Crystal structure and electronic properties of three phenylpropionic acid derivatives: A combined X-ray powder diffraction and quantum mechanical study

Article abstract of DOI:10.1016/j.cplett.2010.12.002

Crystal structures of three derivatives of phenylpropionic acid, 2, 3 and 4 with hydroxyl, methyl and methoxy substitutions at the 2, 4 positions have been determined from X-ray powder diffraction data and their electronic structures were calculated at th

Full text of DOI:10.1016/j.cplett.2010.12.002

Chemical Physics Letters 501 (2011) 351–357
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Crystal structure and electronic properties of three phenylpropionic acid
derivatives: A combined X-ray powder diffraction and quantum mechanical study
a,c
b
b
c,
⇑
Uday Das , Basab Chattopadhyay , Monika Mukherjee , A.K. Mukherjee
a
Department of Physics, Haldia Govt. College, Haldia, Purba Medinipur, WB, India
Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Crystal structures of three derivatives of phenylpropionic acid, 2, 3 and 4 with hydroxyl, methyl and
methoxy substitutions at the 2, 4 positions have been determined from X-ray powder diffraction data
and their electronic structures were calculated at the DFT level. The optimized molecular geometries
Received 18 October 2010
In final form 1 December 2010
Available online 3 December 2010
agree closely to that obtained from the crystallographic analysis. Intermolecular O–H. . .O hydrogen
2
2
bonds generate R (8) rings, which are further connected through O–H. . .O and C–H. . .O hydrogen bonds
into two-dimensional framework in 2 and 4, and a step like architecture in 3. The HOMO–LUMO energy
gap (>4.0 eV) indicates a high kinetic stability of the three compounds.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Phenylpropionic acid (1) derivatives exhibit a strong binding
vides structural information of the bulk material. With the recent
developments in the direct-space approaches for structure solution
[7–9], ab initio crystal structure analysis of organic molecular solids
can now be accomplished from X-ray powder diffraction data.
Crystal structures of several molecular compounds have been
determined from laboratory X-ray powder data using the direct-
space approaches [10–13].
The present letter reports the structural characterization of
three phenylpropionic acid derivatives, 3-(2-hydroxy-4-methyl-
phenyl)propionic acid (2), 3-(2-methoxyphenyl)propionic acid (3)
and 3-(3-methoxyphenyl)propionic acid (4) using X-ray powder
diffraction data, along with the DFT calculation to study the molec-
ular geometry and the electronic structure. An investigation of
close intermolecular interactions in the molecules of 2–4 via Hirsh-
feld surface analysis is also presented.
ability to peroxidases, which catalyze the oxidation of a number
of organic and inorganic substrates [1]. Pharmaceutical composi-
tions containing such compounds can inhibit the chemotactic acti-
vation of neutrophils (PMN leukocytes) induced by the interaction
of Interleukin-8 with CXCR1 and CXCR2 membrane receptors [2].
The propionate metabolites are also useful in the treatment of neu-
trophil-dependent pathologies such as psoriasis, ulcerative colitis,
melanoma, chronic obstructive pulmonary disease, rheumatoid
arthritis and idiopathic fibrosis [3–5]. In vivo and in vitro experi-
ments have demonstrated that phenylpropionates and hydroxy-
phenylpropionates can act as synergistic activators of the MhpR
transcriptional regulators from Escherichia coli [6]. In this respect,
knowledge of the molecular as well as the electronic structures
of phenylpropionic acid derivatives is of key importance for a bet-
ter understanding of their structure–activity correlation.
2
. Materials and methods
Although single crystal X-ray diffractometry is the most widely
used technique for atomic level structural characterization of
molecular compounds, an intrinsic limitation of this approach is
the requirement to grow single crystals of appropriate size and
quality that make them amenable to structure analysis. In such
cases, X-ray powder diffraction offers an alternative option for
structure elucidation. Unlike the single-crystal diffractometry
which reveals crystal structure corresponding to a particular single
crystal selected for data collection, X-ray powder diffraction pro-
2
.1. Synthesis
A solution of 7-methyl-2H-chromene-2-one (1 g, 0.00625 mol)
in 15 ml of EtOH was hydrogenated in presence of palladium char-
coal at 60 psi in a Parr apparatus for 30 h. The reaction mixture was
filtered and concentrated in vacuum to produce 3-(2-hydroxy-4-
methylphenyl)propionic acid (2) as colorless crystalline powder.
Yield 0.78 g (67%). Anal. Calc. for C10
2
12 3
H O : C 66.65, H 6.71, O
6.64%; Found C 67.1, H 6.5, O 26.4%. The compounds, 3-(2-
methoxyphenyl)propionic acid (3) and 3-(4-methoxyphenyl)pro-
pionic acid (4) were purchased from Aldrich, NY, USA (CAS Nos.
6342-77-4 and 1929-29-9), and were used without further
purification.
⇑
Corresponding author. Fax: +91 33 24146484.
E-mail address: akm_ju@rediffmail.com (A.K. Mukherjee).
0
009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.12.002

3
52
U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
2.2. X-ray structure analysis
2.3. Computational details
X-ray powder diffraction data of 2–4 were recorded at ambient
temperature (20 °C) on a Bruker D8 Advance diffractometer oper-
ating in the reflection mode and using CuK radiation (k =
.5418 Å). The indexing of powder patterns of 2–4 carried out
using the program NTREOR [14] indicates monoclinic unit cells
with a = 22.909(3), b = 4.883(7), c = 8.237(4) Å and b = 99.13(4)°
Density functional theory (DFT) calculations have been carried
out in the solid state (periodic) for all three samples with the
DMOL code [20] in the framework of a generalized-gradient
approximation (GGA) [21]. The geometry optimization was per-
formed using the BLYP [22,23] density functional and a double nu-
meric plus polarization (DNP) basis set. The atomic coordinates
were taken from the final X-ray refinement cycle. Geometry opti-
mizations were carried out without any structural constraints.
The net charges of atoms and dipoles, and the electronic structures
of isolated molecules of 2–4 were calculated at the BLYP level.
3
a
1
[
1
(
M(20)=17, F(20) = 35 (0.011605, 50)] for 2, a = 7.803(5), b =
9.596(9), c = 6.724(8) Å and b = 114.46(7)° [M(20) = 34, F(20) = 54
0.007507, 50] for 3, and a = 12.277(4), b = 7.686(3), c = 11.159(4) Å,
and b = 114.26(6)° [M(20) = 19, F(20) = 31 (0.010247, 63)] for 4.
Statistical analysis of powder patterns using the EXPO 2004 [15]
indicated the most probable space groups as P2
and P2 /c for 3, which were used for structure solution. The full
pattern decomposition was performed in each case with EXPO
004 following the Le Bail algorithm and using a pearson VII peak
profile function. Good quality Le Bail fits, R = 0.0490 in 2,
= 0.0377 in 3 and R = 0.0404 in 4 indicated the correctness of
1
/n for 2 and 4,
2
.4. Hirshfeld surface analysis
1
Hirshfeld surfaces [24–26] and the associated 2D-fingerprint
2
plots [27–29] were calculated using CrystalExplorer [30], which ac-
cepts a structure input file in the CIF format. Bond lengths to
hydrogen atoms were set to typical neutron values (C–H =
p
R
p
p
choice of unit cells and the estimates of profile parameters for sub-
sequent structure solution. The structures were solved by global
optimization of structural models in direct-space based on a Monte
Carlo search using the simulated annealing technique (in parallel
tempering mode), as implemented in the program FOX [16]. The
initial molecular geometry input in FOX was determined by the
MOPAC 5.0 program [17].
The atomic coordinates obtained from FOX were used as the
starting model for the Rietveld refinement using the program GSAS
program package [18] with an EXPGUI [19] interface. The lattice
parameters, background coefficients and profile parameters were
refined initially followed by the refinement of positional coordi-
nates of all non-hydrogen atoms with soft constraints on bond
lengths and bond angles, and planar restraints on the phenyl moi-
eties. The background was described by the shifted Chebyshev
function of first kind with 10 points regularly distributed over
1
.083 Å). For each point on the Hirshfeld isosurface, two distances
, the distance from the point to the nearest nucleus external to
the surface, and d , the distance to the nearest nucleus internal to
the surface, are defined. The normalized contact distance (dnorm
d
e
i
)
based on d
e
and d
i
is given by
vdw
vdw
ðd ꢀ r
i
Þ
ðd ꢀ r Þ ;
e
i
e
d
norm
¼
þ
vdw
vdw
r
r
e
i
where r vi dw and r ev dw being the van der Waals radii of the atoms.
The value of dnorm is negative or positive depending on intermolec-
ular contacts being shorter or longer than the van der Waals sepa-
rations. The parameter dnorm displays a surface with a red–white–
blue color scheme, where bright red spots highlight shorter con-
tacts, white areas represent contacts around the van der Waals
separation, and blue regions are devoid of close contacts.
the entire 2h range. A fixed isotropic displacement parameter of
2
0
.04 Å for all non-hydrogen atoms was maintained. Hydrogen
3
. Results and discussion
atoms were placed in the calculated positions with a common Biso
value of 0.06 Å . In the final stage of refinement, preferred orienta-
tion correction was applied using the generalized spherical har-
monic model, and the order of spherical harmonics necessary to
describe the preferred orientation was 12 in 2 (texture in-
dex = 1.32), 8 in 3 (texture index 1.16), and 10 in 4 (texture index
2
3.1. Crystal and molecular structure
The compounds, 2–4, consist of terminal aromatic ring–n–ali-
phatic string–carboxyl group sequence (Figure 1). The essential dif-
ference among the compounds is the position of substituents in the
1
.16). Relevant crystal data for 2–4 are summarized in Table 1.
3
-phenylpropionic acid (1) skeleton (hydroxyl and methyl groups
at the 2 and 4 positions in 2, methoxy group at the 2 and 4 posi-
tions in 3 and 4, respectively). The Rietveld plots and molecular
views with atom labelling scheme for 2–4 are shown in Figures 2
and 3i. The molecules in 2–4 have fully extended propionic side
chain in a trans configuration; the torsion angles C6–C7–C8–C9 as-
Table 1
Crystal data and structure refinement parameters of compounds 2–4.
Compound
C
10
H
12
O
3
(2)
C
10
H
12
O
3
(3)
10 12 3
C H O (4)
Molecular weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
Volume (Å )
180.20
293
Monoclinic
P2 /n
22.8913(14)
4.8938(27)
8.2326(6)
99.227(3)
910.33(14)
4
180.20
293
Monoclinic
P2 /c
7.8243(4)
19.6301(7)
6.7344(4)
114.385(4)
942.08(6)
4
180.20
293
Monoclinic
P2 /n
1
1
1
12.2392(5)
7.6727(2)
11.1229(2)
114.168(3)
952.97(3)
4
3
Z
D
calc (g cm 3
ꢀ
)
1.315
1.270
1.256
Wavelength (Å)
h Interval (°)
No. of parameters
No. of data points
No. of restraints
1.5418
5–100
64
4624
51
1.5418
5–100
55
4780
47
1.5418
5–100
58
4780
46
2
R
R
R(F )
p
0.0527
0.0748
0.1425
5.514
0.0541
0.0780
0.1009
3.204
0.0476
0.0662
0.0669
1.416
wp
2
2
v
Figure 1. Chemical diagrams of compounds 1–4.

U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
353
12 3 12 3
Figure 2. Final Rietveld plots and molecular views of (i) C10H O (2) and (ii) C10H O (3) with atom numbering scheme. Red crosses: observed pattern, green curve:
calculated pattern, magenta curve: difference curve. The intensity in the high-angle region has been multiplied by a factor of 5. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)
sumes a value of ꢀ166.8(2)°, ꢀ170.7(2)° and 175.4(2)° in 2, 3 and
, respectively. While the carboxyl group (C9, O11, O12) in 3 is
(Table 2) and the corresponding crystallographically determined
values are 0.02 Å, 1.6° in 2, 0.02 Å, 0.8° in 3 and 0.02 Å, 0.7° in 4.
The differences between the molecular energies of the X-ray ana-
lyzed and DFT analyzed structures of 2, 3, and 4 are 8.91, 7.98.
and 10.93 eV, respectively.
The molecular packing in 2–4 exhibits intermolecular O–H. . .O
hydrogen bonds and C–H. . .O interactions (Supplementary Table
1). The carboxylic acid group in 2–4 forms O–H. . .O hydrogen
4
coplanar with the phenyl ring (C1–C6), they are almost perpendic-
ular to each other in 4; the dihedral angles between the relevant
least-squares planes in 3 and 4 are 14.2(2)° and 85.9(2)°, respec-
tively. Similar nearly parallel or virtually perpendicular orientation
between the carboxyl acid group and the aromatic fragment has
been reported for analogous compounds, in which the terminal
aromatic ring and the carboxyl group are linked via aliphatic
chains of varying length [31–33]. In compound 2, however, the car-
boxyl group is twisted about the C7–C8 bond with respect to the
phenyl ring by 38.1(1)°.
bonded dimer with O12. . .O11 distances of 2.617(4)–2.761(5) Å
2
2
in an R (8) graph-set motif [34,35], which links the molecules into
pairs around the inversion centers in the unit cell. The adjacent di-
mers in 2 are linked through intermolecular O13–H13. . .O13
hydrogen bonds to form two dimensional corrugated sheets paral-
lel to the (1 1 0) plane (Figure 4). Additional reinforcements within
the two dimensional sheets in 2 are provided by C–H. . .O hydrogen
bonds (Supplementary Table 1). The dimeric units in 3 are joined
through intermolecular C–H. . .O hydrogen bonds forming a step
like framework (Supplementary Figure 1). In 4, however, the inter-
3
The molecular-orbital optimization with DMol also corrobo-
rates an almost trans configuration of aliphatic chain in 2–4, and
the overall molecular geometry in the compounds as established
by the quantum mechanical calculations agrees well with that ob-
tained from the X-ray structure analysis. The r.m.s. deviations be-
tween the geometrically optimized bond lengths and bond angles

3
54
U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
Figure 3. (i) Final Rietveld plots and molecular view of C10
H
12
O
3
(4) with atom numbering scheme. Red crosses: observed pattern, green curve: calculated pattern, magenta
(4)
curve: difference curve. The intensity in the high-angle region has been multiplied by a factor of 5. (ii) A view of two dimensional supramolecular architecture in C10
12 3
H O
2
2
6
6
based on R (8) and R (44) synthons. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2
connection of R (8) rings via C–H. . .O hydrogen bonds generates a
in Figure 5i are due to C. . .H contacts. Other visible spots in Figure
5i correspond to H. . .H interactions. In the 2D fingerprint plots, Fig-
ure 5ii, prominent pairs of sharp spikes of almost equal length in
2
2
two dimensional supramolecular architecture based on R (8) and
R (44) synthons (Figure 3ii).
2
6
6
The Hirshfeld surfaces of 2–4 are illustrated in Figure 5i, show-
ing surfaces that have been mapped over dnorm range of ꢀ0.5 to
the region 1.7 < d
e i
+ d < 2.7 Å are characteristics of nearly equal
O(donor). . .O(acceptor) distances and a cyclic hydrogen bonded
2
1
.5 Å. The dominant interactions between carboxylic O–H and O
2
i e
R (8) synthon [36]. The points in the (d , d ) regions of (1.3,
atoms forming dimeric species in 2–4 can be seen in the Hirshfeld
surface plots as the bright red areas (marked a and b) in Figure 5i.
The phenolic O13. . .O13 interactions in the Hirshfeld surface of 2
are encircled in Figure 5i. The light red spots labelled as c and d
1.0 Å) and (1.0, 1.3 Å) in the fingerprint plot of 2 (Figure 5ii) are
due to O13–H13. . .O13 interactions. The upper spike a in Figure
5ii corresponds to the donor spike (carboxylic H-atoms interacting
with O-atoms of the COOH groups), the lower spike b being an

U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
355
Table 2
Comparison of selected bond lengths (Å) and bond angles (°) for compounds 2–4 as determined from X-ray structure analysis and DFT calculations.
Bonds/angles
C
10
H
12
O
3
(2)
C
10
H
12
O
3
(3)
10 12 3
C H O (4)
X-ray
DFT
1.405
1.508
1.563
1.506
1.342
1.240
1.492
X-ray
DFT
1.381
1.517
1.536
1.510
1.338
1.243
X-ray
DFT
C5–O13
C6–C7
C7–C8
C8–C9
C9–O12
C9–O11
C3–C10
1.369(1)
1.510(1)
1.541(1)
1.521(1)
1.296(2)
1.200(1)
1.511(1)
1.372(2)
1.507(2)
1.537(2)
1.508(2)
1.301(2)
1.221(2)
1.510(1)
1.531(1)
1.501(1)
1.300(1)
1.219(1)
1.515
1.541
1.508
1.338
1.243
O13–C10
C4–C5–C6
C4–C5–O13
C6–C5–O13
C5–C6–C1
C5–C6–C7
C6–C7–C8
C7–C8–C9
C8–C9–O11
C8–C9–O12
O11–C9–O12
1.393(2)
120.7(2)
1.449
120.8
1.421(1)
121.2(1)
1.446
121.4
120.5(2)
121.6
120.8(2)
118.8(2)
120.0(2)
119.9(2)
118.8(2)
113.3(2)
125.3(2)
115.4(2)
119.3(2)
120.1
118.1
116.5
123.2
116.3
115.0
123.3
114.5
122.3
125.1(2)
114.2(2)
118.5(2)
119.6(2)
114.5(1)
112.9(2)
122.7(2)
115.4(2)
121.8(2)
123.8
115.4
118.0
118.6
115.4
114.9
121.9
115.1
123.0
118.2(1)
120.7(1)
112.8(1)
113.2(1)
122.6(2)
113.9(1)
122.5(2)
117.8
121.1
112.7
113.8
123.6
113.0
123.4
Figure 4. Perspective view of crystal packing in C10
H
12
O
3
(2) along [0 1 0] direction.
acceptor spike (O-atoms from propionic acids interacting with the
H atoms of –COOH groups). The wings in the region d = 1.0 Å to
= 1.6 Å and d = 1.0 Å to d = 1.6 Å, marked as c and d in Figure
ii, in the fingerprint plots correspond to C. . .H interactions in 2–
. The difference between the molecular interactions in 2–4 in
ence between O–H. . .O bonded dimers in 2–4 and the sum of the
isolated monomer energies, and the calculation only included the
intermolecular O–H. . .O hydrogen bonds not the C–H. . .O interac-
i
d
5
4
e
e
i
tions [37]. The essentially same
D
E values of ꢀ6.0, ꢀ6.1 and
ꢀ7.0 kcal/mol for compounds 2, 3 and 4, respectively, reveal the
2
2
terms of H. . .H interactions is reflected in the distribution of scat-
tered points in high (d , d ) region of the fingerprint plots Figure 5ii,
and the appearance of small wings around d = 1.3 to d = 2.0 Å and
= 2.0 to d = 1.3 Å in 4 (marked with circles in Figure 5ii). The rel-
robust nature of R (8) synthon.
i
e
i
e
3.2. Electronic structure
d
i
e
ative contributions of different interactions to the Hirshfeld sur-
faces in 2–4 were calculated (Supplementary Figure 2), which
indicated that the H. . .H interactions contributed about 50%
The net charges of atoms and dipoles, and the molecular orbital
energies of 2–4 calculated at the BLYP level (Supplementary Table
2) indicate that all oxygen atoms in the molecules bear negative
charges, while the carbon atoms of the phenyl ring having substi-
tutions (C3, C5, and C6 in 2, C5 and C6 in 3, C3 and C6 in 4), the
methoxy group (C10 in 3 and 4) and the carboxylic group (C9) bear
positive charges. The remaining carbon atoms in the molecules
bear negative charges. The large electron densities at the oxygen
atom (O13) of hydroxyl in 2 and methoxy (in 3 and 4) groups sug-
gest possible protonation of the associated atoms. Due to this
charge redistribution, the dipole of the molecules of 2–4 becomes
1.07, 1.40 and 0.97 a.u., respectively.
(
47.7% in 2, 52.9% in 3 and 50.1% in 4) to the Hirshfeld surface area,
while the remaining 50% contribution were mostly due to O. . .H
31.1% in 2, 26.5% in 3 and 30.4% in 4) and C. . .H (19.4% in 2,
(
1
6.4% in 3 and 15.3% in 4) interactions.
2
The hydrogen bonded R (8) synthon energy in 2–4 was calcu-
2
3
lated with the program DMol at the BLYP level using the DNP ba-
sis set. The synthons were built from the refined structures of 2–4
with O–H bond lengths set to typical neutron value (O–
H = 0.983 Å). The synthon energy(DE) was estimated as the differ-

3
56
U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
Figure 5. (i) Hirshfeld surfaces and (ii) 2D fingerprint plots of compounds 2–4. A colouring scheme red (contacts < vdW separations)-white (contacts ꢁvdW separations)-blue
contacts > vdW separations) used in (i). Close contacts are divided into four regions: a is H. . .O, b is O. . .H, c is H. . .C and d is C. . .H. (For interpretation of the references to
(
colour in this figure legend, the reader is referred to the web version of this article.)
The HOMO (highest occupied molecular orbital) and LUMO
lowest unoccupied molecular orbital) wave functions in mole-
The orbital energy level analysis at the BLYP level shows EHOMO
and ELUMO values of ꢀ5.25 and ꢀ1.21 eV for 2, ꢀ5.13 and ꢀ0.96 eV
for 3, and ꢀ5.08 and ꢀ1.04 eV for 4, respectively. The HOMO–
LUMO energy separation has been used as an indicator of kinetic
stability of the molecule [12]. A large HOMO–LUMO gap implies
a high kinetic stability and low chemical reactivity, because it is
energetically unfavourable to add electrons to a high-lying LUMO
or to extract electrons from a low-lying HOMO [38]. The HOMO–
LUMO energy gaps in 2, 3 and 4 are 4.04, 4.17 and 4.04 eV, respec-
tively. The differences between the orbital energies corresponding
to HOMO-1 and HOMO-2 of 0.34 eV in 2, 0.19 eV in 3 and 0.16 eV
in 4 are much larger than 0.05 eV, which indicate that the HOMO-1
and HOMO-2 energy levels in 2–4 are non-degenerate. Similar con-
clusion can be drawn from the LUMO + 1 and LUMO + 2 orbital en-
ergy calculations in compounds 2–4.
(
cules of 2–4 exhibit some differences, and the charge densities
for the HOMO and LUMO (Figure 6 and Supplementary Figure 3)
indicate very little charge accumulation on the hydrogen atoms.
The HOMO in 2–4 is primarily localized on the oxygen atom of hy-
droxyl (O13 in 2) and methoxy (O13 in 3 and 4) groups as well as
on the C–C bonds of the phenyl ring (C2–C3 and C5–C6 in 2, C2–C3,
C4–C5 and C5–C6 in 3, C1–C6, C2–C3 and C3–C4 in 4) showing
bonding–antibonding patterns characteristic of phenyl ring sys-
tems. The aliphatic side chain in 2–4 had hardly any HOMO popu-
lation. The LUMO in the molecules is populated on the carboxylic
group atoms (C9, O11 and O12) with only minor population local-
ized on the carbon atoms of the phenyl ring (C1–C6) in 2–4.
4
. Conclusion
The crystal and molecular structures of three propionic acid
derivatives, 3-(2-hydroxy-4-methylphenyl)propionic acid (2),
-(2-methoxyphenyl)propionic acid (3) and 3-(3-methoxy-
3
phenyl)propionic acid (4), have been solved using laboratory
X-ray powder diffraction data. The molecular geometry and the
electronic structures of 2–4 have been analysed by the DFT calcu-
lations. The molecular conformations of the compounds as estab-
lished by the X-ray analysis agree well with that obtained from
the quantum mechanical calculations. A comparison of close inter-
molecular interactions in compounds 2–4 using the Hirshfeld sur-
face analysis revealed that about 50% of total contribution of
different interactions to the Hirshfeld surface area can be attrib-
uted to the H. . .H interactions, while the remaining 50% contribu-
Figure 6. Charge density of HOMO (a) and LUMO (b) orbitals in compounds 2 and 3
calculated by DFT method [surfaces in yellow (+) and blue (ꢀ)]. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
tion corresponds to the O. . .H and C. . .H interactions.
2
2
Intermolecular O–H. . .O hydrogen bonds in 2–4 generate R (8)
synthons of nearly equal synthon energy, ꢀ6.0 to ꢀ7.0 eV. The

U. Das et al. / Chemical Physics Letters 501 (2011) 351–357
357
molecular packing in 2 and 3 exhibits a columnar structure built
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Acknowledgements
Financial support from the University Grants Commission, New
Delhi, and The Department of Science and Technology, Govern-
ment of India, New Delhi, through the DRS (SAP) and FIST pro-
grams for purchasing the Bruker D8 Advance X-ray powder
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