Intramolecular resonance assisted N–H???O=C hydrogen bond and weak noncovalent interactions in two asymmetrically substituted geminal amido-esters: Crystal structures and quantum chemical exploration

Article abstract of DOI:10.1016/j.molstruc.2021.131210

Two asymmetrically substituted geminal amido-esters, namely ethyl (2E)-2-[(2,5-dimethoxy phenyl)carbamoyl]-3-[(4-nitrophenyl)amino] prop-2-enoate (I) and ethyl (2E)-2-[(9,10-dioxo-9,10-dihydroanthracen-1-yl)carbamoyl]-3-(phenylamino) prop-2-enoate (II) were synthesized and the nature and strength of intramolecular resonance assisted hydrogen bond (RAHB) and non-RAHB was studied. X-ray analysis revealed that intramolecular N–H???O, and C–H???O interactions lead to the formation of angularly fused pseudo tricyclic (A-C) motif in compound I and fused pseudo pentacyclic (A-E) motif in compound II. Intramolecular RAHB; non-RAHB interactions are characterized and quantified by Bader's quantum theory of atoms-in-molecules approach (QTAIM). In both compounds, ring A was found to exhibit intramolecular RAHB characteristics. Crystal structures of I and II are stabilized by weak intermolecular C–H???O, C–H???π, and π???π interactions. Intermolecular interaction energies for different molecular dimers in I and II have been quantified by using the PIXEL, QTAIM, and DFT methods. The pseudoring stacking interaction is observed only in compound II whereas no such stacking interactions are seen in compound I. Hirshfeld surface (HS) analysis suggested that the H???H and O???H contacts are the first and second dominant contacts in both crystal structures. The theoretical charge density analysis revealed that the C–H???O and C–H???C(π) interactions produce closed-shell characteristics. Further, the crystal packing of compounds I and II analyzed based on the energy frameworks.

Full text of DOI:10.1016/j.molstruc.2021.131210

Journal of Molecular Structure 1246 (2021) 131210
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Intramolecular resonance assisted N–HꢀꢀꢀO=C hydrogen bond and
weak noncovalent interactions in two asymmetrically substituted
geminal amido-esters: Crystal structures and quantum chemical
exploration
Perumal Venkatesana,b,c, Subbiah Thamotharan , M. Judith Percinob,
d,∗
c,∗
Andivelu Ilangovan
a
Department of Chemistry, Srimad Andavan Arts and Science College (Autonomous), T.V. Koil, Tiruchirappalli 620 005, India
b
Unidad de Polímeros y Electrónica Orgánica, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla, Val3, Eco-campus Valsequillo,
Independencia O2 Sur 50, San Pedro Zacachimalpa, Pue. México
c
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamilnadu, India
d
Biomolecular Crystallography Laboratory, Department of Bioinformatics, School of Chemical and Biotechnology, SASTRA Deemed University, Thanjavur 613
4
01, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two asymmetrically substituted geminal amido-esters, namely ethyl (2E)-2-[(2,5-dimethoxy
phenyl)carbamoyl]-3-[(4-nitrophenyl)amino] prop-2-enoate (I) and ethyl (2E)-2-[(9,10-dioxo-9,10-
dihydroanthracen-1-yl)carbamoyl]-3-(phenylamino) prop-2-enoate (II) were synthesized and the nature
and strength of intramolecular resonance assisted hydrogen bond (RAHB) and non-RAHB was studied.
X-ray analysis revealed that intramolecular N–HꢀꢀꢀO, and C–HꢀꢀꢀO interactions lead to the formation of
angularly fused pseudo tricyclic (A-C) motif in compound I and fused pseudo pentacyclic (A-E) motif in
compound II. Intramolecular RAHB; non-RAHB interactions are characterized and quantified by Bader’s
quantum theory of atoms-in-molecules approach (QTAIM). In both compounds, ring A was found to
exhibit intramolecular RAHB characteristics. Crystal structures of I and II are stabilized by weak inter-
molecular C–HꢀꢀꢀO, C–Hꢀꢀꢀπ, and πꢀꢀꢀπ interactions. Intermolecular interaction energies for different
molecular dimers in I and II have been quantified by using the PIXEL, QTAIM, and DFT methods. The
pseudoring stacking interaction is observed only in compound II whereas no such stacking interactions
are seen in compound I. Hirshfeld surface (HS) analysis suggested that the HꢀꢀꢀH and OꢀꢀꢀH contacts are
the first and second dominant contacts in both crystal structures. The theoretical charge density analysis
revealed that the C–HꢀꢀꢀO and C–HꢀꢀꢀC(π) interactions produce closed-shell characteristics. Further, the
crystal packing of compounds I and II analyzed based on the energy frameworks.
Received 25 April 2021
Revised 22 June 2021
Accepted 28 July 2021
Available online 30 July 2021
Keywords:
Geminal amido-esters
Intramolecular Resonance-Assisted
Hydrogen bond
Weak intermolecular interactions
PIXEL
DFT calculations
QTAIM analysis
©
2021 Published by Elsevier B.V.
1
. Introduction
tified that partial charges on the oxygen atom arising out of res-
onance of the carbonyl (C=O) group enhance the proton acceptor
characteristic. As a result, both proton donor and acceptor func-
tionalities have moved closer to each other to yield a strong HB [3].
Thus the strong stabilization energy for the RAHB arises due to the
partial delocalization of the π-electrons [4-6]. Since the proposal
on RAHB, both intramolecular and intermolecular RAHB have been
extensively studied in a wide range of molecules with different
kinds of O–HꢀꢀꢀO, N–HꢀꢀꢀO, O–HꢀꢀꢀN, and O–HꢀꢀꢀS interactions [1,
2,7-15]. Over the past few decades, several experimental and the-
oretical approaches have been explored to understand the origi-
nating point for the RAHB and its importance in the wide range
of molecules established. The RAHB phenomenon has received the
most attention because it drastically influenced molecular prop-
Resonance-assisted hydrogen bond (RAHB) [1,2] is shorter and
more robust than the classical hydrogen bond (HB), observed in
the system that is conjugated with the multiple π−bonds in the
neutral molecules. The concept of RAHB was proposed by Gilli
and co-workers for the first time, using enol form of β-diketone
compounds, and more significant developments took place in the
last two decades of the 20th century [2]. As Gilli and co-workers
stated, the more robust and shorter nature of the RAHB has jus-
∗
Corresponding authors.
E-mail addresses: thamu@scbt.sastra.edu (S. Thamotharan), ilangovan@bdu.ac.in
(A. Ilangovan).
https://doi.org/10.1016/j.molstruc.2021.131210
022-2860/© 2021 Published by Elsevier B.V.
0

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Scheme 1. Chemical structures of symmetrically substituted geminal amido ester (1-8) studied earlier and asymmetrically substituted geminal amido ester (I and II) used
in the present study.
erties, such as electronic energies, geometrical parameters, vibra-
tional frequencies, and downfield shifted 1H NMR signals [16,17].
The RAHB phenomena play a crucial role in protein folding and
DNA pairing [18].
Hirshfeld surfaces, 2D-fingerprint plots, energy frameworks, and
Bader’s QTAIM framework to characterize non-covalent interac-
tions. The energetics of different dimers observed in the crystal
structures of I and II using the PIXEL method is presented. Among
the amido esters I and II, Cole et. al. already reported the molec-
ular hyperpolarizabilities, electron delocalization, and intramolecu-
lar charge transfer properties of compound I [35].
After Gilli’s proposal on RAHB, different researchers proposed
other possible explanations on the originating point for the RAHB.
Fonseca Guerra and co-workers suggested that the donor-acceptor
orbital interactions comparably dominate in the stabilization of
RAHB with π-polarization effects (partial delocalization of the
π-electrons) [19,20]. Further, Gora and co-workers made com-
putational studies (valence bond and atoms-in-molecules com-
putations) and suggested that the partial charges generated by
resonance can enhance RAHB [21]. Several other theoretical ap-
proaches, such as energy decomposition analyses [22], block-
localized wavefunction analyses [23], and coupling constants [24],
suggested that the effects of RAHB originated from geometric con-
straints of the σ-framework [3]. Recently, we demonstrated the
crystal packing modulation of the strength of RAHB using a se-
ries of geminal amido esters 1-8 with symmetrical substituents
2. Experimental
2.1. Synthesis of compounds I and II
Compounds I and II were synthesized using the method re-
ported earlier (Scheme 2) [25,35]. A detailed synthesis procedure
for compounds I and II and their spectral data are given in sup-
porting information.
2.2. Single-crystal X-ray diffraction
(
Scheme 1) [25]. In our previous report, we described the ef-
Single crystals of I and II were grown by slow evaporation
method at room temperature. Single crystals of I were obtained
from DMSO solvent, whereas single crystals of II were obtained
in hexane: ethyl acetate mixture (8:2, v/v). The X-ray intensity
data were collected at room temperature (296 K) on a Bruker
SMART APEX II CCD diffractometer (APEX2, SAINT, and SADABS.
Bruker AXS Inc., Madison, Wisconsin, USA) using Mo Kα radiation
fect of crystal packing on the strength of intramolecular RAHBs
in symmetrically substituted amido-ester derivatives based on X-
ray and theoretical models with the aid of Bader’s QTAIM ap-
proach [25]. Two intramolecular N–HꢀꢀꢀO hydrogen bonds (RAHB
and non-RAHB) and one intramolecular C–HꢀꢀꢀO interaction gen-
erate fused pseudo tricyclic (S(6), S(6), S(6)) motifs in the symmet-
rically substituted geminal amido esters 1 – 8 (Scheme 1). We also
noted that the strength of intramolecular RAHB (N–HꢀꢀꢀO interac-
tion) varied in a wide range (6.9 to 11.4 kcal mol 1) due to crystal
packing constraints arising out of different aromatic ring substi-
tutions. Further, this effect was less significant for non-resonance
assisted intramolecular N–HꢀꢀꢀO hydrogen bond (8.2 to 9.9 kcal
mol 1). Another interesting packing motif observed in our previ-
ous study was the strong resonance-assisted pseudo ring stacking
interaction and this motif acted as a basic motif in geminal amido
ester derivatives [25]. Similar stacking dimers were observed be-
tween an aromatic ring and a pseudo planar five/six-membered
hydrogen-bridged rings; [26,27], between two planar hydrogen-
bridged rings [28-31] and between two resonance-assisted hydro-
gen bond (RAHB) rings [2, 21,30-34].
˚
(λ = 0.71073 A). Both structures were solved by the direct meth-
ods using the SHELXS-2014 program [36]. All non-hydrogen atoms
in I and II were refined by full-matrix least-squares on F2 us-
ing SHELXL-2018/3 [36]. The positions of amine H atoms were lo-
cated from a difference Fourier map and refined freely along with
their isotropic displacement parameters. All the remaining H atoms
were placed at the calculated positions (C–Haromatic and C–Hvinylic
is 0.93 A˚ ; and other C–H=0.96–0.97 A˚ ) using a riding model ap-
−
−
proach. In the case of methyl H atoms, they were constrained to
ride on their parent atoms but were allowed to rotate about the
C–C bond. The ORTEP and crystal packing diagrams were generated
using the PLATON [37], and Mercury [38], respectively.
2.3. Structural optimization
In the present work, we synthesized two asymmetrically sub-
stituted amido ester (I and II; Scheme 1) derivatives to investigate
the effect of crystal packing on the strength of N–HꢀꢀꢀO RAHB and
non-RAHB and to compare the effect of symmetrical and asym-
metrical substituted amido esters on the crystal packing and non-
covalent interactions. We used different theoretical tools such as
All the quantum chemical calculations were performed using
the program Gaussian16 [39]. The gas-phase structural optimiza-
tion was carried out to study the conformational stability of I and
II. The X-ray geometry of I and II was used as an initial model
for the optimization calculation with the M06-2X/cc-pVTZ level
2

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Scheme 2. Synthetic route for the compounds I and II.
of theory [40,41] along with the incorporation of Grimme’s D3
dispersion correction [42]. Furthermore, the vibrational frequen-
cies were calculated for both optimized molecules to confirm the
proper convergence to energy minima on the potential energy sur-
face (PES). There were no imaginary frequencies obtained from op-
timized molecules which indicates that the energy minima on the
PES.
(BCP’s) were used to quantify the strength of intra- and inter-
molecular interactions. Moreover, the dissociation energy (D.E.int
)
for a non-covalent interaction was calculated using an empirical
formula proposed by EML [50].
3. Results and discussion
3
.1. Synthesis, structural description and conformational analysis
2
.4. Hirshfeld surface (HS) analysis
Compounds I and II were synthesized as shown in Scheme 2,
and these compounds were characterized by IR, NMR, and single-
crystal X-ray diffraction techniques. The 1H and 13C–NMR spectra
of I and II are provided in the supporting information (SI, Figures
S1-S4 and Table S1). It should be mentioned that a broad down-
The Hirshfeld surface (HS) and the decomposed 2D fingerprint
(
FP) plots were used to visually analyze the various intermolecular
interactions in I and II. The HS and FP plots were generated with
the aid of the CrystalExplorer17 program [43]. The energy frame-
works were also obtained with the B3LYP/6-31G (d, p) level of the-
ory.
field peak appeared at ∼ δ 12 ppm for the N–H
aniline
in both com-
aniline
pounds. This downfield shift for N–H
is due to the formation
of RAHB (N1–H1 ꢀꢀꢀO=Canilide, Fig. 1) as observed in symmetrically
N
substituted geminal amido esters [25]. A brief discussion on vari-
ous characteristic peaks in the 1H and 13C–NMR spectra are dis-
2
.5. PIXEL energy calculation
cussed under the NMR characterization section in the supporting
information.
Lattice energies for I and II and the intermolecular interaction
energies for molecular pairs in these crystal structures were cal-
culated by the CLP package (version 12.5.2014) with the PIXELC
module [44]. For the PIXEL calculation, the electron density for
As shown in Fig. 1, the aniline ring (ring 1), anilide ring (ring
2
), and ester components are common moieties in compounds I
and II, and these moieties are interconnected via a central olefinic
∗
∗
these molecules has been calculated at the MP2/6-31G level of
theory. The percentage contributions of electrostatic and dispersion
energies were computed as mentioned in our studies [45-47]. Fur-
ther, we also calculated dimerization energy for different molec-
ular pairs identified from the PIXEL calculation using single-point
energy with M06-2X/cc-pVTZ level of theory along with the incor-
poration of Grimme’s D3 correction [42]. This dimerization energy
was further corrected with a basis set superposition error (BSSE)
using the counterpoise method [48].
(
C=C) double bond. However, rings 1 and 2 contain different sub-
stitutions (asymmetrically substituted) in both compounds. In I,
the 4–NO2 group is substituted in ring-1 whereas as 2, 5–OCH3
groups in ring-2. In compound II, the phenyl ring in ring-2 is re-
placed by 1-AAQ moiety. Both compounds crystallize in the mon-
oclinic crystal system with the P2 /c space group. The crystal data
1
and crystallographic refinement statistics are summarized in Table
S2. The ORTEP diagrams of I and II along with the atom-numbering
scheme, and aniline, anilide, and ester components are highlighted
with a different colour in Fig. 1.
2
.6. QTAIM analysis
A slightly twisted molecular conformation is observed for struc-
tures I and II. This molecular twist is confirmed using two torsion
angles (C7–N1–C1–C2, τ ; and C9–N2–C10–C15/C11, τ ) which de-
The topological analysis was performed with the AIMALL pack-
1
2
age [49]. The wave functions were generated at the M06-2X/cc-
pVTZ level of theory with Grimme’s dispersion correction (D3)
from normalized isolated molecules (monomer) and molecular
pairs (dimer). The selected topological parameters such as electron
scribe the rotation of rings 1 and 2 (Table 1). In the case of com-
pound, I, the enantiomerically related structure was used to com-
pare the conformation. Compound 1 in Scheme 1, containing un-
substituted phenyl ring on both the sides, i.e. ring-1 and ring-2,
in the symmetrically unsubstituted geminal amido-ester was taken
for comparison and referred to as the parent compound. Both τ1
and τ2 angles deviated ∼20° from each other and a similar trend is
2
densities (ρ), the Laplacian of the electron density (∇ ρ), local
potential energy density (V), kinetic energy density (G), and total
electronic energy density (H = V+G) at the bond critical points
3

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 1. ORTEP diagrams for the compound I and II are drawn at the 50% probability level. Aniline (blue), anilide (purple), and ester moiety (orange) are highlighted with
three different colors, and rings and pseudo rings are also labeled. The intramolecular interactions (N/C–HꢀꢀꢀO) are shown in dashed lines.
Fig. 2. Structural overlay diagrams, (a, b) crystal structure and their optimized molecule of I and II; (c) crystal structure overlay of compound I and parent compound
(
compound 1in Scheme 1, containing unsubstituted phenyl ring on both the sides, was taken for comparison and referred to as the parent compound); (d) crystal structure
of II and parent compound [Colour codes: green: carbon atoms in I and II crystal structures; purple: carbon atoms in the optimized molecules; gray: carbon atoms in the
parent compound].
Table 1
3
.2. Resonance and non-resonance assisted intramolecular N–HꢀꢀꢀO
Selected torsion angle (θ,°).
hydrogen bonds and intramolecular C–HꢀꢀꢀO interactions
Torsion
Ib
IIa
Parent
Intramolecular N1–H1 ꢀꢀꢀO1(pseudo ring A), N2–H2 ꢀꢀꢀO2
C7–N1–C1–C2 (τ1)
160.81(19)
-3.1(3)
179.2(3)
19.5(4)
157.93(13)
18.3(2)
N
N
C9–N2–C10–C15/C11 (τ2)
(pseudo ring B), and C15–H15ꢀꢀꢀO1 (pseudo ring C) interactions
formed an angularly fused three pseudo S(6), S(6), S(6) ring mo-
tif in compound I, whereas in compound II fused five pseudo
S(6), S(6), S(6), S(6), S(5) ring motifs generated via intramolecu-
lar N1–H1 ꢀꢀꢀO1(pseudo ring A), N2–H2 ꢀꢀꢀO2 (pseudo ring B),
a
C9–N2–C10–C11(t2)
b
Enantiomerically related molecule
N
N
C15–H15ꢀꢀꢀO1 (pseudo ring C) N2–H2 ꢀꢀꢀO4(pseudo ring D), and
also noticed when compared to the parent compound. The struc-
tural overlay diagram shows different combinations of structures
including the optimized molecule of I and II (Fig. 2). The RMSD
N
C7–H7ꢀꢀꢀO3 (pseudo ring E) interactions were observed. Among
these fused pseudo ring motifs in both the compounds I and II,
ring A with S(6) motif, displayed RAHB due to intramolecular N1–
˚
between the X-ray and the optimized molecule of I is 0.02 A and
˚
H1 ꢀꢀꢀO1 interaction similar to what was observed in our earlier
0
.19 A for II. These lower RMSD values indicate both experimental
N
study [25]. The RAHB nature of the pseudo ring motif was as-
and theoretical structures are in good agreement. However, a slight
structural deviation is around bulky substituents.
sessed by the method proposed by Gilli et al [2]. These authors
4

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Table 2
The selected intranuclear distances in the two fused S(6) motifs in I and II.
Bond length in A˚
d1: C7–N1
q1
q2
Q=q1+q2
λ
d2:C7=C8
d3:C8–C9
d4:C9=O1
N1–H1NꢀꢀꢀO1 interaction
I
1.3224
1.3224
1.377
1.377
1.469
1.469
1.238
1.238
0.092
0.079
-0.100
-0.078
-0.008
0.001
0.51
0.50
II
N2–H2NꢀꢀꢀO2 interaction
d1: C9–N2
d2:C8–C9
d3:C8–C16
d4:C16=O2
q1
q2
Q=q1+q2
λ
I
1.352
1.369
1.466
1.458
1.467
1.448
1.219
1.219
0.133
0.150
-0.001
0.010
0.132
0.160
0.29
0.25
II
q1= d1-d4; q2=d2-d3; λ= (1-Q/0.320)/2
Table 3
Selected topological parameters for intramolecular interactions observed in the asymmetric units of I and II. Rij: Bond
path, ρ: electron density (e A˚ ), ∇ ρ: Laplacian electron density (e A ), V: potential energy density (a.u.); G: kinetic
−
3
2
˚ −5
energy density (a.u.). D.E(int): dissociation energy =-V × 0.5 (kcal mol−1).
2
−V
Atoms
Rij
ρ
∇ ρ
V
G
H
DE(int)
|
|
G
I
H1NꢀꢀꢀO1
H2NꢀꢀꢀO2
H15ꢀꢀꢀO1
1.804
1.851
2.228
0.2844
0.2429
0.1292
3.4302
3.3413
2.03063
-0.04240
-0.03465
-0.01388
0.03899
0.03492
0.01747
-0.00341
0.06957
0.00359
13.30
10.87
4.36
1.09
0.99
0.79
II
H1NꢀꢀꢀO1
H2NꢀꢀꢀO4
H2NꢀꢀꢀO2
H7ꢀꢀꢀO3
1.847
1.920
2.068
2.407
2.217
0.2493
0.2174
0.1674
0.1348
0.1369
3.3408
3.2279
2.6471
2.5258
2.1642
-0.03516
-0.02972
-0.02033
-0.01790
-0.01516
0.03316
0.03160
0.02390
0.02205
0.01881
-0.00200
0.00188
0.00357
0.00415
0.00365
11.03
9.33
6.38
5.62
4.76
1.06
0.94
0.85
0.81
0.81
H11ꢀꢀꢀO1
use λ value calculated from the bond lengths of the RAHB ring to
differentiate between RAHB and non-RAHB. The fully delocalized
keto form of enol ring (RAHB) has a λ value of 0.5 and the lower
value λ (in the range of 0.25-0.29) indicates the non-RAHB nature
effect. From Tables 3 and S6, the topological parameters and the
strength of RAHB and non-RAHB interactions are comparable in the
crystal structure and optimized molecule. This result suggests that
the asymmetric substituents in geminal amido-ester do not alter
the strength of the RAHB and there is no effect of crystal pack-
ing modulation on the strength of RAHB. However, as reported by
us earlier, in the case of the geminal amido-esters with symmetri-
cal substituents, the crystal packing played a significant role in the
modulation of the strength of RAHB [25].
(
Table 2).
Further, the nature and strength of RAHB and non-RAHB inter-
actions in I and II were explored using Bader’s QTAIM calculations.
The strength of the RAHB (N1–H1 ꢀꢀꢀO1) interaction is stronger
N
compared to non-RAHB (N2–H2 ꢀꢀꢀO2, N2–H2 ꢀꢀꢀO4), and C–
N
N
HꢀꢀꢀO (C15–H15ꢀꢀꢀO1, C7–H7ꢀꢀꢀO3) interactions. The strength of
RAHB (N1–H1 ꢀꢀꢀO1) interaction falls in the range of 11.03 to
1
N
3.3. Hirshfeld surface (HS) analysis
3.30 kcal mol 1 and these values are comparable with related
−
geminal amido-ester derivatives [25]. The interaction energies
The HS analysis is a wonderful visualization tool to analyze the
intermolecular interactions in the crystal packing [51]. An exten-
sive HS analysis was carried out to qualitatively analyze various
intermolecular interactions in I and II.
(
D.E.int) were calculated using an empirical formula as proposed
2
earlier [50]. The positive value of Laplacian (∇ ρ > 0), H < 0, and
|
-V/G| > 1 suggested that the RAHB, N1–H1 ꢀꢀꢀO1 hydrogen bond
N
shows the intermediate bonding character between shared and
closed-shell interaction (Table 3). The dissociation energy of non-
The potential short (less than the sum of vdW radii of inter-
acting atoms) intermolecular interactions show red spots on the
HS (Fig. 3) and these contacts are also labeled in Table 4. There
are five such short inter-contacts observed in I whereas four inter-
contacts in the case of II. Further, a pattern of convex blue and
concave red triangles (highlighted with the circle in SI figure in
Fig. 3) is present in II which indicated the existence of π-stacking
interaction. No such pattern is observed in I, suggesting the exis-
tence of no significant π-stacking interaction.
RAHB interactions particularly, N2–H2 ꢀꢀꢀO2, N2–H2 ꢀꢀꢀO4 lies
N
N
between 6.38 and 10.87 kcal mol 1. The positive value of Lapla-
−
2
cian (∇ ρ > 0), H > 0 H and |-V/G| < 1 suggesting closed-shell
nature of non-RAHB’s (Table 3). The strength of intramolecular C–
HꢀꢀꢀO interactions are in the range of 4.36 to 5.62 kcal mol 1, and
these interactions are also closed-shell in nature as observed in the
symmetrically substituted amido-ester derivatives [25].
−
To assess the role of crystal packing on the strength of in-
tramolecular interactions, we performed QTAIM calculation for the
optimized structures of I and II. The selected topological parame-
ters for the intramolecular interactions at the gas-phase optimized
molecules are given in Table S3. The molecular graphs showing
the intramolecular interactions are illustrated in Fig. S5. It should
be noted that the intramolecular C7–H7ꢀꢀꢀO3 is absent in an op-
timized molecule (Table S3) which is due to the crystal packing
The fingerprint plots (FP; Fig 4a-b) reveal that the HꢀꢀꢀH con-
tacts are predominant in both crystal structures with the contribu-
tion being in the range of 39.1– 44.4 % to the total HS area. The
characteristic sharp spike for HꢀꢀꢀH contacts located at d +de ≈
i
˚
1
.8 A in II and a blunt spike rather than sharp spike is observed
˚
for at d +de ≈ 2.2 A in I. A remarkable difference (11.3%) is noticed
i
in the contribution of OꢀꢀꢀH/HꢀꢀꢀO contacts in I and II (Fig. 4c).
The variation in the contribution could be due to the presence of
5

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 3. Two different orientations of HS mapped with dnorm and shape index (SI) for I and II.
Table 4
Intermolecular interaction energies (in kcal mol 1) are obtained from the PIXEL and DFT methods for various molecular dimers observed in the crystal structure of I and II.
−
Geometry ( A˚ /°) a
HS
ꢀ Ecpc Label
Motif Possible Interactions
Symmetry
CD( A˚ )
6.983
7.861
Ecoul
Epol
Edispb
Erep
Etot
d(HꢀꢀꢀA) d(DꢀꢀꢀA) ࢬ D–HꢀꢀꢀA
Compound I
M1
C2–H2ꢀꢀꢀO5–N
C6–H6ꢀꢀꢀO1=C9
C20–H20AꢀꢀꢀO1=C9
C18–H18CꢀꢀꢀO6
C7ꢀꢀꢀC3
2.53
2.68
2.62
2.72
3.327
3.254
3.640
3.603
3.355(3)
3.211
3.399
3.469
3.476
3.726
130
113
156
138
x, y, 1+z
-5.4
-2.5
-16.3 (67)
10.2
-14.0
-16.9
1
M2
M3
M4
–x, 1–y, –z
-0.9
-1.2
-1.4
-1.3
-13.9 (87)
-7.0 (57)
-8.8 (69)
7.1
4.8
6.2
-8.9
-7.5
-6.6
-11.8
-8.4
2
3
C16ꢀꢀꢀO4
C6–H6ꢀꢀꢀO5–N3
C5–H5ꢀꢀꢀC5
2.69
2.77
2.44
2.68
2.83
2.55
2.66
2.78
2.97
123
122
160
163
–x, 1–y,1–z
12.778 -3.8
9.225 -2.7
C12–H12ꢀꢀꢀO6–C11
C19-H19AꢀꢀꢀO2=C16
C13–H13ꢀꢀꢀCg2
C20–H20BꢀꢀꢀO2=C16
C17–H17BꢀꢀꢀO7
C3–H3ꢀꢀꢀO4-N3
C17–H17AꢀꢀꢀCg1
x,1/2–y, -1/2+z
-7.6
4
5
3.684 (2) 138
M5
3.532
3.437
3.247
3.860
150
128
106
139
1+x, y,1+z
10.671 -2.2
13.392 -1.7
-0.9
-4.5 (59)
2.4
-5.2
-4.8
M6
M7
–1–x,1–y, –z
-0.8
-0.4
-4.2 (63)
-4.2 (82)
3.1
1.9
-3.6
-3.2
-3.8
-3.5
1+x, y, z
9.363
-0.5
Compound II
M8
C2–H2ꢀꢀꢀO2=C24
Stacking interactions
CgRAHBꢀꢀꢀCgRAHB
Cg1ꢀꢀꢀCg2
2.68
3.408
124
-x, y+1/2, -z+1/2 5.036
-6.6
-2.4
-26.3 (75)
17.5
-17.9
-25.9
6
7
3.996
3.704
3.888
3.292
3.482
3.680
3.907
3.525
Cg1ꢀꢀꢀCg4
M9
C20–H20ꢀꢀꢀO4=C22
C19–H19ꢀꢀꢀO1=C9
C3–H3ꢀꢀꢀO2=C24
C25–H25AꢀꢀꢀCg4
C26-H26CꢀꢀꢀO5=C15
2.59
2.70
2.64
2.95
2.46
1212
129
160
147
168
-x, 1 –y, -z
8.860
-2.5
-1.5
-1.6
-10.5 (72)
-7.3 (65)
5.6
5.3
-8.9
-6.0
-9.00
-4.7
M10
x, 1/2 –y, 1/2+ z
11.423 -2.4
M11
M12
-x, -y, -z
x-1, y, z
9.101
-1.4
-1.0
-0.5
-6.4 (73)
-3.8 (75)
4.5
2.3
-4.2
-2.8
-4.4
-2.1
8
9
13.251 -0.8
CD: distance is the centroid-centroid of the molecules in A˚ ; CgRAHB: the center of gravity for the resonance-assisted hydrogen-bonded ring (N1–H1NꢀꢀꢀO1/C9/C8/C7); Cg1-C2,
and Cg4: the center of gravity for the phenyl rings C1–C6 (ring-1); C10–C15(ring-2) for I, and C10 – C14, and C23(ring -2) for II; and C16 – C21 (ring -4) for II, respectively.
a
Neutron values are given for all D–HꢀꢀꢀA interactions. b Values in parenthesis represent % dispersion energy contribution (%Edisp= ((Edisp/ Eelec) 100) towards the total
∗
stabilization
a greater number of O atoms in I. We also note that the short
3.4. PIXEL energy analysis
˚
OꢀꢀꢀH/HꢀꢀꢀO contacts appeared at d +de ≈ 2.4 A in I whereas the
i
˚
corresponding contacts located beyond 2.5 A in II. The intermolec-
3.4.1. Molecular dimers in the Crystal structure of I
ular CꢀꢀꢀH/HꢀꢀꢀC contacts are third significant in I-II with ∼ 16 –
The PIXEL program was used to extract all possible dimers or
molecular pairs from the crystal structures and these dimers are
held together by different intermolecular interactions. Since the
strong donating units are actively engaged in intramolecular hy-
drogen bonds, the weak intermolecular C–HꢀꢀꢀO, C–HꢀꢀꢀC (π), and
πꢀꢀꢀπ interactions play a key role in the stabilization of crys-
tal structures of I and II as observed in symmetrically substituted
1
9 %. The C–Hꢀꢀꢀπ interactions show as wings like arrangement
˚
in FP and these interactions are located around 2.6-2.7 A in these
structures. The relative contribution of CꢀꢀꢀH/HꢀꢀꢀC contacts is also
comparable in I and II. Another difference in the relative contribu-
tion is noted for CꢀꢀꢀC contacts and these contacts are higher in II
compared to I. It indicates that π-stacking interaction exists in II.
6

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 4. (a-b). Decomposed two-dimensional fingerprint plots for I and II. (c) The relative contribution of various contacts in I and II.
amido esters [25]. The energetically significant dimers (M -M ) in
arrangement of the molecules in the crystal packing and the steric
hindrances of –OMe in the ortho position.
1
7
I were identified from PIXEL energy analysis and the intermolec-
ular interaction energies for these dimers are ranging from -14.0
It is well known that the weak C–HꢀꢀꢀO interactions play an
to -3.2 kcal mol 1 (Table 4). These dimers are also depicted in
Fig. S7. A most stabilized dimer, M1 is formed by three C–HꢀꢀꢀO
interactions. The interaction energy (Etot) for this dimer is -14.0
−
important role in crystal packing of (–NO ) nitro group substi-
2
tuted benzene and aniline derivatives, [52] and the nitro substi-
tuted malonic acid half-ester [53]. Similar to previous studies, the
oxygen atoms, O4 and O5 of the nitro group are participating in
kcal mol 1. Briefly, the O5 atom in –NO2 moiety and O1 atom
−
in –C=O
group act as acceptors, and the H2, H6, and H20A
weak C–HꢀꢀꢀO interactions (dimer M3 and M ) in I. Briefly, one
of the oxygen atoms in the nitro group act as an acceptor for
amide
6
atoms act as a donor for the above three C–HꢀꢀꢀO interactions in
M . Among these interactions, the C2–H2ꢀꢀꢀO5 interaction links
the C6–H6ꢀꢀꢀO5 interactions in M and it forms a centrosymmet-
1
3
2
−1
neighbouring molecules into a C(6) chain which runs parallel to
the c axis (Fig. 5a). Further, this molecular chain stabilized by an-
other two C–HꢀꢀꢀO interactions in M1 and these interactions, C6–
ric R (12) closed-loop with Etot: -7.5 kcal mol . This M3 motif
2
is further stabilized by C5–H5ꢀꢀꢀC5 interaction. The interactions in
M3 motif help to connect two neighbouring molecules with tail to
tail (ring-1 to ring-1, Fig. 5b) fashion. The O4 atom in –NO2 group
H6ꢀꢀꢀO1; and C20–H20AꢀꢀꢀO1, independently generate C(8) and
2
2
C(9) motifs, respectively. At the same time, the R (14) closed-loop
is involved in the C3–H3ꢀꢀꢀO4 interaction (M , the HꢀꢀꢀO distance
6
˚
formed by the combination of C2–H2ꢀꢀꢀO5 and C6–H6ꢀꢀꢀO1 inter-
is 0.08 A longer than the sum of vWd radii), forms a centrosym-
2
−1
actions, and a R (17) closed-loop formed from C6–H6ꢀꢀꢀO1, and
metric dimer with Etot: -3.6 kcal mol . The dimer M3 is stronger
2
C20–H20AꢀꢀꢀO1 interactions. These closed-loops are highlighted in
Fig. 5a. In addition to the above-mentioned interactions, the C18–
than dimer M6 and the difference in strength due to the presence
of additional C–HꢀꢀꢀO/C interactions present in dimer M3.
The O2 and O6 atoms belong to ester carbonyl (C=Oamide) and
˚
H18CꢀꢀꢀO6 (the HꢀꢀꢀO distance is 0.01 A longer than the sum of
vWd radii) interactions provide additional support to the stabiliza-
methoxy (–OCH ) groups act as acceptors for the C–HꢀꢀꢀO inter-
3
tion of dimer M . It is interesting to note that the strong dimer, M
actions (C19–H19AꢀꢀꢀO2 and C12–H12ꢀꢀꢀO6) in M4 with Etot: -6.6
1
1
kcal mol 1. These two C–HꢀꢀꢀO interactions collectively to form
−
in I is totally different from other strong dimers observed in the
symmetrically substituted geminal amido-esters [25]. The stacking
interactions (CgRAHBꢀꢀꢀCgRAHB) between two RAHB’s rings were ob-
served and it was observed as the strongest dimer in the symmet-
rically substituted geminal amido-esters [25]. However, no such
CgRAHBꢀꢀꢀCgRAHB stacking interaction is observed in I. The absence
of CgRAHBꢀꢀꢀCgRAHB stacking interaction in I might be due to the
2
2
a R (14) closed-loop and these interactions contact neighbouring
molecules with head to head (ring-2 to ring-2, Fig. S7) fashion. This
head to head arrangement in M4 is further stabilized by the C13–
H13ꢀꢀꢀCg2 interaction. The ester carbonyl oxygen atom, O2, and
O7 atom belongs to one of the methoxy groups act as acceptors
for C20–H20BꢀꢀꢀO2 and C17–H17BꢀꢀꢀO7 interactions in M5 with
7

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 5. Part of the crystal packing of I (a) three C–HꢀꢀꢀO interactions (in M1) forms a molecular array; (b) the C6–H6ꢀꢀꢀO5 (in M3), C20–H20BꢀꢀꢀO2 and C17–H17BꢀꢀꢀO7 (in
M5) interactions form another molecular array. Different closed loops, 1D molecular chains, and the respective dimers are labeled and highlighted with colors.
Etot: -5.2 kcal mol 1. Both C–HꢀꢀꢀO interactions in M5 combined to
−
2
form a R (8) closed-loop and these interactions link neighbouring
2
molecules into the molecular layer which runs parallel to the crys-
tallographic a axis. The two parallel M5 molecular layers are fur-
ther interconnected by the C6–H6ꢀꢀꢀO5 interaction (M ) and form
3
a molecular array along ab plane (Fig. 5b). It should be mentioned
that all the dimers in I are predominantly dispersive in nature with
>
57 % of contribution towards the stabilization of these dimers.
Among them, the M2 (87 %) and M7 (82 %) dimers have the high-
est dispersion energy contribution could be due to the presence of
CꢀꢀꢀC and CꢀꢀꢀH contacts existing in these dimers.
As shown in Fig. 6, the molecules of I form the zigzag dou-
ble layers in head to head (ring-2 to ring-2) and tail to tail (ring-
1
to ring-1). The head to head (ring-1 to ring-1) groups are in-
terconnected by the interactions in M , and tail to tail (ring-1
4
to ring-1) groups are in contact by interactions in M -M and
2
3
M6 dimers. The C20–H20BꢀꢀꢀO2 and C17–H17BꢀꢀꢀO7 in M5 and
C17–H17AꢀꢀꢀCg1 (M ; Etot: -3.2 kcal mol 1) interactions link two
−
5
neighbouring zigzag double layers. The up and down double lay-
ers are connected by the C–HꢀꢀꢀO interactions in M . Overall, the
1
molecules of I are packed as typical herringbone architecture along
the crystallographic ab plane (Fig. 6)
3
.4.2. Molecular dimers in the crystal structure of II
Five energetically significant molecular pairs (M -M12) were ex-
8
tracted from the crystal packing of II using PIXEL energy analy-
sis (Table 4, Fig. S8). Among them, the most stabilized dimer in
II is formed by the molecular stacking interaction (Cg1ꢀꢀꢀCg2 and
Cg1ꢀꢀꢀCg4) and further supported by the C2–H2ꢀꢀꢀO2 interactions
with Etot: -17.9 kcal mol 1. In this dimer, the CgRAHBꢀꢀꢀCgRAHB in-
teraction is also formed between two pseudo-RAHB S(6) ring mo-
tifs. The strength of this dimer is comparable to that of simi-
lar dimers observed in symmetrically substituted geminal amido-
−
esters [25]. One of the oxygen atoms, O4 of C=O
group acts as
AAQ
an acceptor for the C20–H20ꢀꢀꢀO4 interaction in M . This interac-
9
tion links two neighbouring AAQ moiety using inversion symmetry
Fig. 6. The crystal structure of I viewed down the c axis.
2
and forms a centrosymmetric R (10) closed-loop with Etot: -8.9
2
kcal mol 1, see Fig. S8. Another O5 atom of C=OAAQ group involved
−
8

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 7. Part of the crystal packing of II (a) the C26-H26CꢀꢀꢀO5 interaction (M13 ) links neighbouring molecules into a molecular chain; (b) Two C–HꢀꢀꢀO interactions in M10
link the neighbouring molecules into molecular arrays which run parallel to crystallographically c axis.
Table 5
We performed energy framework analysis for I and II to get fur-
Lattice energy (in kcal mol 1) for compounds I and II.
−
ther insights into interaction topology and mechanical behaviour at
a molecular level [54,55]. The 3D topology of the energy frame-
work for both structures is shown in Fig. 9. The distinct 3D-
topology was observed for compounds I and II. In I, the larger
cylinders are arranged in vertical layers, while smaller cylinders
are arranged in a horizontal layer. Both vertical and horizontal lay-
ers are orchestrated into a ladder-like arrangement that runs par-
allel to the crystallographic b axis. As mentioned earlier, the crystal
packing of I is predominantly driven by the dispersion energy com-
ponent (see Table 4). The magnitude of cylinders in the Coulomb
energy component is smaller than the dispersion, and total inter-
action energy component. The larger cylinders in the vertical lay-
ers are representing the different interactions in M2-M3 and M6
dimers, and the smaller cylinders in the vertical layers are rep-
resenting the different interactions in M4 dimers. The small and
large cylinders are arranged alternately in vertical layers. Further,
the smaller cylinders in the horizontal layers are representing the
different interactions in M5 and M7 dimers. In II, the zig-zag large
green and blue cylinders correspond to molecular stacking in M8
dimer. The adjacent zig-zag large cylinders are interconnected by
small horizontal and diagonal cylinders correspond to the different
interactions in M10-M12. These zig-zag patterns run parallel to the
crystallographic b axis. Due to different substitutions in I and II,
they show distinct 3D- energy topology which in turn suggesting
different mechanical properties of these crystals.
Compound
Ecoul
Epol
EDisp
Erep
Etot
% EDisp
I
-8.50
-5.31
-14.44
-12.28
-50.26
-54.37
19.89
22.01
-53.32
-49.95
69
76
II
as an acceptor for the C26–H26CꢀꢀꢀO5 interaction (M12) with Etot:
−
2.8 kcal mol 1. This interaction links the neighbouring molecules
-
into a molecular chain that runs parallel to a axis (Fig. 7a).
The oxygen atoms, O1 and O2, of C=O
and C=Oester groups,
amide
respectively act as acceptors for the C19–H19ꢀꢀꢀO1=C9 and C3–
H3ꢀꢀꢀO2=C24 interactions (M ). These two interactions collec-
1
0
2
2
−1
,
tively generate a R (21) closed-loop with Etot: -6.0 kcal mol
and it links the neighbouring molecules into molecular arrays
which run parallel to the crystallographically c axis (Fig. 7b). In
addition to C–HꢀꢀꢀO interactions, the C–Hꢀꢀꢀπ interaction (C25–
H25AꢀꢀꢀCg4; M11 , Etot: -4.2 kcal mol−1) also make a dimer. The
dispersion energy component contributes 52-75% towards the sta-
bilization of dimers observed in II. Overall, the molecules of II
are arranged as columnar architecture and it extended along the
crystallographic b axis. This arrangement is favored by the C26-
H26CꢀꢀꢀO5 (M12) interaction. Further, the C19–H19ꢀꢀꢀO1=C9 and
C3–H3ꢀꢀꢀO2=C24 (M ) interactions and stacking interactions in
10
M
link the neighbouring columns into herringbone architecture
9
which is extended parallel to the crystallographic bc plane (Fig. 8)
3
.6. Topological analysis of non-covalent interactions in compound I
3
.5. Lattice energy and topology of energy frameworks
and II
The lattice energies for crystals I and II were calculated using
To evaluate the strength of various intermolecular interactions,
the PIXEL method and the results are summarized in Table 5. The
the topological parameters at their bond critical points (BCPs)
for these interactions were calculated and quantitatively analyzed.
These parameters are summarized in Tables S4-S5, and the molec-
ular graphs of various dimers are illustrated in Figs. S9-S10. There
−
1
results suggest that crystal I is 3.4 kcal mol
more stable than
crystal II. In the latter crystal, there is a 7% increase in the con-
tribution of dispersion energy component towards stabilization of
crystal structure. This increasing contribution of the dispersion en-
ergy component in II is due to the presence of stacking interactions
i.e. Cg ꢀꢀꢀCg ; Cg ꢀꢀꢀCg ; and CgRAHBꢀꢀꢀCgRAHB in M8.
2
are ten weak C–HꢀꢀꢀO interactions observed in I. The ρ, and ∇ ρ
values for these C–HꢀꢀꢀO interactions are observed in the range
˚
−3
˚ −5
of 0.036 – 0.068 e A , and 0.46 – 0.92 e A , respectively. The
1
2
1
4
9

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
Fig. 8. Crystal packing of II viewed two different orientations. All the H atoms have been omitted for clarity.
Fig. 9. Energy frameworks for the compound I-II. (a) molecular packing diagram of I showing the double layer arrangement (viewed down the c axis); (b) molecular packing
diagram of II showing the herringbone arrangement (viewed down the a axis). The red, green, and blue cylinders are representing Coulomb, dispersion, and total interaction
−1
energies. The same cylinder scale of 80 used for all diagrams and the energies with magnitude <15 kJ mol have been omitted for clarity.
10

P. Venkatesan, S. Thamotharan, M.J. Percino et al.
Journal of Molecular Structure 1246 (2021) 131210
electron density value for all C–HꢀꢀꢀO interactions observed in I
Acknowledgements
˚
−3
satisfies the Koch and Popelier limit (0.013 < ρ e A
> 0.236)
suggested for hydrogen bonds [56]. The dissociation energies (D.E.
(int)) for these interactions fall in the range of 0.90 – 1.96 kcal mol
The authors PV and AI thank the DST–India (FIST programme)
for the use of Bruker SMART APEX-II diffractometer and the
400 MHz NMR facility at the School of Chemistry, Bharathi-
dasan University. PV, ST, and MJP would also like to thank Lab-
oratorio Nacional de Supercoìmputo del Sureste (LNS-BUAP) for
the calculus service and to the PEZM NAT17-G (VIEP-BUAP) and
DSA/103.5/15/12684(PRODEP-SEP) projects.
−1,
˚
and the bond path distances (R ) vary from 2.453 – 3.091 A.
ij
2
The positive value of Laplacian (∇ ρ > 0), H > 0, and |-V/G| <
1
suggests that all the C–HꢀꢀꢀO interactions in I are closed-shell
in nature. In II, there are six weak C–HꢀꢀꢀO interactions observed
in different dimers. The positive electron density value, and other
2
three parameters (∇ ρ > 0, H > 0, and |-V/G| < 1) satisfy the C–
HꢀꢀꢀO interactions in II are closed-shell bonding type. There are
five C–HꢀꢀꢀC interactions observed in I-II. These C–HꢀꢀꢀC type in-
teractions also satisfy the above conditions, and these interactions
are in a closed-shell nature. The topology analysis suggested that
not only C–HꢀꢀꢀO interactions, and the C–HꢀꢀꢀC type interactions
also play an important role in the stabilization of these crystal
structures.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.131210.
References
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Two asymmetrically substituted geminal amido-esters, (2E)-
2
-[(2,5-dimethoxyphenyl)
(I) and
dihydroanthracen-1-yl)carbamoyl]-3-(phenylamino) prop-2-enoate
II) were synthesized and intramolecular RAHB and non-RAHB
carbamoyl]-3-[(4-nitrophenyl)amino]
prop-2-enoate
ethyl (2E)-2-[(9,10-dioxo-9,10-
[
(
[
and other weak non-covalent interactions observed in them
were characterized. Angularly fused pseudo tricyclic ring motifs
generated by intramolecular N–HꢀꢀꢀO hydrogen bonds and an in-
tramolecular C–HꢀꢀꢀO interaction were similar in compound I and
symmetrically substituted amido-ester derivatives reported earlier.
In contrast, fused pseudo pentacyclic ring motifs constructed by
intramolecular N–HꢀꢀꢀO, and C–HꢀꢀꢀO interactions in compound II.
The central structural motif in symmetrically substituted amido-
ester derivatives was derived from adjacent resonance assisted
pseudoring stacking. A similar pseudoring stacking interaction was
formed only in compound II. This study also demonstrated that
the crystal packing did not play a larger role in the modulation of
the strength of RAHB in asymmetrically substituted compounds
I and II, in contrast, to symmetrically substituted counterparts.
The PIXEL energy analysis suggested that compound I had slightly
higher stabilization energy than compound II. The contribution
of dispersion energy component towards stabilization of crystal
structure in II was 7 % higher than that of compound I, arising
out of stacking interactions including pseudoring stacking in II
whereas no stacking interactions found in I. The theoretical charge
density analysis reveals that C–HꢀꢀꢀO, and C–HꢀꢀꢀC(π) interactions
are closed-shell in nature. The crystal packing of I and II analyzed
based on the energy frameworks shows that these compounds
have a different 3D topology, suggesting these compounds may
have different mechanical behavior.
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Authorship contribution statement
11962, doi:10.1021/jp021336t.
Perumal Venkatesan: Conceptualization, Chemical synthesis,
Investigation, Writing – review & editing. Subbiah Thamotharan:
Conceptualization, Supervision, Investigation, Writing – review &
editing. M. Judith Percino: Resources, Data curation, Writing- re-
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Declaration of Competing Interest
The authors declare no potential conflict of interest.
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