The zwitterion of N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine

Article abstract of DOI:10.1007/s11224-009-9547-4

The crystal and molecular structures of N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine and its protonated form have been determined. The Schiff base exists as the zwitterion, which is stabilized by the intramolecular ionic hydrogen bond. According to

Full text of DOI:10.1007/s11224-009-9547-4

Struct Chem (2010) 21:131–138
DOI 10.1007/s11224-009-9547-4
ORIGINAL RESEARCH
The zwitterion of N-(2-carboxyphenyl)-
4-dimethylaminebenzylideneimine
Agata Trzesowska-Kruszynska
Received: 7 August 2009 / Accepted: 5 November 2009 / Published online: 20 November 2009
Ó Springer Science+Business Media, LLC 2009
Abstract The crystal and molecular structures of N-(2-
carboxyphenyl)-4-dimethylaminebenzylideneimine and its
protonated form have been determined. The Schiff base
exists as the zwitterion, which is stabilized by the intra-
molecular ionic hydrogen bond. According to quantum-
mechanical calculation results this tautomeric form is
energetically unfavorable but in the solid and liquid state
the intermolecular interactions support zwitterionic form.
(or benzenoid) and the keto-amine (or quinoid) forms [14].
Various studies have been carried out with these systems to
establish the existence of intramolecular hydrogen bond-
ing, O–HꢀꢀꢀN or N–HꢀꢀꢀO, stabilizing one of the tautomeric
forms. Another form of Schiff base compounds is their
zwitterionic tautomer, which is rarely seen for hydroxy
derivatives. The characteristic property of this form is the
?
-
presence of ionic N –HꢀꢀꢀO hydrogen bond.
Considerable attention has previously been given to
amino acid Schiff bases and their complexes since they
exhibit a wide range of biological activities [15, 16].
Amino acid Schiff bases are used as a model compounds in
the study of enzymatic transformations of amino acids
(e.g., decarboxylation, transamination, and racemization),
Keywords Schiff base ꢀ Amino acid ꢀ Zwitterion ꢀ
NBO analysis ꢀ Spectroscopic analysis
Introduction
0
which require pyridoxal-5 -phosphate as a cofactor [17–19].
Schiff base compounds possess antifungal, anticancer,
anticonvulsant, diuretic, and cytotoxic activities [1–4].
These compounds are often used as ligands in coordination
chemistry and some of their first-row transition metal
complexes exhibit enhanced biological properties [5–7].
They also have applications in organic synthesis, catalysis,
biotechnology, and analytical chemistry [8–11]. Aromatic
imines and their derivatives have been used successfully to
study resonance-assisted hydrogen bonds [12].
During this catalytic process, the proton migrates from
the hydroxyl group to the imine nitrogen atom and in
?
consequence the O–HꢀꢀꢀN hydrogen bond becomes the N –
-
HꢀꢀꢀO hydrogen bond. It is known that the tautomerism of
this intramolecular OHN hydrogen bond has a crucial
influence on the enzymatic processes. The equilibrium
between two tautomeric forms (O–H and the N–H) depends
on many factors such as temperature, solvent polarity, and
the presence of additional hydrogen bonds [20, 21].
Tautomerism and isomerism phenomena for these
compounds are of particular chemical and theoretical
interest in the context of their photochromic and thermo-
chromic properties [13]. In general, hydroxy Schiff bases
display two possible tautomeric forms, the phenol-imine
In accordance with above-mentioned findings, crystallo-
graphic and computational studies have been performed on
the model Schiff base compound derived from aromatic
b-amino acid—the anthranilic acid (2-aminobenzoic acid) to
probe the role of the intramolecular OHN hydrogen bond.
This particular amino acid was chosen because the crystals of
anthranilic acid can contain both its neutral and zwitterionic
form [22]. The anthranilicacid-derived Schiff bases and their
transition metal complexes have been studied for their
antifungal and antibacterial activities [23, 24]. The structure
of N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine
A. Trzesowska-Kruszynska (&)
Department of X-Ray Crystallography and Crystal Chemistry,
Institute of General and Ecological Chemistry, Technical
University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland
e-mail: agata.trzesowska@p.lodz.pl
123

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Struct Chem (2010) 21:131–138
pentahydrate (1) reported herein is an example of a Schiff
base compound existing as the zwitterion, in which one of the
carboxylic group H atoms is transferred to the imine N atom.
Since hydrogen bonding is necessary for ligand activity, the
protonated form (2) was obtained to establish the primary
site available for molecular interactions.
Mo Ka radiation leads to relatively weak diffraction
intensities (over 20% of reflections can be considered as
effectively unobserved). This affects both the refinement
weighting scheme and the goodness-of-fit parameter.
Careful inspection of data allows to assume that the lack of
fit is also due to an underestimate of the variances of the
observations, whose relative values have been correctly
assigned. The relatively large values of refinement final R
factors of 1 originate mainly from some disagreement
between calculated and observed reflections intensities at
high angles (for h larger than 30°). It should be outlined
that observed value of goodness-of-fit parameter, as well as
final R factors lie in the range required for correctly refined
structures (according to prerequisites of International
Union of Crystallography). SHELXS97 [26], SHELXL97
[27], and SHELXTL [28] programs were used for all the
calculations. Atomic scattering factors were those incor-
porated in the computer programs. Details concerning
crystal data and refinement are summarized in Table 1,
selected bond lengths and bond angles are given in Table 2.
Experimental
Synthesis
N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimine
pentahydrate (1) and N-(2-carboxyphenyl)-4-dimethylami-
nebenzylideneiminium chloride monohydrate (2) were
prepared by one-pot synthesis of aldehyde with anthranilic
acid in methanol. A hot solution of anthranilic acid
(
5 mmol) in methanol (40 mL) was added dropwise to a
hot solution of p-dimethylaminobenzaldehyde (5 mmol) in
methanol (30 mL). The mixture was heated under reflux
for 4 h. The solution was then reduced by evaporation to
half-volume and allowed to cool. After 7 days, red crystals
of 1 were formed. Yield: 75%. Part of the product was
dissolved in methanol–HCl solution (pH 3) and left to
stand at 278 K. After 3 days, red crystals of 2 were formed.
Theoretical calculations
Geometry optimization and natural bond orbital (NBO)
analysis [29–31] were performed at the B3LYP/6-
31??G(d,p) level of theory [32, 33] using the GAUSS-
X-ray crystallography
IAN03 [34] program package. The geometric parameters
were employed from crystal structure data. The optimized
geometrical parameters were in agreement with those
found from X-ray measurement within three standard
deviations. The NBO analysis has been employed to
evaluate the stabilization energy of the donor–acceptor
interactions. The atomic charges were calculated according
to natural population analysis (NPA) [29–31], Merz–
Kollman–Singh (MKS) [35, 36], and Breneman [37]
schemes.
The crystals were mounted in turn on a KM-4-CCD auto-
matic diffractometer equipped with CCD detector, and
used for data collection. X-ray intensity data were collected
with graphite-monochromated Mo Ka radiation
˚
(
k = 0.71073 A) at temperature 291.0(3) K, with x scan
mode. A 24-s (1), 21-s (2) exposure time was used and
reflections were collected up to 2h = 72.74° (1) and 50.0°
(
2) (scan width 0.45°). The unit cell parameters were
determined from 2,387 (1), 2,588 (2) strongest reflections.
Both the crystals used for data collection did not change
their appearance. Lorentz, polarization, and numerical
absorption [25] corrections were applied. The structures
were solved by direct methods and subsequently completed
by difference Fourier recycling. All the non-hydrogen
atoms were refined anisotropically using full-matrix, least-
Spectroscopic and thermogravimetric measurements
-
1
The IR spectra (400–4,000 cm ) were recorded with
samples prepared as KBr disks in a Magma 560 spectro-
photometer. UV–Vis spectra were recorded on a Jasco 660
spectrophotometer. The protonation constant was estab-
lished using the DL-50 Graphix Titrator (Mettler-Toledo).
The thermal analysis was carried out in a TG/DTA-
SETSYS-16/18 thermoanalyser coupled with ThermoStar
(Balzers) mass spectrometer. The sample (4.67 mg for 1)
was heated using corundum crucibles up to 1,000 °C, at the
2
squares technique on F . The hydrogen atoms were found
by difference Fourier methods and treated as ‘‘riding’’ on
their parent non-hydrogen atoms and assigned isotropic
displacement parameters equal to 1.5 (N imine, carboxylic
group, methyl groups, and water molecules) or 1.2 (rest of
atoms) times the value of equivalent displacement param-
eters of the parent atoms. The geometry of hydrogen atoms
attached to carbon atoms was idealized after each cycle of
least-squares refinement. It must be noted that the combi-
nation of solely light atoms existing in structure of 1 and
-
1
heating rate of 10 K min in air atmosphere. The products
of decomposition were calculated from TG curves. The
temperature ranges were determined by thermoanalyser
Data Processing Module [38].
1
23

Struct Chem (2010) 21:131–138
133
Table 1 Crystal data and
refinement for 1 and 2
Empirical formula
C
16 26
H N
O
2 7
(1)
16 2 3
C H19ClN O (2)
Formula weight
Crystal system
Space group
358.39
Triclinic
322.78
Triclinic
ꢀ
P1
ꢀ
P1
Unit cell dimensions
˚
a (A)
7.2646(7)
9.9152(5)
7.8297(7)
10.3652(7)
11.0897(8)
63.901(7)
84.108(7)
77.185(7)
788.09(11)
2
˚
b (A)
˚
c (A)
13.4794(6)
82.838(4)
82.954(6)
77.286(6)
935.17(11)
2
o
a ( )
o
b ( )
o
c ( )
3
˚
Volume (A )
Z
-
1
)
Absorption coefficient (mm
0.100
0.256
F (000)
384
340
o
Theta range for data collection ( )
Index ranges
1.53–36.37
2.04–25.00
-12 B h B 12
-9 B h B 9
-
-
16 B k B 16
22 B l B 22
-12 B k B 12
-13 B l B 13
1.005
2
Goodness-of-fit on F
0.842
Final R indices [I [ 2r(I)]
R indices (all data)
R
R
1
1
= 0.0682, wR
2
2
= 0.1723
= 0.2143
R
R
1
1
= 0.0483, wR
= 0.0640, wR
2
2
= 0.1272
= 0.1378
= 0.1978, wR
3
Largest difference peak and hole (e A )
˚
0.335 and -0.390
0.385 and -0.340
Table 2 Selected geometrical parameters
˚
Bond length (A)/angle (°)
1
2
C14–O1
C14–O2
N1–C1
1.245(3)
1.266(3)
1.411(3)
1.301(3)
1.353(3)
1.458(4)
1.447(3)
124.1(2)
124.6(2)
128.3(2)
1.314(2)
1.225(2)
1.412(2)
1.306(2)
1.343(2)
1.472(3)
1.457(3)
122.2(2)
124.7(2)
127.8(2)
N1–C7
N2–C11
N2–C15
N2–C16
O1–C14–O2
N1–C7–C8
C1–N1–C7
Fig. 1 The molecular structure of 1 showing the atom and ring
numbering scheme. The displacement ellipsoids are drawn at 50%
probability level and H atoms are shown as spheres of arbitrary radii.
Dashed line indicates the intramolecular hydrogen bond
Result and discussion
The description of the structures
coplanar. The largest deviation from the Schiff base plane
˚
is 0.220(2) A, for atom O2. The dihedral angle between the
The crystallographic asymmetric unit of 1 consists of one
anthranilic acid-derived Schiff base molecule and five
water molecules (Fig. 1). The Schiff base exists in the
zwitterionic form of the carboxy-imine compound. The
proton from the carboxylic group migrates to the azome-
thine group (–N=CH–) forming a di-polar ion structure
similar to that occurring in the pure amino acid. All
non-hydrogen atoms of the organic molecule are almost
two benzene rings in the structure is 2.0(1)°.
The Schiff base molecule shows the configuration E
with respect to the N1=C7 bond, which is stabilized by the
?
-
intramolecular N1 –H1NꢀꢀꢀO2 hydrogen bond. The
occupancies of the lone pair (nO2), antibonding bond
orbital (rN1–H1N*) and the stabilization energy value
(7.94 kcal/mol) indicate medium strength (donor–acceptor)
non-covalent interactions between p lone pairs on the
z
123

1
34
Struct Chem (2010) 21:131–138
˚
oxygen atom and r*N–H bond. This is rather low hydro-
gen-bond energy involving charged donor and acceptor
atoms, but deletion of this interaction leads to increase of
total molecule energy by 11.6 kcal/mol. Moreover, the N
and O atoms of the imine and carboxylate groups, are
negatively charged, according to NPA, Breneman, and
Merz–Kollman–Singh (MKS) charge values (Table 3).
This confirms the existence of an intramolecular hydrogen
bond. The geometrical parameters of this interaction
Table 4 Hydrogen-bond geometry (A, °)
D–HꢀꢀꢀA
D–H
HꢀꢀꢀA
DꢀꢀꢀA
D–HꢀꢀꢀA
1
N1–H1 NꢀꢀꢀO2
O99–H99OꢀꢀꢀO2
0.83
0.86
0.89
0.88
0.88
0.92
0.89
0.86
0.89
0.98
0.82
1.90
2.11
1.89
2.06
1.95
2.00
2.08
2.07
1.99
1.93
2.05
2.574(3)
2.944(3)
2.779(3)
2.852(3)
2.831(2)
2.871(3)
2.924(3)
2.907(3)
2.833(3)
2.818(3)
2.801(3)
138.5
162.0
177.6
148.6
179.8
155.9
158.0
162.3
159.6
150.1
153.9
i
ii
O99–H99PꢀꢀꢀO1
ii
O98–H98OꢀꢀꢀO1
iii
O98–H98PꢀꢀꢀO2
iv
(
Table 4) are close to those of similar anthranilic acid-
O97–H97OꢀꢀꢀO95
v
derived Schiff bases [14, 39], in which the imine N atom
acts as bifurcated donor.
O97–H97PꢀꢀꢀO95
O96–H96OꢀꢀꢀO97
O96–H96PꢀꢀꢀO98
O95–H95OꢀꢀꢀO96
O95–H95PꢀꢀꢀO99
2
The quantum-mechanical calculations performed for the
isolated molecule show that the N-(2-carboxyphenyl)-
4
-dimethylaminebenzylideneimine is less stable in the
zwitterionic form (1) than in the non-ionic form (3)
Fig. 2). The proton transfer from 3 to 1 is an endothermic
(
N1–H1 NꢀꢀꢀO2
O1–H1OꢀꢀꢀO3
0.82
0.92
0.97
0.92
1.94
1.64
2.16
2.16
2.617(2)
2.532(2)
3.123(2)
3.066(2)
139.6
164.3
170.0
166.9
process with the enthalpy change of 40.9 kcal/mol. It is
worth mentioning that during the geometry optimization
process, one H atom of the N imine group migrated to the
carboxylate anion, forming the non-polar molecule stabi-
lized by the intramolecular O–HꢀꢀꢀN hydrogen bond. The
existence of 1 in the zwitterionic form in the solid
state confirms that the presence of solvent molecules
and forming of non-covalent interactions influences the
equilibrium between tautomeric forms.
vi
O3–H3OꢀꢀꢀCl1
vii
O3–H3PꢀꢀꢀCl1
Symmetry codes: (i) -x, -y ? 1, -z ? 1; (ii) x, y ? 1, z; (iii)
x ? 1, -y ? 1, -z ? 1; (iv) x ? 1, y, z; (v) -x ? 1, -y ? 2, -z;
vi) x ? 1, y - 1, z; (vii) -x ? 2, -y, -z ? 1
-
(
imine—electron withdrawing group and dimethylamine—
electron-donating group. According to second-order per-
turbation theory analysis, the stabilization energy of
intramolecular charge transfer interactions between the
C8–C13 bonding orbital (which behaves as donors) and
N1=C7 antibonding orbital (which acts as acceptors) is
equal to 11.51 kcal/mol. The rNC ? rCC* interactions
between the N2–C16 bonding orbital atom and C10–C11
antibonding orbital have the lowest stabilization energy,
equal to 2.38 kcal/mol.
In general, the atomic charges do not depend on the
-
method used for calculation (Table 3). The COO group is
negatively charged, whereas the CH groups are positively
3
charged. The p-phenylene CH groups have different char-
ges. Those ones lying on the side of the electron-rich
group—dimethylamine have negative charge, whereas
those ones lying on the side of the imine group have
positive charge. This can be explained by the high degree
of conjugation, with a strong push–pull effect between the
The NLMO bond orders [29–31] are equal to 1.53 for
the C14–O1 partial double bond, 1.42 for the C14–O2
partial double bond and 1.49 for the N1=C7 bond. The
bond order can be also calculated by means of the bond-
valence method (BVM), according to which the bond
Table 3 The atomic charges for 1
Atom/group
NPA charge
Breneman charge
MKS charge
O1
-0.704
-0.809
-0.768
-0.375
0.029
-0.717
-0.795
-0.658
-0.321
0.010
-0.706
-0.804
-0.640
-0.413
-0.040
0.278
valence (v ) is defined as a number of electron pairs
ij
O2
forming the bond. The BVM can be successfully used for
organic compounds on condition that the mean bond length
˚ ˚
C13O1O2
N1
(1.42 A for the C–O bond and 1.47 A for C–N bond) is
N1H
C7H
C9H
C10H
C12H
C13H
N2
treated as the bond-valence parameter value (R ) [40]. The
ij
0.390
0.234
bond orders calculated from the Brown–Altermatt equation
0.095
0.018
0.002
(
m = exp[(R - d )/0.37] [41]) for bonds of lengths d
ij
ij
ij
ij
-0.065
-0.020
0.114
-0.120
-0.126
0.115
-0.106
-0.134
0.139
are equal to 1.60 v.u. for the C14–O1 bond, 1.52 v.u. for
the C14–O2 bond and 1.58 v.u. for the N1=C7 bond.
Although these values are higher than NLMO bond orders,
they show the same tendency. The C–O bonds of the car-
boxylate group are delocalized, whereas the bond order of
the NC double bond is lower than expected. A CSD [42]
-0.411
0.248
-0.360
0.212
-0.109
0.135
C15H
3
3
C16H
0.250
0.256
0.135
1
23

Struct Chem (2010) 21:131–138
135
Fig. 2 The tautomeric forms of
1
N
H
N
H
O
O
N
O
O
N
zwitterionic form (1)
non-ionic form (3)
?
search for the N=C bond length in Ph–HN =CH–Ph (33
fragments in 30 X-ray structures) and Ph–N=CH–Ph (1,030
fragments in 722 X-ray structures) systems revealed that the
along the crystallographic a axis (Fig. 3), composed of
1
3
fused rings described by N R (8) and N R (12) motifs [43].
2
2
3 3
From an acidic environment, the solid-state compound
N-(2-carboxyphenyl)-4-dimethylaminebenzylideneimi-
nium chloride monohydrate (2), containing the -COOH
group (Fig. 4) can be obtained. The hydrogen atom is
bonded to the less negative O1 atom (Table 3), which does
not take part in forming the intramolecular hydrogen bond.
This hydrogen atom creates the intermolecular O–HꢀꢀꢀO
hydrogen bond with a water molecule (Table 4). As in
compound 1, the intramolecular N–HꢀꢀꢀO hydrogen bond is
observed but the C–O bond lengths (Table 2) suggest that
˚
azomethine bond (–N=CH–, mean value of 1.277(3) A) is
?
significantly shorter than the –HN =CH– bond (1.303(1) A).
˚
?
Such elongation of the –HN =CH– bond can be explained
by partial transfer of the electron density from the N=C bond
?
to the N –H bond and thus the bond order of the NC double
bond implies weaker character of the N=C bond.
The separation distance between almost parallel
(
inclined at 2.0(1)8) benzene rings of adjacent Schiff base
molecules oriented in opposite directions [symmetry code:
˚
-x, -y ? 1, -z ? 1)], 3.628(2) A, indicates a weak
?
(
it involves the iminium N –H group and the C=O group of
aromatic-stacking interaction. The angle between the vector
linking the one ring centroid and the normal to the second
ring plane is 21.6(2)8. An interesting feature is the presence
of a weak C–Hꢀꢀꢀp ring interaction: C16–H16Cꢀꢀꢀring#1 (#1:
the carboxylic moiety. The molecules of N-(2-carboxy-
phenyl)-4-dimethylaminebenzylideneiminium cations are
linked into dimers through water molecules and chloride
ions (Fig. 5). The weak pꢀꢀꢀp stacking interactions between
parallel aromatic rings of adjacent-protonated organic
molecules can be observed. The perpendicular distances
between the first ring centroid and that of the second ring
˚
C1–C6) with Hꢀꢀꢀring-centroid distance of 3.270 A,
˚
Cꢀꢀꢀring-centroid distance of 3.885 A, C–Hꢀꢀꢀring-centroid
angle of 123.78. These weak C–Hꢀꢀꢀp ring and p–p stacking
interactions form piles of Schiff base molecules along the
crystallographic a axis.
˚
are 3.976(3) and 3.869(3) A, and the angles between the
vector linking the ring centroids and the normal to the five-
membered ring plane are 26.9(2) and 25.7 (2)8, respec-
tively. The protonated iminium cations are oriented in
opposite directions [symmetry codes: (-x ? 2, -y, -z)
and (-x ? 1, -y, -z), respectively].
The crystal structure of 1 is stabilized by O–HꢀꢀꢀO
hydrogen bonds between the water molecules and car-
boxylate groups of imine molecules (Table 4). In addition,
the water molecules form a hydrogen-bonded network
Fig. 3 Part of the packing of
molecules in 1. Dashed lines
indicate hydrogen bonds
a
c
b
123

1
36
Struct Chem (2010) 21:131–138
protonated azomethine group and by the delocalization of
the negative charge.
Spectroscopic studies
The vibrational analysis was carried out for the Schiff base
compound (1) and its protonated form (2). The IR spectra
of 1 and 2 contain characteristic bands of the stretching and
bending vibrations of the aromatic CC, NH, OH, and CH
groups (Table 5). An important spectral feature that can be
used to distinguish protonated and neutral forms of the
imine N atom is the C=N stretching vibration that typically
-
1
occurs between 1,640 and 1,690 cm . This frequency
displays remarkable bathochromic shift when the N atom
of the C=N bond is substituted in such a way that it is able
to take on a more polar character [14]. Thus, the band
corresponding to the C=N stretching vibration appears at
Fig. 4 The molecular structure of 2 showing the atom and ring
numbering scheme. The displacement ellipsoids are drawn at 50%
probability level and H atoms are shown as spheres of arbitrary radii.
Dashed line indicates the intramolecular hydrogen bond
-
1
1
,585 cm for 1 and 2. The most significant differences
between vibrational frequencies of 1 and 2 were observed
for the CO stretching vibrations. The IR spectrum of 2
-
1
showed a strong band at 1,680 cm originating from the
stretching vibrations of C=O bonds, whereas the medium
-
1
intensity band at 1,220 cm can be attributed to vibrations
of C–O groups. This confirms the presence of carboxylic
groups in 2. The observed lowering of C=O frequency is
caused by the presence of hydrogen bonds formed by this
group in the solid state. In the IR spectrum of 1, the bands
corresponding to above-mentioned vibrations are absent.
The IR spectrum of 1 showed strong bands at ca. 1,362 and
Table 5 Vibrational frequencies and assignment of Schiff base
compounds
1
2
Assignment
Fig. 5 Part of the packing of molecules in 2. Dashed lines indicate
hydrogen bonds. [Symmetry codes: (i) –x ? 2, -y, -z ? 1; (ii)
x ? 1, y - 1, z; (iii) -x ? 3, -y - 1, -z ? 1]
mexp.
mexp.
3,057–3,650 br
3,340–3,510 s
2,854, 2,927 w
m(CH)aryl, m(OH), m(NH)
2
,929, 3,020 w
3
m(CH ), m(CH)
1
,680 s
m(C=O)
1
1
1
1
,603 w
,585 s
,543 s
,489 w
1,612 s
1,586 s
1,542 s
1,482 s
m(CC)aryl
Protonation constant
-
m
as(COO ) (1), d(NH), d(OH)
d(CH)aryl
m(CC)aryl
m(CC)aryl
The protonation constant of Schiff base (1) was determined
potentiometrically in methanol–water mixture of 40%
methanol (v/v). Titration was performed at 25 °C and the
ionic strength of the medium was maintained at 0.40 mol/
1
,463 m
1,392 m
1,362 m
1,338 m
1,265 s
1,392 m
d(CH)aryl
-
(COO )
m
s
3
dm using sodium chloride. The value of logK is equal to
3
1,352 w
1,263 w
m(CarylN)
m(CN)
.81 ± 0.14. The protonation constant is related to the
protonation of an oxygen atom of the carboxylate group (as
in the solid state, 2). A comparison between the logK
values for the Schiff base and the pure amino acid
1
,220 s
m(CO)
1,169 s
816 m
768 m
1,159 s
820 m
770 m
3
d(CH ), d(CH)aryl
d(CH)aryl
d(CH)aryl
d(NH)
(logK = 2.25 [44]) suggests that the carboxylate group has
a lower acidicity in 1 than in the anthranilic acid, because it
5
30 m
is stabilized by the hydrogen-bonding interaction with the
1
23

Struct Chem (2010) 21:131–138
137
-
,585 cm , which can be assigned to the stretching
1
1
CB2 1EZ, UK; fax: ?44(0)1223-336033; email: deposit@
-
vibrations of carboxylate groups, the m (COO ) and m
ccdc.cam.ac.uk].
s
as
-
COO ), respectively. This confirms the presence of these
(
Acknowledgments This study was financed by funds allocated by
the Ministry of Science and Higher Education to the Institute of
General and Ecological Chemistry, Lodz University of Technology,
Poland. The GAUSSIAN03 calculations were carried out in the
Academic Computer Centre CYFRONET of the AGH University of
Science and Technology in Cracow, Poland (Grant No.: MNiSW/
SGI3700/PŁ o´ dzka/040/2008).
groups in 1.
The UV/Vis spectra of 1 and 2 were recorded in methanol
solution. They are almost identical and exhibit three
absorption bands. The strong, sharp band at 213 (e =
2
2
2
5,000 m /mol) with a shoulder at 244 nm (e = 9,500 m /
mol) can be attributed to p ? p* transitions within the
phenyl moieties. The third band located at 354 nm
2
(
e = 19,000 m /mol) is due to n ? p* transitions of the
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