Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural and spectroscopic investigations

Article abstract of DOI:10.1016/j.tet.2017.03.080

Several bis-terephthalamides based on methyl esters of amino acids including glycine (1), β-alanine (2), γ-aminobutyric acid (3) and ε-aminocaproic acid (4), X[sbnd](CH₂)_n[sbnd]HNOC[sbnd]C₆H₄[sbnd]CONH[sbnd](CH<

Full text of DOI:10.1016/j.tet.2017.03.080

Tetrahedron xxx (2017) 1e12
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Insight into hydrogen bonding of terephthalamides with amino acids:
Synthesis, structural and spectroscopic investigations
,
*
a
b
b
a
Barbara Hachuła a , Anna Polasz , Maria Ksia˛zek , Joachim Kusz , Violetta Kozik ,
_
Marek Matussek a, Wojciech Pisarski a
a Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland
b Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Several bis-terephthalamides based on methyl esters of amino acids including glycine (1), b-alanine (2),
Received 25 January 2017
Received in revised form
11 March 2017
Accepted 27 March 2017
Available online xxx
g-aminobutyric acid (3) and ε-aminocaproic acid (4), Xe(CH2)neHNOCeC6H4eCONHe(CH2)neX (X]
CO2CH3, n ¼ 1, 2, 3 and 5), have been synthesized and characterized by single-crystal X-ray diffraction
and spectroscopic methods: FT-IR, polarized FT-IR, Raman, 1H NMR and 13C NMR. The four structures
assemble via classical NeH/O hydrogen bonds between amide functionalities linking the molecules into
chains parallel to the short axis. The analysis of polarized IR spectra of pure and deuterated compounds
reveals that a weak interchain (“through-space”) exciton coupling involves two closely-spaced hydrogen
bonds belonging to two different adjacent chains. The exciton coupling magnitude decreases with the
addition of methylene groups to the terephthalamide system. Isotope effects in terephthalamides show
that the distribution of protons and deuterons in the crystalline lattice depends on the strength of the
exciton couplings involving hydrogen bonds.
Keywords:
Hydrogen bond
Polarized IR spectra
Exciton coupling
H/D exchange
Terephthalamide
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
acid diamide derivatives in which the nitrogen atoms are mono- or
disubstituted were described.6 All previously reported bis-tereph-
The great interest in the study of aromatic ester amides derived
from natural amino acids is explained by the possibility of using
them in the synthesis of potential families of biodegradable syn-
thetic polymers for such applications as sutures, surgical implants,
controlled-release formulations for drugs and scaffolds for tissue
engineering.1e5 The poly(ester amide)s have ester and amide
groups on their chemical structure which are of a degradable
character and provide good thermal and mechanical properties.
thalamide structures self-assembled to form hydrogen-bonded
supramolecular
b
-sheets, ribbon or helix.6e8
Hydrogen bonding is responsible for the molecular and
macroscopic properties of materials, molecular recognition and
supramolecular structure.9 This kind of molecular interaction is
ubiquitous in biological systems providing stability to the pro-
tein molecule and also stabilizing the structure of DNA by
forming a bridge between the pyrimidine and purine bases. Due
to many applications of hydrogen bonds related to understand-
ing the dynamics of the processes in the hydrogen-bonded
systems, it is important to find out how individual hydrogen
bonds would respond to variations in temperature and to
isotope dilution. The most popular methods to analyze changes
in the intermolecular interactions are vibrational spectroscopy
and X-ray diffraction.
Particularly, poly(ester amide)s containing
risen as important materials in the biomedical field. The presence
of the -amino acid contributes to better cellepolymer in-
a-amino acids have
a
teractions, allows the introduction of pendant reactive groups and
enhances the overall biodegradability of the polymers.3 Thus we
decided it would be interesting to construct model compounds
derived from terephthalic acid, a-amino acids and different diols to
study new polymer structures. Several terephthalamides have been
Polarized IR spectroscopy is considered among the most effi-
cient tools for hydrogen bond research, providing complete infor-
reported so far.6e8 The crystal structures of a series of terephthalic
mation about the coupling mechanisms involving these
10
ꢀ
interactions in crystal lattices. In 1998, Flakus and Banczyk
studying polarized IR spectra of cyclic dimers noticed anomalous
isotopic dilution effects. The authors have supposed the existence
* Corresponding author.
E-mail address: barbara.hachula@us.edu.pl (B. Hachuła).
http://dx.doi.org/10.1016/j.tet.2017.03.080
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Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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2
B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
of a new H/D isotopic recognition (viz. the H/D “self-organization”),
depending on a non-random distribution of protons and deuterons
in the lattices of isotopically diluted crystals. The origin of this
anomalous arrangement of protons and deuterons between the
hydrogen bonds was assigned to the new kind of co-operative in-
teractions involving hydrogen bonds (i.e., the dynamical co-
operative interactions).11 Similar experimental observations have
been done in our several dozen works for systems with different
kind of hydrogen bonded associates in the crystalline lattice, i.e.,
chain, dimer, trimer, tetramer or hexamer.12ae12n In a study made
by Flakus et al.12ae12f and Hachuła et al.12ge12n a direct relationship
between the temperature dependence of IR spectral properties and
the electronic structure for H-bonded crystalline systems have
been recently proved. For systems with aromatic hydrogen bonds
(favourable electronic structure), governed by an intrachain (“tail-
to-head” (TH)) exciton coupling mechanism, the lower-frequency
branch of the nX-H band, recorded in the wide temperature range,
was more intense in relation to the intensity of the higher-
frequency band branch.12g,12k In the case of systems with non-
aromatic hydrogen bonds (unfavourable electronic structure),
quite different spectral behavior can be observed, i.e., the higher-
frequency branch of the band was the most intense fragment of
the spectrum at 293 K and 77 K. In these systems only interchain
(“through-space” (TS)) exciton coupling mechanism involving pairs
of closely-spaced hydrogen bonds is active. This coupling of the
“side-toeside”-type occurred basically via the van-der-Waals
forces.12le12n
Due to the importance of terephthalamides in biomaterials and
tissue engineering, and also our hydrogen bond studies discussed
above, it was considered worthwhile to determine crystal struc-
tures and to make polarized IR measurements on hydrogen- and
deuterium-bonded terephthalamides. In addition to this, temper-
ature dependent and isotopic infrared measurements on tereph-
thalamide derivatives were also performed to investigate the
influence of temperature and deuteration on the vibrational
behavior of terephthalamides and draw some meaningful conclu-
sions from the observed spectral changes. Thus, a considerable part
of the presented work is devoted to analyze crystal structures of
1e4 in order to correlate the structures to the vibrational results. In
the next part we extend the approach of our previous vibrational
studies to terephthalamide systems to elucidate the nature of the
co-operative interaction mechanisms involving hydrogen bonds in
systems like infinite chains via a quantitative interpretation of the
crystalline system spectra. Measurement of the IR spectra of ter-
ephthalamide derivatives were mainly focused on spectroscopic
effects corresponding to the intensity distribution, the influence of
temperature, the linear dichroism, and the isotopic substitution of
deuterium recorded in the frequency range of the proton and the
Fig. 1. (a) Molecular structure of 1 with atom labelling of the asymmetric unit; (b) the
infinite 1D hydrogen bonding chain via NeH/O along the crystallographic b axis; (c) a
part of 2D crystal structure showing the C(4) chains (d) 3D hydrogen-bonded network
of 1 (ac plane).
Table S2. The strongest hydrogen-bonding interactions are shown
in Table 2. The detailed values characterizing all interactions are
collected in Table S3.
All the compounds crystallize with crystallographic inversion
symmetry, with each asymmetric unit consisting of half a molecule
(Figs.1e4). The amide and ester groups and the benzene ring of 1e4
are essentially planar (r.m.s. deviations of 0.0056, 0.0219 and
0.0012 Å of 1, 0.0019, 0.0266 and 0.0003 Å of 2, 0.0141, 0.0104 and
0.0008 Å of 3, 0.0278, 0.0276 and 0.0011 Å of 4 for CipsoeCamide(]
O)eNeCamino acid, C1eO1eC2(]O2)eC3 and CaromeCarom). The
amide groups are rotated slightly out of the aromatic plane i.e. the
interplanar angles between the benzene ring and the plane defined
by Cipso-C(¼O)eNeC atoms range from 25.82ꢀ to 28.02ꢀ (27.17ꢀ 1,
28.02ꢀ 2, 25.82ꢀ 3, 26.89ꢀ 4). The molecular conformation of 1e4
can be also determined by the CaromeCipsoeC(]O) and the Car-
omeCipsoeCamideeN torsion angles which clearly deviated from
180ꢀ (ꢁ150.16(15)ꢀ and 155.30 (14)ꢀ for 1; 150.1 (2)ꢀ and 153.7 (2)ꢀ
for 2; 153.81 (15)ꢀ and 155.38 (14)ꢀ for 3; ꢁ150.98(16)ꢀ
and ꢁ153.47(15)ꢀ for 4) (Table S2). The distances and angles within
the four compounds reported are generally as expected, although
the Camide]O bond length increases and the Camide-N bond length
decreases with the increasing number of carbons in the amino acid
deuterium stretching vibration bands, viz. nNeH and nNeD
,
respectively.
2. Results and discussion
2.1. Molecular geometry of compounds 1e4
The crystal structure of 1 was re-determined in this study and is
similar to that reported by Armelin et al.8 Compounds 2e4 have not
been described earlier in the literature. The atomic numbering
scheme for all four compounds is given in Figs. 1e4, which also
show the contents of the asymmetric unit in each case and the
crystal packing of the molecules. The selected crystallographic data
and the melting points determined by differential scanning calo-
rimetry (DSC) are summarized in Table 1. A full summary of the
refinement details is given in Table S1 in the Supporting
Information. The selected geometrical parameters are reported in
Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
3
Fig. 2. (a) Molecular structure of 2 with atom labelling of the asymmetric unit; (b) the
infinite 1D hydrogen bonding chain via NeH/O along the crystallographic c axis; (c)
the molecular packing viewed along the a axis, showing the R12ð6Þ rings, linked by
CeH/O interactions; (d) 3D supramolecular framework of 2.
side chains of 1e3. Moreover, the Camide]O and the CamideeN
distances in 4 are between these for 1 and 2. The increase in side-
chain length slightly alters the planar geometry of 1e4, i.e., the
ester groups are strongly twisted in 1 and 4 or lie in the plane of the
amide group in 2 and 3 (see torsion angles in Table S2). The dihedral
angle between these planes are 78.832 (75)ꢀ for 1, 28.809 (130)ꢀ for
2, 20.099 (34)ꢀ for 3 and 43.68ꢀ for 4.
Fig. 3. (a) Molecular structure of 3 with atom labelling of the asymmetric unit; (b) the
infinite 1D hydrogen bonding chain via NeH/O along the crystallographic b axis; (c)
2D molecular framework along the crystallographic bc plane; (d) 3D hydrogen-
bonding architecture of 3.
Compounds 1e4 are a disubstituted terephthalamides owning
one NH function at each nitrogen atom. All the compounds self-
assemble in a similar manner in which the molecules are con-
nected by intermolecular NeH/O hydrogen bonds along the
shorter crystallographic axis to form ribbon-like structures
(Figs. 1e4). The crystal packing of compounds 1, 2 and 4 is similar to
each other, i.e., the molecules are stacked atop one another to form
the unusual ladder motif involving rings of graph setR22ð18Þ.13
Neighbouring molecules of 1, 2 and 4 in the ladder are related by
translation and the repeat distance of the chain about 5 Å corre-
sponds to the short b, c and a axis, respectively. On the other hand,
each molecule of 3 is connected with four different adjacent mol-
ecules, two lower and two higher, leading to a crosslinked three-
dimensional pattern (Fig. 3c). The strongest hydrogen bonds
occur between the molecules of 3, whereas the weakest H-bond
interactions are found in 4. The ester groups of 4 are considerably
interlinked, while in terephthalamides 1 and 2 the methyl groups of
eCOOCH3 system are adjacent to each other. Beyond classical
NeH/O hydrogen bonds in 1, 2 and 4, there are also observed
CeH/O interactions, mostly from the amino acid hydrogens. The
“shorter” weak hydrogen bonds with C/O distances of up to 3.5 Å
are described below and shown in Figs. 1c and 4c. The “longer”
CeH/O hydrogen bonds (C/O > 3.5 Å) are listed in Table S3 and
also presented in Figs. 1e4.
The adjacent 1D chains of 1 are interconnected via hydrogen
bonds C3eH3A/O2 to form a two-dimensional (2D) layer parallel
to the ac plane (Fig. 1c). The neighboring molecules also join
together via bifurcated hydrogen bonds, N1eH1/O3 and
C6eH6/O3. In compound 4, the terephthalamide molecules are
bonded by a bifurcated (N1eH1/O3 and C6eH6B/O3) and a weak
C3eH3A/O2 hydrogen bond to form layers parallel to the ab plane.
The 2D sheets in 4 are fused together to yield a 3D network through
the weak CeH/O hydrogen bonds (Fig. 4d). Interestingly, the
structure of 2 exhibits a trifurcated CeH/O interaction motif with
the amide oxygen atom acting as a hydrogen bond acceptor from
two different CeH donors. These interactions involve two methy-
lene CeH (C3eH3A/O3 3.249(3) Å; C3eH3B/O3 3.520(4) Å) and
an amide NeH (N1eH1/O3 2.955(3) Å) acting as hydrogen bond
donors,
from
a
symmetry
generated
molecule
Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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4
B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
Table 1
Crystallographic data and the melting points of the compounds 1e4.
1
2
3
4
CCDC numbers
1005546
1005547
214
1005548
177
1511353
157
Melting point [ꢀ C]a 163
Formula
C
14H16N2O6 C16H20N2O6 C18H24N2O6
C22H32N2O6
420.49
Compound weight
Crystal system
Space group
308.29
336.34
364.39
Monoclinic Monoclinic Orthorhombic Triclinic
P21/c
P21/n
Pbca
P1
Temperature, K
298 (2)
298 (2)
298 (2)
294 (1)
Unit cell parameters:
a, Å
b, Å
c, Å
16.7805(7)
4.9693(2)
8.9798(4)
90
90.898(4)
90
748.71(5)
2
1.367
4.366(4)
36.870(4)
5.0970(15)
90
98.49(6)
90
811.5(8)
2
1.377
12.7857(3)
10.0036(5)
14.1379(6)
90
90
90
1808.28(13)
4
1.338
5.2092 (7)
6.5158 (7)
16.599 (3)
99.154(11)
93.532(11)
98.482(10)
548.06 (13)
1
a, deg
b
, deg
g
, deg
V, Å3
Z
Dx, g cmꢁ3
1.274
a
Determinated by DSC.
1e4 and their corresponding 2D fingerprint plots are shown in
Fig. 5. The relative contributions of each interaction to the Hirshfeld
surface are depicted in Fig. 6. dnorm mapped on Hirshfeld surface for
visualizing the intermolecular interactions between the molecules
of compounds 1e4 is presented in Figs. S1eS4. The detailed
fingerprint plots produced to show the relative contribution of
different intermolecular interactions to the Hirshfeld surface are
shown in Fig. S5.
From Figs. 5 and 6 it results that the major contribution is from
H/H (37e53.9%), O/H (29.3e42.9%) and C/H (10.6e16.9%).
Other intercontacts N/H (0e1.8%), O/O (0.7e1.4%) and C/C
(0e3.6%; 3.6% for 2) contribute considerably less to the Hirshfeld
surfaces. The proportion of H/H interactions has increased from 1
to 4 since the longer amino acid chain affords a higher proportion of
hydrogen atoms in the structure. Thus, compound 1 shows the
lowest proportion of H/H interactions (37%) and the highest O/H
contacts (42.9%). Conversely, the terephthalamide 4 contains the
smallest proportion of O/H contacts (29.3%) and the highest
proportion of H/H contacts (53.9%) reflected in the lowest crystal
density (1.274 g cmꢁ3) compared to the other compounds. The
sharp spike appearing in the middle of scattered points in the
fingerprint map of 1 (de ¼ di ¼ ~1.1 Å) is from H/H interactions of
the hydrogen from methyl ester moieties (H1A$$$H1A 2.281 Å). Of
the compounds 1e4, compound 2 contains the lowest contribution
of C/H contacts (10.6%) due to the considerable distances between
Fig. 4. (a) Molecular structure of 4 with atom labelling of the asymmetric unit; (b) the
infinite 1D hydrogen bonding chains via NeH/O along the crystallographic a axis; (c)
the molecular packing viewed along the c axis, showing the R12ð6Þ rings, linked by
CeH/O interactions (d) a view of the 3D hydrogen-bonded layer of 4.
(x,y,ꢁ1 þ z;ꢁ1 þ x,y,ꢁ1 þ z; x,y,ꢁ1 þ z) (Fig. 2c). Compound 3 also
exhibits one “shorter” weak hydrogen bond, namely C3eH3B/O2
(3.448(2) Å; 1 ꢁ x,ꢁ1/2 þ y, 1/2 ꢁ z), which connects the terminal
regions of the butyl acetate chains of neighbouring molecules
approximately the b axis to form layers parallel to the ab plane. In
total the packing of compounds involves 5 (1) and 6 (2e4) weak
hydrogen bonds and then the overall structures of 1e4 are a
complex three-dimensional network (Figs. 1de4d).
Table 2
Hydrogen-bond geometry in terephthalamides 1e4.
DdH/A
DdH (Å)
H/A (Å)
D/A (Å)
DdH/A (ꢀ)
The adjacent benzene rings of the 2D layers of 1, 2 and 4 are
parallel displaced. The distances between the centers of the two
aromatic rings within one stack are 4.969 Å for 1, 6.202 Å for 2,
1
2
3
4
N1eH1/O3a
C3eH3A$$$O2b
N1eH1/O3c
C3eH3A$$$O3c
N1eH1/O3d
C3eH3B/O2e
N1eH1/O3f
C3eH3A$$$O2g
0.86
0.97
0.86
0.97
0.86
0.97
0.86
0.97
2.08
2.42
2.14
2.52
2.08
2.74
2.22
2.57
2.8688(16)
3.327(2)
2.955(3)
3.249(3)
2.8650(16)
3.448(2)
152.9
155.5
157.4
131.7
151.6
130.5
164.7
164.6
10.004 Å for 3 and 5.209 Å for 4. It is outside of the range of
stacking interactions (3.3e3.8 Å), and we conclude that
p
-
p
p
-p
stacking interactions do not exist. The benzene rings from two
different stacks of 1 make a dihedral angle of 73.07ꢀ with a distance
of 5.13 Å between the central point of them. The angles between
the aromatic rings of molecules belonging to two different stacks in
compounds 2 and 3 are 47.72ꢀ and 24.46ꢀ, respectively.
3.0608(16)
3.511(3)
Symmetry code(s).
a
x,-1 þ y,z.
b
x,1/2 ꢁ y,1/2 þ z.
c
x,y,ꢁ1 þ z.
d
1/2 ꢁ x,ꢁ1/2 þ y,z.
2.2. Hirshfeld surface analysis of 1e4
e
1 ꢁ x,ꢁ1/2 þ y,1/2 ꢁ z.
ꢁ1 þ x,y,z.
f
g
The Hirshfeld surfaces mapped with dnorm for the molecules of
1 þ x,y,z.
Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
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Fig. 5. (left) The Hirshfeld surfaces mapped with dnorm for the molecules of 1e4. (right) The corresponding 2D fingerprint plots for the molecules of 1e4.
neighboring molecules within one stack and between different
stacks. Moreover, the length of b-axis is approximately eight times
larger than those along the a-axis. In addition, compound 2 has the
highest crystal density (1.377 g cmꢁ3).
A difference between the molecular interactions in compounds
1e4 in terms of O/H/H/O interactions is reflected in the length of
spikes (upper left and lower right at the bottom left of the plot).
Thus, the spikes in the fingerprint of 4 reflect the shortest hydrogen
bond (H/O 2.22 Å), while the length of spikes in the fingerprint of
1 and 3 is comparable (H/O 2.08 Å for 1 and 3). These strong
NeH/O interactions between molecules of terephthalamides are
visualized as bright red spots between the donor and acceptor
atoms on the Hirshfeld surfaces in Figs. S1eS4. The characteristic
“wings” in the fingerprints are identified as a result of C/H con-
tacts. The small contribution of a C/H interactions in the structure
of 2 means that the wings are almost absent in its fingerprint 2D
plot. The H/H contacts appear in the middle of scattered points in
the fingerprint map of 1e4.
2.3. Vibrational spectra of 1e4 terephthalamides with amino acids
ATR-FTIR spectra of pure compounds in the range of
4000e400 cmꢁ1 are shown in Fig. S6. The IR spectra of neat and
deuterated solid-state samples 1e4 powdered in KBr pellets in the
vN-H and vN-D band frequency ranges at two different temperatures
are presented in Fig. 7. The IR spectra of the same samples recorded
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Fig. 6. The relative contributions of each interaction to the Hirshfeld surface in 1e4.
in the frequency range of 1800e1000 cmꢁ1 are shown in Fig. S7.
Comparing these IR spectra it can be observed that the procedure of
sample preparation does not affect the IR spectra of tereph-
thalamides. The Raman spectra, allowing to identify of the posi-
tions of the nC-H bands, are shown in Fig. S8.
The IR spectra of all compounds reveal the NeH amide
stretching vibrations at around 3300 cmꢁ1 (amide A) confirming
the presence of amide sequences in the side chains. The relatively
small difference between the positions of the nN-H stretching vi-
bration band of associated amide groups indicates a similar
strength of H-bonds in the crystals, which is confirmed in the
structural data (Table 2). The lowest frequency of the nN-H band is
observed for 1 (3292 cmꢁ1); the highest for 4 (3321 cmꢁ1). Hence, it
is possible to conclude that H-bonds are stronger in the crystals of 1
than in 4. For 3, the frequency nN-H of the amide group at RT in IR
spectra recorded by different procedures is slightly higher than for
1, although the corresponding N/O distance in the case of 3 is
0.04 Å smaller (Table 2). The very small band above 3400 cmꢁ1
should be assigned to NeH out-of-phase stretching vibrations. The
N-D amide stretching vibrations in IR spectra of deuterated com-
pounds occur in the region 2550-2350 cmꢁ1
.
Other characteristic bands for all compounds can be found at
~1730 cmꢁ1 (ester carbonyl stretching vibrations), ~1633 cmꢁ1
(amide carbonyl stretching vibrations; amide I), ~1540 cmꢁ1 (in-
plane NeH bending vibrations; amide II), 1396-1361 cmꢁ1 (CeN
amide stretching), peaks at 1205-1160 cmꢁ1 (CeO stretching of
esters) ~990 cmꢁ1 (CeO torsion), 863-856 cmꢁ1 (CeC stretching),
683-639 cmꢁ1 (OCO bending) (Fig. S6-S7). The peaks at the lower
region in the spectra at ~860 cmꢁ1 suggest that the benzene ring is
para-disubstituted. In the IR spectrum of 1, the absorption bands of
Fig. 7. IR spectra of (left) neat and (right) deuterated terephthalamides measured for a KBr pellets at (red) 293 and at (blue) 77 K in the 3500-2250 cmꢁ1 frequency range.
Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
7
C]O stretching vibrations of the ester group splits into two com-
of a red shift of this band with cooling from 293 K to 77 K is much
ponents at 1760 cmꢁ1 and 1740 cmꢁ1, indicative of interactions by H
noticeable. The maximum shift is observed for 3 (
D
¼ 18 cmꢁ1), the
bonding of the ester group (C1eH1B/O1 3.724(3)
Å
and
minimum shift takes place for 4 (
D
¼ 12 cmꢁ1) (
D
¼ 16 cmꢁ1 for 1;
C1eH1C/O1 3.725(4) Å). According to the literature, the peaks in
the region 3100-3000 cmꢁ1 are assigned to CeH ring stretching
vibrations. In our case, the peaks with very weak intensity are
observed at 3069 and 3032 cmꢁ1 (1), 3079, 3040 and 3010 cmꢁ1 (2)
and 3080 cmꢁ1 (3) attributed to CeH ring stretching vibrations in
the IR spectra of a KBr pellet (Fig. 7). These peaks are not visible in
ATR-IR spectra. The CeH in-plane ring bending vibrations appear in
the region 1300-1000 cmꢁ1 and the CeH out-of-plane bending
vibrations in the range of 1000e750 cmꢁ1 in the aromatic com-
pounds.14e16 The ring aromatic carbon-carbon stretching vibrations
occur in the region 1650-1400 cmꢁ1 in benzene derivatives.14e16
For aromatic six membered rings, there are two or three bands in
this region due to skeletal vibrations, the strongest usually being at
D
¼ 14 cmꢁ1 for 2) (Fig. 7). The frequencies of the stretching vi-
brations of the skeletal CH, side CH2 and terminal CH3 groups also
change with temperature. The absorption bands of the CeH ring
and CeH aliphatic stretching vibrations in compounds 1e3 are
shifted to a higher frequency when the temperature is lowered
from RT to 77 K. In the case of 4, on cooling the crystals to 77 K, the
positions of the maxima of aliphatic nC-H bands shift to the low-
frequency region. Therefore, the CeH bonds in
4 probably
become stronger on cooling. In view of our earlier results corre-
sponding
other
amide
systems
(3-methyl-
and
and
N-methyl-
4-
methylacetanilide,12n
thioacetanilide
thiobenzamide26), the terephthalamide crystal spectra exhibit
similarly minor temperature effects in the region of
3500e2500 cmꢁ1, i.e., the lower-frequency nN-H band branch re-
mains less intense in the wide temperature range in comparison to
the higher-frequency one. A decrease in temperature causes a
decrease (by ~ 4 cmꢁ1) in the C]O ester and C]O amide stretching
vibrations. At room temperature the absorption band of the NeH
about between 1500 and 1600 cmꢁ114e16
The frequencies of the
.
asymmetric stretching for CH2 and CH3 are a magnitude higher
than the frequencies of the symmetric stretching. Therefore, the
asymmetric and symmetric stretching vibrations of CH2 and CH3
groups in the side chains have absorption bands at 2955 (1), 2959
(2), 2961 (3) 2945 cmꢁ1 (4) and 2853 (1), 2895 (2), 2925 (3),
2871 cmꢁ1 (4), respectively. With the growing length of the amino
acid chain in the compounds, a gradual widening, increase in the
relative intensity and shift of absorption bands corresponding to
the vibrations of aliphatic CeH bonds is observed due to the for-
mation of CeH/O hydrogen bonds. Interpretations of the IR
spectra were based on previous works, found in the literature.17e21
The characteristic Raman peaks for all terephthalamides were as
follows: a weak band at ~3300 cmꢁ1 (stretching of the NH group),
peaks with medium and very strong intensity above 3000 cmꢁ1
(CeH ring stretching vibrations: 3073 and 3058 cmꢁ1 for 1; 3078
and 3038 cmꢁ1 for 2; 3073 cmꢁ1 for 3 and 3074 and 3061 cmꢁ1 for
4), peaks in the range of 3000e2850 cmꢁ1 (aliphatic CeH
stretching: 2988 and 2952 for 1; 2959, 2943, 2899 and 2851 cmꢁ1
for 2; 2961, 2927 and 2874 cmꢁ1 for 3; 2951, 2910 and 2874 cmꢁ1
for 4), a peak at 1734e1742 cmꢁ1 (ester carbonyl stretching vibra-
tions), a band at 1645-1633 cmꢁ1 (amide carbonyl stretching vi-
bending vibrations is observed in the IR spectrum at ~1550 cmꢁ1
,
while on cooling it shifts towards the high frequency region by
6 cmꢁ1. Other absorption bands in the IR spectra are also shifted by
a smaller degree. Comparing the changes in the form and position
of the nN-H absorption bands on cooling in the IR spectra of all
compounds, one can conclude that the absorption bands of com-
pounds are similarly sensitive to temperature variation as the side
chain of compounds lengthens.
This is likely to be also explained by the presence of additional
hydrogen bonding CeH/O engaging the carbonyl groups of ter-
ephthalamide which contribute towards the stability of the crystal
packing.
Upon cooling, the higher-frequency branch of the N-D stretch-
ing vibrations of terephthalamides shifted toward lower fre-
quencies, whereas the lower-frequency branch toward higher
frequencies. In addition, the intensity of the shorter-wave branch of
the nN-D band is higher than the longer-wave one for 1, 2 and 4 at
both temperatures. At 77 K, the nN-D stretching band of 3 changes
the intensity ratio of its two spectral branches whose intensities are
comparable (Fig. 7, Fig. S9).
The particular spectral behaviour of terephthalamides upon
cooling results from the non-advantageous molecular electronic
properties of terephthalamides, which prevent the effective acti-
vation of the “TH” mechanism even at low temperatures. Hence,
the TS-type Davydov coupling involving pairs of closely-spaced
hydrogen bonds in adjacent chains dominates at both tempera-
tures. Only compound 3 does not confirm this regularity at whole.
In general, the magnitude of the TS-type exciton coupling
involving the lateral spaced hydrogen bonds decreases when the
number of the methylene groups associated to the terephthalamide
system increases. Then the lower-frequency branch of the nN-H
band reduces its intensity and the number of spectral bands from
multiple peaks to two lines at 2940 and 2870 cmꢁ1. Additionally,
the hydrogen bonding strength in terephthalamides also seems to
influence the interplay of intra- and interchain exciton couplings.
The NeH/O hydrogen bond between molecules in tereph-
thalamide 3 is slightly stronger than in the other derivatives. As a
result compound 3 exhibits an intrachain exciton coupling effect
(TH-type), beyond the lateral exciton couplings of side-toeside-
type, and the lower-frequency branch of the nN-D band appears
with a noticeable high intensity. Compound 3 also displays a more
complex temperature effect, depending on the intensity growth of
the lower-frequency branch of the nN-D bands upon cooling.
Conversely, the IR spectra of compound 4 (with the weakest
brations),
a
peak at ~1615 cmꢁ1 (aromatic CeC stretching
vibrations), a weak peak at ~1570 cmꢁ1 (NeH bending vibrations),
1345-1315 cmꢁ1 (CeN amide stretching), 1163-1147 cmꢁ1 (CeO
ester stretching), below 1000 cmꢁ1 (CeC skeleton vibrations, OCO
and CeCeC bending vibrations) (Fig. S8). A comparison of com-
pounds 1e4 shows the evolution of the band widths and shapes in
the range of 3100e2800 cmꢁ1 with the lengthening of side chain by
a methylene groups. Hence, compound 4 has the highest propor-
tion of aliphatic CeH relative to aromatic CeH in IR and Raman
spectra. The assignment of Raman bands was based on previous
works.19,22,23
2.4. The temperature effects in IR spectra of the hydrogen bond
The temperature effect on the IR spectra of 1e4 as neat and
deuterated solid-state samples powdered in KBr pellets in the vN-H
and vN-D band frequency ranges is shown in Fig. 7. Fig. S9 shows the
temperature dependent IR spectra of the most intense polarized
components of the crystalline spectra in the ranges of the vN-H and
the vN-D bands.
When the temperature changes from 293 K to 77 K, the IR
spectra of terephthalamide samples exhibit a shift, an increase in
the intensity and a decrease in the absorption band half-widths.
After cooling, the absorption bands of the NeH stretching vibra-
tions of the amide group shifted toward lower frequencies, indi-
cating that the hydrogen bonds become shorter and more stable.
For the IR spectra of the terephthalamide pellets with KBr, the effect
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8
B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
NeH/O hydrogen bond) shows a very weak lower-frequency
branch of the nN-H and nN-D bands and also very weak tempera-
ture effects. Thus, only the TS-type coupling dominates in this
particular case.
2.5. Infrared spectroscopy in polarized light of the hydrogen bond
The spectra of 1e4 single crystals in polarized light recorded in
the vN-H and vN-D band frequency ranges for two polarizations of
the electric field vector E, at 293 K and 77 K, are shown in Fig. 8.
Fig. 8. Polarized IR spectra of (left) isotopically neat and (right) isotopically diluted crystals of 1e4 (1 10% H, 90% D; 2 10% H, 90% D; 3 10% H, 90% D; 4 40% H, 60% D), measured at
293 K and 77 K in the nNꢁH and nNꢁD band frequency ranges. (blue) The electric field vector .E is parallel to the direction of hydrogen bond chains in the lattice (Ekb for 1 and 3, Ekc
.
for 2, Eka for 4); (red) E⊥ to the direction of hydrogen bond chains in the lattice.
Please cite this article in press as: Hachuła B, et al., Insight into hydrogen bonding of terephthalamides with amino acids: Synthesis, structural
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B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
9
All terephthalamides exhibit similar IR spectra, i.e., the dis-
tribution of absorption bands, frequencies and half-widths at
maximum. The polarized IR spectra are characterized by fairly
similar nN-H band contour shapes, i.e., the shorter-wave branch of
the band (3400e3150 cmꢁ1) is of higher intensity than the in-
Our earlier results have showed that in the polarized IR spectra
of hydrogen bonds within the zig-zag chains, the intrachain
vibrational exciton coupling leads to the characteristic linear
dichroic effects, differentiating properties of the lower- and the
higher-frequency branches of the nX-H bands (the second kind of
dichroic effect).24,25 Thus, the individual band branch corresponds
to the proton stretching vibrations in the chain exhibiting a
different symmetry, i.e., the proton “in-phase” stretching vibration
band is characterized by polarization of the transition moment
vector parallel to the molecular chain direction; the excitation of
the proton “out-of-phase” vibrations is polarized perpendicularly to
the chain direction. Therefore a light non-proportionality of the
intensity distribution in comparison to the component sub-bands
of each polarized spectrum should be observed. In the case of our
terephthalamides, the second kind of polarization effect is relatively
weak in comparison to other systems with a chain arrangement of
hydrogen bonds in their lattices even though amides molecules
also form infinite, zig-zag-type open hydrogen bond chains. On the
other hand, the IR spectra of the compounds significantly resemble
the analogous spectra of other amides in the solid-state.12n,26
tensity of the longer-wave spectral branch (3150e2500 cmꢁ1
)
(Fig. 8). Similarly, the higher-frequency branch of the nN-D band
(~2460 cmꢁ1) is of higher intensity than the intensity of the
lower-frequency spectral branch (~2400 cmꢁ1). Such intensity
distribution in the absorption band is typical for the systems
with non-aromatic hydrogen bonds.12le12n For each compound,
differences can be observed between the spectra obtained with
parallel and perpendicular polarization of light (the “first” kind of
dichroic effect). When the IR polarization is parallel to the di-
rection of hydrogen bond chains in the lattice (Ekb for 1 and 3,
Ekc for 2, Eka for 4), the peak of the nN-H stretching vibration at
~3300 cmꢁ1 is prominent, and an intensity of the shoulder in the
region 3150e2500 cmꢁ1, corresponding to the nC-H stretching
vibrations, is considerably weaker. When the E vector polariza-
tion is perpendicular to the direction of hydrogen bond chains in
the lattice (E⊥b for 1 and 3, E⊥c for 2, E⊥a for 4), the intensity of
the longer-wave spectral branch (3150-2500 cmꢁ1) is higher or
comparable to the intensity of the shorter-wave branch (3400-
3150 cmꢁ1). Thus the dichroic properties of the band at
~3300 cmꢁ1 is different from that of the other absorption bands
in this region, but similar to that of the amide I band at
1633 cmꢁ1. This clearly contains information about the relative
orientations of the dipole moment changes associated with these
vibrations. The observed intensity distribution of the IR bands
depends upon the actual orientation of the oriented mono-
crystalline film with respect to the electric field vector of the
polarized radiation.
2.6. H/D Isotope effect in the IR spectra of the hydrogen bond
The IR spectra of deuterated terephthalamides displayed a fre-
quency shifts in the positions of some of the IR peaks in addition to
changes in the relative intensities of the IR bands before deutera-
tion (Fig. 9, Fig. S10).
FT-IR spectra of the terephthalamides upon deuteration reveal a
number of spectral changes that include a marked change in the
position of the C]O amide band between 1600 and 1700 cmꢁ1. A
considerable decrease in the intensity of the NeH amide bending
band, which is centered near 1550 cmꢁ1 but it shifts down in
Fig. 9. Isotopic dilution effect in the polarized IR spectra of terephthalamides presented for the most intense polarized component bands at 77 K in the ranges of (left)
3500e2250 cmꢁ1 and (right) 1850e1000 cmꢁ1
.
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10
B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
frequency upon deuteration to ~1170 cmꢁ1 was also observed.
Replacement of a hydrogen atom by deuterium slightly weakens
the H-bond strength (Ubbelohde effect).27,28 The nN-D absorption
spectra of terephthalamide crystals indicate that the strongest
exciton couplings are via the van-der-Waals forces as “through-
space” ((TS)-coupling) engaging the laterally spaced hydrogen
bonds from neighbouring molecular chains. With the length-
ening of the side chain by methylene groups in 1e4 derivatives,
the interchain TS-type coupling involving the closely-spaced
hydrogen bonds, weakens. After cooling, the hydrogen bonds
become strengthened and the nN-H and nN-D bands are shifted
toward lower frequencies. Moreover, the intensity of the lower-
frequency branch of the nN-D bands increases with the growth
of the hydrogen bond strength upon cooling. This fact is con-
nected with the growth of relative contribution of intrachain TH-
type exciton coupling in the spectra generation in deuterated
amide samples at 77 K. The deuteration process of the amide
groups is evidenced by a growth in the intensity of the nN-D band
two-branch structure pattern. The strength and the mutual ratio
of intra-to interchain exciton couplings have a significant impact
on the occurrence of the H/D isotopic “self-organization” phe-
nomenon. In compounds 1e3 identical H isotope atoms, protons
or deuterons, occupy the independent pairs of opposite, closely-
spaced H-bonds, each belonging to a different chain in the
crystals because the exciton couplings are strong enough. For
compound 4 the TS-type exciton coupling is very weak and then
the longer-wave branch of nN-D band nearly disappears. This is
probably due to a fragmented disorder in the arrangement of
proton and deuterons in the lattice of hydrogen bonds of 4. In
this case the H/D isotopic “self-organization” effects in the spectra
of crystals of 4 are absent.
bands in the IR spectra are characterized by relatively simple fine
pffiffiffi
structure patterns of frequencies related by the 1/ 2 factor with
the corresponding frequency values of the spectral branches in the
nN-H bands. This is the well-known H/D isotopic effect in the area of
the IR spectroscopy of the hydrogen bond.
The absorption bands of N-D groups in the deuterated tereph-
thalamides appear at 2477-2395 cmꢁ1 and they also exhibit a two-
branch structure, and qualitatively similar linear dichroic and
temperature properties to those of the nN-H bands. The property of
the nN-D absorption bands is almost proportional to the change in
their intensity distribution when the polarization of the electric
field vector ‘‘E” of the incident beam is changed in the experimental
conditions. When the temperature is lowered from RT to 77 K, the
absorption band of N-D stretching become sharp, well pronounced
and red shifted. Moreover the higher-frequency branch of the nN-D
bands is still more intense than the lower-frequency branch at both
temperatures for compounds 1e3.
A comparison of the IR spectra of the deuterated to the non-
deuterated compounds in Fig. 9 shows the nN-H bands that are
observed in both IR spectra. Most intriguing is the peak at
~3300 cmꢁ1, which gives unequivocal evidence for the presence of
the “residual” protons, not replaced by deuterons during the iso-
topic exchange process. This peak is characterized by the invari-
ability regardless of the growing isotopic exchange rates in the
samples. Based on our publications, this phenomenon is associated
with a non-random distribution of protons and deuterons in the
lattice of hydrogen bonds of isotopically diluted crystals.11 Then, all
the vibrational exciton interactions involving the lateral spaced
hydrogen bonds from the adjacent chains are retained in isotopi-
cally diluted crystals.
4. Experimental section
4.1. Materials and reagents
This spectacular isotope effect in terephthalamides has proved
that the distribution of protons and deuterons depends on the
interplay of intra-to interchain exciton couplings between
hydrogen bonds in the crystalline lattice upon deuteration.
Furthermore, the relative contribution of each individual exciton
coupling mechanism in the spectra generation can be related to the
strength of hydrogen bond. With increasing hydrogen bond
strength the contribution of intrachain exciton coupling also in-
creases (from 4, 2, 1 to 3). Then, the two-branch structure of the nN-
All reagents (Fluka, Merck) were of analytical grade and used
without any further purification.
4.2. Synthesis of bis-terephthalamides
Terephthalamides 1e4 were synthesized by using the general
procedure: the revelant aminoacid methyl ester hydrochloride
(22 mmol) was suspended in CH2Cl2 or CH3Cl (100 ml) under argon
atmosphere at 0 ꢀC. The mixture stirred for 30 min. Triethylamine
(6 ml) was carefully added to the mixture, and stirred for 30 min.
Therephthaloyl chloride (11.7 mmol) in CH2Cl2 or CH3Cl (30 ml) was
slowly added to the mixture, and then stirred for 2 h at room
temperature. The reaction mixture was washed three times with
H2O. The organic residues were dried over anhydrous sodium sul-
fate, filtered and evaporated to give a solid. The crude product was
recrystallized from CH2Cl2. The final compounds were character-
ized by IR and Raman spectroscopy, 1H NMR and 13C NMR spec-
troscopy and mass spectrometry. Evaporation of a D2O solution of
each compound at room temperature and under reduced pressure
allowed us to obtain its deuterium-bonded derivative. It appeared
that the deuterium substitution rates for the nN-H groups for the
title systems varied in the following ranges: 90e99% D and 10-1% H
for 1e3 and 60% D and 40% H for 4.
band appears, the distribution of hydrogen isotopes between
D
hydrogen bonds in deuterated 1e3 crystals is non-random and the
dynamical co-operative interactions exist. Conversely, the distri-
bution of protons and deuterons is random or partly random when
only one very weak exciton coupling predominates in deuterated
samples. Thus, the H/D isotopic “self-organization” effects in the
spectra of 4 crystals are probably absent due to very weak TS-type
exciton coupling involving hydrogen bonds in the deuterated
crystal lattice.
3. Conclusion
We have synthesized and structurally evaluated new bis-ter-
ephthalamides 1e4 to obtain the non-conventional molecular
systems whose spectral properties could deliver arguments for
testing the contemporary models of the exciton interactions
involving hydrogen bonds. The NeH/O and CeH/O hydrogen
bond interactions were found in four individual molecular sys-
tems. It is evident from our spectral results that the nN-H band of
terephthalamides is sensitive to the substituent, the temperature
and the isotopic dilution. The two-branch structure of the nN-H
and nN-D bands with their unique intensity distribution patterns,
the polarization and temperature effects observed in the IR
Terephthalamide 1. White powder, Yield (1.893 g, 65%), mp
165 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.85 (s, 4H), 6.82 (br s, 2H),
4.25 (d, J ¼ 5.1 Hz, 4H), 3.81 (s, 6H); 13C NMR (101 MHz, CDCl3)
d:
170.4, 166.5, 136.6, 127.4, 52.5, 41.8; IR (KBr):
n
¼ 3288 (s), 2988(w),
2954(w), 2852(w), 1740(s), 1633(s), 1539(s), 1502(w), 1434(w),
1422(w), 1374(m), 1329(w), 1296(w), 1272(w), 1025(m), 1183(m),
1167(m), 1073(w), 998(w), 872(w), 863(w), 745(w), 665(m),
532(w), 515(w) cmꢁ1; MS (ESI) m/z calculated for C14H16N2O6:
309.29, found m/z: 309.52 [MHþ].
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B. Hachuła et al. / Tetrahedron xxx (2017) 1e12
11
Terephthalamide 2. White powder, Yield (1.674 g, 52%), mp
Mass spectra were carried out on an Agilent (Varian) 500-MS IT
217 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.81 (s, 4H), 6.91 (br s, 2H),
Mass Spectrometer. Melting points were determined on a Boetius
€
3.76e3.72 (m, 4H), 3.72 (s, 6H), 2.67 (t, J ¼ 5.9 Hz, 4H); 13C NMR
PHMK (VEB Analytik Dresden) apparatus.
(101 MHz, CDCl3) d: 173.4, 166.4, 137.0, 127.3, 51.9, 35.4, 33.6; IR
(KBr):
n
¼ 3312(s), 3012(w), 2959(w), 2895(w), 1735(s), 1632(s),
4.4. X-ray crystallography
1547(s), 1502(m), 1433(s), 1341(m), 1305(m), 1196(m), 1177(m),
1159(m), 1092(w), 998(w), 891(w), 856(m), 732(w), 646(w),
425(w) cmꢁ1; MS (ESI) calculated for C16H20N2O6 m/z: 337.35,
found m/z: 337.63 [MHþ].
Single crystals used for X-ray measurements were obtained
from acetone (1) and methanol (2, 3) by slow evaporation of solvent
at room temperature. Slow crystallization from CHCl3eethanol
solution over a period of several days at 5 ꢀC (refrigerated) afforded
single crystals of 4.
Terephthalamide 3. White powder, Yield (2.915 g, 84%), mp
184 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.83 (s, 4H), 6.72 (br s, 2H),
3.67 (s, 6H), 3.55e3.50 (m, 4H), 2.47 (t, J ¼ 6.9 Hz, 4H), 2.01e1.95
(m, 4H); 13C NMR (101 MHz, CDCl3)
: 174.3, 166.7, 137.0, 127.2, 51.9,
39.9, 31.8, 24.3; IR (KBr):
¼ 3296(s), 2960(w), 2926(w), 1729(s),
1629(s), 1544(s), 1439(m), 1375(w), 1319(s), 1287(m), 1267(m),
The data were collected using an Oxford Diffraction kappa
diffractometer with a Sapphire3 CCD detector and SuperNova
kappa diffractometer with an Atlas CCD detector. The crystals were
mounted on a quartz glass capillary and measured at 298 K. Ac-
curate cell parameters were determined and refined using the
program CrysAlis CCD.29,30 For the integration of the collected data
the program CrysAlis RED was used.29,30 Structures were solved
using direct methods with the SHELXS-2013 program and then the
solutions were refined with the SHELXL-2014/6 program.31 Non-
hydrogen atoms were refined with anisotropic displacement fac-
tors. Hydrogen atoms were treated as ‘‘riding’’ on their parent
carbon atoms with d(CeH) ¼ 0.96 Å and Ueq ¼ 1.5 Ueq(C) for methyl
groups or with d(CeH) ¼ 0.97 Å and Ueq ¼ 1.2 Ueq(C) for methylene
groups or with d(CeH) ¼ 0.93 Å and Ueq ¼ 1.2 Ueq(C) for aromatic H
and NeH atoms. The details relating to the atomic coordinates,
complete geometry of molecules and all crystallographic data are
deposited at the Cambridge Crystallographic Data Center (CCDC No.
1005546e1005548 (1e3) and 1511353 (4)).32
d
n
1253(m), 1196(m), 1162(m), 933(w), 883(w), 860(w), 683(w) cmꢁ1
;
MS (ESI) calculated for C18H24N2O6 m/z: [MHþ] 365.40, found m/z:
365.72.
Terephthalamide 4. White powder, Yield (3.085 g, 77%), mp
147 ꢀC); 1H NMR (400 MHz, DMSO-d6)
d: 8.55 (t, J ¼ 5.6 Hz, 2H), 7.89
(s, 4H), 3.57 (s, 6H), 3.27e3.23 (m, 4H), 2.31 (t, J ¼ 7.4 Hz, 4H),
1.59e1.50 (m, 8H), 1.34e1.29 (m, 4H); 13C NMR (101 MHz, DMSO-
d6)
d:173.8, 165.9, 137.2, 127.5, 51.6, 39.5, 33.7, 29.2, 26.4, 24.6; IR
(KBr):
n
¼ 3316(s), 2943(mw), 2870(w), 1733(s), 1623(s), 1537(s),
1497(w), 1436(w), 1361(w), 1278(w), 1235(w), 1165(m), 1106(w),
861(w), 732(w), 639(w) cmꢁ1; MS (ESI) calculated for C22H32N2O6
m/z: 421.51, found m/z: 421.94 [MHþ].
4.3. Characterization
The FT-IR spectra using ATR technique were recorded on a
Nicolet™ iS™ 50 FT-IR (Thermo Scientific) spectrometer with a
diamond attenuated total reflectance (ATR) module. One hundred
scans were collected for each measurement over the spectral range
of 4000e400 cmꢁ1 with a resolution of 4 cmꢁ1. Absorptions are
denoted as follows: strong (s), medium (m), and weak (w) in the
synthesis section. FT-IR spectra (4000-400 cmꢁ1) using the KBr
pressed disk technique were conducted using a FT-IR Nicolet
Magna 560 spectrometer with a resolution of 4 cmꢁ1 at two tem-
peratures (293 K and 77 K). For each sample, 3 mg terephthalamide
and 150 mg dried KBr were weighted and then were ground in an
agate mortar. Polarized IR spectra (4000-400 cmꢁ1) were also
recorded on a FT-IR Nicolet Magna 560 spectrometer between 4000
and 400 cmꢁ1 (200 scan, resolution 4 cmꢁ1) with the use of an IR
polarizer (The Spectra-Tech IR Polarizer). The spectra of isotopically
pure and deuterated compounds, in KBr pellet and as mono-
crystalline samples, at low temperature were taken using a stan-
dard cryostat system. For the aim of the experimental study, melts
of very small quantities of the sample between CaF2 windows were
cooled by applying a suitable temperature gradient to obtain ori-
ented films. Monocrystalline fragments were selected from the
crystalline films and spatially oriented, using a polarization mi-
croscope (Nikon Eclipse E200). Samples were then exposed for the
experiment by placing them on a metal plate diaphragm with a
1.5 mm diameter hole. Measurements of the polarized spectra of
the compounds were performed for two different orientations of
the electric field vector “E” with respect to an individual crystal
lattice (the polarization angle 0 and 90) and were repeated for ca.
ten different single crystals of a given compound. Raman mea-
surements were made using a Thermo Scientific™ DXR™2xi
Raman imaging microscope. The data were collected using a
532 nm laser and the power on samples was a 10 mW. A Raman
4.5. Hirshfeld surface analysis
The close contacts in the crystals of terephthalamides 1e4 were
quantified using computational Hirschfeld surface analysis and
their associated 2D fingerprint plots with the help of the Crysta-
lExplorer 3.1 software on the basis of crystal structures.33 Molecular
Hirshfeld surfaces in the crystal structures were constructed on the
basis of the electron distribution calculated as the sum of spherical
atom electron densities.34,35 The graphical plots of the molecular
Hirschfeld surfaces mapped with dnorm used a red-white-blue
colour scheme, where white regions represented molecular con-
tacts around the van der Waals distance, red highlighted shorter
contacts, and blue was for contacts longer than the van der Waals
distance.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2017.03.080.
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