Supramolecular self-assembly of a new multi-conformational Schiff base through hydrogen bonds: Crystal structure, spectroscopic and theoretical investigation

Article abstract of DOI:10.1016/j.molstruc.2016.11.071

The compound 4-(4-dimethylaminobenzylidene)aminoacetophenone was synthesized by condensation of 4-aminoacetophenone and 4-(dimethylamino) benzaldehyde in ethanol. This compound was characterized by CG-MS, infrared, Raman, UV–Vis, ¹H and ¹³

Full text of DOI:10.1016/j.molstruc.2016.11.071

Accepted Manuscript
Supramolecular self-assembly of a new multi-conformational Schiff base through
hydrogen bonds: Crystal structure, spectroscopic and theoretical investigation
Mariana Rocha, A. Di Santo, G.A. Echeverría, O.E. Piro, F.D. Cukiernik, S.E. Ulic,
Diego M. Gil
PII:
S0022-2860(16)31257-1
10.1016/j.molstruc.2016.11.071
MOLSTR 23172
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 August 2016
Revised Date: 20 October 2016
Accepted Date: 23 November 2016
Please cite this article as: M. Rocha, A. Di Santo, G.A. Echeverría, O.E. Piro, F.D. Cukiernik, S.E. Ulic,
D.M. Gil, Supramolecular self-assembly of a new multi-conformational Schiff base through hydrogen
bonds: Crystal structure, spectroscopic and theoretical investigation, Journal of Molecular Structure
(2016), doi: 10.1016/j.molstruc.2016.11.071.
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Graphical Abstract

ACCEPTED MANUSCRIPT
Supramolecular self-assembly of a new multi-conformational Schiff base through
hydrogen bonds: Crystal structure, spectroscopic and theoretical investigation.
Mariana Rocha,1 A. Di Santo,1 G.A. Echeverría,2,+ O.E. Piro,2,+ F. D. Cukiernik,3,+ S.E. Ulic4,5,+,* and Diego
M. Gil1,+,
*
1 INQUINOA (CONICET-UNT). Instituto de Química Física. Facultad de Bioquímica, Química y Farmacia.
Universidad Nacional de Tucumán. San Lorenzo 456. T4000CAN. Tucumán. Argentina.
2
Departamento de Física. Facultad de Ciencias Exactas. Universidad Nacional de La Plata e IFLP
(CONICET, CCT- La Plata). C.C. 67. 1900. La Plata. Argentina.
3
DQIAQF/INQUIMAE – Facultad de Ciencias Exactas y Naturales – Universidad de Buenos Aires.
Pabellón II. Ciudad Universitaria. Núñez. C1428EHA, CABA. Argentina.
4
CEQUINOR (CONICET-UNLP). Facultad de Ciencias Exactas. Universidad Nacional de La Plata. 47 and
115. 1900. La Plata. Argentina.
5 Departamento de Ciencias Básicas, Universidad Nacional de Luján, Rutas 5 y 7, (6700) Luján, Buenos
Aires, República Argentina.
+ Members of the Research Career of CONICET.
*
Corresponding authors: Dr. Diego M. Gil (e-mail: dmgil@fbqf.unt.edu.ar), Dr. Sonia E. Ulic (e-mail:
sonia@quimica.unlp.edu.ar)
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Abstract
The compound 4-(4-dimethylaminobenzylidene)aminoacetophenone was synthesized by
condensation of 4-aminoacetophenone and 4-(dimethylamino) benzaldehyde in ethanol. This
compound was characterized by CG-MS, infrared, Raman, UV-Vis, 1H and 13C NMR spectroscopy. The
crystal structure was solved by single-crystal X-ray diffraction methods. The crystallographic data
reveals that there are four independent molecules per asymmetric unit, that mainly differ from one
another in rotations around the σ-bond of the azomethine N-atom with the phenyl ring. A detailed
analysis of the intermolecular interactions in the four conformers of the compound has been
performed using Hirshfeld surfaces and their associated two-dimensional fingerprint plots. The
optimized geometrical parameters and calculated spectroscopic features obtained by quantum
chemical calculations at B3LYP method show a very good agreement with the experimental data.
Moreover, Natural Bond Orbital (NBO) analysis confirms the strong hyper-conjugative LP N(n1)→ σ*
C(n9)-H interaction between the lone pair located in the N-atom of the azomethine group and the C-
H bond. Liquid crystalline properties of the Schiff base were studied by differential scanning
calorimetry (DSC), polarizing optical microscopy (POM) and Powder X-ray diffraction techniques.
Mesomorphic behaviour was observed in this unsymmetrical azomethine. Based on POM and DSC
measurements, the hexatic Smetic B phase was detected.
Keywords: Schiff bases; Crystal structure; Quantum chemical calculations; IR and Raman spectroscopy;
Hirshfeld surface analysis; Mesomorphic properties.
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1. INTRODUCTION
Azomethines, generally known as Schiff bases, were first reported in 1864 by Hugo Schiff [1].
They can be obtained by condensation of primary amines with carbonyl compounds (aldehydes or
ketones). The common structural feature of these compounds is the azomethine functional group
with the general formula RHC=N-R1, where R and R1 are alkyl, aryl, cycloalkyl or heterocyclic groups.
Benzylidene anilines are an important class of Schiff bases which have been widely used in
coordination, medicinal and biological chemistry and therefore experimental and theoretical studies
on their molecular and crystal structure have been of considerable interest. Generally, Schiff bases
exhibit a wide array of biological activities, including anti-fungal, anti-bacterial, anti-malarial, anti-
proliferative, anti-inflammatory, anti-viral and anti-pyretic properties [2,3]. Furthermore, imine or
azomethine group is present in various natural, naturally derived and non-natural compounds, and
the –C=N– moiety has proved to be critical for biological activity [4-6]. Schiff bases are also of interest
as corrosion inhibitors, catalyst carriers, thermo-stable materials, metal complexion agents and in
biological systems they have a wide range of applications, such as dyes and pigments [7-11]. A large
number of Schiff bases and their metal complexes have been investigated because their ability to bind
reversibly to oxygen [12], catalytic activity in the hydrogenation of olefins [13,14], photochromic
properties [15] and complexing ability towards some toxic metals [16].
Schiff bases containing 4-(dimethyamino)benzaldehyde, as precursor, are reported in
literature [17-22] and some were used as ligand in coordination compounds [17] and as novel third
order non-linear optical materials [18-21]. Recently, Fang et al have reported the relationship
between the molecular conformation and spectroscopic properties of unsymmetrical 4,4'-
disubstituted benzylidene anilines [23]. The Schiff bases are also used in the design of liquid crystals
and prompted research in establishing structure-property relationships, which are elemental in
selecting molecular modifications for the synthesis of mesogens with desirable properties [24,25].
Indeed, a high number of liquid crystals based on Schiff bases have been reported, either as organic
compounds or as ligands of metal-containing mesogens (metallomesogens) [26]. In most cases,
calamitic mesophases, either nematic (N) or smectic (Sm) have been detected for Schiff bases bearing
long aliphatic chains as terminal substituents. Reports on mesogenic character with short terminal
aliphatic chains or small non-aliphatic substituents remain scarce. Particularly, for the compound
under study, the ability of the terminal dimethylamino group to act as a charge-transfer donor in the
formation of conjugative interactions and contributing to the polarizability of the molecule, prompted
several recent studies on the potential applications of Schiff bases containing dimethylamino group as
liquid crystals [27,28].
In the present work, the synthesis, structure, conformational and spectroscopic analysis of 4-
(4-dimethylaminobenzylidene)aminoacetophenone are reported. The compound was characterized
1
by H and 13C Nuclear Magnetic Resonance (NMR), Infrared (IR), Raman, UV-Vis and fluorescence
spectroscopy, GC-MS spectrometry, thermo-gravimetric and differential thermal (TG and DTA)
analysis and differential scanning calorimetry (DSC). These measurements were complemented with
quantum chemical calculations using the DFT/B3LYP method and different basis sets. Interestingly,
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the presence of non classical intermolecular interactions, mainly such as H···H and C···H, allow the
stabilization of four independent conformers in the unit cell, which were detected by X-ray diffraction
methods.
A detailed analysis of the intermolecular interactions on the crystal packing was evaluated
using Hirshfeld surface analysis. The stability of the different observed conformations was estimated
by means of the NBO analysis. The liquid crystalline properties were also evaluated.
2. MATERIALS AND METHODS
2.1 Synthesis of 4-(4-dimethylaminobenzylidene)aminoacetophenone
A solution of 4-(dimethylamino) benzaldehyde (1) and 4-aminoacetophenone (2) in absolute
ethanol in molar ratio 1:1 was heated under reflux for 12 h. The reaction mixture was then cooled
and the obtained yellow solid was isolated by filtration and washed with cold ethanol. The solid was
re-crystallized from hot ethanol to give yellow fine crystals. The purity of the substance was checked
by 1H and 13C NMR, IR and Raman spectroscopy and GC-Mass spectrometry. Single crystals of suitable
quality for X-ray diffraction analysis were obtained by slow evaporation of the solution in methanol.
1
4-(4-dimethylaminobenzylidene)aminoacetophenone (3): Yield: 70 %; MP: 178 °C; H NMR (200 MHz,
CDCl3): δ= 8.35 (s, H-C=N), 8.02 (d, H-8, H-9), 7.82 (d, H-24, H-26), 7.26 (d, H-7, H-10), 6.78 (d, H-28, H-
29), 3.12 (s, N(CH3)2), 2.65 (s, COCH3). 13C NMR (50.3 MHz, CDCl3): δ= 197 (C-11), 161 (C-17), 157 (C-
27), 153 (C-6), 134 (C-3), 130 (C-22, C-21, C-2, C-4), 129 (C-20), 121 (C-1, C-5), 112 (C-23, C-25), 40 (C-
31,C-35), 27 (C-13) ppm (For atoms numbering scheme see Figure 1). EIMS: m/z (rel. int.) [M+] 266
(90), 251 (75), 223 (40), 145 (15), 120 (5). Experimental spectra are presented in Supplementary
Information.
O
O
O
H+/
H3C
H3C
N
∆
H
CH3
N
CH3
- H2O
H3C
+
N
H2N
(1)
(2)
(3)
CH3
Scheme 1: Synthesis of the Schiff base
2.2 Instrumentation
1
NMR spectra: The H (200 MHz) and 13C (50.3 MHz) NMR spectra were recorded on a Varian Unity
1
200 spectrometer. The sample was dissolved in CDCl3 in a 5 mm NMR tube. Chemical shifts (δ) for H
and 13C NMR spectra are given in ppm relative to TMS (δ= 0 ppm) and are referenced by using the
residual non-deuterated solvent signal. Coupling constants, J, are reported in Hz, being the singlet and
doublet identified as s and d, respectively.
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GC-MS spectrometry: The analysis was carried out using a Model Trace GC Ultra gas chromatography
Thermo electron coupled to a Polaris Q mass spectrometer with an ion-trap analyzer using a DB-5
capillary column. Split-less injection (10µL) was used for this study, the initial temperature of 60 °C
was kept for 4 minutes, and then was increased at a rate of 15 °/min to reach 200 °C. The final
temperature was maintained for 6 minutes and the total run time was 13.8 minutes, using Helium as
carrier gas. The mass spectrometer was operated in the electronic ionization scan mode (range, m/z:
40-450). Quantification of the peaks was based on peak area.
UV-visible spectroscopy: The spectra of the substance in dimethylformamide (DMF) and chloroform
(10-4 M) were recorded on a Beckman/DU 7500 spectrophotometer in the spectral region of 200 - 800
nm using a quartz cell (10 mm optical path length).
Fluorescence spectra: Fluorescence measurements were recorded on a Perkin-Elmer LS-50B
luminescence spectrometer (Beaconsfield, England) equipped with a quartz cell of 1 cm path length, a
pulsed xenon lamp (half peak height < 10 m s, 60 Hz), an R928 photomultiplier tube. Both excitation
and emission slits were 10 nm wide.
IR and Raman spectroscopy: The IR absorption spectra of the solid were recorded in KBr pellets, with
a resolution of 2 cm-1 on a FTIR Perkin Elmer GX1 in the 4000 and 400 cm-1 range. The Raman
spectrum of the solid was measured in the 3500-50 cm-1 interval with a Thermoscientific DXR Raman
microscope. The Raman dispersion data were collected using a diode-pump, solid state laser of 532
nm (at 5 cm-1 spectral resolution), a confocal aperture of 25 µm pinhole and 10x objective. The
sample was placed on gold-coated sample slide. To improve the signal to noise ratio, 30 spectral scans
of 2 seconds each were accumulated during the measurements with the laser power maintained at 10
mW.
Thermal analysis: Thermo-gravimetric (TGA) and differential thermal analysis (DTA) measurements
were performed with a Shimadzu DTG-60 thermo-balance in the temperature range of 25 - 800 ºC at
a heating rate of 5°C /min under air flow.
Mesomorphic studies: Mesomorphic properties have been studied by means of variable
temperature polarizing optical microscopy (POM), Differential Scanning Calorimetry (DSC) and room
temperature powder X-ray diffraction (PXRD) techniques. POM measurements have been carried out
between crossed polarizers using a Leitz DMRX microscope equipped with a Leitz 1350 hot-stage.
DSC experiments have been performed either on a Shimadzu DSC-50 calorimeter or a Perkin Elmer
Pyris DSC 6 calorimeter, with heating and cooling rates of 2 or 10 °C /min, respectively. PXRD
measurements on powdered samples were performed on a Siemens D5000 apparatus using the Cu-
Kα1 line (λ= 1.5418 Å) source radiation; the samples were contained on planar glass sample holders.
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2.3 X-ray diffraction data
The measurements were performed on an Oxford Xcalibur, Eos, Gemini CCD diffractometer
with graphite-monochromated CuKα (λ= 1.54184 Å) radiation. X-ray diffraction intensities were
collected (ω scans with ϑ and κ-offsets), integrated and scaled with CrysAlisPro [29] suite of
programs. The unit cell parameters were obtained by least-squares refinement (based on the angular
settings for all collected reflections with intensities larger than seven times the standard deviation of
measurement errors) using CrysAlisPro. Data were corrected empirically for absorption employing
the multi-scan method implemented in CrysAlisPro.
In a first attempt, it was intended to solve the structure in P-1 space group with a standard
run of direct methods implemented in SHELXS of the SHELX suite of programs [30]. Though it
provided an electron density map which could be interpreted in terms of the expected molecules,
however the appearance of spurious peaks mainly at the centers of a honey comb-like structure
make it difficult the interpretation of the map in terms of four molecules per asymmetric unit. It is
well known that frequently direct methods works better by referring the cell to a non center-
symmetric triclinic space group. In fact, a standard run in P1 cell revealed all eight molecules in the
unit cell though still showing the honey-comb centering mentioned above. At variance with the P-1
run, now the map can be easily interpreted in terms of the molecules atomic constituents and the
structural problem reduced to find the shift of the constellation of atoms to refer them to one
inversion center of P-1 group, then eliminate one of the inversion related set of molecules and finally
proceed with the refinement of the remaining ones in the correct center-symmetric space group. The
above problems were easily overcome with the recently available SHELXT program (see Ref. [31] and
references therein). The new development is a radical departure of standard structure determination
procedures where normally the space group is determined first and the crystal structure afterward.
Now, with the only prior knowledge (besides cell constants) of the Laue group and the atom species
present in the solid:
1) The X-ray diffraction data set is expanded to the subgroup P1 of all space groups where the
structure is solved from an initial trial constellation of peaks provided by Patterson superposition
methods. This is followed by dual-space recycling to obtain optimal electron density and phases.
2) The phases are first subjected to a center-symmetric test and a measure of the phase error (α0) is
calculated. This should be small for a constellation of atoms that possess an inversion center.
3) The phases are then employed to determine both the correct space group and the translation
necessary to refer the electron density to the proper unit cell origin.
4) The phases are then averaged in every possible space group compatible with the known Laue
group and used to calculate improved maps.
5) The integrated electron density around the peaks of the maps is assigned to atomic species and
thus the chemical formula is determined.
6) The correct space group and structure solution is selected among the trials on the basis of several
2
figures of merit, including the standard agreement R1-factor, Rweak (average of calculated
for the
Ecalc
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10% of unique reflections with the smallest observed normalized structure factors E ), and the phase
o b s
error (α), all of which should be the smallest for the right choose.
Assuming the Laue group P-1 (Ci) and the presence in the solid of O, N, and C atomic species, the
structure now yielded easily to the above procedure producing a clear and complete electron density
map (no spurious peaks) with an essentially correct interpretation of its maxima in terms of the
expected non-H atoms except for the miss assignment of a nitrogen as a carbon atom, a minor problem
that probably arises because the closeness in the number of atomic electrons for both elements.
The initial molecular model was refined by full-matrix least-squares procedure with SHELXL of
the SHELX package [30]. Most H-atoms were found in a Fourier difference map phased on the heavier
atoms. However, they were positions on stereo-chemical basis and refined with the riding model. The
methyl H-atoms were refined as rigid groups allowed to rotate around their corresponding C-C and N-C
bonds such as to maximize the sum of the observed residual electron density at their calculated
positions. As a result all –CH3 groups converged to staggered conformations. Crystal data and structure
refinement results are summarized in Table 1. Crystallographic structural data have been deposited at
the Cambridge Crystallographic Data Centre (CCDC). Any request to the Cambridge Crystallographic
Data Centre for this material should quote the full literature citation and the reference number CCDC
1411260.
2.4 Quantum chemical calculations
Theoretical calculations were performed using the Gaussian 03 program [32]. The employed
methods are based on the gradient corrected Density Functional Theory (DFT) with the three-
parameter hybrid functional (B3) [33] for the exchange part and the Lee-Yang-Parr (LYP) correlation
function [34]. Scans of the potential energy curves were carried out at the B3LYP/6-31G(d,p)
approximation. Final optimizations and vibrational frequencies were computed employing the 6-
311++G(d,p) basis set. The calculated vibrational properties correspond, in all cases, to potential
energy minima with no imaginary values for the frequencies. The Potential Energy Distribution (PED)
analysis has been calculated using the VEDA4 program [35,36]. Electronic transitions were calculated
within the Time-Dependent Density Functional Theory (TD-DFT) [37] in gas phase and taking into
1
account implicitly the solvent effect (DMF and CHCl3) at B3LYP/6-311+G(2d,p) approximation. The H
and 13C NMR chemical shifts were calculated at the B3LYP/6-311+G(2d,p) level by the Gauge-including
atomic orbital (GIAO) method [38] using the corresponding TMS shielding, calculated at the same
level of theory. The Natural Bond Orbital (NBO) analysis for the molecule was performed at B3LYP/6-
311++G(d,p) approximation by means the NBO 3.1 program implemented in the Gaussian 03 package.
2.5 Hirshfeld surface analysis
Hirshfeld surfaces and their associated 2D fingerprints for all conformers of the Schiff base
were generated using CrystalExplorer3.1 program [39]. The dnorm (normalized contact distance)
surface and the breakdown of 2D fingerprint plots [40-42] are a novel visual simultaneously
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representation of all the intermolecular interactions, and they are unique for a given crystal structure
and polymorph. Fingerplots provide numerous applications to a wide variety of molecular crystals and
intermolecular interactions, including polymorphs systems, as well as crystals with more than one
molecule in the asymmetric unit (Z > 1). Distances from points on the surface to a nucleus (atom)
inside (di) and outside (de) the mean surface are determined by the differing van der Waals (vdW)
radii of atoms, whereby the contact distances di and de can be normalized (dnorm). Therefore,
intermolecular interactions (short, moderate, long) in the crystal structure resulting from hydrogen
bond donors/acceptors can be visually represented by Hirshfeld surfaces. The 3D dnorm surfaces are
mapped over a fixed color scale of -0.08 au (red) to 0.6 au (blue). The 2D fingerprint plots are
displayed by using the standard 0.57-2.7 Å view and including reciprocal contacts, with the de and di
distance scales showed on the graph axes.
3. RESULTS AND DISCUSSION
3.1 Crystallographic structural results
Crystal data and structure refinement results are summarized in Table 1. The compound 4-(4-
dimethylaminobenzylidene)aminoacetophenone crystallizes in the triclinic P-1 space group with a =
9.9357(4) Å, b = 17.1016(9) Å, c = 18.1945(9) Å, α = 78.347(4)°, β = 77.169(4)°, γ = 76.996(4)°, and Z= 8
molecules per unit cell. Figure 1 shows the molecular structure of the four independent conformers
per asymmetric unit observed in the solid state structure of the compound which have an E
conformation about the central -C=N- bond. These molecules differ from each other mainly in
rotations around the σ-bond of the azomethine N-atom with the phenyl ring (Cn3-Cn4-Nn1-Cn9
torsion angles of 42.1(3)°, -37.7(3)°, 130.3(2)°, and -140.2(2)° for n = 1(CI), 2(CII), 3(CIII), 4(CIV)
conformers, respectively), a fact that reflects the conformational freedom of the molecule around this
single bond [N-C(ph) distances in the 1.403(3)-1.406(3) Å range]. This link contrasts with the much
shorter and partially double N(azo)=CH bond [N-C distances in the 1.265(3)-1.275(3) Å interval], which
is part of an extended π-delocalization that renders N-(CH)-(ph)-N(CH3)2 skeleton nearly planar. The
(ph)-N(CH3)2, N-CH3 and carbonyl C=O bond lengths are in the range of 1.358(3)-1.370(3) Å , 1.436(4)-
1.452(3) Å and 1.212(3)-1.221(3) Å, respectively. Bond distances within the phenyl rings vary from
1.364(3) to 1.415(3) Å, as expected for the resonant bond structures of these aromatic groups. The
corresponding bond lengths, angles and selected dihedral angles for each conformer found in the
crystal are given in Table 2, where they are compared with the corresponding computed values at
B3LYP/6-311++G(d,p) approximation. All geometrical parameters determined experimentally are
presented in the Supplementary Information. The above bond distances are in general agreement
with the corresponding reported ones for the closely related 1-(4-{[(E)-(4-Diethylamino)-2-
hydroxyphenyl)-methylene]amino}phenyl)ethanone [43]. The main difference between both
structures is the intra-molecular O-H···N hydrogen bond observed for the last compound, which
stabilizes the planar molecular conformation around the azomethine group. The crystal packing is also
stabilized by π-π stacking interactions between neighboring molecules[43], which are absent in the
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title compound.
Figure 1: View down the crystal c-axis of the four different Schiff base molecules in the asymmetric
unit, showing the labeling of the non-H atoms and their displacement ellipsoids at the 40% probability
level. For clarity, only one molecule is fully labeled and a few representative atoms of the other
molecules are indicated to show their numbering scheme. Intermolecular CH3···O bonds are indicated
by blue lines.
Besides their mutual interaction through van der Waals forces, the molecules are arranged in a
pair of dimers (see Fig. 2b) involving molecules 1 (CI) and 4 (CIV) and molecules 2 (CII) and 3 (CIII), where
they are linked to each other by weak CH3···O bonds [d(C38···O2)=3.550(5) Å, (C38-H···O2)=173.2°
and d(C48···O1)=3.584(4) Å, (C48-H···O1)=167.4°]. At the same time, the molecules are organized
into columns along the crystal b-axis. The columns project onto the crystal a*c*-plane as a distorted
hexagonal, centered wasp net-like, arrangement (see Supplementary Information). As will discussed
below, in the sub-section 3.9, DSC and Polarized Optical Microscopy (POM) experiments show, that
upon heating, the solid undergoes a crystal‰liquid crystal (LC) phase transition to an (hexatic) smectic
B phase. The crystallographic results suggest the possible structure of the LC phase. In fact, upon
increasing temperature, when partial disorder sets in, the system could undergo, by small topological
changes, a crystal‰LC transformation to a smectic phase with a slightly tilted pseudo-hexagonal
structure. Further structural studies on the phase transition are in progress and will be reported
elsewhere.
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Table 1: Crystal data and structure refinement results for the Schiff base 4-(4-
dimethylaminobenzylidene)aminoacetophenone.
Compound (3)
Empirical formula
Formula weight
Temperature
C17H18N2O
266.33
293(2) K
Wavelength
Crystal system
Space group
1.54184 Å
Triclinic
P-1
Unit cell dimensions
a = 9.9357(4) Å
b = 17.1016(9) Å
c = 18.1945(9) Å
α = 78.347(4)°
β = 77.169(4)°
γ = 76.996(4)°
3
Volume
2899.2(2) Å
3
Z, density (calculated)
8, 1.220 Mg/m
-1
Absorption coefficient
F(000)
0.604 mm
1136
3
Crystal size
ϑ-range for data collection
Index ranges
0.354 x 0.154 x 0.109 mm
3.38 to 71.00°.
-6 ≤ h ≤ 12, -20 ≤ k ≤ 20, -19 ≤ l ≤ 22
Reflections collected
Independent reflections
Observed reflection
Completeness to ϑ = 71.00°
23254
11154 [R(int) = 0.0290]
7116
99.6 %
2
Refinement method
Full-matrix least-squares on F
Data / restraints / parameters
11154 / 0 / 733
2
Goodness-of-fit on F
1.019
Final R indices [I>2σ(I)]
R indices (all data)
R1 = 0.0557, wR2 = 0.1423
R1 = 0.0889, wR2 = 0.1715
-3
Largest diff. peak and hole
0.220 and -0.165 e.Å
2
aR1=Σ||Fo 2)2/Σw( 2)2]1/2
|-|Fc||/Σ|Fo|, wR2=[Σw(|Fo| -|Fc| |Fo|
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Table 2: Selected structural parameters (calculated and experimental) for the Schiff base 4-(4-
dimethylaminobenzylidene)aminoacetophenone.
Experimental b
Calculated c
Parameters a
CI
CII
CIII
CIV
CI
CII
CIII
CIV
Bond lengths (Å)
Cn16-Nn2
1.439(3)
1.449(3)
1.364(3)
1.441(3)
1.265(3)
1.405(3)
1.485(3)
1.500(4)
1.217(3)
1.448(3)
1.452(3)
1.362(3)
1.442(3)
1.275(3)
1.406(3)
1.488(3)
1.497(4)
1.212(3)
1.445(4)
1.436(4)
1.358(3)
1.447(3)
1.272(3)
1.403(3)
1.469(3)
1.499(3)
1.221(3)
1.440(4)
1.448(4)
1.370(3)
1.449(3)
1.274(3)
1.406(3)
1.484(3)
1.494(4)
1.210(4)
1.456
1.455
1.377
1.454
1.282
1.397
1.493
1.519
1.219
1.455
1.455
1.377
1.455
1.282
1.396
1.493
1.519
1.219
1.455
1.456
1.379
1.455
1.287
1.398
1.493
1.521
1.226
1.455
1.456
1.379
1.455
1.287
1.398
1.493
1.521
1.226
Cn17-Nn2
Nn2-Cn13
Cn9-Cn10
Cn9-Nn1
Nn1-Cn4
Cn1-Cn7
Cn7-Cn8
Cn7-On
Angles(˚)
Cn16-Nn2-Cn17
Cn10-Cn9-Nn1
Cn9-Nn1-Cn4
Nn1-Cn4-Cn5
Nn1-Cn4-Cn3
Cn1-Cn7-Cn8
Cn1-Cn7-On
Cn8-Cn7-On
Dihedral angles (˚)
Cn11-Cn10-Cn9-Nn1
Cn15-Cn10-Cn9-Nn1
Cn9-Cn10-Nn1-Cn4
Cn9-Nn1-Cn4-Cn3
Cn9-Nn1-Cn4-Cn5
116.9(2)
122.7(2)
121.4(2)
118.0(2)
123.8(2)
118.7(2)
120.6(3)
120.7(3)
117.5(2)
123.3(2)
119.9(2)
117.5(2)
124.5(2)
118.2(2)
120.9(3)
120.9(3)
116.9(2)
124.3(2)
118.6(2)
123.1(2)
118.9(2)
119.5(2)
121.3(2)
119.2(3)
117.3(2)
123.9(2)
120.0(2)
123.2(2)
118.3(2)
118.9(3)
121.4(3)
119.7(3)
118.6
123.5
120.3
118.3
123.1
118.8
121.0
120.2
119.3
123.2
120.1
118.0
123.4
118.8
120.9
120.2
118.8
123.5
120.3
123.1
118.2
119.1
120.9
-120.0
118.8
123.5
-120.3
-123.1
118.2
119.1
120.9
120.0
164.6(2)
-171.9(2)
-2.9(3)
6.0(3)
178.4
-178.8
-1.6
1.6
-11.7(3)
179.1(2)
42.1(3)
7.6(3)
179.8(2)
-37.7 (3)
143.4(2)
176.1(2)
179.6(2)
130.3(2)
-51.5(3)
-174.4(2)
-176.5(2)
-140.2(3)
-42.2(3)
1.4
176.4
42.0
1.1
-176.5
39.7
178.1
176.3
141.1
-42.0
-178.0
-176.3
-141.1
42.0
-142.5(2)
141
-143.2
a n = 1, 2, 3, 4 for CI, CII, CIII and CIV conformers, respectively
b Experimental parameters obtained from X-ray diffraction measurements.
c Calculated geometrical parameters at B3LYP/6-311++G(d,p) approximation.
3.2 Structural properties
The conformational properties of the Schiff base have been investigated through quantum
chemical calculations using B3LYP/6-31G(d,p) level of approximation. The scan was obtained by
minimizing the potential energy in all geometrical parameters by varying the torsion angle around the
C-N bond, in steps of 10°, in the range of 0-360°. The potential energy curve (see Supplementary
Information) shows four minima, two of them located approximately at 40° and 140°(global). From
the curve, two stable conformations are predicted for the compound, the CI conformer possesses the
lowest energy corresponding to the most stable conformation. The relative energy difference
between CI and CII conformers is predicted to be 0.5 kJ mol-1. The minima located at 220 and 325º
correspond to mirror images (enantiomers) of CI and CII conformers, respectively.
Furthermore, the geometry of the four conformers detected experimentally by X-ray
11

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diffraction methods were fully optimized including frequency calculations at the B3LYP/6-311++G(d,p)
approximation (See Supplementary Information). The molecule possesses an E conformation
determined both experimental and calculated. The 4-(dimethylamino)benzylidene ring is nearly co-
planar with the C-C=N chain, whereas the acetophenone ring is twisted significantly. Some
conformational discrepancies between the crystal structure and the optimized counterpart were
observed in conformer CIII. The most significant structural differences are found in the orientation of
the 4-aminoacetophenone ring (Ring 2). This structural disparity is defined by the following torsion
angles C(39)-N(31)-C(34)-C(33) and C(39)-N(31)-C(34)-C(35) with experimental values 130.3(2)° and -
51.5(3)°, respectively for CIII. The computed values at B3LYP/6-311++G(d,p) level were 141.1° and -
42.0°, respectively. The angles C(n3)-C(n5)-N(n1) and N(n1)-C(n10)-C(n9) are in the range 118.2-
123.4° and 123.2-123.5°, respectively for all conformers. The increase respect to the usual values
(122.3 Å) [22] should reduce steric repulsions between the azomethine group and the phenyl ring.
The observed bond lengths C(n9)-N(n1) are between 1.265 and 1.275 Å, indicating a double bond
character (calculated values: 1.282-1.287 Å).
The Natural Bond Orbital (NBO) analysis is an efficient method to evaluate the intra- and
intermolecular bonding and interactions. Some relevant interactions between Lewis and non-Lewis
orbitals along with their stabilization energies are listed in Table 3. For the Schiff base, the most
important interaction is LP N(n1) → σ* C(n9)-H (between the C=N azomethine and the C-H bond). The
stabilization energy associated with this hyper-conjugative interaction is in the range 12.27-12.71 kcal
mol-1 for all conformers, and increases the electron population of the σ* C(n9)-H orbital (0.043e). The
calculated energy values for the LP N(n2) → π* C(n13)-C(n12) interactions ranges between 42.81 and
46.66 kcal mol-1 as shown in Table 3. This strong donor-acceptor overlap can be associated with a
resonance structure with a partial double bond character for the N(n2)-C(n13) bond.
Table 3: Second-order perturbation theory analysis of the Fock matrix for all the observed conformers of 4-
(dimethyamino)benzylidene acetophenone calculated at B3LYP/6-311++G(d,p) approximation.
E(2)b, kcal mol-1
Interaction (donor → acceptor)a
CI
CII
CIII
CIV
LP O(n1) → σ* C(n1)-C(n7)
LP O(n1) → σ* C(n7)-C(n8)
LP N(n2) → π* C(n13)-C(n12)
LP N(n1) → π* C(n1)-C(n5)
LP N(n1) → σ* C(n1)-C(n2)
LP N(n1) → σ* C(n9)-H
Total
20.33
21.41
46.66
6.32
20.31
21.42
45.28
6.30
19.49
20.55
42.82
6.68
19.49
20.55
42.81
6.68
6.10
6.00
6.22
6.22
12.71
113.5
12.27
111.6
12.68
108.4
12.68
108.4
a LP indicates the lone pair on the specified atom. See Fig. 1 for atom numbering scheme. n= 1, 2, 3, 4 for CI, CII,
CIII and CIV, respectively.
b E(2) is the energy of hyper-conjugative interactions.
12

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3.3 Hirshfeld surface analyses
The Hirshfeld surface in the crystal is representative of the region in space where molecules
come into contact. Therefore, its analysis gives the possibility of obtaining quantitative insights into
the nature of intermolecular interactions in the crystalline state. In order to understand the nature of
intermolecular contacts and their quantitative contributions to the crystal packing of the Schiff base,
the Hirshfeld surfaces and the associated 2D fingerprint plot were calculated. Figure 2 shows the
surfaces mapped over dnorm for the four conformers observed in the crystal lattice. As expected, they
reveal the close contacts between donors hydrogen bonds and some acceptors, but other close
contacts are also evident. For conformer I, the two red regions labeled 1 and 2 in the dnorm map are
attributed to C-H···O hydrogen bonds. The remaining visible red region on dnorm surface of CI is
assigned to N···H-C interactions (labeled 3) involving the N atom from the azomethine group and the
hydrogen at the meta position on dimethylaminophenyl ring of conformer CIV. The pale blue to white
spots (labeled 4) represent C···H contacts associated to C-H···C interactions. The packing of the
compound is further controlled by a T-shaped C-H···π interaction (labeled 5 in the dnorm map) involving
the H18 atom of the methyl group bounded to the carbonyl of CI and the C36-C35 benzene ring
[centroid Cg(3); symmetry: 1-x,1-y,1-z]. The shorter interaction H18···Cg(3) found was 2.90 Å. C-H···O
hydrogen bonds are also present in CII conformer, and can be observed as deep-red spots labeled 1
and 2 (See Fig. 2). Similar results were obtained for CIII conformer where the deep-red regions labeled
1 and 2 are attributed to C-H···O interactions. The red area marked as 1 on the CIV conformer surface
is associated to C-H···O interactions.
Fingerprint plots [40-42] of the main intermolecular contacts for all conformers are shown in
Figure 3. The shortest interactions labeled 1 correspond to the close H···H contacts. The O···H (labeled
2) and N···H (labeled 3) interactions, with sharp pairs of spike centered near a (de+di) sum of 2.5 and
2.8 Å, respectively, correspond to O···H-C and N···H-C hydrogen bonds. A pair of broad wings labeled
4 at around (de+di) of 2.8 Å is evidence of C···H contacts. In accordance with the fingerprint plots, no
π-π stacking interactions were observed. These results are in accordance with reported values for the
hydrogen bonding parameters (Supplementary Information). This study emphasizes the importance of
Hirshfeld surface and fingerprint plots for a full understanding of non-classical hydrogen bonds and
short intermolecular contacts in Schiff bases.
The relative contributions of the intermolecular interactions to the Hirshfeld surface area for
all conformers are shown as a histogram (see Supplementary Information). For CI conformer, the H···H
interactions have the major contribution to the crystal packing (51.4 %) and the C···H interactions
comprise the 34 % of total Hirshfeld surface area. Some interactions such as O···H and N···H are also
observed with contributions of 8.7 % and 5.1 %, respectively. For CII and CIV conformers, the O···H
interactions have higher values of contribution compared with those CI and CIII conformers.
13

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Conformer I
Conformer III
Conformer II
Conformer IV
Figure 2: Views of Hirshfeld surfaces of the four Schiff base conformers with thermal ellipsoids plotted
at the 50% level of probability. For Conformer I, the surface in column 2 is rotated 180° around the
horizontal axis of the plot.
14

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Figure 3: Fingerprint plots for conformers I, II, III and IV showing (1) H···H, (2) O···H, (3) N···H and (4)
C···H.
3.4 Vibrational analysis
The experimental IR and Raman spectra of the compound (solid) are shown in Figure 4. The
assignment of the observed bands was performed based on theoretical calculations (B3LYP/6-
311++G(d,p)) and values reported for related molecules [44-52]. Table 4 displays experimental and
calculated frequencies and a tentative assignment of the most important vibration modes. In order to
approximate the theoretical values to experimental ones, the scale factor of 0.9608 was used, in
accordance with results previously reported [44,45]. The simulated IR and Raman spectra are
presented in Supplementary Information. Only some characteristic vibration modes of the Schiff base
will be discussed.
Methyl vibrations: The molecule possesses one methyl group linked to the C=O group and two others
connected to the amino group. The bands located at 2993, 2946 and 2847 cm-1 in the IR spectrum are
assigned to the CH3 asymmetric stretching modes of the methyl groups attached to the nitrogen
atom, in good agreement with the computed values (3009, 2966 and 2872 cm-1), whereas the Raman
15

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spectrum shows only a very weak band at 2926 cm -1. The low intensity band located at 2821 cm -1 in
the IR spectrum can be attributed to the CH3 symmetric stretching mode (computed value 2866 cm-1),
while the CH3 stretching modes of the acetyl group were not observed.
The weak bands located at 1485 and 1447 cm-1 in the IR spectrum (1447 cm-1 in Raman) are assigned
to the CH3 asymmetric and symmetric bending modes of the N(CH3)2 group, respectively, whereas
those for the CH3 linked to the carbonyl group are observed at 1416 and 1365 cm-1, respectively.
These values are in accordance with calculated (See Table 4) and previously reported results [44,46].
C=O vibrations: The strong IR band located at 1670 cm-1 (1668 cm-1 in Raman) is assigned to the C=O
stretching mode, while the predicted value is 1665 cm-1. The red shift, relative to other ketones, can
be due to the conjugation effect of the C=O bond with the aromatic ring. The frequency of the
carbonyl stretching mode is not particularly influenced by the ring substituents [47-49]. For the
parent Schiff base, 1-(4-((4-bromo-benzylidene)amino)phenyl)ethanone, the C=O vibration is
observed as a strong IR band at 1668 cm-1 [47].
The medium-intense bands located at 600 and 592 cm-1 in the IR spectrum are assigned to the out-of
plane and in-plane C=O bending modes, respectively, and are in good agreement with the calculated
values (see Table 4).
Phenyl ring vibrations: The very weak IR bands located at 3053 and 3031 cm-1 are assigned to the C-H
stretching modes of both aromatic rings. The C-H in-plane bending modes of both rings are attributed
to the IR bands at 1316, 1208, 1175 and 1165 cm-1 (1317, 1208, 1177 and 1166 cm-1 in Raman). The C-
H out-of-plane bending modes are observed at 956, 819 and 720 cm-1 in IR and in Raman at 959, 952,
941, 825, 818 and 724 cm-1, in accordance with reported [44,50] and calculated values (See Table 4).
PED calculations show clearly that these vibrations are coupled with C-C stretching modes of the
aromatic rings and with other vibration modes of the substituents.
The C-C ring stretching modes are prominent in the vibrational spectra of benzene and its derivatives
and are assigned in the range of 1650-1200 cm -1 [44,51]. For the title molecule, they are attributed to
the medium and strong IR bands located at 1607, 1578, 1550, 1436 and 1408 cm-1(1578, 1553, 1434
and 1411 cm-1 in Raman), most of them are coupled. This spectral region is well reproduced with
quantum chemical calculations with the corresponding frequencies computed to occur at 1649, 1616,
1581, 1471 and 1441 cm-1.
C=N vibrations: The azomethine bond is very sensitive to the charge transfer between donor and
acceptor groups, and therefore, the C=N stretching mode becomes relevant in the synthesis and study
of Schiff bases and coordination compounds. The weak band located at 1623 cm-1 in the IR spectrum
(1624 cm-1 in Raman) is assigned to the C=N stretching mode. The calculated value for this mode is
predicted at 1679 cm-1 with a contribution of 63%. For related compounds this vibration was reported
at 1609 cm-1 and 1582 cm- [47,52].
16

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The H-C=N in-plane bending mode appears as a strong band located at 1268 cm-1 in the IR spectrum
(1269 cm-1 in Raman), and the weak IR band at 956 cm-1 with a Raman counterpart at 959 cm-1 can be
assigned to the out-of-plane bending of the H-C=N group.
Figure 4: Experimental IR and Raman spectra (solid) of the Schiff base.
17

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Table 4: Experimental and calculated wavenumbers of the Schiff base and tentative assignment of main fundamental vibrational modes
Experimental
IR (solid) a Raman (solid )b
Calculated c
B3LYP/6-311++G(d,p)
Approximate description of modes, PED (%) d,e
ѵ C23-H (75) + ѵ C21-H(13) + ѵC25-H (11)
ѵ C2-H (76) + ѵ C5-H (18)
Unscaled
Scaled
3082
3053
3157
3009
2966
2926
2879
2872
2866
1665
1612
1583
1551
1518
1497
1471
1428
1412
1408
1383
1329
1288
1229
1185
1152
1137
1092
1045
1036
984
3053 vvw
3031 vvw
-
-
3210
3180
3157
3134
3090
3048
2999
2992
2985
1734
1679
1649
1616
1581
1559
1532
1488
1471
1467
1441
1384
1342
1280
1234
1200
1184
1138
1089
1079
1025
1018
1006
996
-
-
ѵ C22-H (97)
ѵaCH3(N)(96)
2993 vw
2946 sh
-
-
-
ѵa CH3(N ) (100)
ѵa CH3(N) (86)
2926 (˂1)
-
2898 w
2847 w
2821 w
1670 s
1623 w
1607 m
1578 vs
1550 s
1528 m
1485 w
1447 w
1436 m
1416 vw
1408 m
1365 s
1316 m
1268 s
1208 w
1175 sh
1165 s
1123 w
-
ѵ C17-H (97)
ѵa CH3 (N) (83)
-
-
ѵa CH3(N) (83)
ѵ C=O (87)
1668 (4)
1624 (9)
-
ѵ C17=N19 (63)
ѵ C21-23 + ѵ C22-25 (53) + δ CCH (11)
ѵ C4-C5 (58)
1578 (100)
1553 (38)
-
ѵ C20-C21 + ѵ C20-C22 (50)
ѵ C27-N30 (20) + δ CCH (26)
δa CH3(N)(59) + δa CH3(N)(21)
δs CH3(N) (75)
-
1447 (3)
1434 (5)
1417 (3)
1411 (2)
1365 (7)
1317 (1)
1269 (6)
1208 (5)
1177 (9)
1166 (48)
1126 (3)
1075 (5)
-
ѵ C22-25 + ѵ C21-23 (40) + δ CCH (27)+ δs CH3(N)(11)
δa CH3(CO)(94)
ѵ C2-C1 + ѵ C5-C4(47) + δ CCH (29)
δs CH3(CO)(83)
δ CCH (40)+ ѵ C3-C4 + ѵ C1-C6 (21)
δ N19-C17-H (16) + ѵ C20-C17 (13) + ѵ C6-N19 (11)
ѵ C6-N19 (24)+ δ CCH (R2)(60)+ ѵ C4-C5 (11)
δ CCH (R1) (54)
δ CCH (56)
ρ CH3 (N) (62) + δa CH3(N) (27)
ѵ C-C (R2)(31)+ ѵ C13-C11 (10)+ δ CH3(CO)(22)
ѵ N30-C35 + ѵ N30-C31(28) + ρ CH3(N) (67)
δ CCC (R2)(69)
1070 w
-
987 (5)
980 (4)
959 (2)
952 (2)
941 (2)
888 (˂1)
848 (˂1)
825 (2)
-
978
966
δ CCC (R1)(74)
γ C17-H (70)
956 w
-
956
945
γ CCH-R2(78)
γ CCH-R1(75)
-
984
900
887 w
843 w
-
864
813
ѵ C-C(R1) (11) + δ C17-N19-C6(10) + δ C20-C17-N19 (14) + γ CCH (R2) (10)
ѵ C-C (R1) + ѵ C-C (R2) (20) + γ CCH-R2 (14) + γ CCH-R1 (12 )+δ CCC (R2)(10)
γ CCH -R2 (82)
847
841
807
18

ACCEPTED MANUSCRIPT
819 m
747 vw
818 (3)
748 (4)
-
832
759
746
738
704
647
605
600
555
539
498
487
460
436
423
331
288
250
198
160
110
87
799
729
717
709
676
621
581
576
533
518
478
468
442
419
406
318
276
240
190
154
106
84
γ CCH -R1 (70)
δ CCC-R1 (23)
732 vw
720 vw
695 w
δ CCC-R1 (34)
γ CCH-R1(58)
724 (˂1)
697 (˂1)
634 (˂1)
600 (˂1)
-
ѵ C11-C3 (16) + ѵ C13-C11 (14)
δ CCC (R1) + δ CCC (R2)(70)
γ C=O (41) + ρ CH3(CO) (10)
ѵ C13-C11 (10)+ δ O12-C11-C13 (38)
δ O12-C11-C13 (15) + δ CCC (R1)(15) + δ C31-N30-C27 (10)
γ CCCC (R1) (58)
-
600 m
592 m
-
532 w
546 (1)
528 (2)
492 (˂1)
482 (˂1)
458 (˂1)
427 (˂1)
409 (3)
333 (1)
290 (2)
232 (2)
210 (3)
163 (10)
131 (6)
90 (4)
495 vw
485 vw
γ NCCC(R1) (14)
δ CCC (R2)(13) + γ CCCC (R2)(10) + γ OCCC(10)
δ C31-N30-C27(24 )+ δ C13-C11-C3(4)
δ C31-N30-C27(29)
453 vvw
426 vvw
-
-
-
-
-
-
-
-
-
γ CCCC (R1)(57)
ѵ C11-C3 (15) + δ O12-C11-C13 (16)
δ N19-C6-C1(17) + τ CCCC(R2) (14)
τ CCCC(R2) (16) + γ CCCC (R2)(15) + δ C31-N30-C27 (10)
δN30C27C25 (21) + δ C11-C3-C4 (16) + τ C31H3(21)
τ C31H3 (38) + δ N30C27C25 (11)
τ CH3(CO)(69)
τ C6-N19-C17-C20 (16)
τ CCCC(R2)(27)+ τ CH3(N) (10)
70 (6)
76
73
a sh, shoulder; s, strong; w, weak; m, medium; vw, very weak; vvw, extremely weak.
b Relative band heights in parentheses.
c Calculated at B3LYP/6-311++G(d,p) level of theory. Scale factor: 0.9608.
d ν: stretching, δ: in-plane deformation, γ: out-of-plane deformation, ρ: rocking, ω: wagging, τω: twisting, τ: torsion modes.
e See Fig. S7 for the atom numbering . R1: Dimethylaminophenyl ring; R2: Acetophenone ring.
19

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3.5 1H and 13C NMR analysis
Chemical shifts are recognized as an imperative part of the information contained in NMR
spectra. They are valuable for structural interpretation due to their sensitivity to conformational
variations. The combined use of NMR experimental spectra and computational simulation methods
offers a powerful way to interpret and predict the structure of a large organic molecules and
biomolecules. The observed and calculated 1H and 13C NMR chemical shifts for 4-(4-
dimethylaminobenzylidene) aminoacetophenone are presented in Table 5 (TMS was used as internal
reference). The experimental 1H and 13C NMR spectra are shown in Supplementary Information.
Table 5: Comparison between experimental and B3LYP calculated NMR chemical shifts (in ppm) for
4-(4-dimethylaminobenzylidene)aminoacetophenone.
B3LYP/6-311+G(2d,p) b
Gas Phase CHCl3
B3LYP/6-311+G(2d,p) b
Gas Phase CHCl3
Exp.a
Jc
-
Exp.
3.01 (0.11) 3.08(0.04) C23, C25
112
121
114 (-2.00) 114 (-2.00)
121 (0.00) 119 (2.00)
N-CH3
C(O)-CH3
H28,H29
H7,H10
H24,H26
H8
3.12 (s)
2.65 (d)
6.78 (d)
7.26 (d)
7.82 (d)
8.02 (d)
8.35 (s)
8.82 (d)
-
2.46 (0.19) 2.58 (0.07) C1, C5
6.75 (0.03) 6.84(-0.06) C20
8.96
129
130
130 (-1.00) 128 (1.00)
136 (-6.00) 136 (-6.00)
C22, C21
C2, C4
C3
-
-
-
-
-
7.27 (-0.01) 7.38(-0.12)
7.88 (-0.06) 8.1(-0.28)
8.02 (0.00) 8.24(-0.22)
8.44 (-0.09) 8.53(-0.18)
8.53 (0.29) 8.46 (0.36)
130
134
153
157
161
197
134 (-4.00) 137 (-7.00)
137 (-3.00) 137 (-3.00)
165 (-12.0) 164 (-11.0)
156 (1.00) 157 (0.00)
164 (-3.00) 166 (-5.00)
200 (-3.00) 204 (-7.00)
C=N-C
C27
H18
H9
C=N
C(O)-CH3
N-CH3
27
40
-
-
27 (0.00)
41 (-1.00)
28(-1.00)
41 (-1.00)
C(O)
a Multiplicity between parentheses; s: singlet; d: doublet.
bΔ=δexp–δcalc values in parentheses predicted in vacuo and taking into account implicitly the solvent. The
standard numbering scheme adopted for atoms labeling is presented in Supplementary Information.
c Coupling constants in Hz.
1
The chemical shifts calculation (δ) of H and 13C was performed with the GIAO method [38],
after full geometry optimization with the GAUSSIAN 03 program. Comparing the experimental and
theoretical data for protons, a good agreement is observed with Δ=δexp-δcalc deviation ranging from
0.29 to -0.09 in gas phase and from 0.36 to -0.22 in CHCl3.
The Δ-values found for carbon atoms rise up to 12.0 ppm. The greatest discrepancy was found
in the prediction of the C=N-C chemical shift; Δ = -12.0 ppm,( -11,0 in CHCl3). This fact can be
explained taking into account that calculations only consider the inductive effect (-I) that is negative
for the corresponding carbon atom, excluding the resonance effect, and consequently the shielding
contribution of the nitrogen atom.
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3.6 Thermo-gravimetric and differential thermal analysis (TG-DTA)
The thermal behaviour of the title compound was evaluated by means of TG and DT analysis
(Supplementary Information). The experiment was carried out under air atmosphere at a heating rate
of 5°/min. The molecule resulted stable up to 250 °C and then thermal decomposition takes place.
The high thermal stability of this compound probably arises because the chains are further stabilized
in the crystal packing by hydrogen bonds between adjacent molecules as was deduced from the X-ray
structure discussed previously. The first step finished at 400 °C with a decrease in mass of 60.2 %
(theoretical value: 58 %), which is associated with the loss of the phenyl groups. From the DT analysis
(see Supplementary Information), two endothermic peaks were observed at 153 and 179 °C, without
changes in mass. Both peaks are attributed to phase transitions (see section 3.9), but the second
corresponds to the melting point of the sample. The second step in the thermal decomposition occurs
between 415-640 °C, with an experimental mass loss of 39.8 % (theoretical value: 42%) and
corresponds to the complete decomposition of the sample. The DTA curve shows an exothermic peak
located at 608 °C, corresponding to the complete degradation of the sample into volatile compounds.
3.7 Electronic spectra
The experimental and calculated electronic spectra of the Schiff base in chloroform and
dimethylformamide are shown in Figure 5. The transitions with major contribution are shown in Table
6, and only the dominant excitations (chosen in accordance with their oscillator strength) are used to
assign the observed bands. Based on these results, it can be concluded that the correlation between
them is a good.
Calculated CHCl3
100000
Calculated DMF
Experimental CHCl3
Experimental DMF
80000
60000
40000
20000
0
300
350
400
450
500
Wavelength (nm)
Figure 5: Experimental and calculated (B3LYP/6-311+G(2d,p)) UV-visible spectra for Schiff base
solutions in CHCl3 and DMF.
The electronic spectrum in CHCl3 presents a band located at 375 nm (calculated: 403 nm)
attributed to a dominant one-electron transiꢀon, with π → π* character (see Supplementary
Information), from HOMO to LUMO orbitals (98 % of contribution). The absorption observed at 335
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nm is assigned to the HOMO-2 → LUMO excitaꢀon, with minor contribuꢀon of HOMO-2 → LUMO+1
transition and the band at 323 nm corresponds to the HOMO-1 → LUMO excitaꢀon. The behaviour of
the Schiff base in DMF resulted similar and these results are in accordance with the computed values
reported in Table 6.
Table 6: Calculated and experimental absorption wavelengths (nm) and oscillator strengths for the most
significant transitions in 4-(4-dimethylaminobenzylidene)aminoacetophenone.
Wavelength (nm)
Oscillator strength
Assignment
Experimental
Calculateda
Chloroform
375
403
1.0869
HOMO → LUMO (98%)
HOMO-2 → LUMO (74%)
HOMO-2 → LUMO+1 (16%)
HOMO-1 → LUMO (59%)
HOMO → LUMO+1 (27%)
335
323
328
0.0248
325
0.1755
Dimethylformamide (DMF)
378
320
415
330
1.0461
0.2004
HOMO → LUMO (98%)
HOMO -1 → LUMO (48%)
HOMO → LUMO+1 (27%)
HOMO-2 → LUMO (65%)
-
324
0.0734
a Calculated at B3LYP/6-311+G(2d,p) approximation.
The frontier molecular orbitals mainly involved in the electronic transitions used to assign the
observed bands are depicted in Supplementary Information. The HOMO corresponds to a π bonding
system localized over phenyl groups and the non-bonding character of the oxygen and both nitrogen
atoms. The HOMO-1 involves a π-bonding system localized over both phenyl rings and a p-type orbital
strongly located on the N atoms. HOMO-2 contains π bonding electronic density located in the phenyl
ring (R2) and a p system over acetyl group. Both LUMO and LUMO+1 orbitals exhibit π anti-bonding
character delocalized over both phenyl rings and non-bonding character of nitrogen, oxygen and
carbon atoms of methyl groups.
3.8 Fluorescence Spectra
4-(4-dimethylaminobenzylidene)aminoacetophenone was checked for fluorescence, even
though some information was found in the literature [53] and the emission spectrum excited at 430
nm (λexc) is shown in Figure 6. The broad fluorescence band extends from 450 to 650 nm with the
maximum located at 507 nm. Taking into account the UV absorption spectrum, the emission takes
place in a region (red-shifted) where there is no absorption, and accordingly no overlapping between
both signals was observed. This could be due, probably, to differences in the geometry of the excited
and ground states.
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Initial
16.3 nmoles
700
600
500
400
300
200
100
0
3 x 16.3 nmoles
4 x 16.3 nmoles
5 x 16.3 nmoles
450
500
550
600
650
Wavelength (nm)
Figure 6: Fluorescence spectra of the Schiff base in CH3CN excited at 430 nm.
To determine the influence of concentration on the fluorescence, moles of the Schiff base
were added to the initial solution in CH3CN (8.25 10-4M). As observed, the fluorescence raised to the
third addition (3 x 16.3 nmoles added), then began to go down up to (5 x 16.3 nmoles added) and
subsequently an erratic behaviour was noted. In principle, the auto-absorption could be discarded
since the negligible UV absorption cross-section at the wavelength of fluorescence. Therefore, it could
be inferred, at least at an early stage, that the most probable process would be intermolecular
interaction to form dimers. This is supported by the observed crystal packing, (see, Figure 1), in which
the molecules are linked through hydrogen bonds as was discussed previously.
3.9 Mesomorphic behavior
DSC experiments performed at 2o/min (see Figure 7) showed two endothermic peaks on
heating: the first one from 149 to 153 oC (ΔH= 3 kJ mol-1); the second one from 175 to 177 oC (ΔH= 27
kJ mol-1). On cooling, their exothermic counterparts were observed at lower temperatures: 134 to 131
o
oC (ΔH= -27 kJ mol-1) and 36 to 32 C (ΔH= -1 kJ mol-1). The same values have been obtained in
subsequent heating/cooling cycles; moreover, the powder XRD pattern recorded at room
temperature on a sample heated in the DSC up to the end of the second endothermic peak agreed
(except for the relative intensities of some peaks) with that corresponding to a virgin sample.
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Figure 7: DSC measurements for the Schiff base with the corresponding POM images.
o
o
POM observations showed subtle texture changes at ca. 150 C; at 180 C the sample cleared
to the isotropic state (fluid opaque texture). On cooling, a well defined fan-shaped + focal-conic
texture has been detected below 135 oC. This texture, characteristic of smetic phases, is displayed in
Figure 7. Mosaic textures have been observed in other cooling cycles. Fan-shaped textures are often
associated to SmA, SmC or SmB mesophases; mosaic textures are more typical of SmB phases.
Moreover, transformations from fan-shaped into mosaic textures after annealing for long time have
been reported as characteristic of SmB phases [54]. The relative values of the enthalpy changes
associated to both phase transitions (ΔH for the crystal to Sm transition lower than that of the Sm to
isotropic transition) also point to an ordered (hexatic) SmB phase.
Schiff bases have been widely used in the field of liquid crystals. A search in the LiqCryst5.1
database [55] retrieved 10184 entries for the specific core under study (-Ph-CH=N-Ph-) from aprox.
100000 total cases. In most cases, mesogenic compounds involve long aliphatic chains in at least one
of the two terminal positions. Most of the compounds exhibiting the -Ph-CH=N-Ph- core and "small"
substituents in the terminal positions (i.e. halogen atoms groups, nitro groups, alkoxy groups with no
more than 2 carbon atoms) are not mesogenic. Among the few examples of mesogenic compounds
inside this group, O2N-Ph-CH=N-Ph-OCH3 (N monotropic) and CH3O-Ph-CH=N-Ph-CONH2 (Sm
enanthiotropic) deserve to be mentioned. A search restricted to derivatives exhibiting one terminal
group identical to those of the compound under study in this work (excluding long-chain compounds)
retrieved less than 15 cases of R-Ph-CH=N-Ph-COCH3 compounds and about 20 cases of (CH3)2N-Ph-
CH=N-Ph-R compounds. Among the first group, the only mesogenic compounds were those with R=
Ph-, NC-, CH3CH2S- (unidentified phases), CH2=CH-COO- (N), CH3OCOO- (N), CH3CH2OCOO- (N),
CH2COO- (N), CH3O- (N) and CH3CH2O- (N). Among the second group, only those in which -R= -OCH3
(N), -OCH2CH3 (N), -CH=CH-COOCH2CH(CH3)2 (Sm), -COO-Ph-X for X= Br, CN, and CH3 (N) were
mesogenic.
The absence of a mesogenic character for most of this kind of compounds deprived from long
terminal chains agrees with the ordered character of the SmB mesophase detected for our
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compound. It bears very short aliphatic groups in both extremes, but the number of terminal methyl
groups and the nature of the functional groups they are attached to, seem to be just enough to
warrant its mesogenic character -very likely as a delicate balance of geometry anisotropy [56], axial
and lateral interactions [27, 57] which manifest in a hexatic SmB phase.
4. CONCLUSIONS
The synthesis of the Schiff base 4-(4-dimethylaminobenzylidene)aminoacetophenone was
achieved by the reaction between 4-(dimethylamino) benzaldehyde and 4-aminoacetophenone in
1
ethanol. The product was characterized by IR, Raman, UV-Vis, H and 13C NMR and fluorescence
spectroscopy. To support the interpretation of the experimental results, the analysis was
complemented with quantum chemical calculations at B3LYP method and different basis sets. The
crystal structure was determined by means of single-crystal X-ray diffraction methods. This compound
crystallizes in the triclinic P-1 space group with a = 9.9357(4) Å, b = 17.1016(9) Å, c = 18.1945(9) Å, α =
78.347(4)°, β = 77.169(4)°, γ = 76.996(4)°, and Z= 8 molecules per unit cell. The crystallographic data
reveals that there are four independent molecules per asymmetric unit that mainly differ from one
another in rotations around the σ-bond of the azomethine N-atom with the phenyl ring, hence
reflecting the conformational degree of freedom of the molecule. The Hirshfeld surfaces and
fingerprint plots were an important tool in the analysis of the intermolecular interactions and their
quantitative contributions to the crystal packing of the Schiff base. These results indicate that the
H···H contributions are more remarkable than other contacts. The crystal packing showed noticeably
stabilized by non-conventional C-H···O, C-H···N and C-H···C hydrogen bonds and no π-π stacking
o
interactions were observed. The substance is thermally stable up to 250 C, and the vibration and
electronic properties of the substance were fully determined and supported by quantum chemical
calculations. The Fluorescence spectrum of the molecule provided measurable signals when irradiated
with λexc.= 430 nm. The formation of dimers proposed due to intermolecular interactions, when Schiff
base concentration was increased, is consistent with the short contacts found in the crystal packing,
and seems also in line with the detected hexatic SmB mesophase.
ACKNOWLEDGEMENTS
This work was supported by CONICET (PIP 1529, PIP 0205, and PIP 0359), ANPCyT (PME06 2804,
PICT06 2315, and PICT-2013-0697), CIUNT, UNLP, and UNLu of Argentina. SEU thanks Deutscher
Akademischer Austauschdienst Germany (DAAD) for an equipment grant and financial support. M.R.
and A.D.S. thank CONICET for doctoral fellowships.
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SUPPLEMENTARY INFORMATION
All bond lengths and angles, atomic coordinates and equivalent isotropic displacement parameters,
anisotropic displacement parameters, hydrogen coordinates and isotropic displacement parameters,
1
and torsion angles and hydrogen bonding geometrical parameters are given in Tables S1-S6. The H
and 13C NMR spectra are shown in Figs. S1 and S2, respectively. The mass spectrum, major
fragmentation pattern, crystal packing of the Schiff base projected perpendicularly to the crystal b-
axis, potential energy curve versus dihedral C(n9)N(n1)C(n4)C(n3) angle, optimized molecular
structure of the four conformations, relative contributions of intermolecular interactions to the
Hirshfeld surface, experimental and theoretical IR spectra of the Schiff base, experimental and
theoretical Raman spectra of the Schiff base, TGA and DTA curves and plots of the frontier molecular
orbitals involved in the electronic transitions are given in Figs. S3-S12.
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