Molecular structure investigation of organic cocrystals of 1,10-phenanthroline-5,6-dione with aryloxyacetic acid: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.saa.2014.05.020

Two organic cocrystals namely, 1,10-phenanthroline-5,6-dione:2- naphthoxyacetic acid [(phendione)(2-naa)] (1) and 1,10-phenanthroline-5,6-dione: 2-formylphenoxyacetic acid [(phendione)(2-fpaa)] (2) were synthesized and studied by single crystal XRD, FT-IR, NMR, thermogravimetric, and powder X-ray diffraction analysis. The molecular properties of cocrystals were studied using density functional theory (DFT), basis set B3LYP/6-31G(d,p). Both cocrystals are stabilized through intermolecular hydrogen bonding (OH-N). The total electron density and molecular electrostatic potential surfaces of the cocrystals were constructed by NBO analysis using B3LYP/6-31G(d,p) method to display the electrostatic potential (electron + nuclei) distribution. The energy gap between HOMO and LUMO was measured for both cocrystals.

Full text of DOI:10.1016/j.saa.2014.05.020

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Molecular structure investigation of organic cocrystals
of 1,10-phenanthroline-5,6-dione with aryloxyacetic acid: A combined
experimental and theoretical study
b
c
b
d
G.S. Suresh Kumar a, , A. Antony Muthu Prabhu , N. Bhuvanesh , X.A.V. Ronica , S. Kumaresan
⇑
a Faculty of Chemistry, Department of Science and Humanities, Dr. Mahalingam College of Engineering and Technology (Dr. MCET), Udumalai Road, Pollachi 642 003, Tamil Nadu, India
b Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamil Nadu, India
c Department of Chemistry, Texas A & M University, College Station, TX 77842, USA
d Department of Chemistry, Noorul Islam University, Kumaracoil, Thuckalay 629 180, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Cocrystals were synthesized via
solvent mediated crystallization and
neat grinding methods.
ꢀ
ꢀ
ꢀ
ꢀ
Cocrystal 1 is stabilized through
H-bonding as well as p–p interaction.
Cocrystals 1 and 2 are stable up to
210 °C.
Binding energy of cocrystal 2 is higher
than that of cocrystal 1.
Formation of H-bonding in cocrystals
1 and 2 is confirmed by MEP and NBO
analysis.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two organic cocrystals namely, 1,10-phenanthroline-5,6-dione:2-naphthoxyacetic acid [(phendione)
(2-naa)] (1) and 1,10-phenanthroline-5,6-dione:2-formylphenoxyacetic acid [(phendione)(2-fpaa)] (2)
were synthesized and studied by single crystal XRD, FT-IR, NMR, thermogravimetric, and powder X-ray
diffraction analysis. The molecular properties of cocrystals were studied using density functional theory
(DFT), basis set B3LYP/6-31G(d,p). Both cocrystals are stabilized through intermolecular hydrogen bond-
ing (OAHꢁ ꢁ ꢁN). The total electron density and molecular electrostatic potential surfaces of the cocrystals
were constructed by NBO analysis using B3LYP/6-31G(d,p) method to display the electrostatic potential
(electron + nuclei) distribution. The energy gap between HOMO and LUMO was measured for both
cocrystals.
Received 20 February 2014
Received in revised form 30 April 2014
Accepted 9 May 2014
Available online 17 May 2014
Keywords:
1,10-Phenanthroline-5,6-dione
2-Naphthoxyacetic acid
2-Formylphenoxyacetic acid
Ó 2014 Elsevier B.V. All rights reserved.
DFT
MEP
NBO
Introduction
cocrystal is a single crystalline solid that incorporates two neutral
molecules, one being an active pharmaceutical ingredient (API) and
Cocrystal technology has shown great promise in rectifying the
undesirable properties of a drug substance. A pharmaceutical
the other a cocrystal former [1]. Once an API has been selected for
cocrystallization studies, non-toxic cocrystallizing agent should be
chosen so as to result in a pharmaceutically acceptable product. In
recent times, cocrystal formation using the crystal engineer-
ing approach has been shown to be effective for altering the
⇑
Corresponding author. Tel.: +91 9003180667.
E-mail address: shunsureshkumar@gmail.com (G.S. Suresh Kumar).
http://dx.doi.org/10.1016/j.saa.2014.05.020
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

466
G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
physicochemical properties such as melting point [2], solubility
[3], thermal [4], and photostability [5] as well as mechanical prop-
erties [6] of active pharmaceutical ingredients (APIs).
wavelength 1.54060, 1.54443, and 1.39225 Å respectively. Powder
X-ray diffraction data were recorded in the range 10° 6 2h 6 80°.
The step size was 0.0170°. Elemental analyses were performed
on a Perkin Elmer 2400 Series II Elemental CHNS analyzer.
The recent progress in density functional theory (DFT) has pro-
vided a very useful tool for understanding molecular properties
and for explaining the behavior of atoms in molecules. DFT meth-
ods have become very popular in the past decade due to their accu-
racy and less computational time [7]. The calculation of a wide
range of molecular properties with DFT allows a close connection
between theory and experiment, and often leads to important
clues about the geometric, electronic, and spectroscopic properties
of the systems being studied [7].
1,10-Phenanthroline-5,6-dione (phendione), displays signifi-
cant anticancer activity, both with and without a coordinated
metal [8]. It is also an excellent anti-Candida agent [9]. Phenoxy-
acetic acid moiety is associated with potent antimicrobial, antidia-
betic, antibiotic, anti-obesity, antiplatelet aggregation activities,
etc. [10a,b].
In continuation of our work in organic cocrystals [10], herein we
report the syntheses of binary cocrystals of 1,10-phenanthroline-
5,6-dione (phendione) with 2-naphthoxyacetic acid (2-naa) and
2-formylphenoxyacetic acid (2-fpaa) through solvent mediated
crystallization as well as neat grinding methods (Scheme 1). The
structure of the newly synthesized cocrystals were analyzed using
FT-IR spectroscopy, powder X-ray diffraction (PXRD), NMR, and
single crystal X-ray diffraction (SXRD). The molecular properties
of the cocrystals [(phendione)(2-naa)] (1) and [(phendione)
(2-fpaa)] (2) were studied using DFT method.
Syntheses of cocrystals [(phendione)(2-naa)] (1) and [(phendione)
(2-fpaa)] (2) via solution crystallization method
1,10-Phenanthroline-5,6-dione (1.0 mmol) and 2-naphthoxy-
acetic acid (1.0 mmol) or 2-formylphenoxyacetic acid (1.0 mmol)
were dissolved individually in aqueous ethanol (1:1 v/v, 10 mL)
in two Erelenmeyer flasks (25 mL) at room temperature. The mix-
tures were stirred and warmed until the starting materials com-
pletely dissolved. These mixtures were filtered to avoid the
inclusion of any undissolved starting materials. Slow evaporation
of the filtrate under ambient conditions over 3–4 days yielded
crystals of [(phendione)(2-naa)] (1) (yield: 80%) and [(phendi-
one)(2-fpaa)] (2) (yield: 84%) suitable for X-ray diffraction. Anal.
Calcd. for [(phendione)(2-naa)] (1) C24H16N2O5: C, 69.90; H, 3.91;
N, 6.79%. Found: C, 69.84; H, 3.98; N, 6.72% and for [(phendi-
one)(2-fpaa)] (2) C21H14N2O6: C, 64.62; H, 3.62; N, 7.18%. Found:
C, 64.60; H, 3.66; N, 7.10%.
Solvent-free syntheses of [(phendione)(2-naa)] (1) and
[(phendione)(2-fpaa)] (2) via neat grinding method
1,10-Phenanthroline-5,6-dione (1.0 mmol) and 2-naphthoxy-
acetic acid (1.0 mmol) or 2-formylphenoxyacetic acid (1.0 mmol)
were ground well individually at 30 °C under dry condition. After
30 min, red/brown colored solids of [(phendione)(2-naa)] (1) and
[(phendione)(2-fpaa)] (2) were found to have formed (PXRD,
Figs. s1 and s2, vide Supporting information).
Experimental
General details
1,10-Phenanthroline-5,6-dione (phendione) [11], 2-naphthoxy-
acetic acid (2-naa) and 2-formylphenoxyacetic acid (2-fpaa) [12]
were synthesized as per the literature methods. FT-IR spectra were
recorded on a JASCO FT-IR-410 spectrometer in the range 4000–
400 cmꢂ1 on KBr discs. The 1H NMR and 13C NMR spectra were
recorded on a Bruker (Avance) 400 MHz NMR instrument using
TMS as internal standard and DMSO-d6 as solvent. Thermogravi-
metric analysis (TGA) experiments were performed on a Diamond
Thermal Analyzer in the temperature range of 25–700 °C under
nitrogen atmosphere at a heating rate of 10 °C/min. Powder
X-ray diffraction data were collected using a XPERT-PRO diffrac-
X-ray structure determination
A BRUKER APEX 2 X-ray (three-circle) diffractometer was
employed for crystal screening, unit cell determination, and data
collection. Integrated intensity information for each reflection
was obtained by reduction of the data frames with the program
APEX2 [13]. A solution was obtained readily using SHELXTL (XS)
[14]. Absence of additional symmetry and subcell were verified
using PLATON [15] and CELL_NOW respectively. Olex2 was
employed for the final data presentation and structure plots [16].
tometer system with Cu K 1, Cu K 2, and Cu Kb with radiation of
a
a
Scheme 1. Syntheses of cocrystals 1 and 2.

G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
467
Table 1
Computational methodology
Crystal data and structure refinement parameters of cocrystals 1 and 2.
Theoretical studies on cocrystals 1 and 2 in gas phase were per-
formed using the analytical gradient methods of DFT with Becke’s
three parameters (B3) exchange functional together with the
Lee–Yang–Parr (LYP) non-local correlation functional, symbolized
B3LYP [17] by means of 6-31G(d,p) basis set implemented in
Gaussian 09 software package [18]. The optimized structural
parameters in gas phase were used to calculate energies and ther-
modynamic properties of the compounds. The energies of the high-
est occupied (HOMO) and lowest unoccupied (LUMO) molecular
orbital, and HOMO–LUMO energy gap have also been measured.
Gauss-View 5.0.8 visualization program [19] was used to construct
the molecular electrostatic potential (MEP) map and the shape of
the frontier molecular orbitals. The natural charges of the atoms
are determined by natural bond orbital (NBO) analysis using
B3LYP/6-31G(d,p) method. Isoelectronic molecular electrostatic
potential surfaces (MEP) and electron density surfaces [20]
mapped with electrostatic potential were calculated.
Cocrystal 1
Cocrystal 2
CCDC no.
899820
899821
Empirical formula
Formula weight
Appearance
C
24H16N2O5
C21H14N2O6
390.34
Brown block
412.39
Red block
Crystal size
0.19 ꢃ 0.06 ꢃ 0.06 0.40 ꢃ 0.40 ꢃ 0.08
Wavelength (Å)
Crystal system
Space group
0.71073 Å
0.71073 Å
Monoclinic
Cc
Triclinic
P – 1
Unit cell dimensions
a = 7.107(3) Å
b = 17.121(8) Å
c = 30.825(13) Å
a = 7.7654(3) Å
b = 9.558(4) Å
c = 12.757(5) Å
a
=
c
= 90°
a
= 84.072(4)°
b = 96.412(4)°
b = 85.611(4)°
c
= 71.416(4)°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
3727(3) Å3
8
878.9(6) Å3
2
1.470 mg/m3
0.105 mmꢂ1
1712
1.475 mg/m3
0.110 mmꢂ1
404
Theta range for data collection 2.38–27.50°
1.61–27.50°
ꢂ9 6 h 6 9,
ꢂ12 6 k 6 12,
16 6 l 6 16
10,187
Results and discussion
Index ranges
ꢂ9 6 h 6 9,
ꢂ22 6 k 6 22,
ꢂ40 6 l 6 39
21,146
Crystal structure
Reflections collected
Independent reflections
8339
[R(int) = 0.0215]
99.3%
Semi-empirical
from equivalents equivalents
3954 [R(int) = 0.0223]
The crystallographic data and structure refinement parameters
of the cocrystals 1 and 2 are presented in Table 1. The bond lengths
and angles are given in Table 2. The hydrogen bond geometry is
shown in Table 3. Crystallographic data for the cocrystals 1 and 2
have been deposited with the Cambridge Crystallographic Data
Centre (CCDC) (deposition numbers: 899820 and 899821
respectively).
Completeness to theta
Absorption correction
97.9%
Semi-empirical from
Max. and min. transmission
Refinement method
0.9937 and 0.9804 0.9912 and 0.9572
Full-matrix least- Full-matrix least-
squares on F2
8339/2/561
1.030
squares on F2
3954/0/263
1.068
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R1 = 0.0334,
wR2 = 0.0825
R1 = 0.0367,
wR2 = 0.0850
0.232 and
R1 = 0.0377,
wR2 = 0.0970
R1 = 0.0444,
wR2 = 0.1018
Single crystal XRD of cocrystal 1
R indices (all data)
Fig. 1 shows the ORTEP diagram of cocrystal 1 along with the
adopted atomic numbering scheme. The asymmetric unit of
cocrystal 1 contains two molecules of 1,10-phenanthroline-5,6-
dione (phendione) and two molecules of 2-naphthoxyacetic acid
(2-naa). The compound crystallizes in noncentrosymmetric space
group Cc, with Z = 8 (Z0 = 2). Absence of additional symmetry and
smaller unit cell were verified using Platon and CELL_NOW respec-
tively. The crystal structure shows that there is no proton transfer
from the carboxyl group of 2-naa to phendione in the asymmetric
Largest diff. peak and hole
0.308 and ꢂ0.199 e Åꢂ3
ꢂ0.170 e Åꢂ3
(phendione) and one molecule of 2-fpaa. The compound crystal-
lizes in noncentrosymmetric space group P ꢂ 1 with Z = 2. In the
asymmetric unit, the crystal structure shows that there is no pro-
ton transfer from the carboxyl group of 2-fpaa to the phendione.
From the CAO bond lengths [C8AO3: 1.2115(16) Å, C8AO2:
1.3177(15) Å] of the carboxyl acid moiety (Table 2), the non-
deprotonation of the ACOOH group could be inferred. Each 2-fpaa
(carbonyl OAH) combines with one unit of phendione (pyridyl
N-atom) through O2AH2ꢁ ꢁ ꢁN2 (O2AH2ꢁ ꢁ ꢁN2 bond distance is
2.783 Å) hydrogen bonding with Etter’s [21] graph set designator
D forming discrete molecules.
The N2ꢁ ꢁ ꢁH5 bond length of cocrystal 1 (1.999 and 1.981 Å) is
longer than the N2ꢁ ꢁ ꢁH2 bond length of cocrystal 2 (1.957 Å)
(Table 3). This reveals that hydrogen bond of cocrystal 2 is stronger
than cocrystal 1 and this gives extra stability to cocrystal 2.
The dihedral angle between the carboxyl group (O3AC8AO2) of
2-fpaa and the phendione (hydrogen bonded pyridyl ring) is
38.07°, and that of phendione (hydrogen bonded pyridyl ring)
unit.
A
predicted intermolecular hydrogen bonding motif
O5AH5ꢁ ꢁ ꢁN2 is observed between the carboxylic OAH of 2-naa
and the nitrogen of phendione. From Table 2, the CAO bond
lengths [C24AO4: 1.197(2), C24AO5: 1.328(2)] indicate that the
acid moiety is present as ACOOH. Each 2-naa combines with one
unit of phendione through O5AH5ꢁ ꢁ ꢁN2 (O5AH5ꢁ ꢁ ꢁN2 distance is
2.834 Å) hydrogen bonding with Etter’s [21] graph set designator
D forming discrete molecules. O5AH5ꢁ ꢁ ꢁN2 is essentially linear,
with an angle 173.08° (Table 3).
The dihedral angle between the carboxyl group (O4AC24AO5)
of 2-naa and the pyridyl ring of phendione is 20.84°. Phendione lies
almost on the same plane of the naphthalene ring of 2-naa, since
its dihedral angle is 5.55°. Two types of
p–p stacking forces were
observed (Fig. 2) between non-hydrogen bonded pyridyl ring of
phendione and substituted phenyl ring of 2-naa (3.524 and
3.584 Å). Fig. 3 depicts the crystal packing arrangement of cocrys-
tal 1 along b-axis.
and aryl ring of 2-fpaa is 63.17°. Extra stabilization through
p–p
interactions could not be visualized as the short contacts are more
than 4.1 Å. Fig. 5 shows the crystal packing arrangement along a⁄
axis.
Single crystal XRD of cocrystal 2
FT-IR spectra of cocrystals 1 and 2
The ORTEP diagram of the cocrystal 2 is shown in Fig. 4. The
geometrical parameters of 2 are listed in Table 2. The asymmetric
unit of 2 contains one molecule of 1,10-phenanthroline-5,6-dione
The formation of a cocrystal could be confirmed by the changes
in the carbonyl frequencies in the crystals. The change in carbonyl

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G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
Table 2
Selected bond lengths and bond angles of experimental and theoretical studies on cocrystals 1 and 2.
Bond
Cocrystal 1
Bond
Cocrystal 2
SCXRD
PM3
HF
DFT
SCXRD
PM3
HF
DFT
Bond lengths (Å)
C13AO3
C24AO4
C24AO5
O5AH5
C10AN2
C5AN2
C9AN2
1.378 (19)
1.197 (2)
1.328 (2)
0.840
1.342 (2)
1.345 (2)
1.333
1.379
1.220
1.337
0.974
1.354
1.361
1.350
1.357
1.349
1.187
1.313
0.961
1.316
1.320
1.319
1.323
1.365
1.214
1.330
0.999
1.333
1.339
1.343
1.339
C1AO1
C8AO3
C8AO2
O2AH2
C21AN2
C18AN2
C10AN1
C14AN1
1.366 (15)
1.211 (16)
1.318 (15)
0.840
1.339 (17)
1.341 (16)
1.335 (17)
1.341 (16)
1.385
1.220
1.341
0.975
1350
1.357
1.354
1.362
1.346
1.187
1.310
0.962
1.316
1.320
1.319
1.323
1.360
1.214
1.328
1.001
1.334
1.339
1.339
1.344
C4AN2
1.347 (2)
Bond angles (°)
C13AO3AO23
C23AC24AO4
O4AC24AO5
C24AO5AH5
C6AC1AC2
115.36 (12)
125.40 (15)
125.78 (15)
109.50
118.72 (14)
117.22 (15)
116.70
126.35
118.71
113.05
114.19
113.96
119.47
125.15
125.43
112.40
117.50
117.58
118.02
124.47
126.26
111.39
117.47
117.64
C1AO1AC7
C7AC8AO3
O3AC8AO2
C8AO2AH2
C17AC16AC15
C13AC15AC16
117.66 (9)
124.47 (11)
125.27 (12)
109.50
118.02 (11)
117.40 (11)
117.11
128.52
118.16
113.02
113.93
114.18
120.82
124.73
125.76
112.59
117.49
117.59
119.48
123.97
126.57
111.48
117.47
117.64
C3AC2AC1
spectra confirming the presence of nonionized carboxylic acids
[22]. Two intense carbonyl bands at 1755 and 1685 cmꢂ1 are dis-
played in the IR spectrum of cocrystal 1. The band at 1755 cmꢂ1
is due to the carbonyl stretching frequency of 2-naa and
1685 cmꢂ1 is due to the carbonyl stretching frequency of the phen-
dione in the cocrystal 1.
Table 3
Hydrogen bond geometries (Å and °) in the crystal structure of 1 and 2.
d(Hꢁ ꢁ ꢁA)
d(DAH)
d(DAHꢁ ꢁ ꢁA)
DAHꢁ ꢁ ꢁA
Cocrystal 1
O5AH5ꢁ ꢁ ꢁN2
1.999
1.981
0.840
0.840
2.834
2.816
172.30
173.08
O5aAH5aꢁ ꢁ ꢁN2a
In the IR spectrum of cocrystal 2, two bands at 1724 and
1693 cmꢂ1 are due to the carbonyl stretching frequencies of 2-fpaa
and 1682 cmꢂ1 is due to the carbonyl stretching frequency of
phendione. Carbonyl stretching frequencies of phendione is not
much affected in cocrystals 1 and 2 which confirm the noninvolve-
ment of phendione carbonyl in hydrogen bonding. The change in
carbonyl frequency (COOH) in cocrystals (1 and 2) is due to the
involvement of carboxyl group in hydrogen bond formation.
From single crystal XRD data, the carbonyl (C@O) bond length of
cocrystal 1 (1.197 Å) is lower than 2-naa (1.212 Å) [22e–f]. The
decrease in bond length of carboxyl group decreases the carbonyl
stretching frequency of cocrystal 1. But in cocrystal 2 (1.211 Å),
the carbonyl bond length is higher than that of 2-fpaa (1.203 Å)
[22g]. The increase in bond length of carboxyl group increases
the carbonyl stretching frequency of cocrystal 2.
Cocrystal 2
O2AH2ꢁ ꢁ ꢁN2
1.957
0.840
2.783
167.06
A – acceptor, D – donor, and H – hydrogen atom.
frequencies in cocrystals is due to the involvement of carboxyl
group in hydrogen bond formation because of the hydrogen bond
interactions that disturb the electron distribution on carboxyl
group [22]. The IR spectra of phendione, 2-naa, 2-fpaa, and cocrys-
tals (1 and 2) are given in the Supporting information (vide
Supporting information, Figs. s3–s7).
Table 4 shows the carbonyl stretching frequencies of starting
materials and cocrystals (1 and 2). The carbonyl stretching fre-
quency for cocrystals 1 and 2 is above 1600 cmꢂ1 in the infrared
Fig. 1. ORTEP diagram of [(phendione)(2-naa)] (1) along with the adopted atomic numbering scheme.

G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
469
Fig. 2. H-bonding and p–p stacking interactions in [(phendione)(2-naa)] (1).
Fig. 3. Packing arrangement of [(phendione)(2-naa)] (1) along b axis.
Fig. 4. ORTEP diagram of [(phendione)(2-fpaa)] (2) along with the adopted atomic numbering scheme.

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G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
consistent with the elimination of one molecule of naphthol moi-
ety. At higher temperatures, the weight loss is experienced at
about 325 °C. The loss in mass at this point is about 14.55% (calcu-
lated: 13.90%) and may be attributed to the loss of one molecule of
acetic acid. At still higher temperatures, the decomposition speeds
up again and at about 380 °C, a weight loss of 50.92% (calculated:
50.60%) is observed, which may be due to the decomposition of
the remaining phendione moiety (vide Supporting information,
Fig. s12).
Cocrystal 2 is stable up to 210 °C at which point it loses about
41.02% (calculated: 40.5%) of its weight. This may be due to the
elimination of one molecule of formaldehyde, one molecule of car-
bon dioxide, one molecule of methanol, and two molecules of car-
bon monoxide. At 270 °C, there is a weight loss of about 20%
(calculated: 20.5%) which may be attributed to the loss of one mol-
ecule of phenyl unit. Further increase in temperature speeds up the
decomposition and at about 370 °C, a weight loss of 38.97% (calcu-
lated: 39%) was noticed which may be due to the decomposition of
the remaining phendione moiety (vide Supporting information,
Fig. s13).
Powder XRD of cocrystals 1 and 2
Powder XRD is useful for the fast identification of the new
phases of compounds. The cocrystal formed between phendione
and 2-naa/2-fpaa at a molar ratio of 1:1 is further supported by
powder XRD studies. Powder XRD pattern of the 1:1 ground mix-
ture of the starting materials displayed similar pattern of the
cocrystals of both 1 and 2 obtained from solution crystallization
(vide Supporting information, Figs. s1 and s2).
Fig. 5. Packing arrangement of [(phendione)(2-fpaa)] (2) along a⁄ axis.
Table 4
Carbonyl stretching frequencies mC@O (cmꢂ1) of starting materials and cocrystals 1
and 2.
Theoretical studies
Compounds
Stretching frequencies mC@O (cmꢂ1
)
Phendione
2-naa
2-fpaa
1686
1738
1760 (COOH)
1720 (CHO)
1755
1685
1724
1693
1682
Gas phase geometry optimizations were performed starting
from the crystal structures of cocrystals 1 and 2 at the B3LYP/6-
31G(d,p) level of theory using Gaussian 09W software [18]. In most
theoretical studies, optimizing the geometry of the species under
investigation is the first step. Gas phase geometrical parameters
of the optimized structures of cocrystals 1 and 2 at the PM3, HF/
3-21G(d), and DFT/6-31G(d,p) levels are listed in Table 2. The con-
formational analysis based on the geometrical parameters such as
bond length, bond angle, and planarity of cocrystals 1 and 2
(Table 2) reveals that DFT method represents a good correlation
between the calculated geometrical parameters and the single
crystal XRD data. Fig. 6 shows the optimized structures of cocrys-
tals 1 and 2. The calculated geometrical parameter displays good
correlation with experimental values that can lead to the calcula-
tion of other parameters for the cocrystals 1 and 2.
Cocrystal 1
Cocrystal 2
1H NMR and 13C NMR spectra of cocrystals 1 and 2
Liquid 1H NMR spectra of cocrystals are used to find out the
stoichiometric ratio of individual molecules in cocrytal [10a,10b].
1H NMR (in DMSO-d6) spectra of cocrystals 1 and 2 confirm that
the 1:1 stoichiometric ratio of phendione with 2-naphthoxyacetic
acid and 2-formylphenoxyacetic acid (vide Supporting information,
1H NMR and 13C NMR spectra, Figs. s8–s11 and peak analysis of
cocrystals (1 and 2), Table s1 and s2).
The molecular properties of phendione, 2-naa, 2-fpaa, cocrystal
1, and cocrystal 2 are summarized in Table 5. The total energy of
both cocrystals is more negative in magnitude than that of individ-
ual molecules. The binding energy of the cocrystals is given as,
The chemical shift (d ppm) of AOCH2 protons of cocrystals 1
(H23A and H23B) and 2 (H7A and H7B) are observed at 4.81 and
4.90 ppm whereas AOCH2 carbon of cocrystal 1 (C23) and cocrystal
2 (C7) resonate at 64.5 and 65.1 ppm respectively.
D
E ¼ Ecocrystal ꢂ ðEphendione þ E2-naa or2-fpaa
Þ
ð1Þ
where Ecocrystal, Ephendione, and E2-naa or 2-fpaa represent the energy of
the cocrystal, phendione, 2-naa, and 2-fpaa respectively. Cocrystal 1
has more negative binding energy than cocrystal 2. The binding
energies of cocrystals are negative, indicating that the cocrystals
can be formed thermodynamically. Generally, the binding energies
of the cocrystals increase with the increase in the number of the
weak bonds (H-bonds, van der Waals, etc.) formed [23]. H-bonding,
1H NMR and 13C NMR spectra of cocrystals 1 and 2 show that
proton and carbon atoms of phendione have similar chemical shift
value in both cocrystals. This indicates that in liquid NMR spectra
the cocrystals show the chemical shift of individual molecules.
p
–
p
, and CAHꢁ ꢁ ꢁ
p interactions are confirmed from both the theo-
TGA/DTA of cocrystals 1 and 2
retical binding energy and SCXRD studies of cocrystal 1.
Theoretically calculated Gibbs free energy changes (DG) using
Cocrystal 1 is stable up to 210 °C at which point it quickly
suffers about 34.43% weight loss (calculated: 35.50%) which is
DFT calculations is of a negative magnitude (Table 5) suggesting
that the formation of cocrystals was a spontaneous and feasible

G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
471
(a) [(phendione)(2-naa)] (1)
(b) [(phendione)(2-fpaa)] (2)
Fig. 6. Optimized structures of cocrystals (a) 1 and (b) 2.
process. The higher
lower degree of randomness than the cocrystal 1 in gas phase.
D
S values indicate that the cocrystal 2 has
interaction between the two molecules leads to the extension of
the conjugation system and decrease in the energy gaps. The
efficient H-bonding causes the flow of electrons from the proton-
donor to the proton-acceptor, which intensifies the electron den-
sity on the proton-acceptor and widens the energy gap of the
cocrystals [23].
It is clear from Fig. 7 that while HOMO is localized on the aro-
matic rings of 2-naa/2-fpaa, LUMO is localized on the whole mole-
cule of phendione (vide Supporting information, HOMO and LUMO
diagram for phendione, 2-naa, and 2-fpaa). The HOMO to LUMO
transition implies an electron density transfer from the aromatic
rings of 2-naa/2-fpaa to the electron deficient phendione. Cocrys-
tals 1 and 2 possess lower ionization potential data than that of
the phendione, and thus they are ready to lose electrons.
Frontier molecular orbital (FMO) analysis
Molecular orbitals (HOMO and LUMO) and their properties are
very useful for physicists, chemists, and biochemists; and are very
important parameters for quantum chemistry. This is also used by
the frontier electron density for predicting the most reactive
position in
p-electron systems [24]. The conjugated molecules
are analyzed by the highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) separation, which
is the result of a significant degree of intramolecular charge trans-
fer from the end-capping electron-donor groups to the efficient
Absolute hardness and softness are important properties to
measure the molecular stability and reactivity. A hard molecule
has a large energy gap and a soft molecule has a narrow energy
gap. Soft molecules are more reactive than hard ones because they
could easily offer electrons to an acceptor. For the simplest transfer
of electrons, adsorption could occur at the part of the molecule
where softness has the highest magnitude and hardness has the
lowest [28]. The thermal stabilities of cocrystals 1 and 2 display
electron-acceptor groups through
p-conjugated path [25]. Both
the highest occupied and lowest unoccupied molecular orbitals
are the main factors taking part in chemical stability [26]. HOMO
and LUMO energy of cocrystals 1 and 2 and their formers were cal-
culated by B3LYP/6-31G(d,p) method (Table 5). This electronic
absorption corresponds to the transition from the ground to the
first excited state and is mainly described by one-electron excita-
tion from HOMO to LUMO.
higher
r values than that of the phendione since several intermo-
The energy of HOMO is directly related to the ionization poten-
tial while that of ELUMO is directly related to the electron affinity.
The energy gap between HOMO and LUMO is a critical parameter
in determining molecular electrical transport properties [27]. The
lecular interactions exist in the two cocrystals. These indicate that
cocrystals 1 and 2 are more reactive than phendione and 2-naa/
2-fpaa (Table 5).

472
G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
Table 5
Theoretical studies on cocrystals 1 and 2.
Parameters
Phendione
2-naa
2-fpaa
Cocrystal 1
Cocrystal 2
Total energy, E (kcal/mol)
ꢂ452434.79
–
ꢂ432352.29
–
ꢂ407150.88
–
ꢂ884732.37
ꢂ54.71
ꢂ859396.66
ꢂ189.01
ꢂ6.3184
ꢂ3.6331
ꢂ2.6853
6.0968
6.3184
3.6331
4.9757
Binding energy,
EHOMO (eV)
DE (kcal/mol)
ꢂ7.8591
ꢂ3.5169
ꢂ4.3422
2.6879
7.8591
3.5169
5.6880
ꢂ5.6880
2.1711
0.4606
7.4509
ꢂ452330.12
–
ꢂ7.6517
ꢂ0.9588
ꢂ6.6929
2.1316
7.6517
0.9588
4.3052
ꢂ4.3052
3.3464
0.2988
2.7693
ꢂ432222.05
–
ꢂ6.8721
ꢂ1.8546
ꢂ5.0175
5.8165
6.8721
1.8546
4.3633
ꢂ4.3633
2.5087
0.3986
3.7945
ꢂ407044.78
–
ꢂ6.4537
ꢂ3.9329
ꢂ2.5208
2.5767
ELUMO (eV)
EHOMO–ELUMO (eV)
Dipole moment (Debye)
Ionization potential, I (eV)
Electron affinity, A (eV)
6.4537
3.9329
5.1933
Electronegativity,
Electrochemical potential,
Absolute hardness, (eV)
Softness, (eV)
Nucleophilicity,
Enthalpy, H (kcal/mol)
H (kcal/mol)
Gibbs free energy, G (kcal/mol)
G (kcal/mol)
v
(eV)
l
(eV)
ꢂ5.1933
ꢂ4.9757
g
1.2604
1.2318
r
0.7934
0.8118
x
(eV)
10.6991
ꢂ884495.55
56.62
10.0493
ꢂ859183.68
191.22
ꢂ859236.83
ꢂ202.14
178.308
0.00
D
ꢂ452361.50
ꢂ432255.48
ꢂ407077.47
ꢂ884550.27
D
–
–
–
ꢂ66.71
Entropy, S (cal/mol K)
(a) Electronic
105.248
0.00
112.104
0.00
109.536
0.00
183.438
0.00
(b) Translational
(c) Rotational
(d) Vibrational
41.930
32.025
31.292
–
97.45065
0.85555
0.53806
0.33032
ꢂ452337.68
0.00
41.815
32.349
37.940
–
122.31905
1.84669
0.26013
0.22834
ꢂ432230.31
0.00
41.471
31.661
36.404
–
98.49396
1.36622
0.46322
0.34668
ꢂ407052.70
0.00
43.939
37.461
43.776
36.944
97.588
ꢂ36.476
197.69819
0.39094
0.05609
0.04907
ꢂ859199.68
0.00
102.038
ꢂ33.914
220.35554
0.38751
0.04281
0.03856
ꢂ884511.99
0.00
D
S (cal/mol K)
Zero-point vibrational energy (kcal/mol)
Rotational constant (GHz)
Zero-point energy (kcal/mol)
Mullikan charges
ELUMO = -3.9329 eV
ELUMO = -3.6331 eV
Energy
EHOMO = -6.4537 eV
EHOMO = - 6.3184 eV
(a)
(b)
Fig. 7. HOMO and LUMO energy diagrams of cocrystals (a) 1 and (b) 2.
The nucleophilicity measures the electrophilic power of a mol-
The dipole moment in a molecule is mainly used to study the
intermolecular interactions involving the non-bonded type
dipole–dipole interactions, because higher dipole moment values
indicate stronger intermolecular interactions [29]. Cocrystal 2 has
ecule. It has been reported that the lower the value of
x, the lower
the capacity of the molecule to donate electrons [28]. Table 5
reveals that the 2-naa and 2-fpaa donate electron to phendione.

G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
473
(a) Cocrystal 1
(b) Cocrystal 2
Fig. 8. Molecular electrostatic potential map of cocrystals (a) 1 and (b) 2.
higher dipole moment than its former-compounds and cocrystal 1
(Table 5).
and pyridyl nitrogen atom of phendione with a minimum value of
ꢂ0.06244 and ꢂ0.05883 a.u. for cocrystals 1 and 2 respectively.
However, maximum positive (blue) region is localized on the car-
boxylic acid hydrogen atoms, with a maximum value of 0.06244
and 0.05883 a.u. respectively. The proton/electron donating/accept-
ing behaviors are confirmed by the MEP map of their parent com-
pounds (vide Supporting information).
Therefore, MEP maps of cocrystals 1 and 2 (Fig. 8) confirm the
existence of an intermolecular OAHꢁ ꢁ ꢁN bonding. The pale blue
and yellow regions reveal the presence of weak interactions. Hence
it can be said these color differences give the information about the
Analysis of molecular electrostatic potential (MEP)
Molecular electrostatic potential (MEP) is related to the elec-
tronic density and is a very useful descriptor in understanding sites
for electrophilic attack and nucleophilic reactions as well as hydro-
gen bonding interactions. This is correlated with dipole moments,
electronegativity, partial charges, and chemical reactivity of the
molecules [29,30].
MEP maps allow us to visualize variable charged regions of a
molecule. Knowledge of the charge distributions can be used to
determine how molecules interact with one another. Different val-
ues of the electrostatic potential are represented by different col-
ours: red represents regions of most negative electrostatic
potential, blue region represents regions of most positive electro-
static potential, and green represents regions of zero potential.
Potential increases in the following order: red < orange < yel-
low < green < blue [31].
region from where the compounds can have
p
–
p
and CAHꢁ ꢁ ꢁ
p
intermolecular interactions in the gaseous phase. In cocrystal 1,
the pale blue region is located on the aromatic ring of phendione
and the yellow region is located on the aromatic ring of 2-naa.
The pale blue region is localized on both the aromatic rings of
phendione which confirms the formation of two types of
interactions in cocrystal 1. This was confirmed from the SCXRD
studies (Fig. 2). For cocrystal 1, H-bonding,
, and CAHꢁ ꢁ ꢁ
interactions were confirmed from both theoretical and SCXRD
studies. But in cocrystal 2, the interaction was not observed
p–p
p
–p
p
p–p
Fig. 8 shows the MEP maps of cocrystals 1 and 2 determined
using DFT/B3LYP 6-31G(d,p) method. As can be seen in Fig. 8, the
negative (red1) region is localized on the carbonyl-oxygen atom
in SCXRD which is confirmed by theoretical studies. The MEP
map of cocrystal 2 shows that the aromatic rings of both phendi-
one and 2-fpaa have the pale blue region. SCXRD data confirmed
the formation of CAHꢁ ꢁ ꢁ
p
interactions. H-bonding and CAHꢁ ꢁ ꢁ
p
interactions of cocrystal 2 are confirmed by both theoretical and
1
For interpretation of color in ‘Figs. 2 and 8’, the reader is referred to the web
SCXRD studies.
version of this article.

474
G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
Table 6
NBO results showing the formation of Lewis and non-Lewis orbitals of cocrystal 1 and 2.
Bond
orbital
Occupancy Atoms Contribution from
parent NBO (%)
Atomic hybrid
contributions (%)
Bond
orbital
Occupancy Atoms Contribution from
parent NBO (%)
Atomic hybrid
contributions (%)
Cocrystal 1
C1AO1
Phendione
s(31.19) + p2.20(68.70) C2AN1
s(42.44) + p1.35(57.22)
s(31.21) + p2.20(68.69) C2AN1
s(42.44) + p1.35(57.22)
s(30.98) + p2.23(68.97) C12AN1
s(35.77) + p1.79(64.05)
s(30.75) + p2.25(69.20) C5AO1
s(36.46) + p1.74(63.37)
s(31.17) + p2.21(68.76) C5AO1
s(35.98) + p1.77(63.84)
1.99558
1.99560
1.98303
1.98431
1.98600
1.69995
C1
O1
C2
O2
C4
N1
C5
N2
C9
N1
C9
N1
C10
N2
C10
N2
C13
O3
C23
O3
C24
O4
C24
O4
C24
O5
O5
H5
34.41
65.59
34.42
65.58
41.76
58.24
41.14
58.86
40.15
59.85
40.41
59.59
39.38
60.62
37.54
62.46
32.39
67.61
32.41
67.59
34.02
65.98
29.78
70.22
32.03
67.90
79.45
20.55
1.98585
1.69657
1.98312
1.99559
1.95317
C2
N1
C2
N1
C12
N1
C5
O1
C5
O1
C11
N10
C6
O2
C6
O2
C9
N10
C9
40.43
59.57
41.08
58.92
58.15
41.85
34.41
65.59
35.46
64.54
41.85
58.15
34.41
65.59
35.46
64.54
59.57
40.43
58.92
41.08
s(31.35) + p2.19(68.58)
s(35.56) + p1.81(64.24)
s(0.00) + p1.00(99.93)
s(0.00) + p1.00(99.74)
s(35.72) + p1.79(64.09)
s(31.08) + p2.22(68.87)
s(31.14) + p2.21(68.75)
s(42.43) + p1.35(57.23)
s(0.00) + p1.00(99.87)
s(0.00) + p1.00(99.67)
s(31.08)p + 2.22(68.87)
s(35.72) + p1.79(64.09)
s(31.14) + p2.21(68.75)
s(42.43) + p1.35(57.23)
s(0.00) + p1.00(99.87)
s(0.00) + p1.00(99.67)
s(35.56) + p1.81(64.24)
s(31.35) + p2.19(68.58)
s(0.00) + p1.00(99.74)
s(0.00) + p1.00(99.93)
C2AO2
C4AN1
C5AN2
C9AN1
C9AN1
s(0.00) + p1.00(99.92)
s(0.00) + p1.00(99.76)
C11AN10 1.98312
C10AN2 1.98603
C10AN2 1.71145
C13AO3 1.98999
C23AO3 1.98991
C24AO4 1.99794
C24AO4 1.99237
C24AO5 1.99613
s(30.07) + p2.32(69.86) C6AO2
1.99559
1.95317
1.98585
1.69657
s(36.24) + p1.76(63.60)
s(0.00) + p1.00(99.91)
s(0.00) + p1.00(99.79)
C6AO2
s(24.77) + p3.03(75.02) C9AN10
s(32.84) + p2.04(67.09)
s(21.32) + p3.68(78.43) C9AN10
s(28.90) + p2.46(71.04)
s(34.28) + p1.91(65.61) 2-naa
s(41.65) + p1.39(57.97)
N10
s(0.00) + p1.00(99.80)
s(0.00) + p1.00(99.68)
C1AO1
1.98954
1.99003
1.99606
1.99169
1.99593
1.98507
C1
O1
O1
32.20
67.80
66.81
33.19
34.14
65.86
32.45
67.55
31.15
68.85
74.95
25.05
s(24.44) + p3.08(75.35)
s(32.73) + p2.05(67.20)
s(28.95) + p2.45(70.99)
s(22.11) + p3.51(77.66)
s(34.44) + p1.90(65.45)
s(42.40) + p1.35(57.20)
s(0.00) + p1.00(99.82)
s(0.00) + p1.00(99.64)
s(27.66) + p2.61(72.07)
s(33.64) + p1.97(66.29)
s(21.67) + p3.61(78.25)
s(99.78) + p0.00(0.22)
s(29.36) + p2.40(70.39) O1AC11
s(33.99) + p1.94(65.95)
s(27.62) + p2.62(72.32) C12AO3
s(99.73) + p0.00(0.27)
C11
C12
O3
C12
O3
C12
O2
O2
O5AH5
1.98454
C12AO3
C12AO2
O2AH2
H2
Cocrystal 2
C8AO2
2-fpaa
s(29.65) + p2.36(70.11) C8AO2
s(33.06) + p2.02(66.88)
s(34.35) + p1.91(65.54) C8AO3
s(41.60) + p1.39(58.02)
1.99606
1.99792
1.99226
C8
O2
C8
O3
C8
O3
C15
O5
C15
O5
C16
O6
C16
O6
C14
N1
C10
N1
C10
N1
C18
N2
C18
N2
C21
N2
32.12
67.88
34.04
65.96
29.83
70.17
34.42
65.58
35.64
64.36
34.42
65.58
35.64
64.36
41.75
58.25
40.14
59.86
40.39
59.61
41.11
58.89
39.20
60.80
39.35
60.65
1.99603
1.99812
1.99255
1.98613
C8
O2
C8
O3
C8
O3
O2
H2
31.22
68.78
34.12
65.88
31.40
68.60
76.11
23.89
s(27.89) + p2.58(71.85)
s(33.89) + p1.95(66.04)
s(34.52) + p1.89(65.37)
s(41.92) + p1.38(57.68)
s(0.00) + p1.00(99.81)
s(0.00) + p1.00(99.64)
s(21.41) + p3.67(78.52)
s(99.77) + p0.00(0.23)
C8AO3
C8AO3
s(0.00) + p1.00(99.80)
s(0.00) + p1.00(99.67)
C8AO3
C15AO5 1.99560
C15AO5 1.95269
C16AO6 1.99558
C16AO6 1.95281
C14AN1 1.98303
C10AN1 1.98600
C10AN1 1.70077
C18AN2 1.98432
C18AN2 1.70249
C21AN2 1.98605
s(31.22) + p2.20(68.68) O2AH2
s(42.43) + p1.35(57.23)
s(0.00) + p1.00(99.87)
s(0.00) + p1.00(99.67)
s(31.20) + p2.20(68.70)
s(42.43) + p1.35(57.22)
s(0.00) + p1.00(99.87)
s(0.00) + p1.00(99.67)
s(30.97) + p2.23(68.98)
s(35.78) + p1.79(64.04)
s(31.17) + p2.21(68.76)
s(35.99) + p1.77(63.83)
s(0.00) + p1.00(99.92)
s(0.00) + p1.00(99.76)
s(30.74) + p2.25(69.21)
s(36.48) + p1.74(63.36)
s(0.00) + p1.00(99.94)
s(0.00) + p1.00(99.80)
s(30.07) + p2.32(69.86)
s(36.24) + p1.75(63.59)
Mulliken charge and natural atomic charge distribution
protons (H5 and H2) have a tendency to accept electrons. In
phendione, the strong negative charge is distributed on N-atoms,
N1 and N2. This reveals that both N-atoms can act as electron
donors. But in the cocrystals, one of the N-atoms (N2 for both 1
and 2) has a higher negative value than the other N-atom (N1
for both 1 and 2) in the phendione moiety. This confirmed that
the hydrogen bonding is formed between the N-atom (N2 for
both 1 and 2) of phendione moiety and acidic H-atom of 2-naa
(H5) or 2-fpaa (H2) moiety.
Mulliken and natural atomic charge distribution of atoms in
cocrystals 1 and 2 are given in Supporting information (vide
Supporting information, Tables s-3 and s-4). As can be seen from
Tables s-3 and s-4, the positive charges on hydrogen (H5 for
2-naa and H2 for 2-fpaa) bound to the O-atoms of 2-naa (O5)
and 2-fpaa (O2) are found to be much higher than those for other
H-atoms in 2-naa and 2-fpaa. This shows that these acidic

G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
475
Natural bond orbital (NBO) analysis
stabilization energy of 22.84 kJ/mol, whereas in cocrystal 2, the
electron donation from N2 LP(1) to O2AH2 results in a stabilization
energy of 33.14 kJ/mol.
It is well-known that NBO analysis provides an efficient method
for studying intra- and inter-molecular bonding and interactions
among bonds and also provides a convenient basis for investigating
charge transfer or conjugative interaction in molecular systems
[32]. Delocalization of electron density between occupied Lewis
type (bonded or lone pair) NBO orbitals and formally unoccupied
(antibonded or Rydgberg) non-Lewis NBO orbitals correspond to
a stabilizing donor–acceptor interaction (Table 6).
The larger the E(2) value, the more intensive is the interaction
between electron donors and electron acceptors and greater the
extent of conjugation of the whole system. Therefore, we conclude
from the NBO analysis that the H-bond is formed strongly between
N-atom of phendione and H-atom of 2-naa in cocrystal 2.
Conclusion
BD(1) C10AN2 orbital with 1.98603 electrons has 39.38% C10
character in an sp2.32 hybrid and 60.62% N2 character in an sp1.76
hybrid. The sp2.32 hybrid on C-atom has 69.86% p-character and
the sp1.76 hybrid on N-atom has 63.60% p-character in cocrystal
1. BD(1) O5AH5 orbital with 1.98454 electrons has 79.45% O5
character in an sp2.62 hybrid and 20.55% H5 character in an sp0.00
hybrid. The sp2.62 hybrid on O-atom has 72.32% p-character and
the sp0.00 hybrid on H-atom has 0.27% p-character in cocrystal 1.
The selected CAN (phendione) and OAH (2-naa/2-fpaa) bond orbi-
tal, sp character, and occupancy values are slightly changed in
cocrystals 1 and 2. These results also indicate that the hydrogen
bonding is formed between N-atom of phendione moiety and
H-atom of 2-naa or 2-fpaa moiety.
Two new cocrystals, [(phendione)(2-naa)] (1) and [(phendi-
one)(2-fpaa)] (2) have been synthesized using solvent mediated
crystallization as well as neat grinding method. The single crystal
X-ray diffraction data collection and refinement explored the com-
plete molecular structure of the cocrystals stabilized through the
hydrogen bonding. The OAHꢁ ꢁ ꢁN bond length of cocrystal 2 is
lower than that of cocrystal 1 which shows cocrystal 2 is more sta-
ble than cocrystal 1. TGA results showed that cocrystals 1 and 2
were stabilized up to 210 °C. Binding energy, MEP, Mulliken
charge, natural charge distribution, and NBO analysis confirmed
the formation of intermolecular interactions in cocrystals 1 and
2, and the stability of cocrystal 2 is higher than that of the cocrystal
1. Dipole moment of cocrystal 2 is higher than those of its formers
and hence cocrystal 2 is more polar and soluble in polar solvents.
In order to investigate the intermolecular interactions, the sta-
bilization energies of cocrystals 1 and 2 were computed using sec-
ond-order perturbation theory. For each donor NBO(i) and acceptor
NBO(j), the stabilization energy E(2) associated with electron delo-
calization between donor and acceptor is estimated [33,34] as,
Acknowledgements
One of the authors (G.S. Suresh Kumar) gratefully thanks CDRI,
Lucknow, India for spectral studies. Authors thank Dr. K. Veluraja,
Department of Physics, Manonmaniam Sundaranar University,
Tirunelveli, Tamilnadu, India for PXRD data.
2
Fði; jÞ
Eð2Þ
¼ DEij ¼ q
i ej
ꢂ
ei
where qi is the donor orbital occupancy, ei and ej are diagonal ele-
ments (orbital energies), and F(i, j) is the off-diagonal NBO Fock
Matrix element. The results of second-order perturbation theory
analysis of the Fock Matrix at the B3LYP/6-31G(d,p) level of theory
are presented in Table 7.
Appendix A. Supplementary material
CCDC 899820 and 899821 contain the supplementary crystallo-
graphic data for [(pnendione)(2-naa)] (1) and [(phendione)
(2-fpaa)] (2) respectively. This can be obtained free of charge from
The most important interaction energy in cocrystal 1 is electron
donation from N2 LP(1) to the antibonding (O5AH5) resulting in a
Table 7
Second order perturbation theory analysis of Fock Matrix of cocrystals 1 and 2 from NBO analysis using B3LYP/6-31G(d,p) method.
Donor (i)–acceptor (j) interaction
Cocrystal 1
Donor (i)–acceptor (j) interaction
Cocrystal 2
E(2)a
E(j)–E(i)b
(a.u.)
F(i, j)c
(a.u.)
E(2)a
E(j)–E(i)b
(a.u.)
F(i, j)c
(a.u.)
(kJ molꢂ1
)
(kJ molꢂ1
)
1,10-Phenanthroline-5,6-dione
LP(1)N1 ? p⁄(C3AC4)
LP(1)N2 ? p⁄(C5AC6)
LP(1)O1 ? RY⁄(1)C1
1,10-Phenanthroline-5,6-dione
LP(1)N1 ? p⁄(C28AC29)
LP(1)N2 ? p⁄(C32AC33)
LP(1)O5 ? RY⁄(1)C15
10.54
9.63
0.89
0.90
1.55
0.62
0.71
1.55
0.62
0.71
0.088
0.085
0.133
0.109
0.106
0.133
0.109
0.106
10.52
9.58
0.89
0.90
1.55
0.62
0.71
1.55
0.62
0.71
0.087
0.085
0.133
0.109
0.106
0.133
0.109
0.106
14.30
23.85
19.48
14.30
23.88
19.45
14.30
23.88
19.48
14.30
23.85
19.52
LP(1)O1 ? p⁄(C1AC2)
LP(1)O1 ? p⁄(C1AC6)
LP(1)O2 ? RY⁄(1)(C2)
LP(1)O2 ? p⁄(C1AC2)
LP(1)O2 ? p⁄(C2AC3)
LP(1)O5 ? p⁄(C15AC16)
LP(1)O5 ? p⁄(C15AC13)
LP(1)O6 ? RY⁄(1)(C16)
LP(1)O6 ? p⁄(C16AC17)
LP(1)O6 ? p⁄(C16AC17)
2-Naphthoxyacetic acid
LP(2)O3 ? p⁄(C13AC14)
LP(1)O4 ? RY⁄(1)C24
LP(1)O4 ? r⁄(C24AC23)
LP(1)O4 ? p⁄(C24AO4)
LP(2)O5 ? p⁄(C24AO5)
2-Formylphenoxyacetic acid
LP(1)O2 ? LP⁄(1)H2
LP(3)O2 ? RY⁄(1)H2
LP⁄(1)H2 ? RY⁄(1)H2
LP(1)N2 ? LP⁄(1)H2
31.54
14.67
22.04
28.80
57.56
0.35
1.43
0.62
0.66
0.32
0.097
0.130
0.107
0.125
0.123
13.66
18.62
29.19
33.14
0.71
1.71
1.05
0.53
0.099
0.178
0.324
0.130
1,10-Phenanthroline-5,6-dione to 2-naphthoxyacetic acid
1,10-Phenanthroline-5,6-dione to 2-formylphenoxyacetic acid
LP(1)N2 ? p⁄(O2AH2)
33.14 0.53
LP(1)N2 ? p⁄(O5AH5)
22.84
0.80
0.122
0.130
ED – Electron density.
LP – Lone pair.
RY – Rydberg.
a
E(2) – Energy of hyper conjugative interactions.
Energy difference between donor and acceptor i and j NBO orbitals.
F(i, j) – Fock Matrix element between i and j NBO orbitals.
b
c

476
G.S. Suresh Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 132 (2014) 465–476
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