Vibrational and theoretical study of the 2',6'-dimethoxyflavone cis-formic acid inclusion compound.

Article abstract of DOI:10.1016/S1386-1425(00)00296-1

The FT-infrared and Raman microscopy spectra of the 2',6'-dimethoxyflavone and its 1:1 complex with formic acid in solid state have been recorded and analysed. Some vibrational components appear as specific to the cis-rotamer of formic acid in the crystalline sample, especially the CH group stretching vibration feature. The broad and intense infrared absorption observed in the range 3400-1900 cm(-1) and assigned to the hydrogen bonded OH group stretching vibration exhibits the characteristic ABC structure of strong hydrogen bonded complexes. This ABC pattern corroborates previous X-ray crystallographic data showing that cis-formic acid is strongly hydrogen bonded to the flavonic compound. The inclusion complex is quite unstable and the infrared spectrum clearly shows that formic acid disappears after a period of a few months. In order to get some information on the stability criterions of the intermolecular hydrogen bonded complex, semiempirical AM1 calculations have been investigated. The comparison of the calculated heats of complexation (deltacH) for chelates involving the cis- and trans-conformers of formic acid suggests that the reaction of hydrogen bonding complexation with the cis-rotamer is surely favoured.

Full text of DOI:10.1016/S1386-1425(00)00296-1

Spectrochimica Acta Part A 56 (2000) 2439–2450
www.elsevier.nl/locate/saa
Vibrational and theoretical study of the
2%,6%-dimethoxyflavone cis-formic acid inclusion compound
Laurence Vrielynck a,*, Jean-Claude Wallet b, Jean-Claude Merlin a
a Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516), Centre d’Etudes et de Recherches Lasers et Applications,
Uni6ersite´ des Sciences et Technologies de Lille, Baˆt. C5, 59655 Villeneu6e d’Ascq Cedex, France
b Laboratoire de Phytochimie, Case 412, Faculte´ des Sciences et Techniques de Saint-Je´roˆme, 13397 Marseille Cedex 20, France
Received 8 February 2000; accepted 25 April 2000
Abstract
The FT-infrared and Raman microscopy spectra of the 2%,6%-dimethoxyflavone and its 1:1 complex with formic acid
in solid state have been recorded and analysed. Some vibrational components appear as specific to the cis-rotamer of
formic acid in the crystalline sample, especially the CH group stretching vibration feature. The broad and intense
infrared absorption observed in the range 3400–1900 cm−1 and assigned to the hydrogen bonded OH group
stretching vibration exhibits the characteristic ABC structure of strong hydrogen bonded complexes. This ABC
pattern corroborates previous X-ray crystallographic data showing that cis-formic acid is strongly hydrogen bonded
to the flavonic compound. The inclusion complex is quite unstable and the infrared spectrum clearly shows that
formic acid disappears after a period of a few months. In order to get some information on the stability criterions
of the intermolecular hydrogen bonded complex, semiempirical AM1 calculations have been investigated. The
comparison of the calculated heats of complexation (DcH) for chelates involving the cis- and trans-conformers of
formic acid suggests that the reaction of hydrogen bonding complexation with the cis-rotamer is surely favoured.
© 2000 Elsevier Science B.V. All rights reserved.
Keywords: Dimethoxyflavone; Cis-formic acid; Hydrogen-bonded complex; Vibrational spectroscopy; Semi-empirical calculation
1. Introduction
low-temperature argon matrix [1] by infrared
spectroscopy and the second one dealt with its
Recently two papers have been published al-
identification in
a cocrystal with hydrogen
most at the same time about the ‘unusual’ cis-
conformer of formic acid. The first one reported
on the characterisation of the cis-isomer in a
fluoride [2] by difference thermal analysis and
temperature-dependent X-Ray powder diffrac-
tion. The cis-conformer was also found a few
years ago in
a
cocrystal with the 2%,6%-
* Corresponding author. Tel.: +33-3-20436988; fax: +33-
3-20436755.
dimethoxyflavone [3]. This flavone which is a
weak base, gives numerous inclusion compounds
with alkyl [3–5] and aryl [6] carboxylic acids as
E-mail
address:
laurence.vrielynck@univ-lillel.fr
(L.
Vrielynck).
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00296-1

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L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
structure and stability criterions of the intermolec-
ular hydrogen bonded complex.
well as inorganic [7,8] and Lewis acids [9]. With
formic acid, it provides one example of two achi-
ral molecules with hydrogen bonding which crys-
tallised in a non centrosymmetric space group
(P212121).
2. Experimental
Single-crystal X-ray analysis of formic acid
[10,11] has been reported as well as the 1:1 addi-
tion compound with formamide [12]. Both struc-
tures display the trans-rotamer and involve the
participation of both the carbonyl oxygen and the
hydroxyl group of formic acid molecules. At the
isolated molecule level, the cis-rotamer of formic
acid has been calculated to be less stable than the
trans-rotamer [13]. In the title complex, pairs of
molecules are linked through the carbonyl group
of the flavone and the hydroxyl group of the
cis-rotamer of formic acid (Fig. 1).
The main purpose of the present paper con-
sisted in confirming and characterising the pres-
ence of the cis-conformer of formic acid hydrogen
bonded in the cocrystal by vibrational (Raman
and infrared) spectroscopic techniques. Semiem-
pirical AM1 calculations have also been per-
formed in order to obtain information on the
2.1. Synthesis
2%,6%-dimethoxyflavone was prepared by addi-
tion of excess thionyl chloride to 2,6-dimethoxy-
benzoic acid. The flask was warmed until reaction
ceased. After removal of unreacted thionyl chlo-
ride on a rotary evaporator, dry pyridine and
o-hydroxyacetophenone were added. The mixture
was heated to reflux over 1 h and poured onto
crushed ice. After filtration, the o-hydroxyace-
tophenone was dried and submitted to transposi-
tion with powdered sodium hydroxide disssolved
in dimethyl sulfoxide to give the corresponding
o-hydroxy-dibenzoylmethane. The cyclisation was
realised in glacial acetic acid solution with a few
drops
of
sulphuric
acid.
The
2%,6%-
dimethoxyflavone was recrystallised twice in etha-
nol (the first recrystallisation with black carbon).
The 2%,6%-dimethoxyflavone was dissolved in a
minimum of pure formic acid and left in the hood
to evaporate. Pale yellow crystals were obtained
after a few days which decomposed a few months
later.
2.2. Spectroscopic measurements
The Fourier transform infrared (FT-IR) mi-
croscopy spectra were measured with a Bruker
FT-IR IFS88 spectrometer supplied with a A590
Bruker microscope accessory. The sample pow-
ders were put on IR transparent BaF2 slices and
analysed in transmission mode through a 36×
microscope objective. The lighted crystalline area
was 2500 mm2 for the 2%,6%-dimethoxyflavone and
600 mm2 for its complex. The spectra were
recorded in the 4000–650 cm−1 spectral range
with a spectral resolution of 4 cm−1 and averaged
on 500 scans.
The Raman microscopy spectra were measured
with
a
DILOR-Instruments
S.A.-Horiba
Fig. 1. Numbering and structure of the 1:1 hydrogen bonded
complex formed by the 2%,6%-dimethoxyflavone and cis-formic
acid.
LABRAM spectrometer supplied with an OLYM-
PUS BX40 microscope accessory. The excitation

L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
2441
line was a red radiation of a He–Ne laser emitting
at 632.8 nm. The sample powders were put on
glass slices and analysed through a 80X U.L.W.D.
microscope objective (numerical aperture: 0.75)
and an irradiation power of 16 mW at the sample
was used. The spectra were recorded in the 4000–
250 cm−1 spectral range with a spectral resolution
of 8 cm−1, an integrating time of 10 s and were
averaged on 20 scans.
ponents in the Raman and infrared spectra. The
experimental wavenumbers together with a tenta-
tive assignment are presented in Table 1 and
discussed below in detail.
The prominent band extending over the region
3400–1900 cm−1 range in the IR spectrum of the
complex is attributable to the stretching vibration
of the hydrogen bonded OH group, w(OH…O)
referred as w1. The presence of such a broad and
low frequency infrared absorption pattern, ex-
hibiting three relatively wide components which
are situated approximately at 2850 (A), 2500 (B)
and 2050 (C) cm−1, proves the existence of a
strong intermolecular hydrogen linkage between
the hydroxyl hydrogen of formic acid molecule
and the carbonyl oxygen of the flavonoid. Indeed,
the shape of the w(OH…O) absorption with the
characteristic ABC structure is a well known fea-
ture for strong hydrogen bonded systems in the
solid state [19–21]. The corresponding Evans win-
dows between A, B and C sub-bands are located
near 2200 and 2600 cm−1 and could well be due
to the Fermi resonances between 6(OH…O) and
two bands combinations, respectively (1560+990
cm−1) and (1207 cm+990 cm−1); the assignment
of these components involving especially
l(OH…O) and k(OH…O) vibrations will be dis-
cussed later in the text. The w(OH…O) band is
also overlapped by multiple overtones and combi-
nations of bands occurring near 2000 cm−1 and
2.3. Calculations
The Austin method 1 (AM1) molecular orbital
calculation was performed at the RHF level by
using the HYPERCHEM program (version 5.0) [14].
Because of the size of the system, ab initio geome-
try optimisations were not easily computationally
feasible. However, previous theoretical studies
[15–17] have shown that the AM1 semiempirical
method, which has been developed with a proper
description of hydrogen bonding [18], is well suit-
able for the structural investigation of flavonoids
compounds. The global minimum search of the
free 2%,6%-dimethoxyflavone molecule and the two
conformers of formic acid was started from stan-
dard geometrical parameters. The geometry opti-
misations of the hydrogen bonded systems were
computed from the crystallographic data of the
1:1 2%,6%-dimethoxyflavone cis-HCOOH com-
pound. The termination conditions of calculation
for the energy minimisations were imposed by the
CH stretching vibrations near 3000 cm−1
.
,
set of a root-mean-square gradient to 0.008 kJ/(A
The CH stretching vibration of the formic acid
molecule, w2, clearly appears at 2899 cm−1 in the
IR spectrum of the complex. The observed value
exactly corresponds to that obtained for the cis-
rotamer isolated in a low-temperature solid argon
matrix [1]. Then, it seems that no peculiar interac-
tion involving that CH stretching vibration occurs
in the crystal and allows us to confirm the pres-
ence of the cis-conformation of formic acid in the
complex. In that way, this feature can be consid-
ered as a relevant fingerprint of the cis-rotamer
(for comparison purpose, the CH stretching vibra-
tion for the trans-rotamer has been characterized
in the spectral range 2940–2960 cm−1) [22–26].
The carbonyl stretching vibration of the formic
acid molecule, w3, appears as a strong infrared
band but as a weak Raman line near 1705 cm−1
mol−1).
3. Results and discussion
3.1. Vibrational spectroscopy
The microscopy FT-IR and Raman spectra of
2%,6%-dimethoxyflavone and its hydrogen bonded
complex with formic acid in the solid state are
shown, respectively, on Fig. 2 (a, b) and Fig. 3 (a,
b). As the 3500–2000 cm−1 spectral range of the
Raman spectra is weak in intensity, only the
1800-500 cm−1 region of these spectra is reported.
The strong hydrogen-bonded complex could be
characterized by several specific vibrational com-

2442
L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
Fig. 2. Microscopy FT-IR spectra of the 2%,6%-dimethoxyflavone (a) and its complex 2 weeks after the synthesis (b), 6 months later
(c) in the range 3500–700 cm−1. The characteristic bands of formic acid discussed in the text are indicated by stars.

L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
2443
Fig. 3. Microscopy Raman spectra of the 2%,6%-dimethoxyflavone (a) and its complex 2 weeks after the synthesis (b) in the range
1750–500 cm−1. The characteristic lines of formic acid discussed in the text are indicated by stars.
in the spectra of the complex. As no direct inter-
action occurs in the crystal between the formic
CꢀO bond and the flavone molecule, this value is
surprisingly low compared to those obtained for
most CHO groups (which generally strongly ab-
sorb between 1740 and 1720 cm−1) and consider-
ing the strong double bond character of this bond
pling interactions between the two vibrators
mentioned above, together with a strong crystal
field effect, may induce this significant decreasing
of the carbonyl bond frequency in the crystal.
In the IR spectrum of the complex, the notable
enhancement and broadening of the absorption
signal observed in the 1600–1500 cm−1 range
may arise from the hydrogen bonded OH in-plane
deformation mode, l(OH…O) referred as w5. In-
deed, if in solid argon matrix the corresponding
wavenumber is reported at 1244 cm−1 implying a
partial coupling with the CꢁO bond stretch [1],
nevertheless in hydrogen bonding interaction, the
OH in-plane bending vibration can be normally
expected at higher frequencies as it is the case for
,
(1.19 A). In Argon matrix, the w3 vibrational
component of the isolated molecule is reported at
1808 and 1767 cm−1 for the cis- and trans-ro-
tamers, respectively [1,27] but appears partially
mixed with the CH in-plane bending vibration
and the CꢁO bond stretch according to results of
ab initio calculations [28] and isotopic substitu-
tion effect [29]. A weakening of mechanical cou-

2444
L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
partially mixed with the OH group in-plane defor-
mation motion. Such a high frequency shift has
already been observed in the case of the trans-iso-
mer dimer but also in solid state [25,29] and could
be mainly explained by an increase of the corre-
sponding vibrator force constant. The intermedi-
ate single-double character of the CꢁO bond (1.28
the trans-rotamer in solid state [25]. However, a
consequent crystal lattice effect in addition with a
more or less strong mechanical coupling with the
CꢁO bond stretch may be responsible for such a
high frequency shift in the condensed phase. In
the Raman spectrum, the w5 vibrational mode is
also observed at about 1550 cm−1 in the bottom
of the pattern involving vibrations specific to aro-
matic carbon-carbon bond stretches of the flavone
molecule.
The comparison of the FT-IR spectra of the
2%,6%-dimethoxyflavone and its complex in the
1400–1300 cm−1 region allows us to clearly asso-
ciate the minimum observed at 1388 cm−1 (juxta-
posed to the intense band at 1369 cm−1
characteristic of the k-pyrone ring) to the CH
in-plane bending mode of formic acid, w4. In the
Raman spectrum of the complex, a corresponding
very weak line arises at 1384 cm−1. This assign-
ment is based on the fact that this hydrogen does
not seem to undergo any severe restraint in the
crystal. Then its frequency is probably weakly
affected and remains very close to the value re-
ported for the isolated molecule [1].
,
A) in the crystal is consistent with the previous
suggestion.
The CH out-of-plane bending mode, w8, proba-
bly occurs in the 1100–1000 cm−1 range but its
localisation is not obvious. The very weak band
appearing at 1068 cm−1 in the IR spectrum of the
complex could finally be characteristic of this
deformation but the assignment remains
uncertain.
The OH out-of-plane deformation vibration,
k(OH…O) referred as w9, previously observed at
503 and 635 cm−1, respectively, in the IR spectra
of the cis- and trans-rotamers in solid argon
matrix, may be expected in a higher frequency
range considering the hydrogen bond effect [30];
then, only the weak infrared band arising at 990
cm−1 and not observed in the spectrum of the
free flavone molecule can be proposed.
The significant broadening of infrared absorp-
tion signal near 1220–1190 cm−1 in the spectrum
of the complex may be induced essentially by the
CꢁO bond stretching vibration, w6, that is surely
According to previous works performed on the
trans-rotamer [22], we can propose the weak Ra-
man line at 696 cm−1 as representative to the w7
Table 1
Raman and infrared wavenumbers (cm−1) of the formic acid molecule included in the complexa
trans-HCOOHa
cis-HCOOHb
cis-HCOOH (this work)
Solid
Ar matrix
Infrared
2958
Infrared
2899
Assignment
w(CH)
Infrared
Raman
Tentative assignment
w2
w1
2899
ꢀ2850
ꢀ2500
ꢀ2050
1703
ꢀ1560
1388
1207
m
w(CH)
s,L
s,L
s,L
vs
m
s
m
vw
w
w(OH…O) A
w(OH…O) B
w(OH…O) C
w(CꢀO)
l(OH…O)+w(CꢁO)
l(CH)
w(OꢁC)+l(OH…O)
k(CH)
k(OH…O)
2532
3618
w(OH)
w3
w5
w4
w6
w8
w9
w7
1609/1703
1560
1370/1381
1224/1255
1083
1808
1244
1396
1108
?
w(CꢀO)
l(OH)+w(CꢁO)
l(CH)
w(OꢁC)+l(OH)
k(CH)
k(OH)
1705
1548
1384
w
m
vw
1068
985
974
721
503
661
991
696
vw
w
i(OꢁCꢀO)
i(OꢁCꢀO)
a The notation w, l, k and i refer, respectively, to stretching, in-plane bending, out-of-plane bending and in-plane angular
deformation motions.

L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
2445
mode which involves the in-plane angular defor-
mation, i(OꢁCꢀO).
significant frequency shift of the carbonyl line
which retains the same value than in 3-
methoxyflavone and flavone in a given apolar
organic medium (CCl4 solution) [16]. The differ-
ence of +10 cm−1 observed between the w(CꢀO)
of the 2%,6%-dimethoxyflavone and the w(CꢀO) of
the non-substituted flavone molecule [31] may be
explained by the higher q angle value in the
2%,6%-dimethoxyflavone that strongly limits the y
electron delocalisation between the chromone part
and the B phenyl ring. The great enhancement of
the Raman relative intensities observed in the
complex spectrum clearly suggests a large distur-
bance in the electronic environment which in-
creases the polarizability of the flavone carbonyl
bond. As in the case of the stretching mode, the
out-of-plane deformation of the carbonyl bond
k(CꢀO) of the flavone molecule, giving rise to an
intense Raman line near 640 cm−1 in the spectra
of both complexed and free molecules, does not
seem to be particularly sensitive to the hydrogen
bonding.
The micro-IR spectrum presented on Fig. 2 (c)
corresponds to that of the complex being kept 6
months at room temperature. It clearly brings to
the fore the disappearance of formic acid which
probably escapes out of the crystal because of a
sensitive room temperature increasing during this
period. It can be noticed that the loss of the
formic acid molecule leads to a change in the
relative intensities of some infrared bands of the
flavonic compound compared to those observed
in the FT-IR spectrum of 2%,6%-dimethoxyflavone.
It can be suggested that many heterogeneities
observed in the analysed crystal but crystal field
may also contribute to these main differences.
The spectral region from 340 to 33 cm−1 is well
known to contain vibrations proper to the hydro-
gen bond. In the low frequency range of the
Raman spectrum of the complex (data not
shown), a weak but quite large line appears at 317
cm−1 which could arise from the stretching mode
characteristic of the intermolecular hydrogen
bridge, w(O…H). This low frequency value consti-
tutes an additional argument to the involvement
of the acidic group of formic acid in a quite
strong intermolecular hydrogen bond.
Many vibrational modes related to the flavone
molecule, specific to the aromatic rings, are re-
ported in Table 2 but not discussed in detail.
Most of them are attributed from the results
obtained on the pattern flavone molecule [31] and
reference data collected in Varsanyi’s book [32]
(the Wilson’s notation adapted by Varsanyi is
used for the description of vibrations characteris-
tic of the aromatic rings) with the support of
normal vibrational modes calculations. By com-
plexation, significant modifications in relative in-
tensity occur while the wavenumbers are not
drastically perturbed. These changes may be ex-
plained by the effect of crystal field that is surely
completely
different
in
the
free
2%,6%-
dimethoxyflavone and in the complex. Our atten-
tion was however put on the vibrational modes
characteristic of the carbonyl bond liable to be
influenced by the intermolecular hydrogen bond.
The carbonyl stretching vibration of the flavone
molecule, w(CꢀO), located at 1645 cm−1, surpris-
ingly is not affected in wavenumber by the forma-
tion of the intermolecular hydrogen bond with the
cis-formic acid and remains quite similar to the
frequency of the non-hydrogen bonded molecule.
The complexation does not involve peculiar
change of the carbonyl bond force constant of the
2%,6%-dimethoxyflavone although a mesomeric ef-
fect could be expected. Nevertheless, the profile of
the respective FT-IR and Raman components is
particularly modified by the presence of the acid.
Such effect has already been observed in the
spectra of some flavonic compounds. Indeed, the
presence of a quite strong intramolecular hydro-
gen bond in 3-hydroxyflavone does not produce a
3.2. Molecular modelling
The main geometrical parameters for 2%,6%-
dimethoxyflavone, free and complexed with for-
mic acid, obtained from energy optimisation with
the semiempirical method AM1 are presented in
Table 3. To enable a comparison between the
computational results and the X-ray data, the
corresponding crystallographic molecular parame-
ters of the complex are also listed. When compar-
ing X-ray structures with calculated optimised

2446
L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
Table 2
Raman and infrared wavenumbers (cm−1) of the 2%,6%-dimethoxyflavone molecule included in the complexa
2%6%(OMe)2 flavone
Free
Complex
Tentative assignment
Infrared
Raman
Infrared
Raman
3100–3000
2961
2840
1647
1635
w
w
w
vs
m
m
m
3100–3000
2965
2847
1644
1630
1615–1560
1615–1560
1615–1560
1615–1560
1475
w
w
w
s
m
vs
vs
vs
vs
vs
vs
s
w(CH) aromatic
was(CH3)
ws(CH3)
w(C4ꢀO)
w(C2ꢀC3)
w(CC) 8a B
w(CC) 8b A
w(CC) 8a A+8b B
w(CC) 8b B+8a A
w(CC) 19b A+l+as(CH3)
w(CC) 19a A+l+as(CH3)
1641
ꢀ1632
1608
1588
1588
1570
1478
1463
1441
1432
1362
1333
1301
vs
m
w
w
w
1643
1630
1611
1594
1579
1568
1481
1464
1441
m
s
w
m
m
s
w
w
vw
1608
1589
1572
1474
1465
m
vs
vs
w
vw
vw
vw
vw
vw
m
vw
m
m
1457
1439
1431
1367
1333
1299
1288
1254–1242
1220
s
vs
ms
m
m
1369
1336
1302
1288
1254
vs
m
s
m, sh
s
w(C9–O1–C2)+ls(CH3)
w(C2–C1%)+ls(CH3)
l(CH) 14 A
l(CH) 3 A+3B
l(CH) aromatic
l(CH) 13 A
1333
1300
1288–1292
1254
1221
vs
w
m
m
m
1280–1288
1243
1221
s
ms
m, sh
m
1206
1207
m
1184
w
1185–1190
w
m
w
m
vw
m
w
m
vw
m
w(OꢁCH3)
w(OꢁCH3)
l(CH) 9b B
l(CH) 18a A
w(OꢁCH3)
l(CH) 18a B
l(CH) aromatic
l(CH) aromatic+l-as(CH3)
l(CH) 18b A+l-as(CH3)
l(CH) aromatic+l-as(CH3)
k(CH) 5 B
k(CH) 17b A+Y(O–C2–C3)
k(CH) 17a B
k(CH) 17a A
1176
1154
1123
1107
m
w
vs
vs
1177
1151
1126
1109
m
w
m
s
1153
1126
1107
1093
1060
1033
1023
1012
957
vw
w
vw
w
vw
w
vw
m
1155
1129
1107
1098
1056
1040
1025
1015
958
1067
1041
1025
1015
957
1036
1025
1013
958
s
s
ms
w
vs
s
m
m
w
m
vw
vw
vw
vw
vw
vw
w
911
910
899
914
910
900
866
876–865
845
781
757–751
740
725
m
ms
vs
s
m
m
870
m
847
776
759–749
737
721
689
vw
vw
m,vw
vw
vw
850
785–779
m
m
847
772
m
m
vw
750
k(C3–H)
k(CH) 11 B
k(CH) 11 A
Z(CC) 12 B
Y(CC) 4 B
Y(CC) 4 A
760–752
m
w
743
726
724
689
670
643
616
566
521
512
m
w
w
vs
w
w
w
m
vw
672
641
616
566
520
505
m
w
w
vw
m
k(CꢀO) flavone
Z(CC) 1 B
Z(CC) 6a A
Z(CC) 6b A
a The notation w, l, k, Z and Y refer, respectively, to stretching, CH in-plane bending, CH out-of-plane bending, radial skeletal
vibrations and skeletal out-of-plane bending modes.

L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
Table 3 (Continued)
2447
Table 3
Geometrical parameters of the 2%,6%-dimethoxyflavone and the
cis-formic acid molecules free and engaged in the complex
2%,6%-(OMe)2 flavone
AM1 optimisation
Free C(1)
X-ray
2%,6%-(OMe)2 flavone
AM1 optimisation
Free C(1)
X-ray
Intermolecular parameters
Bond lengths (A)
ꢀO…HO
ꢀO…OC
ꢀO…HC
2.096
2.890
2.330
1.570
2.601
2.926
,
Bond lengths (A)
O1ꢁC2
C2ꢁC3
C3ꢁC4
C4ꢁC10
C10ꢁC5
C5ꢁC6
C6ꢁC7
C7ꢁC8
CꢁC9
C9ꢁC10
C9ꢁO1
C2ꢁC1%
C1%ꢁC2%
C2%ꢁC3%
C3%ꢁC4%
1.388
1.385
1.353
1.458
1.467
1.405
1.388
1.402
1.387
1.408
1.404
1.385
1.466
1.411
1.402
1.392
1.392
1.402
1.411
1.245
1.343
1.352
1.460
1.471
1.404
1.388
1.401
1.388
1.408
1.404
1.384
1.463
1.413
1.402
1.391
1.391
1.402
1.413
1.240
1.346
1.447
1.420
1.418
1.366
1.404
1.375
1.393
1.390
1.381
1.477
1.394
1.389
1.359
1.387
1.396
1.398
1.239
Bond angles (°)
C4ꢀO…HO
ꢀO…HꢁO
113.9
137.8
127.4a
149.0
Dihedral angles (°)
C4ꢀO…HꢁO
ꢀOꢁH…OꢁC
ꢀO…OꢁCꢁH
−179.1
−0.8
−0.4
130.3
63.4
7.3
a Considering the C3ꢁH and OꢁH bond lengths equal 1.08
,
and 0.98 A, respectively.
b Restrainted value for energy minimization during calcula-
tion.
geometries, one has to keep in mind that calcula-
tion is performed on a molecular system in vac-
uum devoid of external constraints whereas the
structure obtained in solid state is subjected to
more or less important crystal lattice effects, then
some discrepancies may be expected. However,
considering standard deviations in the X-ray data,
calculations carried out for the complex appear in
rather good agreement with the crystallographic
structure. The bond lengths, valence angles and
dihedral angles are close to those previously deter-
mined for other flavone derivatives [15–17].
Moreover, the comparison with the calculation
performed for the free molecule indicates that the
structure of 2%,6%-dimethoxyflavone is not drasti-
cally disturbed by the presence of formic acid.
The principal structural feature is the large
value of the dihedral angle (q) between the
chromone moiety and the B phenyl ring calcu-
lated at 48° while the crystallographic data [3]
reveal the existence of three stable conformers
having q values, respectively, equal to 86, 84 and
68° in the solid state. The results of the AM1
energy minimisation of the free 2%,6%-
dimethoxyflavone molecule show that the rotation
around the C2ꢁC1% bond is a low-energy process
C4%ꢁC5%
C5%ꢁC6%
C6%ꢁC1%
C4ꢀO
Bond angles (°)
C9ꢁO1ꢁC2
O1ꢁC2ꢁC3
C2ꢁC3ꢁC4
C3ꢁC4ꢁC10
C3ꢁC4ꢀO
C3ꢁC2ꢁC1%
C2ꢁC1%ꢁC2%
C4ꢁC10ꢁC5
C8ꢁC9ꢁO1
C4ꢁC3ꢁH
Dihedral angles (°)
C2ꢁC3ꢁC4ꢀO
C4ꢁC3ꢁC2ꢁC1%
C3ꢁC2ꢁC1%ꢁC2%
116.8
117.0
119.0
123.6
122.2
114.0
123.0
125.2
120.9
121.8
115.2
117.3
123.7
121.9
114.4
122.5
125.1
120.9
121.7
115.3
117.5
122.9
121.2
115.2
122.1
125.9
121.6
122.7
116.0
119.4a
179.9
179.2
48.3
179.9
179.7
78.2b
−178.8
−179.5
79.9
Cis-formic acid
,
Bond lengths (A)
CꢀO
CꢁO
OꢁH
1.230
1.230
1.196
1.357
0.971
1.103
1.363
0.970
1.107
1.282
0.980a
1.080a
CꢁH
Bond angles (°)
HꢁOꢁC
OꢁCꢁH
OꢁCꢀO
110.6
112.3
117.6
109.1
118.4
114.1
109.5
116.9
126.1

2448
L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
above 9 kJ mol−1 and, if quite similar values were
obtained for flavone [33] (ca. 6.5 kJ mol−1) and
7,8-benzoflavone [34] (ca. 2.9 kJ mol−1), the
larger energy difference observed in the present
calculation is a direct consequence of the steric
hindrance of the methoxyl groups on the exocyclic
phenyl ring.
In order to better understand why the cis-for-
mic acid complex preferentially occurs during the
crystallisation process, we investigated AM1 cal-
culations on several 1:1 hydrogen bonded systems
involving the 2%,6%-dimethoxyflavone molecule and
the cis- or trans-rotamer of formic acid. The
results of the energy minimisation of four systems
(two with the cis-isomer and two with the trans-
one) are presented in Table 4. The result of ge-
ometry optimisation performed by using the
X-ray data as starting structure leads to a more
stable structure C(1) than the C(2) which exhibits
a different relative position of the cis-rotamer
towards the 2%,6%-dimethoxyflavone molecule. In
the crystal, the complex presents a strong inter-
Fig. 4. Rotational energy barrier around the C2ꢁC1% bond of
the 2%,6%-dimethoxyflavone molecule. The reported calculated
values, D(DfH) (kJ mol−1), are relative to the enthalpy of
formation of the fully optimised global minimum conforma-
tion. The arrows with number locate the three stable conform-
ers in the solid state (1: q=68°, 2: q=84° and 3: q=86°).
in the q angle range 40–140° (Fig. 4). The height
,
of the calculated rotational energy barrier is
molecular hydrogen bond, dO…H=1.57 A, so that
Table 4
Structural features and relative heats of formation of the four investigated complexes calculated by the AM1 calculation methoda
a The DcH values are obtained from the calculated DfH values for the free 2%,6%-dimethoxyflavone (−267.4 kJ mol−1), cis-formic
acid (−376.2 kJ mol−1) and trans-formic acid (−407.2 kJ mol−1) molecules. The q angle value was constrained to 80° for energy
minimisation during calculation.

L. Vrielynck et al. / Spectrochimica Acta Part A 56 (2000) 2439–2450
2449
the formic acid molecule slightly goes out of the
plane of the chromone moiety. In the calculated
C(1) structure, the formic acid molecule lies in the
plane of the g-pyrone ring and two intermolecular
hydrogen bonds are formed between the carbonyl
room temperature it will transform into the trans-
complex. The hydrogen bond then is broken and
formic acid escapes from the crystal.
,
oxygen of the flavone and the OH (2.10 A) and CH
Acknowledgements
,
(2.33 A) groups of formic acid. This discrepancy is
The Laboratoire de Spectrochimie Infrarouge et
Raman is Unite´ Mixte de Recherche de l’Univer-
site´ de Lille 1 et du CNRS. The Centre d’Etudes
et de Recherches Lasers et Applications (CERLA)
is supported by the Ministe`re charge´ de la
Recherche, the Re´gion Nord-Pas de Calais and the
Fonds Europe´en de De´veloppement Economique
des Re´gions.
probably due to the environment of the formic acid
molecule in the crystal, not considered in our
calculation.
The comparison of the heats of formation (DfH)
between the C and T structures suggests that the
trans-HCOOH.2%,6%-dimethoxyflavone complexes
have a higher relative stability than the cis-
HCOOH.2%,6%-dimethoxyflavone complexes about
15–20 kJ mol−1. Nevertheless, in terms of hydro-
gen bonding interaction energy, it appears that
complexation with the cis-form of the acid is more
favourable than with its trans-rotamer as it is
supported below. The complexation enthalpy
change of the system {OH+O=UOH…O=},
noted DcH, can be calculated as the difference
between the DfH(complex) and the sum DfH(free
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