An insight into the synthesis, crystal structure, geometrical modelling of crystal morphology, Hirshfeld surface analysis and characterization of N-(4-methylbenzyl)benzamide single crystals

Article abstract of DOI:10.1107/S1600576717012316

A versatile approach for the synthesis of N-(4-methylbenzyl)benzamide, C₁₅H₁₅NO, using CuI as catalyst has been reported. Single crystals of the synthesized compound were grown using the slow evaporation solution technique. The crystal structure of the N-(4-methylbenzyl)benzamide crystals has been determined by single-crystal X-ray diffraction. The compound crystallizes in an orthorhombic lattice, noncentrosymmetric space group Pna2₁. The crystal structure is stabilized by intermolecular N-H?O hydrogen bonds and weak C-H?π interactions to form layers parallel to the a axis. A user-friendly approach based on centre of mass propagation vector theory was used to predict the crystal morphology. The framework developed here utilizes the concept of intermolecular bond strength to discern the crystal morphology. Fourier transform IR, NMR and high-resolution mass spectrometry analytical techniques were used for the identification of functional groups and confirmation of the structure of the title compound. All of the intermolecular interactions present in the crystal structure, including the C-H?π, C-H?O and N-H?O interactions, were investigated and confirmed by molecular Hirshfeld surface analysis. From linear optical spectroscopy, the transmittance, optical band gap and UV cutoff wavelength were determined. The photoluminescence emission spectrum was recorded for a grown crystal. Dielectric measurements were performed at room temperature for various frequencies. The mechanical strength of the (001) plane of the title compound was measured using the Vickers micro-hardness technique. A piezo-coefficient of 15pCN^-1 was found along the (001) plane of the title crystals. The thermal stability and melting point were also investigated. In addition, density functional theory simulations were used to calculate the optimized molecular geometry and the UV-vis spectrum, and to determine the highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap. The results show that N-(4-methylbenzyl)benzamide is a potential candidate for multifunctional optical and piezoelectric crystals.

Full text of DOI:10.1107/S1600576717012316

research papers
An insight into the synthesis, crystal structure,
geometrical modelling of crystal morphology,
Hirshfeld surface analysis and characterization of
N-(4-methylbenzyl)benzamide single crystals
ISSN 1600-5767
a
a
b
c
c
Sahil Goel, Harsh Yadav, Nidhi Sinha, Budhendra Singh, Igor Bdikin,
d
d
a
Devarapalli Chenna Rao, Kovuru Gopalaiah * and Binay Kumar *
Received 16 May 2017
Accepted 24 August 2017
a
b
Crystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India, Department of
c
Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India, TEMA-NRD, Mechanical Engineering
d
Edited by F. Meilleur, Oak Ridge National
Laboratory, USA, and North Carolina State
University, USA
Department, Aveiro Institute of Nanotechnology (AIN), University of Aveiro, Aveiro 3810-193, Portugal, and Organic
Synthesis and Catalysis Laboratory, Department of Chemistry, University of Delhi, Delhi 110007, India. *Correspondence
e-mail: gopal@chemistry.du.ac.in, b3kumar69@yahoo.co.in
Keywords: N-(4-methylbenzyl)benzamide;
crystal structure; crystal morphology; centre of
mass propagation vector; Hirshfeld surface
analysis; photoluminescence; piezoelectricity.
A versatile approach for the synthesis of N-(4-methylbenzyl)benzamide,
C H NO, using CuI as catalyst has been reported. Single crystals of the
synthesized compound were grown using the slow evaporation solution
technique. The crystal structure of the N-(4-methylbenzyl)benzamide crystals
has been determined by single-crystal X-ray diffraction. The compound
crystallizes in an orthorhombic lattice, noncentrosymmetric space group
15
15
CCDC reference: 1524601
Supporting information: this article has
Pna2 . The crystal structure is stabilized by intermolecular N—Hꢀ ꢀ ꢀO hydrogen
1
supporting information at journals.iucr.org/j
bonds and weak C—Hꢀ ꢀ ꢀꢀ interactions to form layers parallel to the a axis. A
user-friendly approach based on centre of mass propagation vector theory was
used to predict the crystal morphology. The framework developed here utilizes
the concept of intermolecular bond strength to discern the crystal morphology.
Fourier transform IR, NMR and high-resolution mass spectrometry analytical
techniques were used for the identification of functional groups and
confirmation of the structure of the title compound. All of the intermolecular
interactions present in the crystal structure, including the C—Hꢀ ꢀ ꢀꢀ, C—Hꢀ ꢀ ꢀO
and N—Hꢀ ꢀ ꢀO interactions, were investigated and confirmed by molecular
Hirshfeld surface analysis. From linear optical spectroscopy, the transmittance,
optical band gap and UV cutoff wavelength were determined. The photo-
luminescence emission spectrum was recorded for a grown crystal. Dielectric
measurements were performed at room temperature for various frequencies.
The mechanical strength of the (001) plane of the title compound was measured
ꢁ
1
using the Vickers micro-hardness technique. A piezo-coefficient of 15 pC N
was found along the (001) plane of the title crystals. The thermal stability and
melting point were also investigated. In addition, density functional theory
simulations were used to calculate the optimized molecular geometry and the
UV–vis spectrum, and to determine the highest occupied molecular orbital/
lowest unoccupied molecular orbital energy gap. The results show that N-(4-
methylbenzyl)benzamide is a potential candidate for multifunctional optical and
piezoelectric crystals.
1
. Introduction
In the field of molecular electronics, the enormous demand for
low power consumption, cheap and flexible electronics, and
the design of energy-efficient harvesting devices is attracting
the attention of researchers to synthesize and develop energy-
efficient organic molecular crystals (Gundlach, 2007; Kippelen
&
Br e´ das, 2009; Klauk et al., 2007). The ability to exhibit high
symmetry, which allows monomeric organic molecules to form
large supramolecular chains of specific symmetry, qualifies
#
2017 International Union of Crystallography
1
498 https://doi.org/10.1107/S1600576717012316
J. Appl. Cryst. (2017). 50, 1498–1511

research papers
compounds based on such molecules as better alternatives to
organic polymer-based materials, helping to reduce the cost of
real world consumer electronics (Brooks, 2010). In organic
crystals, the intermolecular interactions present in the mol-
ecular packing greatly influence their optoelectronic and
piezoelectric properties. In natural as well as synthetic
chemicals, amide functionality constitutes an important scaf-
fold (Saha et al., 2014). The amide group acts as a basic
skeletal linkage in the peptide chain of proteins, pharmaceu-
ticals, polymers, agrochemicals and bio-potent organic mol-
ecules (Han & Kim, 2014; Nageswara Rao et al., 2014).
Because of the relatively inert and poor electrophilic nature of
the amide group, the nucleophiles need either harsh reaction
conditions or complex reaction mechanisms, which lead to the
transamidation of the amides with amines under high-
temperature conditions (Durgaiah et al., 2016). Although the
Ritter reaction is a well established and extensively used
method for the preparation of a wide range of amides, lactams
and other heterocycles, the classical experimental procedure is
cumbersome. Moreover, amides could be synthesized by
coupling of carboxylic acids, alcohols and aldehydes with
amines, hydroamination of alkynes, and hydration of nitriles
using stoichiometric quantities of reagents and harsh reaction
conditions (Nageswara Rao et al., 2014). Therefore, the cata-
lytic approach is in high demand to improve the method for
the synthesis of carboxamide derivatives (Nammalwar et al.,
gations have been carried out to determine the growth,
dielectric, piezoelectric, optical and mechanical properties of
the N-(4-methylbenzyl)benzamide crystal.
In the present work, we synthesized N-(4-methylbenzyl)-
benzamide (C H NO; MBB) from p-methyl benzylamine
15
15
and benzaldehyde under mild conditions using CuI as a
catalyst and tert-butylhydroperoxide (TBHP) as an oxidant.
The synthesized compound was crystallized by the slow
evaporation technique using hexane/ethyl acetate (EtOAc)
(5:1) solution and the obtained crystals were employed for
further experimental measurements. Herein, systematic
studies have been conducted to determine the growth and
crystal structure and to perform conformational and spectro-
scopic analysis of the MBB single crystals. The crystals were
characterized using Fourier transform infrared spectroscopy
1
13
(FTIR), H and C nuclear magnetic resonance, high-reso-
lution mass spectrometry (HRMS), UV–vis–NIR spectro-
scopy, photoluminescence spectroscopy, and dielectric,
piezoelectric, mechanical and thermal studies. In addition, a
visual identification of all the intermolecular interactions
present in the crystal packing was achieved using three-
dimensional Hirshfeld surfaces and two-dimensional finger-
print plot analysis. The theoretically optimized molecular
geometry, UV–vis absorbance spectrum and highest occupied
molecular orbital/lowest unoccupied molecular orbital
(HOMO–LUMO) energy levels were simulated using density
functional theory (DFT) and were compared with the
experimentally observed results.
2
015).
The molecular structure of N-(4-methylbenzyl)benzamide,
C H NO, consists of p-methylbenzylamine and benzoyl
In one of our previous reports, we described a simple
algorithm to predict the crystal morphology and the growth
rate of the crystal plane, derived from the propagation vector
of the centre of mass of the molecular basis present in the
crystal lattice (Yadav, Sinha & Kumar, 2016). The proposed
theory was successfully employed to predict the morphologies
of l-prolinium tartrate, benzophenone and benzil crystals,
which were found to be in good agreement with the experi-
mentally observed morphologies (Yadav, Sinha & Kumar,
2016). In the present report, the experimentally observed
morphology of the MBB crystals was studied on the basis of
our centre of mass propagation vector theory, denoted HNB
vector (Harsh–Nidhi–Binay). The morphology derived from
this theory was compared with the morphology predicted by
existing models [BravaisꢁFriedelꢁDonnayꢁHarker (BFDH)
law (Docherty et al., 1991) and HartmanꢁPerdok (HP) theory
(Hartman & Perdok, 1955)].
1
5
15
scaffolds. Benzamide compounds show various pharmacolo-
gical activities like antiarrythmic, antipsychotic and antiemetic
properties (B u¨ lb u¨ l et al., 2015). In the case of N-substituted
derivatives of benzamide, anticonvulsant activity is present
(Revathi et al., 2015). However, some other benzamide
compounds act as potent Kv1.3 inhibitors and Kv1.3 ion
channel blockers (Haffner, Thomson, Guo, Petrov et al., 2010;
Haffner, Thomson, Guo, Schaller et al., 2010). Recently,
Elbagerma et al. (2010) investigated the formation of co-
crystals from equimolar quantities of salicylic acid and
benzamide for the enhancement of the physiochemical prop-
erties of the active pharmaceutical ingredient without chan-
ging the pharmaceutical activity.
However, there are only a few reports available on the
synthesis of N-(4-methylbenzyl)benzamide (Karabulut &
Kacan, 2002; Kerdphon et al., 2015; Shen et al., 2008). In many
of these reports, harsh reaction conditions were employed for
the optimization of the reaction, using heavy metals as cata-
lysts. The literature also reports several similar crystal struc-
tures, such as N-(4-methylphenyl)benzamide (Gowda,
Tokar cˇ ı´ k, Ko zˇ ı´ sˇ ek & Sowmya, 2008), 2,6-dichloro-N-(4-
methylphenyl)benzamide (Wei et al., 2012), N-phenyl-
benzamide (Bowes et al., 2003), 2-methyl-N-(4-methyl-
phenyl)benzamide (Gowda, Tokar cˇ ı´ k, Ko zˇ ´ı sˇ ek, Sowmya &
Fuess, 2008), 2-methyl-N-p-tolylbenzamide (Saeed et al., 2010)
etc. Luo & Huangn (2004) have reported the solid phase
synthesis and crystal structure of N-(4-methylbenzyl)-
benzamide. However, to our knowledge, no further investi-
2
. Experimental
2.1. Synthesis and crystal growth
To a suspension of 4-methylbenzylamine (1.0 mmol)
dissolved in dry dimethyl sulfoxide in a round-bottomed flask,
benzaldehyde (1.0 mmol) and CuI (5 mol%) were added at
room temperature (RT) under nitrogen atmosphere. The
reaction mixture was stirred at 353 K for 6 h. Then, the flask
was cooled to room temperature, and TBHP (1.0 mmol) was
added dropwise. The resulting mixture was heated at 353 K for
ꢂ
J. Appl. Cryst. (2017). 50, 1498–1511
Sahil Goel et al.
N-(4-Methylbenzyl)benzamide 1499

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1
0 h. The reaction flask was cooled to room temperature and
aid of the TONTO program using a STO-3G basis set
(Hartree–Fock theory) (Spackman et al., 2008). The empty
space in the crystal lattice was analysed using void mapping
(CrystalExplorer 3.1). The FTIR spectrum for the MBB
the crude material was extracted with ethyl acetate (15 ml)
and water (25 ml). The organic layer was separated and dried
over anhydrous sodium sulfate, filtered off, and concentrated
under reduced pressure. The crude compound was purified by
silica gel column chromatography using hexane/EtOAc as
mobile phase, resulting in N-(4-methylbenzyl)benzamide in
ꢁ
1
crystals was recorded in the infrared range 4000–600 cm
employing a PerkinElmer RXI FTIR spectrophotometer in
1
dichloromethane solution. The H NMR spectrum was
8
2% yield as a colourless solid. The scheme below is a
recorded on a JEOL 400 MHz spectrometer (JNM-EXCP
400) in CDCl ; chemical shifts (ꢃ p.p.m.) and coupling
graphical representation of the reaction described above. The
product isolated by column chromatography was dissolved in
a hexane/EtOAc (5:1) solvent system. The solution was kept
for slow evaporation. After a time span of 2 d, fine needle-
shaped crystals suitable for our studies were harvested.
Photographs, a scanning electron microscope image and the
indexed graphical morphology are shown in Fig. 1.
3
constants (Hz) are reported in the standard fashion with
reference to internal standard tetramethylsilane (ꢃ
=
H
13
0.00 p.p.m.). The C NMR spectrum was recorded on a
100 MHz spectrometer (JNM-EXCP 400); chemical shifts (ꢃ
p.p.m.) are reported relative to CDCl (ꢃ = 77.00 p.p.m.,
3
C
central line of triplet). The high-resolution mass spectrum was
recorded on an electrospray quadrupole time-of-flight mass
spectrometer [Agilent, G6530AA (LC-HRMS-Q-TOF)]. The
UV–vis transmittance spectrum was measured using a
SHIMADZU UV-2501PC in the wavelength range 200–
1100 nm. The photoluminescence (PL) emission spectrum of
the solid material was measured on a Horiba Jobin Yvon
Fluorolog modular spectro-fluorometer using a continuous-
wave xenon lamp source. An Agilent E4980A LCR meter was
used to measure the dielectric properties at room temperature,
with a frequency range of 20 Hzꢁ2 MHz. The piezo-coeffi-
cient of the MMB crystals was measured using a piezometer
(PM 300, Piezotest) at a tapping force and tapping frequency
of 0.25 N and 110 Hz, respectively. A Vickers micro-hardness
MVH-I (Omni Tech) tester was used to characterize the
mechanical strength of the material under a fixed dwell time of
5 s. Thermogravimetric analysis on the MBB crystals was
performed using a PerkinElmer Diamond thermogravimetric/
differential thermal analysis (TG/DTA) analyser in a nitrogen
2
.2. Characterization techniques
A single-crystal X-ray diffraction (SCXRD) analysis of the
MBB crystals was performed in the ! scan mode on an Oxford
Xcalibur diffractometer (Mo Kꢁ X-ray source, ꢂ = 0.71073 A).
A single crystal of dimension 0.5 ꢃ 0.5 ꢃ 0.5 mm was selected
for SCXRD measurement. Hirshfeld surfaces for the title
crystal mapped over normalized contact distance, shape index,
curvedness and electrostatic potential were generated using
CrystalExplorer 3.1 (Wolff et al., 2012). The electrostatic
potentials were mapped over the Hirshfeld surface with the
˚
ꢁ
1
atmosphere at a heating rate of 10 K min in the temperature
range from 308 to 723 K.
2.3. Computational details
All DFT calculations were performed using the GAMESS
software with a B3LYP hybrid exchange functional and the
-31G(d,p) basis set for all atoms in the MBB molecule
Schmidt et al., 1993). No geometrical constraints were
6
(
imposed during the optimization of the MBB molecule. The
UV–vis spectroscopic data (oscillator strength) of the MBB
molecule were computed with the time-dependent (TD-DFT)
approach. Furthermore, the HOMOꢁ1, HOMO, LUMO and
LUMO+1 energy levels, their shapes, and their energy gaps
were simulated using the B3LYP/6-31G(d,p) basis set.
3
. Results and discussion
3.1. Single-crystal X-ray diffraction analysis
The crystallographic study of MBB was performed by an
SCXRD analysis. A transparent single crystal of dimensions
Figure 1
(a), (b) Photographs of as-grown MBB single crystals. (c) High-
0.5 ꢃ 0.5 ꢃ 0.5 mm was chosen to collect the structural
magnification scanning electron microscope image of an MBB crystal
along with Miller indexing of various crystal faces. (d) Indexed graphical
morphology of the MBB crystal.
information. The structure of the title compound was
solved by direct methods using the OLEX2 software and
ꢂ
1
500 Sahil Goel et al.
N-(4-Methylbenzyl)benzamide
J. Appl. Cryst. (2017). 50, 1498–1511

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Table 1
Crystal data and structure refinement for the MBB single crystal.
probability level and H atoms shown as capped sticks for
clarity, is given in Fig. 2. The crystal structure of the title
compound includes intramolecular C—Hꢀ ꢀ ꢀO hydrogen
bonds. The –NH—(C O)– amide group subtends dihedral
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
15
C H15NO
225.28
293 (2)
ꢄ
angles of 19.52 (15) and 97.5 (3) with the benzoyl ring and the
para-substituted benzyl ring, respectively. The two aromatic
Orthorhombic
Pna2
1
˚
ꢄ
a (A)
9.5277 (8)
11.1555 (11)
11.7848 (10)
90
90
90
1252.6 (2)
4
1.195
0.075
480.0
rings are noncoplanar [dihedral angle = 83.65 (17) ]. These
˚
b (A)
˚
values of dihedral angles for the title crystal differ from those
for several aryl amide crystals reported in the literature. The
reported molecular structures of various aryl amide crystals
differ in the inclination of their central amide group with
c (A)
ꢁ
ꢄ
( )
ꢅ ( )
ꢄ
ꢄ
ꢆ ( )
Volume (A )
˚
3
ꢄ
Z
respect to the benzoyl ring [dihedral angles of 19.52 (15) for
ꢄ
ꢁ
3
ꢇcalc (g cm
)
the title crystal (present work), 20.5 (1) for N-(4-methyl-
ꢁ
1
ꢈ (mm
F(000)
)
ˇ
phenyl)benzamide (Gowda, Tokar cˇ ı´ k, Ko zˇ ´ı sek & Sowmya,
3
ꢄ
Crystal size (mm )
Radiation
0.5 ꢃ 0.5 ꢃ 0.5
2008), 59.96 (11) for 2-methyl-N-(4-methylphenyl)benzamide
(
Gowda, Tokar cˇ ı´ k, Ko zˇ ı´ sˇ ek, Sowmya & Fuess, 2008) and
˚
Mo Kꢁ (ꢂ = 0.71073 A)
6.602–58.964
ꢄ
2ꢉ range for data collection ( )
ꢄ
55.99 (7) for 2-methyl-N-p-tolylbenzamide (Saeed et al.,
2010)] and in the relative orientations of the two aromatic
Index ranges
ꢁ12 ꢅ h ꢅ 11
ꢁ
14 ꢅ k ꢅ 14
16 ꢅ l ꢅ 16
ꢁ
ꢄ
ꢄ
rings [97.5 (3) for the title crystal (present work), 63.41 (5)
for N-(4-methylphenyl)benzamide (Gowda, Tokar cˇ ı´ k, Ko zˇ ı´ sˇ ek
Reflections collected
Independent reflections
Data/restraints/parameters
16 911
3189 (Rint = 0.1078, Rsigma = 0.1256)
ꢄ
3189/1/156
0.972
& Sowmya, 2008), 60.9 (3) for 2,6-dichloro-N-(4-methyl-
ꢄ
2
Goodness of fit on F
Final R indexes [I ꢆ 2ꢄ(I)]
phenyl)benzamide (Wei et al., 2012), 81.44 (5) for 2-methyl-
N-(4-methylphenyl)benzamide (Gowda, Tokar cˇ ı´ k, Ko zˇ ı´ sˇ ek,
R
R
1
1
= 0.0633, wR
= 0.1902, wR
2
= 0.0763
= 0.1079
Final R indexes (all data)
2
ꢁ
3
)
ꢄ
˚
Largest difference peak/hole (e A
Flack parameter
0.12/ꢁ0.12
Sowmya & Fuess, 2008) and 88.67 (8) for 2-methyl-N-p-
tolylbenzamide (Saeed et al., 2010)].
ꢁ1.9 (10)
A portion of the molecular network for the MBB
compound is depicted in Fig. 3. From the figure, it is clear that
MBB molecules are connected through intermolecular N—
Hꢀ ꢀ ꢀO hydrogen bonds, which link the molecules into infinite
subsequently refined by the SHELX97 program using the full-
matrix least-squares technique (Sheldrick, 2015; Dolomanov
et al., 2009). The non-H atoms were refined using anisotropic
displacement parameters and H atoms were treated as riding
with isotropic displacement parameters. The final full-matrix
2
least-squares refinement on F with minimization of the
2
P
function w(|F | ꢁ |F |) converged to R = 0.0633 [for I >
o
c
1
2
2
ꢄ(I)] and wR = 0.1079 (F , all data). The crystallographic
2
parameters of the grown crystal with discrepancy indices are
listed in Table 1. The MBB compound crystallizes in a
noncentrosymmetric space group (Pna2 ) of an orthorhombic
1
lattice, which is a prerequisite for nonlinear optical and
piezoelectric characteristics. A displacement ellipsoid style
plot of the title compound, with ellipsoids shown at the 50%
Figure 2
Displacement ellipsoid representation of the MBB compound with
atomic labelling scheme. Displacement ellipsoids for non-H atoms are
drawn at the 50% probability level. H atoms are represented as green-
capped sticks for clarity. C, N and O atoms are displayed as dark-grey,
blue and red ellipsoids, respectively.
Figure 3
C—Hꢀ ꢀ ꢀꢀ interactions (red dashed lines) and N—Hꢀ ꢀ ꢀO hydrogen bonds
(black dashed lines) in the MBB compound, linking the molecules in
head-to-head fashion along a. The yellow and pink spheres represent ring
centroids.
ꢂ
J. Appl. Cryst. (2017). 50, 1498–1511
Sahil Goel et al.
N-(4-Methylbenzyl)benzamide 1501

research papers
Table 2
Experimentally observed and theoretically optimized bond lengths for
MBB.
Table 3
Experimentally observed and theoretically optimized bond angles for
MBB.
˚
Length (A) (SCXRD) Length (A) (DFT) Deviation (A)
˚
˚
ꢄ ꢄ ꢄ
Angle ( ) (SCXRD) Angle ( ) (DFT) Deviation ( )
C5—C6—C7
C1—C6—C7
C1—C6—C5
C9—C14—C13
C14—C9—C8
C10—C9—C14
C10—C9—C8
C7—N1—C8
122.7 (5)
119.1 (5)
118.2 (5)
121.3 (5)
119.6 (5)
117.8 (5)
122.5 (5)
123.1 (4)
123.4 (6)
117.4 (6)
119.2 (6)
120.9 (6)
121.0 (6)
118.2 (6)
120.8 (6)
122.4 (5)
121.1 (6)
121.4 (6)
122.2 (6)
116.3 (6)
120.4 (6)
120.4 (6)
111.5 (6)
120.8 (6)
120.9 (6)
117.8 (6)
121.3 (6)
121.2 (6)
119.8 (5)
120.1 (5)
120.1 (6)
0.7
ꢁ1.7
1.0
O1—C7
C6—C7
C6—C5
1.227 (5)
1.498 (6)
1.385 (6)
1.361 (6)
1.376 (6)
1.390 (6)
1.515 (6)
1.370 (6)
1.331 (5)
1.462 (5)
1.369 (6)
1.381 (6)
1.386 (6)
1.383 (6)
1.367 (6)
1.506 (6)
1.371 (6)
1.364 (6)
1.232 (6)
1.507 (5)
1.403 (5)
1.404 (5)
1.401 (6)
1.398 (6)
1.515 (5)
1.402 (5)
1.368 (6)
1.462 (6)
1.402 (5)
1.397 (6)
1.395 (5)
1.396 (6)
1.403 (6)
1.512 (6)
1.399 (6)
1.398 (6)
0.005
0.009
0.018
0.043
0.025
0.008
0.000
0.032
0.037
0.000
0.033
0.016
0.009
0.013
0.036
0.006
0.028
0.034
ꢁ0.4
C6—C1
1.4
0.4
C14—C9
C14—C13
C9—C8
C9—C10
N1—C7
N1—C8
C13—C12
C5—C4
ꢁ1.7
ꢁ0.7
0.3
C12—C13—C14 120.8 (5)
O1—C7—C6
O1—C7—N1
N1—C7—C6
C4—C5—C6
C6—C1—C2
N1—C8—C9
C9—C10—C11
C13—C12—C15 121.7 (5)
C11—C12—C13 117.4 (5)
C11—C12—C15 120.9 (6)
C12—C11—C10 122.4 (5)
119.7 (5)
122.6 (5)
117.7 (4)
120.6 (5)
121.3 (5)
112.2 (4)
120.3 (5)
1.7
ꢁ0.4
ꢁ1.4
ꢁ0.2
ꢁ0.9
ꢁ0.7
0.5
C1—C2
C10—C11
C12—C11
C12—C15
C3—C2
ꢁ0.8
0.4
0.4
C3—C4
ꢁ1.2
C4—C3—C2
C3—C2—C1
C3—C4—C5
119.2 (5)
120.1 (5)
120.7 (5)
0.6
0.0
supramolecular chains running along the a axis. Also, the
crystal structure is stabilized by weak intermolecular C—
Hꢀ ꢀ ꢀꢀ interactions. From Fig. 4, we can see that there are no
directional interactions present between any two supramol-
ecular layers stacked along the a axis. The FTIR, H NMR, C
NMR and HRMS spectra for the obtained MBB crystals are
available as supporting information (Figs. S1–S3).
The displacement parameters and fractional atomic posi-
tions for the grown crystal are provided in CIF format in the
supporting information. Tables 2 and 3 summarize the bond
lengths and bond angles of the title molecule, respectively. The
hydrogen-bonding geometry is summarized in Table 4.
ꢁ0.6
Table 4
Hydrogen-bonding parameters of MBB (A, ).
1
13
˚
ꢄ
Bond
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
i
C5—H5ꢀ ꢀ ꢀN1
0.930
0.930
0.970
0.930
0.930
0.860
0.930
2.617
2.503
2.395
2.774
2.610
2.076
2.577
2.894
2.793
2.795
2.981
3.468
2.882
3.374
97.82
98.31
104.15
93.62
153.65
155.69
143.99
i
i
C1—H1Aꢀ ꢀ ꢀO1
C8—H8Aꢀ ꢀ ꢀO1
i
C10—H10ꢀ ꢀ ꢀN1
ii
C5—H5ꢀ ꢀ ꢀO1
ii
N1—H1ꢀ ꢀ ꢀO1
iii
C2—H2ꢀ ꢀ ꢀO1
1
2
3
2
1
2
Symmetry codes: (i) x, y, z; (ii) x + , ꢁy ꢁ , z; (iii) ꢁx, ꢁy ꢁ 1, z + .
Table 5
Morphological importance of the MBB crystal based on the BFDH law
and the HNB propagation vector theory.
Calculated
relative
growth rates
Morphological
importance
Morphological Relative
importance growth rate by propagation
Faces
(
d
hkl
˚
hkl) (A)
from BFDH law by BFDH law by Rhkl
vector
(001) 11.7848 1.0000
(010) 11.1555 1.0564
(100) 9.5277 1.2369
1.0000
0.9466
0.8085
1.0000 1.0000
4474.2729 0.0002
53.2653 0.0188
3.2. Morphological studies
The correct prediction of the crystal morphology helps one
to explore its efficiency and applicability in sensor and device
applications (Singh et al., 2012). According to the BFDH law,
the growth rate of a crystal plane (hkl) is inversely dependent
on its interplanar spacing (dhkl) (Docherty et al., 1991). On the
basis of the BFDH law, Table 5 displays the relative growth
rate and morphological importance (MI) of various crystal
planes of the MMB crystal. Fig. 1(d) shows the indexed
Figure 4
A molecular packing view of the MBB compound with one layer in space-
fil i ln l ie ns .g mode. The N—Hꢀ ꢀ ꢀO hydrogen bonds are shown as blue dashed
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1
morphology of the MBB crystal simulated using the WinX-
Morph software (Kaminsky, 2005).
Rhkl
¼
;
ð1Þ
jR ꢀ ðh ^i þ k ^j þ l k^ Þj
P
Besides the geometrical (BFDH) explanation, Hartman &
Perdok (1955) proposed a theoretical model based on bond
energies that successfully explains discrepancies in the BFDH
law and correctly determines the crystal morphology. In HP
theory, the uninterrupted periodic chains present in the crystal
system are identified using information on the internal
bonding present between the molecules of the crystal. These
uninterrupted periodic chains of strong bonds are known as a
periodic bound chain (PBC) vectors. The various faces present
in the crystal system are classified into three types: F faces
where R is the propagation vector of the centre of mass and
P
h, k and l are the Miller indices of the crystal plane. Fig. 6
shows two supramolecular chains (layer 1, L1, and layer 2, L2)
in the MBB crystal lattice running parallel to the a crystal-
lographic axis. The N—Hꢀ ꢀ ꢀO, C—Hꢀ ꢀ ꢀO and O—Hꢀ ꢀ ꢀO
intermolecular interactions are depicted using blue dashed
lines in Fig. 6.
The arrangement of the MBB molecule is different in these
two layers (L1 and L2). In layer L1 (blue ellipse), the mol-
ecular basis consists of the two MBB molecules shown in
yellow (see Figs. 6 and 7). The molecular basis of layer L2
(brown ellipse) consists of the two MBB molecules shown in
purple in Figs. 6 and 7. The centres of mass of molecules 1 and
2 of layer L1 are depicted by orange and green balls, respec-
tively, in Fig. 7 and their coordinates are computed to be
(flat; parallel to two or more PBC vectors), S faces (stepped;
parallel to one or more PBC vector) and K faces (kinked; do
not contain any PBC vectors). The order of MI associated with
the occurrence of crystal faces is F > S > K. In the case of MBB
crystals, the (001) plane contains the largest number (nine) of
PBC vectors, which are depicted by light-blue, light-green,
dark-blue, red, purple, orange, yellow, light-pink and dark-
green arrows in Fig. 5. Therefore, the (001) plane belongs to
the F face, which makes it the most visible in the crystal
morphology. The projections of the light-green, red, light-blue,
yellow and dark-green arrows are more likely to be along the
^
^
^
r11 ¼ 0:259989i þ 0:840827j þ 0:118092k;
r12 ¼ 0:759985i þ 0:659161j þ 0:122566k:
ð2Þ
^
^
^
The value of the propagation vector of layer L1 is computed to
be
(010) plane, which indicates that the (010) plane belongs to the
S-face category. The (100) crystal plane also belongs to the S-
face category owing to the projection of the dark-blue, purple,
light-pink and orange arrows. Therefore, HP theory predicts
the (001) plane of the MBB crystals to be of greater
morphological importance than the other faces.
r
¼ r12 ꢁ r11
;
1
ð3Þ
^
^
^
r ¼ 0:499996i ꢁ 0:181666j þ 0:004474k:
1
The dynamics of a rigid body are connected with its centre
of mass. In our previous report, we proposed a theory (HNB
vector) to predict the crystal morphology (Yadav, Sinha &
Kumar, 2016). According to the HNB model, the propagation
of the centre of mass of the molecular basis of a crystal system
allows us to define the crystal morphology (Yadav, Sinha &
iKs ud me fia nr ,e 2d 0 1a 6s ). The growth rate ðRhklÞ of the crystal plane (hkl)
Figure 6
Packing diagram of the MBB crystal along the a axis. Brown and blue
ellipses delimit the two supramolecular chains running parallel to the a
axis. The molecular basis units of layers 1 and 2 are shown in yellow and
purple, respectively.
Figure 5
Molecular packing diagram of the MBB crystal along with PBC vectors. a,
b and c denote the three crystallographic axes.
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Figure 8
The theoretically optimized molecular structure of the MBB compound.
3.4. Analysis of the Hirshfeld surfaces
Hirshfeld surface analysis is a three-dimensional graphical
tool which helps one understand all the intermolecular inter-
actions present in the crystal lattice (Spackman & Byrom,
Figure 7
Unit-cell packing diagram of the MBB crystal. Calculation of propagation
vector R based on the centre of mass of the molecular basis.
P
1997; Hirshfeld, 1977). The colour code information on the
Hirshfeld surface provides an easy way to identify various
kinds of different intermolecular interactions (Spackman &
Jayatilaka, 2009). Generally, the Hirshfeld surface covers both
the electron density isosurface (0.002 a.u.) and the Corey–
Pauling–Koltun hard-sphere model (Yadav et al., 2014). In this
analysis, the structure partitioning scheme is based on dividing
the crystal electron density distribution into discrete mole-
cular fragments. The weight function for a molecular fragment
of interest is defined as
In order to estimate the value of the propagation vector in
layer L2, the same formalism as described above was used, and
its value is computed to be
^
^
^
r ¼ ꢁ0:500080i þ 0:181665j þ 0k:
ð4Þ
2
Finally, the resulting propagation vector is evaluated as
promolecule
procrystal
wðrÞ ¼ ꢇ
ðrÞ=ꢇ
ðrÞ;
ð6Þ
R ¼ r þ r ;
P
1
2
ð5Þ
ꢁ4
ꢁ4
ꢁ4
^
procrystal
^
^
R ¼ ꢁ0:84 ꢃ 10 i ꢁ 0:01 ꢃ 10 j þ 44:74 ꢃ 10 k:
where ꢇ
ðrÞ denotes the sum of spherical atom electron
P
promolecule
densities for the whole crystal (procrystal) and ꢇ
the same sum for a molecule (promolecule). Thus,
ðrÞ is
Table 5 lists the relative growth rate and MI of various crystal
planes of the MBB crystal based on the BFDH law and HNB
propagation vector theory. The experimentally observed
morphology of the MBB crystal suggests that the (001) plane
is of the highest MI, which is also explained by the BFDH law,
HP theory and HNB propagation vector theory.
P
P
wðrÞ ¼
ꢇiðrÞ=
ꢇiðrÞ;
ð7Þ
i2molecule A
i2crystal
where ꢇ
density located at the ith nucleus (Yadav, Sinha, Goel et al.,
016; Spackman & Byrom, 1997). The weight function w(r) is a
ðrÞ is the spherically averaged atomic electron charge
i
2
continuous function and its value lies in the range 0 < w(r) < 1.
The molecular surfaces are generated across the molecule by
integrating over the weighted electron density, w(r)ꢇ(r). The
isosurface which is constructed by considering w(r) = 0.5 and
covers the 0.002 a.u. electron density isosurface is known as
the Hirshfeld surface.
3
.3. Optimized geometry
The optimized molecular geometry of MBB was calculated
by solving the self-consistent field equation for attaining true
minima on the potential energy surface and is shown in Fig. 8.
Tables 2 and 3 list the calculated bond length and bond angle
values along with the experimental SCXRD data for the MBB
compound. From Table 2, we can conclude that the C6—C1
bond has the largest deviation in bond length value (0.043 A).
Likewise, the maximum deviation in bond angle value was
found as 1.7 for the C10—C9—C8, C1—C6—C7 and O1—
C7—C6 bond angles (Table 3). This small discrepancy
between the optimized molecular structure and its XRD
counterpart may be attributed to packing interactions within
the crystal lattice, which were not taken into account during
computational studies.
d and d represent the contact distances of nearest atoms
i
e
inside and outside, respectively, the Hirshfeld surface. In order
to enable both donor and acceptor interactions on the same
molecular Hirshfeld surface, the concept of normalized
contact distance (dnorm) was incorporated (McKinnon et al.,
˚
ꢄ
2007). dnorm is expressed as a function of d , d and the van der
Waals (vdW) radii of atoms as follows:
i
e
vdW
i
vdW
vdW
e
d ꢁ r
d ꢁ r
i
e
dnorm
¼
þ
;
ð8Þ
vdW
r
r
e
i
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vdW
vdW
where re and ri denote the vdW radii of atoms external
and internal to the molecular surface, respectively. A specific
colour code is chosen to represent the intermolecular contacts
on the molecular Hirshfeld surface as follows: red for vdW <
dnorm, blue for vdW > dnorm and white for intermediate
distance. In this fixed colour scale scheme, the dnorm value is
acceptor and donor of the dominating N—Hꢀ ꢀ ꢀO hydrogen
bonds. These are seen as red and blue regions in the Hirshfeld
surface mapped over electrostatic potential in Fig. 11 and
correspond to negative and positive potentials, respectively.
The other four faint red spots in Figs. 9 and 10 are due to the
presence of comparatively weak C—Hꢀ ꢀ ꢀO interactions. The
blue region on the Hirshfeld surface depicts the comparatively
weak intermolecular interactions like Hꢀ ꢀ ꢀH.
0
.0 for white, 1.10 (positive) for blue and ꢁ0.75 (negative) for
red.
Figs. 9 and 10 represent two different views of the Hirshfeld
surface mapped over dnorm for the MBB crystal along with
close contacts with the neighbouring molecules. In each figure,
the bright-red spot on the Hirshfeld surface appearing near
carbonyl oxygen O1 and amine hydrogen H1 represents the
Figure 11
A view of the Hirshfeld surface mapped over the electrostatic potential.
Figure 9
A view of the Hirshfeld surface mapped over dnorm, depicting N—Hꢀ ꢀ ꢀO
ea nn vd ir Co n— m He ꢀnꢀt.ꢀO interactions of the MBB molecule with its immediate
Figure 12
(
a) A two-dimensional fingerprint plot depicting all the intermolecular
interactions present in the MBB crystal structure. (b)–(e) The individual
contributions from different contacts to the Hirshfeld surface of the title
compound.
Figure 10
A different view of the Hirshfeld surface mapped over dnorm
.
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The local shape efficiency of three-dimensional Hirshfeld
surfaces can be assessed by using tools that measure surface
curvature such as the shape index and curvedness map. The
shape index (S) is a function of the principal curvatures (kmax
and kmin) and is expressed as (Roberts, 2001)
ꢀ
ꢁ
2
kmax þ k
kmax ꢁ k
min
min
S ¼ arctan
:
ð9Þ
ꢀ
The map of the shape index on the Hirshfeld surface can be
used to represent the local morphology in terms of colour-
coded information, i.e. complementary bumps (blue) and
hollow regions (red). It provides a visual identification of the
regions where two molecular Hirshfeld surfaces touch one
another. Fig. 14(a) illustrates the front and rear sides of the
molecular Hirshfeld surface of the title molecule mapped over
shape index. The bright red sites identified with arrows 1, 2
ra en sdp e 3c t ii vn ed l iyc .ate the Cꢀ ꢀ ꢀH, Oꢀ ꢀ ꢀH and Hꢀ ꢀ ꢀH interactions,
Figure 13
Relative contributions of various intermolecular contacts for several aryl
amide crystals. (1) N-(4-Methylbenzyl)benzamide (present work), (2)
0
(E)-4-amino-N -[1-(4-methylphenyl)ethylidene]benzohydrazide (Sivajey-
anthi et al., 2017), (3) N-(2-aminophenyl)-2-methylbenzamide (Nagaraju
et al., 2016), (4) 4-methyl-N-(2-oxido-1,3,2-diazaphosphinan-2-yl)benza-
mide (Saneei et al., 2017) and (5) N-{bis[(3,4-dimethylphenyl)amino]-
phosphoryl}-4-methylbenzamide (Saneei et al., 2017).
The map of curvedness (C) gives a measure of the total
curvature present within the surface and is defined by
A fingerprint plot of d versus d (Spackman & McKinnon,
e
i
ꢀ
ꢁ
1=2
2002) provides a convenient two-dimensional summary of the
2
max
2
min
2
C ¼ ln
ꢀ
k
þ k
:
ð10Þ
intermolecular interactions in the crystal system. Fig. 12(a)
displays the two-dimensional fingerprint plot for the title
molecule. It involves all the intermolecular interactions
present within the grown crystal structure. The overall two-
dimensional fingerprint plot is decomposed into Hꢀ ꢀ ꢀH,
Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC, Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO and Nꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀN to quantify
their relative contributions within the structure (Figs. 12b–
2
The magnitude of C depends on the value of r.ms. curvature
of the surface and on the length scale (McKinnon et al., 2004;
Inan & Halici, 2012). Maps of C are typically characterized by
large flat regions of green colour separated by blue edges.
From Fig. 14(b), we see that small segments of flat green
regions are delineated by blue outlines, confirming the
absence of ꢀ–ꢀ stacking interactions. This is also evident from
the fact that there is zero contribution from Cꢀ ꢀ ꢀC contacts to
the Hirshfeld surface (see Fig. 12).
The porous properties of crystalline materials play a
considerable role in deciding various anisotropic properties of
single crystals. Void mapping for the as-grown crystals was
performed to visualize and locate the void space present in the
1
2e). Fig. 12(b) shows that the Hꢀ ꢀ ꢀH interactions make the
most significant contribution (57.4%) to the Hirshfeld surface.
These interactions play an important role in the molecular
packing of the MBB crystal. The 31.4% contribution from
Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC contacts (Fig. 12c) is due to the presence of short
C—Hꢀ ꢀ ꢀꢀ intermolecular interactions. In the packing of the
molecular crystal, C—Hꢀꢀꢀꢀ interactions play an important
role and can affect the optical properties. The pair of long
˚
spikes with tips at d + d ’ 1.95 A in
e
i
Fig. 12(c) are the result of short inter-
atomic N—Hꢀ ꢀ ꢀO contacts (Table 4).
This significant contribution from
Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO contacts (10.3%) to the
Hirshfeld surface is also due to the
presence of C—Hꢀ ꢀ ꢀO and N—Hꢀ ꢀ ꢀO
hydrogen bonds. These interactions
decide the electric properties of the
crystal, such as dielectric constant,
piezoelectricity etc. Hence, the struc-
tural geometry of the crystal is
connected to its physical properties and
also indicates the potential future role
of the material in various related
applications. The relative contributions
of various intermolecular contacts for
several aryl amide crystals reported in
the literature are compared in a bar
chart in Fig. 13.
Figure 14
a) Hirshfeld surfaces of the MBB molecule with shape index mapped onto the surface. Blue and red
regions denote bumps and hollow regions, respectively. (b) Hirshfeld surfaces with curvedness
(
mapped onto the surface to recognize the curved and planar regions in the MBB molecule.
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crystal lattice and to compute volumes and surface areas of
void space and porosity (Turner et al., 2011). Fig. S4 in the
supporting information displays the unit-cell packing diagram
of the MBB compound with voids mapped at the 0.002 a.u.
isosurface along the crystallographic a axis. The void channels
were found to occupy 13.5% of the unit-cell volume with a
Table 6
The calculated quantum molecular descriptors of the MBB molecule.
Properties
b3lyp/6-311g**
E
E
HOMO (eV)
LUMO (eV)
ꢁ6.5144
ꢁ1.2001
5.314
6.514
1.200
Energy band gap |EHOMO ꢁ ELUMO| (eV)
2
Electron affinity (I = ꢁE
) (eV)
˚
HOMO
surface area of 547.57 A . From void analysis, we can conclude
Electron affinity (A = ꢁELUMO) (eV)
Chemical hardness [ꢋ = (I ꢁ A)/2] (eV)
Electronegativity [ꢌ = (I + A)/2] (eV)
Chemical potential [ꢈ = ꢁ(I + A)/2] (eV)
that cavity channels are uniformly distributed in the crystal
lattice, which shows the scope of binding of the title compound
with metal ions to alter its physical properties.
2.657
3.857
ꢁ3.857
ꢁ
1
)
Softness (ꢍ = 1/2ꢋ) (eV
0.188
2.799
2
Electrophilicity index (! = ꢌ /2ꢋ) (eV)
3.5. UV–vis–NIR spectral and HOMO–LUMO analysis
Fig. 15(a) illustrates the experimental UV–vis–NIR absor-
by the TD-DFT method and is displayed in Fig. 15(b). The
theoretically calculated value of the strong absorption peak
was computed as 248 nm/5.01 eV/0.102 (wavelength/excitation
energy/oscillator strength), which agrees well with the
experimental value.
bance spectrum of an MBB crystal in the wavelength range
from 200 to 800 nm. The grown crystal was observed to exhibit
minimum optical absorbance in the visible region and the
cutoff wavelength was found to be at 260 nm. This absorption
band observed at 260 nm can be assigned to n–ꢀ* transitions
and to excitation in the C O group and the aromatic rings
The optical band gap of the grown crystal was estimated
n
using Tauc’s relation: (ꢁhꢊ) = A(hꢊ ꢁ E ), where ꢊ represents
g
(
Crasta et al., 2004). Intriguingly, we observed from the
the incident frequency, h denotes Planck’s constant, E is the
g
absorbance spectrum that the title crystal does not absorb
light in the region between 300 and 800 nm, which makes it a
potential candidate for optical window applications. The
simulated UV–vis spectrum of the MBB crystal was evaluated
band gap, A is a constant independent of photon energy, n
determines the nature of electronic transitions (n = 1/2 for
indirect allowed transitions and n = 2 for allowed direct
transitions) and ꢁ is the absorption coefficient measured in
cm (Tauc & Menth, 1972). Its value was calculated using the
expression ꢁ = ln(1/T)/t, where T is the transmittance and t is
the thickness of the specimen. In the Tauc plot [see inset of
Fig. 15(a)], the value of the energy band gap can be obtained
from the intersection point of the extrapolated linear portion
ꢁ1
Figure 15
The (a) experimental and (b) simulated UV–vis–NIR spectra of the MBB
crystal. The inset shows the Tauc plot of (ꢁhꢊ) against hꢊ; the value of
the direct optical band gap is equal to 4.28 eV.
Figure 16
2
The HOMOꢁ1, HOMO, LUMO and LUMO+1 energy levels, their
shapes, and the energy gaps of the MBB molecule.
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of the absorption edge with the energy (hꢊ) axis. The value of
E was calculated to be 4.28 eV. The low cutoff wavelength
g
and large optical transmittance window confirm the absence of
grain boundaries and the presence of low defect concentra-
tions in the crystal lattice (Weber, 2003). This behaviour
indicates the potential utility of MBB in the upgrade and
design of various optoelectronic applications. Zedan et al.
(2015) have reported a band gap of 2.9 eV for N-{5-[(anti-
pyrinylhydrazono)cyanomethyl][1,3,4]thiadiazol-2-yl}benz-
amide.
In quantum chemistry, both the molecular orbitals (LUMO
and HOMO) and their energy gap (E ) are very useful para-
g
meters in determining the intermolecular charge transfer
interactions in the molecule and its chemical stability. In the
case of the MBB crystals, the distributions of the LUMO+1,
LUMO, HOMO and HOMOꢁ1 energy levels, their shapes,
and their energy gaps were computed using the B3LYP/6-
Figure 17
PL emission spectrum of the MBB crystal at an excitation of 290 nm.
Three broad emission peaks were found at 347, 375 and 410 nm.
311G** basis set in the GAMESS program (Schmidt et al.,
1
993) and are depicted in Fig. 16. In the MBB molecule, the
the generation of only singlet excitons is favoured by elec-
tronic dipole transitions. In order to excite triplet excitons,
strong exciton spin-orbit interaction is needed to flip the spin
of the triplet configuration in the excited state of the molecule
(Ompong & Singh, 2016). In the case of aromatic carbonyls,
spin-orbit coupling exhibited at the carbonyl O atom allows
intrinsic triplet generation through intersystem crossing
(Bolton et al., 2011). Fig. 17 shows the PL spectrum of an MBB
crystal at room temperature. Broad emission was observed in
the region of 350–450 nm at an excitation of 290 nm. Lumi-
nescence spectra are generally blurred and diffuse at room
temperature and this may be attributed to variation in the
strength of the crystalline field occurring from thermal
disturbances (Pant, 1943). A second possible cause is a high
contribution from weak intermolecular interactions present in
the grown crystal system, which leads to sheer congestion: the
rovibronic transitions may be too numerous to separate
(Byrne & Ross, 1965). On the spectral deconvolution of a
main broad peak, three emission bands were identified at 347,
HOMO is localized on the para-substituted benzyl ring and
the –NH—(C O)– amide group, while the LUMO is entirely
located on the benzoyl ring. The HOMO–LUMO energy band
gap (ꢀE = LUMO ꢁ HOMO) of the MBB molecule is esti-
mated to be 5.31 eVand is used to determine the values of the
various quantum molecular descriptors summarized in Table 6.
The optical band gap of the MBB crystal was evaluated from
the experimental UV–vis analysis to be equal to 4.28 eV,
which is in agreement with the theoretically predicted value
from LUMO–HOMO analysis (5.31 eV).
3
.6. Photoluminescence studies
Generally, pure organic crystals belong to the category of
non-phosphorescent material. However, bright phosphores-
cence is also observed from the organic matrix because of
features like aromatic carbonyls, halogen bonding and the
heavy-atom effect (Bolton et al., 2011). In organic materials,
Figure 18
(a) Plot of dielectric constant versus log(F) at room temperature (RT) for the MBB crystal. (b) Plot of dielectric loss with log(F) at room temperature for
the MBB crystal.
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3
75 and 410 nm (UVand blue regions). Thus, the MBB crystal
bond length are required for a large piezoelectric effect
(Werling et al., 2014). The value of the piezoelectric coefficient
on the (001) face of the MBB crystal was found to be
becomes a potential candidate for radiation detector and laser
applications.
ꢁ
1
15 pC N . The promising piezoelectric nature of the MBB
crystal makes it useful for energy-harvesting, sensor and
actuator applications.
3
.7. Dielectric studies
Fig. 18(a) shows the variation of the dielectric constant with
frequency at room temperature for the MBB crystal. It was
observed that the value of the dielectric constant suddenly
decreased from 6.5 to 3.5 in the low-frequency region of 20–
3.9. Vickers micro-hardness testing
1
00 Hz. Owing to the effect of all four polarizations (space
The micro-hardness of the MBB crystal was determined
charge, orientational, electronic and ionic) at low frequency,
the value of the resulting dielectric constant becomes high.
However, the value of the dielectric constant attains a
constant value of ꢇ3.5 in the high-frequency region (100 Hz–
using the Vickers micro-hardness technique. Here, the
dimensions of the impression resulting from the penetration of
an indenter are monitored under constant applied load. The
indenter load was varied from 5 to 40 g for static indentation
with a fixed dwell time of 5 s. The Vickers hardness number
2
MHz). A possible consequence of the constant value of
dielectric constant is the progressive loss of significance of
various polarizations in the higher-frequency region. Fig. 18(b)
shows a plot of dielectric loss against frequency at room
temperature. Initially, at lower frequencies, the value of the
dielectric loss was found to be 0.25. Subsequently, in the high-
frequency region, the value of dielectric loss becomes tanꢃ =
(
VHN) is expressed as (Lopez, 1993) H = (1.8544P/
v
2
ꢁ2
d ) N m , where P is the applied indenter load in N, d is the
average diagonal length of indentation in m and 1.8544 is a
constant geometrical factor for the diamond pyramid indenter.
Fig. 19(a) shows the variation of VHN against indenter load
and also displays the indentation marks at various applied
loads on the MBB crystal plane. Initially, the micro-hardness
value increases with indenter load up to 20 g, because at low
values of applied load only the top surface layers of the crystal
are pierced. This variation in hardness number reflects the
reverse indentation size effect (RISE) (Goel et al., 2016). The
maximum value of the VHN was computed to be 1.5 ꢃ
0.002, which indicates that there are few defects and the title
compound has a highly crystalline nature. Moreover, the low
dielectric loss at higher frequency in the grown crystal is
directly related to the optical quality of the crystal with fewer
grain boundaries.
3
.8. Piezoelectricity
6
ꢁ2
1
0 N m for a 20 g indenter load. Beyond 20 g, the value of
Piezoelectric crystals can convert mechanical vibrations and
VHN starts to decrease and it attains a constant value (1.0 ꢃ
ꢁ2
6
movements into electric charge and vice versa. In organic
crystals, the collective responses of hydrogen bonds and C—
Hꢀ ꢀ ꢀꢀ interactions to the applied electric field decide the
piezoelectric behaviour of the grown crystal. For the design of
highly piezoelectric organic materials, both the dipole moment
and the polarizability of hydrogen-bonded systems need to be
considered carefully (Gagrai et al., 2016). In hydrogen-
bonding systems, a bond or vibration mode with low force
constant and a large change in dipole moment with a change in
10 N m ) after 50 g. This type of nonlinear decrease in
hardness with indenter load is due to release of stress gener-
ated in the form of cracks. This variation in hardness number
shows the normal indentation size effect (ISE).
Mayer’s law is used to understand the variation of load and
n
is defined as (Iurchenko et al., 2016) P = Ad , where A is the
material constant and n is called Meyer’s index. The value of n
can be estimated from a logarithmic plot of ln(p) against ln(d)
(Fig. 19b). The computed value of n (1.88) was observed to be
Figure 19
v
(a) Plot of H versus indenter load (P) for the MBB crystal and photographs of the indentation marks at various loads (5, 10, 20, 30 and 40 g) on the
crystal plane (001). (b) The variation of ln(P) against ln(d) for Meyer’s index evaluation.
ꢂ
J. Appl. Cryst. (2017). 50, 1498–1511
Sahil Goel et al.
N-(4-Methylbenzyl)benzamide 1509

research papers
molecular Hirshfeld surface analysis depicted the immediate
environment of the molecule in the crystal packing and
identified the close contacts between neighbouring molecules.
The relative contributions from Hꢀ ꢀ ꢀH, Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC, Oꢀ ꢀ ꢀH/
Hꢀ ꢀ ꢀO and Nꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀN interactions were found to be 57.4,
31.4, 10.3 and 0.9%, respectively. The significant contributions
from Cꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀC and Oꢀ ꢀ ꢀH/Hꢀ ꢀ ꢀO contacts confirm the
presence of C—Hꢀꢀꢀꢀ interactions and C—Hꢀ ꢀ ꢀO and N—
Hꢀ ꢀ ꢀO hydrogen bonds, respectively. The topology of the
three-dimensional molecular surface was analysed using the
shape index and curvedness mapping. A void analysis study
shows that the MBB crystal possesses uniformly distributed
void channels, which contribute further scope for tailoring its
anisotropic properties. The FTIR, NMR and HRMS analyses
confirm the presence of all the functional groups that contri-
bute to the molecular structure of the grown crystals. The UV–
vis analysis results showed that the crystal exhibits a lower
Figure 20
TG/DTA curves of the MBB single crystal.
cutoff wavelength of 285 nm and E = 4.28 eV, which makes
g
the MBB crystal a promising and competitive candidate for
future optoelectronic devices. The PL emission spectrum
shows UV and blue richness (347, 375 and 410 nm) for the
MBB crystal, which makes it applicable for laser and photonic
devices. The MBB crystal has shown potential for microelec-
tronic and piezoelectric applications, with a low value of
dielectric constant (ꢇ3.5) and high value of the piezo-coeffi-
greater than 1.6, which suggests the softer nature of the grown
crystals.
3.10. Thermal analysis
Thermogravimetric and differential thermal analysis are
useful tools which help in obtaining information about the
thermal stability and melting point of a material. The TG/DTA
measurements were conducted on an MBB crystal at a heating
ꢁ1
cient (15 pC N ). A Vickers micro-hardness study revealed
the softer nature of the grown crystals. The melting point of
the MBB crystal was found to be 419 K and the compound was
thermally stable up to 473 K. The optimized molecular
geometry, UV–vis spectrum and HOMO–LUMO energy gap
for the MBB molecule were found to be consistent with
experimental results.
ꢁ
1
rate of 10 K min under nitrogen atmosphere. Fig. 20 displays
the TG spectrum recorded in the temperature range 308–
723 K for the MBB crystal. The results clearly show that there
is no weight loss up to 473 K, indicating that the MBB crystal
is thermally stable in the temperature range of room
temperature to 473 K, which further emphasizes its potential
use for optoelectronic devices at higher temperatures. Above
Acknowledgements
473–560 K, 97% of the MBB material decomposes. In the
DTA curve of the MBB crystal (see Fig. 20), there occurs an
endothermic peak (melting point) at 419 K.
HY and SG would like to thank UGC and CSIR for scholar-
ships. NS expresses her gratitude to the Principal, SGTB
Khalsa College.
4
. Conclusion
Funding information
In summary, N-(4-methylbenzyl)benzamide was synthesized
using a simple and versatile approach. The slow evaporation
method was used to harvest single crystals of the synthesized
material. SCXRD analysis on a selected crystal confirms that
The following funding is acknowledged: Armament Research
Board, DRDO (award No. ARMREB/MAA/ 2015/163);
SERB, DST (award No. EMR/2015/000385). BS would like to
express his personal thanks to Funda c¸ a˜ o para a Ci eˆ ncia e a
Tecnologia for postdoctoral research grant SFRH/BPD/76184/
2011. IB wishes to acknowledge the Portuguese Foundation
for Science and Technology for financial support (grant IF/
00582/2015).
the it belongs to the orthorhombic space group Pna2 . The two
1
aromatic rings are almost perpendicular to each other,
ꢄ
forming a dihedral angle of 83.65 (17) . The crystal packing of
the MBB crystal shows that the high propensity for formation
of N—HꢀꢀꢀO hydrogen bonds and C—Hꢀꢀꢀꢀ interactions leads
to infinite supramolecular chains running parallel to the a axis.
The geometrical considerations of the HNB propagation
vector of centre of mass explain the morphological importance
of the most visible (001) crystal plane, which is in agreement
with the results of other existing theoretical models. An
analysis of the molecular Hirshfeld surface provided a
summary of all the intermolecular contacts in the crystal. Also,
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