0
C1–C6
C1–C12
C2–C3
C2–O7
C3–C4
C4–C5
C4–O8
C5–C6
C5–C9
C9–O10
C9–C11
C12–O13
C12–C14
C14–C15
C15–C16
C16–O17
C16–C20
O17–C18
C18–C19
C19–C20
Average
1.134/1.269
0.722/0.320
1.309/1.519
1.577/1.272
0.492/1.490
1.102/0.658
1.774/1.774
0.148/0.294
2.632/2.324
1.642/2.372
1.240/1.254
1.134/1.269
0.401/0.0
a
The atom numbering is given
in Fig. 9S
b
2.132/2.348
2.252/1.946
1.195/2.200
0.661/0.219
2.882/2.882
0.517/0.073
3.835/3.523
1.927/2.660
1.525/1.513
A and B denote two
structurally similar but
crystallographically
independent molecules in the
asymmetric unit of the
compound (Fig. 9S)
c
For a1 structure, see Figs. 1S,
4S and 6S
d
d=|X-ray bond length–DFT or
MP2 bond length|/X-ray bond
length 9 100 (in %)
Table 3 Comparison of experimental (X-ray) and predicted
(DFT(B3LYP)/6–31G** and MP2/cc-pVDZ levels of theory) lengths
˚
of C–C, C–O and C=O bonds (in A) in FDC
Fig. 11S in the Supplementary data). The molecular
structure of FDC is also stabilized by two S(6) type
intramolecular O–HꢀꢀꢀO H-bonds (Table 8S in the Sup-
plementary data). In the crystal structure, the parallel FDC
molecules interacting via C=Oꢀꢀꢀp contacts (Table 9S in
the Supplementary data) are arranged in columns along the
a axis; the inverted columns in which molecules are in
contact via C–HꢀꢀꢀO interactions (Table 10S in the Sup-
plementary data) form pairs of columns, each of which
interacts dispersively with four adjacent pairs of columns
stabilizing the crystal lattice (Figs. 12S, 13S in the Sup-
plementary data).
Bonda
X-ray
B3LYPb
dc
MP2b
dc
C1–C2, C1A–C2A
C1–C4, C1A–C4
C1–C6, C1A–C6A
C2–C3, C2A–C3
C2–O5, C2A–O5A
C6–O7, C6A–O7A
C6–C8, C6A–C8A
C8–C9, C8A–C9A
C9–C10, C9A–C10A
1.417 (4) 1.442 1.764 1.437 1.411
1.384 (3) 1.398 1.012 1.406 1.590
1.461 (4) 1.470 0.616 1.480 1.300
1.384 (3) 1.393 0.650 1.401 1.228
1.346 (3) 1.330 1.189 1.339 0.520
1.248 (3) 1.255 0.561 1.251 0.240
1.457 (4) 1.472 1.030 1.482 1.716
1.331 (4) 1.352 1.578 1.362 2.329
1.429 (4) 1.430 0.070 1.440 0.770
The experimentally determined structural parameters of
the a1 forms of the compounds investigated compared well
with the predicted values (Tables 2, 3) according to the
criteria we applied earlier [25]. The lowest energy forms
(a1) of both FC and FDC (C2v symmetry) should be planar
in the gaseous phase (Table 4). In the crystalline solid
phase both compounds are less (FC) and more (FDC) non-
planar. Other tautomers/rotamers of FC and FDC are pre-
dicted to be either planar (a2 and c1* of FC, and a2 and a3
of FDC) or non-planar (Tables 4 and 11S in the Supple-
mentary data). Low energy rotamers resulting from the
rotation of furanyl fragment(s) around a C–C bond (a1 and
a2 of FC, and a1, a2 and a3 of FDC) are predicted to be
C10–O11, C10A–O11A 1.367 (3) 1.375 0.585 1.370 0.219
C10–C14, C10A–C14A 1.337 (4) 1.377 2.992 1.392 4.114
O11–C12, O11A–C12A 1.372 (4) 1.357 1.093 1.362 0.729
C12–C13, C12A–C13A 1.320 (4) 1.365 3.409 1.381 4.621
C13–C14, C13A–C14A 1.412 (4) 1.424 0.850 1.428 1.133
Average
1.243
1.566
a
The atom numbering is given in Fig. 10S
b
For a1 structure, see Figs. 2S, 5S and 6S
c
d=|X-ray bond length–DFT or MP2 bond length|/X-ray bond
length 9 100 (in %)
123

974
Struct Chem (2014) 25:969–977
Table 4 Structural features of
selected (Table 1) tautomers/
rotamers of FC and FDC
Tautomera
Methodb
Ac
Bond lengthsd
C–O
B
C=O
D
B
D
FC
a1
X-ray
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
0.0352 (2); 0.0465 (2)e
0.0002
1.346 (4)
1.340
1.330
1.340
1.330
1.362
1.358
1.349
1.342
1.347
1.339
1.365
1.362
1.361
1.359
1.245 (4)
1.247
1.249
1.246
1.248
1.251
1.256
1.241
1.240
1.248
1.251
0.0000
a2
0.0000
0.0000
a1(i)
b1*
c1*
d1
0.1026
0.0948
0.1600
1.347
1.342
1.343
1.338
1.237
1.234
1.237
1.234
1.229
1.223
1.232
1.227
0.1764
0.0000
0.0000
0.0841
0.0813
a
e1
0.4159
For canonical structures, see
Figs. 1 and 1S–3S in the
Supplementary data
0.4149
FDC
b
MP2 (MP2/cc-pVDZ), DFT
(DFT(B3LYP)/6–31G**) levels
of theory
a1
X-ray
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
MP2
DFT
0.1774 (2)
0.0000
0.0000
0.0000
0.0000
0.3144
0.3223
0.2682
0.2446
0.3782
0.3929
1.346 (3)
1.339
1.330
1.339
1.329
1.330
1.323
1.348
1.340
1.365
1.363
1.248 (3)
1.251
1.255
1.251
1.254
1.248
1.251
1.248
1.251
c
A—average deviation from
planarity of the molecular
system involving all C and O
atoms
a2
a1(ii)
b1*
c1
d
Mean lengths of the C–
O(H) and C=O bonds (in A)
˚
1.348
1.343
1.237
1.234
1.232
1.228
when O atoms are engaged
(B) and non-engaged (D) in
intramolecular H-bonds
e
Values for molecules A (first)
and B (second) in Fig. 3
planar, whereas rotamers arising from the rotation of the
OH group around a C–O bond (b1*, b2* and c2* of FC,
and b1*, b2*, b3*, and b4* of FDC) ought to be non-
planar. It is thus evident that the disappearance of one of
the H-bonds in the latter rotamers is accompanied by a loss
of planarity. Another reason for the loss of planarity is the
partial disappearance of resonance conjugation from the
molecular systems of FC [in tautomers a1(vi), a1(vii),
a1(x), a1(xi), a1(xiv), and a1(xv)] and FDC [a1(iii) and
a1(iv)] (Fig. 3S in the Supplementary data). The thermo-
dynamic measure of this effect can be given by the dif-
ference in Gibbs free energies of the relevant tautomers of
the series a1(i)–a1(xv) of FC and a1(i)–a1(iv) of FDC. The
lowest such differences [i.e. relevant to a1(x)–a1(viii) and
a1(xv)–a1(xiii) of FC, and a1(iii)–a1(ii) of FDC], ranging
from 24 to 32 kJ/mol, reflect the extent of the thermody-
namic destabilization of both molecules caused by the
partial disappearance of conjugation in them.
C=O bonds in molecules in which O atoms are involved
in (C)OꢀꢀꢀH–O interactions, whose geometry is described in
Tables 6S and 8S (Supplementary data) and demonstrated
in Figs. 2 and 4S–6S (Supplementary data), are usually
longer than those not engaged in such interactions
(Tables 4 and 11S in the Supplementary data). On the
other hand, the C–O bond length, if the O atom is a part
of the hydroxyl group, is not affected by the involvement
or non-involvement of this group in H-bonding interac-
tions. The involvement of the above mentioned fragments
of FC and FDC molecules in intramolecular O–HꢀꢀꢀO
bonds is confirmed by IR and NMR spectral data: the
wavenumbers of the C=O and O–H stretching vibrations
determined experimentally agree qualitatively with the
1
predicted values (Table 5) [25, 26], and the H signals of
H atoms from hydroxyl groups engaged in such interac-
tions appear at relatively high chemical shifts (Table 6)
[25, 27].
123

Struct Chem (2014) 25:969–977
975
Table 5 Infrared absorption data (in cm-1) for FC and FDC
Method
FC
Vibration/wavenumbera
C(9)=O(10)
1,630
C(12)=O(13)
O(7)–H
O(8)–H
Exp
Theorb
FDC
1,610
1,651
3,420 (broad)
1,663
3,283
3,349
C(6/6A)=O(7/7A)
1,699
O(5/5A)–H
3,440 (broad)
3,297
Exp
Theorb
1,702
Exp—experimental values from IR spectra recorded in KBr pellets;
Theor—values predicted for isolated molecules at the MP2/cc-pVDZ
level of theory
Fig. 5 The UV–vis absorption (solid line) and deconvoluted (using
Spectral Data Lab software [35]) (dashed lines) spectra of FC
(dissolved in methanol) together with predicted (at the
TDDFT(B3LYP)/6–31G** level of theory for structure a1) electronic
absorption transitions (bars)
a
The atom numbering (in parentheses) is given in Figs. 9S and 10S
b
Stretching vibrations in structures a1
Extension of the electronic absorption of FC and FDC to
the visible region (Figs. 5, 6) confirms that both molecular
systems are conjugated and planar. The long wavelength
absorption accompanied by an electron density shift from
4,6-dihydroxy-1,3-phenylene (in both FC and FDC) to the
one (FC) or two (FDC) (1E)-1-(furan-2-yl)-3-oxoprop-1-
en-3-yl fragments, as reflected by changes in the sum of the
atomic partial charges of these fragments (Table 7) and the
contours of the HOMO and LUMO orbitals of molecules in
the ground and electronically excited states (Figs. 14S, 15S
in the Supplementary data), is similar in both compounds
(falling within the 370–380 nm range). This implies that
the extent of conjugation does not affect the spectral
behaviour of FDC. Electronic excitation is also accompa-
nied by loss of planarity in both FC and FDC (Fig. 16S in
the Supplementary data). Such substantial structural and
electron density changes accompanying electronic excita-
tion make these compounds potential absorption spectral
Fig. 6 The UV–VIS absorption (solid line) and deconvoluted (using
Spectral Data Lab software [35]) (dashed lines) spectra of FDC
(dissolved in methanol) together with predicted (at the
TDDFT(B3LYP)/6-31G** level of theory for structure a1) electronic
absorption transitions (bars)
Table 6 1H chemical shifts (in ppm) for FC and FDC
Method
FC
Hydrogen atom numbera/chemical shift
3
6
7/8
11
14
15
18
19
20
Exp
Theorb
FDC
6.39
6.23
8.52
8.41
13.04
2.69
7.63
6.95
7.77
8.24
7.95
7.57
6.72
6.44
7.16
6.85
14.46/13.25
2.35/2.19/2.34
3
4
5/5A
8/8A
9/9A
12/12A
7.95
13/13A
6.72
14/14A
7.16
Exp
Theorb
6.44
6.25
8.66
8.71
13.24
7.63
7.77
14.46/14.46
7.10/7.10
8.24/8.25
7.61/7.61
6.45/6.45
6.84/6.84
Exp—experimental values from 1H spectra recorded in DMSO-d6; Theor—values predicted for isolated molecules at the GIAO MP2/cc-pVDZ
level of theory [20–22, 28–31]
a
Identical with the carbon or oxygen atom number to which it is attached (the atom numbering is given in Figs. 9S and 10S)
b
Values for structures a1
123

976
Struct Chem (2014) 25:969–977
Supplementary data
Table 7 Sum of Mulliken [32, 33] and MEP fitted [34] atomic partial
charges of selected fragments of FC and FDC in the ground and
excited electronic states
CompleteX-ray datahavebeendeposited with the Cambridge
Crystallographic Data Centre (CCDC) as supplementary
publications Nos. 854359 (FC) and 854360 (FDC); copies of
these data can be obtained, free of charge, on application to
CCDC, 12 Union Raod, Cambridge CB2 1EZ, UK (fax: ?44-
(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
Fragment
Sum of atomic partial charges
Ground state
Excited state
Mulliken
Mulliken
MEP fit
MEP fit
FC
A
0.1620
-0.0922
-0.0698
-0.1641
0.0940
0.0701
0.3625
-0.0461
-0.3164
0.1380
0.1075
Acknowledgments This study was financed by the State Funds for
Scientific Research (Grant DS/530-8220-D184-12) and from Grants
Nos. 538-8221-1067-12 and 538-8220-B004-13. M.W. acknowledges
financial support from the European Social Fund within the project
‘‘Educators for the elite—integrated training program for PhD stu-
dents, Post-docs and Professors as academic teachers at the University
B
C
-0.2455
FDC
A
0.1570
-0.0785
-0.0785
-0.0935
0.0392
0.0543
0.3266
-0.1633
-0.1633
0.1465
-0.0732
-0.0733
B
´
of Gdansk’’, and the Human Capital Operational Programme Action
C
4.1.1, ‘‘Improving the quality on offer at tertiary educational insti-
tutions’’. This publication reflects the views only of the authors: the
sponsor cannot be held responsible for any use which may be made of
the information contained herein. The calculations were carried out
on computers of the Tri-City Academic Network Computer Centre in
indicators for physicochemical and analytical investiga-
tions. On the other hand, the compounds investigated do
not luminesce, so they cannot be used as fluorescent
probes.
´
Gdansk (TASK) and the Wroclaw Centre for Networking and Su-
percomputing (WCSS).
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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