Extensive intramolecular and intermolecular interactions in two quaternary salts of the pyridoxal oxime

Article abstract of DOI:10.1007/s10870-012-0312-y

Two derivatives of pyridoxal oxime (1) were prepared by quaternization of pyridoxal oxime with phenacyl bromide and 2-methoxyphenacyl bromide. The crystal structures of the 1-phenacyl-3-hydroxy-4-hydroxyiminomethyl- 5-hydroxymethyl-2-methylpyridinium brom

Full text of DOI:10.1007/s10870-012-0312-y

J Chem Crystallogr (2012) 42:752–758
DOI 10.1007/s10870-012-0312-y
ORIGINAL PAPER
Extensive Intramolecular and Intermolecular Interactions in Two
Quaternary Salts of the Pyridoxal Oxime
•
Mario Cetina Ante Nagl Dajana Gaso-Sokac
•
•
ˇ
ˇ
•
Spomenka Kovac Valentina Busic
•
ˇ
ˇ ´
´
Dijana Saftic
Received: 8 November 2011 / Accepted: 31 March 2012 / Published online: 27 April 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Two derivatives of pyridoxal oxime (1) were
prepared by quaternization of pyridoxal oxime with
phenacyl bromide and 2-methoxyphenacyl bromide. The
crystal structures of the 1-phenacyl-3-hydroxy-4-hydro-
xyiminomethyl-5-hydroxymethyl-2-methylpyridinium bromide
(2) and the novel 3-hydroxy-4-hydroxyiminomethyl-5-hydro-
xymethyl-2-methyl-1-(20-methoxyphenacyl) pyridinium bro-
mide (3) have been determined by X-ray diffraction method.
The conformation of these two pyridoxal oxime phenacyl
derivatives is locked by numerous intramolecular hydrogen
bonds. The O–HꢀꢀꢀBr, C–HꢀꢀꢀBr and C–HꢀꢀꢀO hydrogen bonds
in 2 and 3 form complex three-dimensional network. Supra-
molecular structure of 3 also contains C–Hꢀꢀꢀp and pꢀꢀꢀp
interactions which link neighbouring cations.
Introduction
The organophosphorus compounds are widely used in
agriculture as insecticides, in industry and technology, as
well as in military technology as chemical warfare agents
(sarin, soman, tabun). They are extremely potent inhibitors
of the enzyme acetylcholinesterase (AChE) that is
responsible for the termination of the action of acetylcho-
line at cholinergic synapses [1, 2]. There are many com-
monly used reactivators of inhibited AChE, such as
2-pralidoxime, trimedoxime and toxogonin [3–6]. Unfor-
tunately, none of the currently used oximes is sufficiently
effective against all inhibitors and there is no single reac-
tivator having the ability to reactivate inhibited enzyme
regardless of the chemical structure of the inhibitor [7, 8].
Commonly used reactivators are characterised by the
presence of several structural features: functional oxime
group, quaternary nitrogen atom and different length of
linking chain between two pyridinium rings in the case of
bispyridinium reactivators [9, 10]. Pyridoxal oxime, a
derivative of B6 vitamin, meets some of these structural
features and can be used for synthesis of compounds
Keywords Quaternary pyridinium salts ꢀ
Phenacyl bromides ꢀ Pyridoxal oxime derivatives ꢀ
X-ray diffraction ꢀ Supramolecular assembling
M. Cetina (&) ꢀ A. Nagl
´
structurally similar to common antidotes. Milatovic et al.
Department of Applied Chemistry, Faculty of Textile
Technology, University of Zagreb, Prilaz baruna Filipovica 28a,
´
[11] synthesized three new dioximes which combine the
structure features of pyridoxal oxime and toxogonine.
10000 Zagreb, Croatia
e-mail: mario.cetina@ttf.hr
ˇ
ˇ
Gaso-Sokac et. al. [12] synthesized a series of novel py-
ridinium oximes and tested them as reactivators of AChE
inhibited by organophosphorus compounds tabun and
paraoxon.
ˇ
ˇ
ˇ
ˇ ´
D. Gaso-Sokac ꢀ S. Kovac ꢀ V. Busic
Department of Chemistry, J.J. Strossmayer University of Osijek,
ˇ
Kuhaceva 20, 31000 Osijek, Croatia
In this paper, we report the structures of two pyri-
doxal oximes, 1-phenacyl-3-hydroxy-4-hydroxyiminomethyl-
5-hydroxymethyl-2-methyl pyridinium bromide (2) [12]
and 3-hydroxy-4-hydroxyiminomethyl-5-hydroxymethyl-2-
methyl-1-(20-methoxyphenacyl) pyridinium bromide (3). Pyr-
idoxal oxime quaternary salts were prepared by quaternization
ˇ
ˇ
ˇ
D. Gaso-Sokac (&) ꢀ S. Kovac
Faculty of Food Technology, J.J. Strossmayer University
ˇ
of Osijek, Kuhaceva 20, 31000 Osijek, Croatia
e-mail: dajana.gaso@ptfos.hr
´
D. Saftic
Ruder Boskovic Institute, Bijenicka 54, 10000 Zagreb, Croatia
ˇ
´
ˇ
¯
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J Chem Crystallogr (2012) 42:752–758
753
of the pyridoxal oxime (1) with phenacyl bromide and
2-methoxylphenacyl bromide (Scheme 1). In this work we
particularly pay attention on supramolecular assembling of
these two compounds as bromides and cations, which possess
three hydroxyl groups, can form an intricate network of
hydrogen bonds.
product was collected by filtration under reduce pressure
and recrystallized from methanol.
Brown solid, mp: 217–219 °C. Yield: 12 %. IR (KBr,
cm-1): 3309, 3103–2935, 1676, 1597–1437, 1254,
1
1049–979. H NMR (DMSO-d6) d 13.03 (bs, 1H, NOH);
11.90 (bs, 1H, OH); 8.66 (s, 1H, H-6); 8.62 (s, 1H, H-300);
7.85 (d, 1H, H-500); 7.74 (d, 1H, H-600); 7.35 (d, 1H, H-400);
7.15 (s, 2H,CH2CO); 6.28 (bs, 1H, CH2OH). 13C NMR
(DMSO-d6) d 189.89 (CO); 152.54 (C-3); 145.56 (C-40);
145.51 (C-2); 139.71 (C-4); 137.35 (C-6); 135.11 (C-100);
132.28 (C-200, C-600); 130.50 (C-300, C-500); 128.18 (C-5);
64.52 [CH2(C-50)]; 58.53 (CH2CO0); 13.37 [(CH3 (C-20)].
Elemental analysis for C17H19N2O5Br: Found (Calculated):
C 49.60 (49.65), H 4.65 (4.66), N 6.58 (6.81) %.
Experimental Section
Solvents and reagents were purchased from Fluka and
Aldrich and used without further purification. IR spectra
were measured on Paragon 500 FT-IR spectrophotometer
1
with KBr pellets. H NMR and 13C NMR spectra were
measured on a Varian XL-GEM 300 spectrophotometer in
DMSO-d6 solutions and chemical shifts are reported in d
values downfield from TMS as an internal standard.
Melting points were determined with capillary melting
point apparatus Stuart melting point apparatus SMP3. The
purity of compound was determined by 1H NMR, 13C
NMR and elemental analysis. The single crystals were
obtained at room temperature by slow evaporation of a
methanol solution.
X-ray Crystal Structure Determination
The intensities were collected at 295 K on a Oxford dif-
fraction Xcalibur2 diffractometer with a Sapphire 3 CCD
detector using graphite-monochromated MoKa radiation
˚
(k = 0.71073 A). The data collection and reduction were
carried out with the CrysAlis programs [13]. The intensities
were corrected for absorption using the multi-scan
absorption correction method by CrysAlis RED [13].
Details of crystal data, data collection and refinement
parameters are given in Table 1. The crystal structures
were solved by direct methods [14]. All non-hydrogen
atoms were refined anisotropically by full-matrix least-
squares calculations based on F2 [14]. The hydrogen atoms
attached to the O1 and O2 atoms in 2, as well as O1A,
O2A, O3A, O1B and O2B in 3 were found in a difference
Fourier map and their coordinates and isotropic thermal
parameters have been refined freely. All other hydrogen
atoms were treated using appropriate riding models, with
SHELXL97 defaults [14]. PLATON program was used for
analysis and molecular and crystal structure drawings
preparation [15]. CCDC 756361 and 756362 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis
Synthesis of 1-phenacyl-3-hydroxy-4-hydroxyiminomethyl-
5-hydroxymethyl-2-methyl pyridinium bromide (2) has
been described previously [12].
Synthesis of 3-hydroxy-4-hydroxyiminomethyl-
5-hydroxymethyl-2-methyl-1-(20-methoxyphenacyl)
pyridinium bromide (3)
Pyridoxal oxime (1) (0.36 g; 2 mmol) was dissolved in
acetone (300 mL) and stirred 20 min at 50 °C. The reac-
tion mixture was cooled to room temperature, and
2-methoxyphenacyl bromide was added (0.46 g; 2 mmol).
The reaction mixture was stirred for 24 h at room tem-
perature, and left in dark for 3 weeks. The crystalline crude
Scheme 1 Synthesis of the
oximes 2 and 3
CH=NOH
CH=NOH
CH2OH
R
HO
CH2OH
HO
+
Br
Br
H3C
N
1
H3C
N
O
CH2C
O
R
2 R=H
3 R=OCH3
123

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J Chem Crystallogr (2012) 42:752–758
Table 1 X-ray crystallographic data for 2 and 3
Compound
2
3
Formula
C16H17BrN2O4
381.23
C17H19BrN2O5
411.25
Formula weight
Crystal size [mm]
Crystal colour, shape
Crystal system
Space group
0.10 9 0.33 9 0.40 0.22 9 0.45 9 0.49
Yellow, prism
Monoclinic
P 21/c
Yellow, irregular
Triclinic
P1
Unit cell dimensions
˚
a [A]
9.3672 (3)
11.1758 (3)
16.6843 (6)
90
8.6597 (2)
14.4279 (4)
15.4781 (4)
70.202 (3)
78.376 (2)
86.411 (2)
1782.20 (8)
4
˚
b [A]
˚
c [A]
a [°]
b [°]
c [°]
111.097 (3)
90
3
˚
V [A ]
1629.54 (9)
4
Z
D
calc. [g cm-3
Absorption coefficient
l [mm-1
]
1.554
1.533
2.544
2.336
]
Scan mode
h range [°]
Index ranges
x scans
x scans
3.87–27.99
3.90–28.00
-11 B h B 11
-19 B k B 19
-20 B l B 20
41,626
Fig. 1 A molecular structure of cation of 2, with the atom-numbering
scheme. Displacement ellipsoids for nonhydrogen atoms are drawn at
the 30 % probability level. Intramolecular hydrogen bonds are shown
dashed
-12 B h B 12
-14 B k B 14
-21 B l B 22
Collected reflections no. 20,541
Independent reflections 3924/0.0387
no./Rint
8588/0.0322
and 3 are very close to already published similar structures,
3-hydroxy-4-hydroxyiminomethyl-5-hydroxymethyl-1,2-
dimethylpyridinium iodide and 3-hydroxy-4-hydro-
xyiminomethyl-5-hydroxymethyl-1,2-dimethylpyridinium
chloride monohydrate [16]. A survey of Cambridge struc-
tural database [17] revealed that above mentioned struc-
tures are the only two 4-(hydroxyiminomethyl) pyridinium
derivatives which have bonded methyl group to C-2 atom,
hydroxyl group to C-3 and hydroxymethyl group to C-5
atom of the pyridinium ring.
.
Reflections no.
I C 2r(I)
2,464
4,566
Data/parameters ratio
3924/218
8588/487
Transmission factors,
0.62687/1.0000
0.80803/1.0000
Tmin/Tmax
Goodness-of-fit on F2, S 0.989
0.902
R [I C 2r(I)]/R [all
data]
0.0325/0.0580
0.0333/0.0780
wR [I C 2r(I)]/wR [all
data]
0.0767/0.0795
0.0733/0.0777
In both structures, one intramolecular hydrogen bond of
O–HꢀꢀꢀN type, O1ꢀꢀꢀN2, forms S(6) [18] ring (Table 2).
Furthermore, the C7ꢀꢀꢀO1 intramolecular hydrogen bond
generates five-membered ring of S(5) type. There are lot of
other intramolecular hydrogen bonds that lock conforma-
tion of 2 and 3 through five-membered and six-membered
rings. Orientation of the hydroxymethyl group with respect
to the pyridinium ring represents the first conformational
difference between 2 and 3 and between two independent
cations of 3. The conformation defined by torsion angle
C6–C5–C9–O3 is synperiplanar in 2 [-8.7(3)°] and A
cation of 3 [–3.4(3)°]. Such conformation enables C6ꢀꢀꢀO3
intramolecular hydrogen bond formation. On the contrary,
major component of the hydroxyl oxygen atom O3 in B
cation of 3 is in anticlinal disposition towards C6 atom of
Max./min. electron
-3
0.554/-0.677
0.723/-0.440
˚
density [e A
]
Results and Discussion
Compound 2 crystallizes in monoclinic space group P 21/c
(Fig. 1), while compound 3 crystallizes in triclinic space
group P1 with two independent cations, denoted as A and
B (Fig. 2), and two bromides in the asymmetric unit. The
O3B atom in B cation of compound 3 is disordered over
two sites, with the occupancies refined to 0.691(5) and
0.309(5), respectively. The geometrical parameters of 2
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J Chem Crystallogr (2012) 42:752–758
755
Fig. 2 The molecular structures
of two independent cations of 3
(A and B), with the atom-
numbering schemes. For clarity,
only major component of
disordered O3B atom is shown.
Displacement ellipsoids for
nonhydrogen atoms are drawn
at the 30 % probability level.
Intramolecular hydrogen bonds
are shown dashed
Table 2 Hydrogen-bonding geometries of intramolecular hydrogen bonds in 2 and 3
˚
D–H (A)
˚
˚
D–HꢀꢀꢀA
HꢀꢀꢀA (A)
DꢀꢀꢀA (A)
D–HꢀꢀꢀA (°)
2
3
O1–H1ꢀꢀꢀN2
0.73 (3)
0.93
1.91 (3)
2.35
2.561 (2)
2.727 (3)
2.740 (3)
2.604 (3)
2.710 (3)
2.745 (3)
2.658 (2)
2.752 (3)
2.551 (3)
2.751 (3)
2.981 (5)
2.621 (3)
149 (3)
104
C6–H6ꢀꢀꢀO3
C7–H7BꢀꢀꢀO1
0.96
2.28
109
O1A–H1AꢀꢀꢀN2A
C6A–H6AꢀꢀꢀO3A
C7A–H72AꢀꢀꢀO1A
C10A–H10BꢀꢀꢀO5A
C17A–H17AꢀꢀꢀO4A
O1B–H1BꢀꢀꢀN2B
C7B–H72BꢀꢀꢀO1B
C8B–H8BꢀꢀꢀO3B
C10B–H10CꢀꢀꢀO5B
0.81 (2)
0.93
1.88 (2)
2.31
149 (3)
105
0.96
2.34
104
0.97
2.30
101
0.93
2.44
100
0.86 (2)
0.96
1.78 (2)
2.29
148 (2)
109
0.93
2.47
115
0.97
2.27
100
the pyridinium ring [C6B–C5B–C9B–O3B = 119.2(3)°],
and it participates as acceptor in the C8BꢀꢀꢀO3B intramo-
lecular hydrogen bond (Table 2).
-77.3(2)° in 2 and B cation of 3, respectively. The same
torsion angle in A cation of 3 amounts 75.5(2)°, which
means that phenacyl ring is at the opposite side of the
pyridinium ring plane. Finally, there is one more intra-
molecular hydrogen bond in A cation of 3, C17AꢀꢀꢀO4A,
formed between phenyl ring hydrogen atom and carbonyl
oxygen atom. This hydrogen bond is missing in 2 and B
cation of 3, because of slightly different phenyl ring ori-
entation towards carbonyl group which is accompanied
with longer HꢀꢀꢀO distance and smaller C - HꢀꢀꢀO angle.
From Figs. 1 and 2 is obvious that phenacyl moiety has
different orientation in 2 and two independent cations of 3.
The conformation in two cations of methoxy derivative 3 is
locked by the same, C10ꢀꢀꢀO5, intramolecular hydrogen
bond. Furthermore, phenacyl moiety in 2 and B cation of 3
is positioned in front of the pyridinium ring plane defined
by the C2–N1–C10–C11 torsion angle of -87.8(2) and
123

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J Chem Crystallogr (2012) 42:752–758
Table 3 Hydrogen-bonding geometries of intermolecular hydrogen bonds in 2 and 3
D–HꢀꢀꢀA (°)
Symmetry codes
˚
D–H (A)
˚
˚
D–HꢀꢀꢀA
HꢀꢀꢀA (A)
DꢀꢀꢀA (A)
2
O2–H2ꢀꢀꢀBr1
0.73 (3)
0.82
2.42 (3)
2.47
3.146 (2)
3.272 (2)
3.726 (3)
3.259 (3)
3.301 (3)
3.217 (4)
3.154 (2)
3.232 (2)
3.767 (2)
3.701 (2)
3.436 (3)
3.166 (2)
3.274 (5)
3.702 (2)
3.186 (3)
3.634 (3)
175 (3)
166
x, 3/2 - y, - 1/2 ? z
x, - 1 ? y, z
O3–H3ꢀꢀꢀBr1
C10–H10AꢀꢀꢀBr1
C6–H6ꢀꢀꢀO2
0.97
2.88
146
-x, - 1/2 ? y, 1/2 - z
x, 1/2 - y, 1/2 ? z
-x, 1/2 ? y, 1/2 - z
1 - x, - 1/2 ? y, 1/2 - z
-x, 1 - y, 1 - z
x, - 1 ? y, 1 ? z
x, y, 1 ? z
0.93
2.43
149
C13–H13ꢀꢀꢀO3
0.93
2.38
171
C17–H17ꢀꢀꢀO1
0.93
2.46
138
3
O2A–H2AꢀꢀꢀBr2
O3A–H3AꢀꢀꢀBr1
C10A–H10AꢀꢀꢀBr2
C14A–H14AꢀꢀꢀBr1
C9A–H92AꢀꢀꢀO4B
O2B–H2BꢀꢀꢀBr2
O3B–H3BꢀꢀꢀBr1
C6B–H6BꢀꢀꢀBr1
C8B–H8BꢀꢀꢀO4A
C9A–H92AꢀꢀꢀCg1a
0.77 (3)
0.83 (2)
0.97
2.39 (3)
2.41 (2)
2.90
168 (3)
175 (2)
149
0.93
2.83
157
-x, 1 - y, 2 - z
1 ? x, y, z
0.97
2.56
150
0.83 (3)
0.82
2.34 (3)
2.49
173 (3)
161
-1 ? x, y, 1 ? z
x, - 1 ? y, 1 ? z
-x, 1 - y, 1 - z
0.93
2.87
149
0.93
2.55
126
0.97
2.92
131
1 ? x, y, z
a
Cg1 is centroid of N1B–C6B ring
It can be predicted that such differences in conformation
angle value being almost identical as those in 2 and A
cation of 3, and 119.52(15)° in structure of pyridinium
chloride monohydrate, which is almost identical to the
value of angle in B cation of 3. The difference in confor-
mation of the hydroxylmethyl group in similar structures is
easy to explain as pyridinium chloride derivative is
are caused by intermolecular interactions, especially
hydrogen bonds in 2 and 3 (Table 3). E.g., conformation of
hydroxymethyl group in two closely related structures [16]
also differ. The C6–C5–C9–O3 torsion angle amounts
7.4(5)° in structure of pyridinium iodide derivative, the
Fig. 3 a Part of the crystal structure of 2, showing O-HꢀꢀꢀBr, C-HꢀꢀꢀBr and C-HꢀꢀꢀO hydrogen bonds; b A crystal packing diagram of 2,
viewed along the c axis. Hydrogen bonds are indicated by dashed lines
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J Chem Crystallogr (2012) 42:752–758
757
monohydrate and hydroxyl oxygen atom of the hydroxy-
methyl group points to the oxygen atom of water molecule.
A pyridoxal oxime cation and bromide ions in 2 are
linked by two O–HꢀꢀꢀBr hydrogen bonds, O2ꢀꢀꢀBr1 and
O3ꢀꢀꢀBr1, so forming finite pattern (Fig. 3a). The C10ꢀꢀꢀBr1
hydrogen bond participates also in cation/anion connection.
The cations are further link by three C–HꢀꢀꢀO hydrogen
bonds, all of them forming different types of chains. The
C6ꢀꢀꢀO2 hydrogen bond generates C(7) [18] chains parallel
to the c axis. Other two C–HꢀꢀꢀO hydrogen bonds, in which
phenyl ring hydrogen atoms participate, C13ꢀꢀꢀO3 and
C17ꢀꢀꢀO1, form C(10) and C(9) spirals parallel to the b axis.
All this ensemble of hydrogen bonds forms three-dimen-
sional network (Fig. 3b). On the crystal packing diagram
along the c axis one can see very small channels (Fig. 3b).
Some of them are partially occupied by bromides (at
x & 1/4 and x & 3/4), but those at x & 1/2 are not
occupied at all. It should be also mentioned that the
pyridinium and phenyl rings do not participate in C–Hꢀꢀꢀp
and pꢀꢀꢀp stacking interactions.
interconnect two independent cations, thus forming huge
ring consists of four cations and four anions (Fig. 4a;
indicated by ellipse). Thus, different conformation of the
hydroxymethyl group in B cation could be a consequence
of the O3BꢀꢀꢀBr1 hydrogen bond, as such orientation is
necessary for ring formation. Bromide ions participate also
in cations connection by three C–HꢀꢀꢀBr hydrogen bonds
(Table 3). All hydrogen bonds in 3 form finite pattern,
whereas C–HꢀꢀꢀO hydrogen bonds in 2 form infinite chains.
One C–Hꢀꢀꢀp interaction between carbon atom of
hydroxymethyl group and pyridinium ring of B cation
participates also in supramolecular aggregation (Table 3).
Finally, supramolecular structure of 3 contains also two
weak aromatic pꢀꢀꢀp stacking interactions [19], the first one
being between phenyl rings of the B cations. An interplanar
angle between the rings is 0°, interplanar spacing is ca
i
3.75 A, a centroid separation 4.1971(16) A, and corre-
˚
˚
˚
sponding centroid–centroid offset ca 1.88 A [symmetry
code: (i) - 1-x, 1 - y, 1 - z]. Second aromatic pꢀꢀꢀp
stacking interaction is accomplished between phenyl rings
of two different independent cations, B and A. In this
interaction, an interplanar angle between the rings amounts
The supramolecular assembling of 3 is completely dif-
ferent. There are only two C–HꢀꢀꢀO hydrogen bonds that
mutually link A and B cations of 3. The cations and bro-
mides are linked by four O–HꢀꢀꢀBr hydrogen bonds in
which O2 and O3 atoms participate as protondonors in both
independent cations. Both bromides, Br1 and Br2,
°
˚
8.84(13) , interplanar spacings are ca 3.83 and 3.72 A,
ii
respectively, a centroid separation 3.9660(16) A, and
˚
˚
corresponding centroid–centroid offset ca 1.37 A [sym-
metry code: (ii) x, y, - 1 ? z]. pꢀꢀꢀp stacking interactions
Fig. 4 a A crystal packing diagram of 3, viewed along the a axis. The ellipse shows a ring formed by four O-HꢀꢀꢀBr hydrogen bonds; b A
crystal packing diagram of 3, viewed along the c axis. Hydrogen bonds are indicated by dashed lines
123

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J Chem Crystallogr (2012) 42:752–758
2. Bajgar J (2004) Adv Clin Chem 38:151–216
link four by four cations along the b axis in a zig-zag finite
chains approximately at c = 1/2 (Fig. 4a). These C-Hꢀꢀꢀp
and pꢀꢀꢀp interactions completes three-dimensional network
(Fig. 4b).
ˇ
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To conclude, a lot of intramolecular hydrogen bonds
lock the conformation of these two pyridoxal oxime
phenacyl derivatives. In derivative 3 that has methoxy
group attached to phenyl ring, besides O–HꢀꢀꢀBr, C–HꢀꢀꢀBr
and C–HꢀꢀꢀO hydrogen bonds, cations are additionally
linked by C–Hꢀꢀꢀp and pꢀꢀꢀp interactions. We believe that
O–HꢀꢀꢀBr hydrogen bonds that form ring of four cations
and anions, and pꢀꢀꢀp interactions which are lacking in 1,
are responsible for the different orientation of phenacyl
moiety and conformation of this structure. In compound 2,
phenyl ring is included in intermolecular interactions
exclusively through C–HꢀꢀꢀO hydrogen bonds. Thus, the
conformation of these molecules is defined by discrete
interplay between intramolecular and intermolecular
interactions during the process of crystallization.
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