Similarities and differences between azomethines and ketimines: Synthesis, materials characterization and structure of novel imines compounds

Article abstract of DOI:10.1016/j.saa.2006.05.016

Imines (ketimines and azomethines) derived from p-dibenzoylbenzene (DB) and terephthalic aldehyde (TA) and two aromatic amines: aniline and 2,6-dimethylaniline have been investigated. Compounds were synthesized via condensation of amines with carbonyl monomers in DMA or amine solution. When using DMA as a solvent, azomethines with high yields were obtained. On the other hand, the amines used as a monomers served also as an effective solvent for the synthesis of the ketanils. This different reactivity of the aldehyde and ketone groups in DMA and in amine depends on the dehydration mechanism being dominated by a kinetic process or thermodynamic one. On the basis of FTIR, ¹³C and ¹H NMR, UV-vis spectra, thermal characteristic and theoretical calculations conclusions are drawn regarding the similarities and differences between azomethines and ketimines. {A figure is presented}.

Full text of DOI:10.1016/j.saa.2006.05.016

Spectrochimica Acta Part A 66 (2007) 1030–1041
Similarities and differences between azomethines and ketimines:
Synthesis, materials characterization and structure of
novel imines compounds
Agnieszka Iwana,∗, Bozena Kaczmarczyka, Henryk Janeczeka,
Danuta Seka, Slawomir Ostrowskib
a Centre of Polymer Chemistry, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland
b Industrial Chemistry Research Institute, 8 Rydygiera Street, 01-793 Warsow, Poland
Received 30 March 2006; received in revised form 9 May 2006; accepted 11 May 2006
Abstract
Imines (ketimines and azomethines) derived from p-dibenzoylbenzene (DB) and terephthalic aldehyde (TA) and two aromatic amines: aniline
and 2,6-dimethylaniline have been investigated. Compounds were synthesized via condensation of amines with carbonyl monomers in DMA or
amine solution. When using DMA as a solvent, azomethines with high yields were obtained. On the other hand, the amines used as a monomers
served also as an effective solvent for the synthesis of the ketanils. This different reactivity of the aldehyde and ketone groups in DMA and in
1
amine depends on the dehydration mechanism being dominated by a kinetic process or thermodynamic one. On the basis of FTIR, 13C and H
NMR, UV–vis spectra, thermal characteristic and theoretical calculations conclusions are drawn regarding the similarities and differences between
azomethines and ketimines.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Azomethines; Ketimines; Spectroscopy
1. Introduction
cations in the field of electronics, opto-electronics and photonics
[10–12].
The study of the properties and the formation of imines
has great interest due to its proceeding in several important
chemical and biological processes, for example in the synthe-
sis of polyazocycles and other nitrogened compounds, or in
the synthesis of penicillin, cefaloporfirin and many other bio-
logically active compounds [1–9]. Additionally, imines have
attracted much attention because of their wide variety of appli-
There has been considerable interest in aromatic compounds
that can be used as prototype of thermostable, luminescence
materials. Most of the papers describe properties of the azome-
thines. Azomethines are known as a class of thermally stable
materials with semiconducting properties [13–16]. Contrary to
azomethines, ketimines in which hydrogen atom in azome-
thine group is replaced by an aliphatic or aryl group, have not
been widely investigated. However, an interest in these poly-
mers is growing up because of their thermal stability, interest-
ing optical and electronic properties and good processability
[17–19].
∗
Corresponding author. Tel.: +48 32 271 60 77
E-mail address: aiwan@cchp-pan.zabrze.pl (A. Iwan).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.05.016

A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
1031
Yield
Table 1
However, we did not found in the literature comparison of the
Synthesis of ketimines and azomethines
azomethines and ketimines properties. In this reason we inves-
tigated the similarities and differences between azomethine and
ketimine compounds.
Code
K1
Solventa
p-Xylene
DMA
Decaline
Aniline
0
0
0
Herein, we report the synthesis of novel azomethines and
ketimines, which have an alternate structure of phenylene and
C N bond. Therefore, an imines chain should be a thermostable
aromatic compound and have electronic functionality because
the structure is analogous to poly(phenylene vinylene), which is
a well known conductive polymer [20–23]. We have succeeded
in the selective synthesis of ketimines with a high yield.
A novel azomethines and ketimines can be used not only as
thermostable materials. Because of the basic centers in these
90
K2
p-Xylene
DMA
Decaline
2,6-Dimethylaniline
0
0
0
90
A1
A2
DMA
DMA
90
90
a
¨
compounds, they can easily bind species of Lewis or Bronsted
p-Toluenesulfonic acid used as a catalyst.
acids, what let tuning their properties and also allow use them as
catalysts supports. On the other hand, the compounds properties
found, can be helpful in the case of proper polymers synthesis
and applications.
2. Results and discussion
2.1. Synthesis of azomethines and ketimines
In order to examine whether the diketone (DB) could react
with aromatic amines or not, compounds were synthesized by
condensing the diketone with respective amines. Two ketimines
(K1, K2) were synthesized: one from amine without substituents
(K1) and the other having methyl substituents in the amine orig-
inating sub-unit (K2) (see Fig. 1). To compare the properties of
ketimines and azomethines compounds two azomethines (A1,
A2) were synthesized by condensing the dialdehyde with respec-
tive amines: one without substituents (A1) and the other having
methyl substituents in the amine originating sub-unit (A2) (see
Fig. 1).
Fig. 2. Synthesis of an azomethines and ketimines.
amines with ketone did not proceed in these conditions (see
Table 1, Fig. 2). Similar phenomenon, was also observed for
polyketimines synthesis [17–19].
However, when the amine (aniline or 2,6-dimethylaniline)
was used as a monomer and as a solvent the condensation with
the diketone gave ketimine compounds with high yield.
In general, the condensation of amines with aldehydes effi-
ciently takes place in different solvent in the presence of p-
toluenesulfonic acid (PTS) as the catalyst and in our case was
successfully carried out in N,N-dimethylacetamide (DMA) solu-
tion to give azomethines compounds The condensation of the
2.2. Spectroscopic analysis of the compounds
The results of the elemental analysis, 1H, 13C NMR and FTIR
spectroscopic investigations are fully consistent with the chemi-
cal structure of the compounds schematically depicted in Fig. 1.
Fig. 1. Chemical structure of synthesized compounds.

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A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
Table 2
Solution 13C and 1H NMR characterizations of ketimines and azomethines studied (CDCl3)
Structure
NMR, ␦ [ppm]
13C NMR (CDCl3, 75 MHz): 130.83 (C1); 136.37, 135.89, 135.74 (C4); 167.63, 167.53 (C5);
141.56, 139.46, 139.36, 139.05, 138.52 (C6); 151.03, 150.98, 150.91 (C10); 120.86, 120.80
(C11); 123.30 (C13); 129.45, 129.37, 129.25, 129.11, 129.01, 128.79, 128.69, 128.64,
128.59, 128.45, 128.23, 127.93 (C7,C3,C2,C12). 1H NMR (CDCl3, 300 MHz): 7.64–7.77
(4H, s, 4xH-7); 6.63–7.51 (20H, m, 2xH-11,12,13,1,2,3).
13C NMR (CDCl3, 75 MHz): 130.63 (C1); 137.20, 136.47 (C4); 166.61 (C5); 141.44, 139.87,
138.90 (C6); 148.70 (C10); 125.67 (C11); 122.80 (C13); 127.84, 128.36, 129.23, 127.74
(C7,C3,C2,C12); 18.53 (CH3). 1H NMR (CDCl3, 300 MHz): 7.61–7.84 (4H, s, 4xH-7);
6.76–7.51 (16H, m, 2xH-12,13,1,2,3); 2.03–2.08 (12H, s, 4xCH3).
13C NMR (CDCl3, 75 MHz): 159.45 (C5); 138.03 (C6); 129.21 (C7); 151.73 (C10);
120.92(C11); 129.13 (C12); 126.32 (C13). 1H NMR (CDCl3, 300 MHz): 8.71 (2H, s, 2xCH);
8.08 (4H, s, 4xH-7); 7.20–7.50 (10H, m, 2xH-11,12,13).
13C NMR (CDCl3, 75 MHz): 161.89 (C5); 138.53 (C6); 129.71 (C7); 150.92 (C10); 123.88
(C11); 128.08 (C12); 126.96 (C13); 18.27 (CH3). 1H NMR (CDCl3, 300 MHz): 8.29 (2H, s,
2xCH); 8.05 (4H, s, 4xH-7); 6.92–7.15 (6H, m, 2xH-12,13); 2.16 (12H, s, 4xCH3).
2.2.1. NMR spectroscopy analysis
more stable. A splitting of the signals in 13C NMR spectrum
The proposed assignments of the NMR lines registered for
azomethines and ketimines are listed in Table 2.
seems to confirm the coexistence of the isomers (see Table 2) and
therefore the carbon NMR spectra revealed that K1 and K2 have
isomerswhileA1andA2existonlyintheoneisomericform. The
splitting of the signals observed for carbon atoms C4 and C6 in
the ketimine structures may indicate the presence of both Z and
E structures in the compound K1 and K2. Ketimine carbon atom
(see C5 in Table 2) is splitted only in the model K1. However, the
signals of carbon atoms of central aromatic ring in the diketone
(see C6 in Table 2) and the signal of carbon atom of pendant ring
bonded to ketimine carbon (see C4 in Table 2) are splitted in the
K1 and K2. On the other hand signals of carbon atoms of amine
ring (see C10 in Table 2) are splitted only in K1 in which there is
no steric hinderance that can be caused by methyl substituents.
No splitting signals were detected in the azomethine NMR
spectra.
In particular the 13C NMR signals in the range of
159–167 ppm (see C5 in Table 2), present in the spectra of all
azomethines and ketimines, confirm the existence of the imine
group carbon atoms and are up-field shifting for the compounds
with the diketone sub-units. For example, in the spectrum of
K1 a clear signal characteristic of the carbon atom in the C N
group (see C5 in Table 2) is seen at ca. 167 ppm, while in the
case of A1 a clear signal characteristic of the carbon atom in
the azomethine group is observed at ca. 159 ppm. On the other
hand, the presence of the methyl groups in the amine sub-unit
of ketimine K2 results in a down-field shift of this signal by
ca. 1.5 ppm in comparison with K1, while the presence of the
methyl groups in A2 causes an up-field shift of this signal by
ca. 2.5 ppm in comparison to A1 (see Table 2). The changes in
the chemical shift, observed upon the modification of the chem-
ical constitution of the amine/carbonyl compound originating
sub-unit, are consistent with the assignment to another carbon
atoms and are proposed in Table 2. Thus, in the case of K2, the
presence of methyl groups in the ortho position with respect to
nitrogen results in a down-field shift of the line related to carbon
number 10 and 13 in comparison to K1. On the other hand in the
case of A2, the presence of methyl groups in the ortho position
with respect to nitrogen results in a down-field shift of the line
related to carbon number 10 and a slightly smaller up-field shift
of the line ascribed to carbon number 13 in relation to A1.
Additionally it should be pronounced that in the case of the
imine structure two isomers are possible (E and Z), even though
the E structure is always supposed to be thermodynamically
Summary 13C NMR study of the compounds we would like
to emphasize that contrary to azomethines (A1, A2), which
exist only in the one isomeric form (see Table 2), the split-
ting of the signals in 13C NMR spectrum of ketimines K1
and K2 are achieved by the different isomeric form of K1 and
K2. Therefore, a novel feature of the ketimines can be com-
pared to azomethines. The extensive data relating to the azome-
thines and ketimines have shown that the substituent effects on
the carbon atom in imine group signal chemical shifts in 13C
NMR can be explained by considering the different resonance
structures.
Additional informations concerning the compounds struc-
tures provides analysis of their 1H NMR spectra. The proposed
1
assignments of the H NMR lines registered for azomethines
and ketimines are listed in Table 2.

A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
1033
Fig. 3. 1H NMR lines of the methyl groups of K2 in DMSO-d6: (a) 22 ◦C, (b) 80 ◦C, (c) 22 ◦C.
1
Solvent effect is manifested in the H NMR spectra of the
ketimines and azomethines. The proton NMR measurements of
ketimines and azomethines in DMSO-d6 and CDCl3 solution
were investigated. Signal of methyl group in the K2 detected in
CDCl3 is observed as three separated peaks which are located at
2.08, 2.05, 2.03 ppm and are up-field shifted in comparison with
signal of methyl group in the compound detected in DMSO-
d6 which were seen at 1.94, 1.97 and 2.01 ppm as is shown in
Fig. 3a. To contrary signal of methyl group in the A2 detected in
CDCl3 is observed as one peak which is located at 2.16 ppm and
is up-field shifted in comparison with signal of methyl group in
A2 detected in DMSO-d6 (2.08 ppm).
In this part of our work we investigated the possible conform-
ers in ketimine K2 structure. By warming the model compound
K2 at 80 ◦C in DMSO-d6 solution, the three methyl signals in
1H NMR spectrum couple into one broad (see Fig. 3b).
Subsequent lowering temperature to 22 ◦C causes the reap-
pearance of well resolved signals (see Fig. 3c). Another
behaviour was observed for the A2. Heating to 80 ◦C caused
no change of the 1H NMR spectrum.
Table 3
Approximate assignment of IR bands of azomethines and ketimines investigated
Assignment
A1
K1
A2
K2
νCHr
νCHr
νCHr
νCHr
νCHr
νCHr
νCH3
νaCH3
νaCH3
νCH
3080
3059
3078
3062
3078
3060
3045
3066
3037
3030
3020 sh
3030
3020
3025
3018sh
2962
2941
2913
3018
2963
2945
2918
2880
2855
1635
1591
2877
νsCH3
2850
1619
1591
1490
νC
N
1617
1585
1578 sh
1481
1613
1592
1578
1482
νPh
νPh
νPh
δaCH3
νPh
νPh
δsCH3
δCH
1470
1441
1465
1444
1402
1378
1449
1414
1449
1405
1372
1354
2.2.2. Infrared spectroscopy analysis
1367, 1355
Furthermore, FTIR spectroscopic characterization to be pre-
sented below provides additional evidence for the imine groups
in these four model compounds. The presence of imine groups
(ketimine, azomethine)isconfirmedbyFTIRspectroscopysince
in each case the band characteristic of the C N stretching vibra-
tions is detected. Assignment of particular bands appearing
in FTIR spectra of azomethines and corresponding to these
ketimines are shown in Table 3.
Imine (C N) group stretching vibration band of unsubsti-
tuted K1 and A1 do not shift significantly being only 4 cm−1
shifted towards higher wavenumbers in the case of ketimine
compound. In the methyl substituted compounds the most con-
siderable changes are observed in the case of C N group stretch-
ing vibrations band for azomethines A1 and A2 being at 1617
and 1635 cm−1, respectively. At ketimines this band shifts also
towards higher wavenumbers, but the difference between K2 and
K1 equals 4 cm−1. It can be proved that strong positive inductive
effect of methyl groups in azomethine is notably diminishing in
ketimine. It causes that differences in position of these bands
in ketimines K1 and K2 are distinctly lower than in the case of
corresponding azomethines.
1318
1300
1314
1299
δCH
1312
1301
1283
δCHr
ωCH
δCHr
δCHr
1299
1277
1253
1178
1150
1111
1090
1040
1017
1000
980
1265
1179
1153
1112
1072
1256
1183
νC
N
1190
1166
1106
1074
δCHr mono
δCHr mono
δCHr 1,4
δCHr 1,2,3
δCHr 1,4
δCHr mono
γCHr 1,4
γCHr mono
γCHr mono
γCHr 1,4
γCHr 1,4
γCHr
1088
1041
1018
1023
969
1025
1000
977
983
958
962
948
914
860
912
868
919
861
850
820
776
767
γCHr 1,4
γPhmono
γPh1,4
830
836
827
775
767
760, 756
696
770
733
γPh
γPhmono
γPh1,2,3
γPh
692
672
553
698
677
Similarly, the position of the bands assigned to stretching
vibrations of C N group shifts towards lower wavenumbers in

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A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
ketimines but contrary to the stretching vibrations of C N group,
difference in band position is higher in the case of the unsubsti-
tuted azomethine than ketimine one (A1 and K1, 11 cm−1; A2
and K2, 3 cm−1). The region of 3300–3000 cm−1 is character-
istic for stretching vibrations of C H group in aromatic ring.
Comparing the FTIR spectra of azomethines investigated in that
region allows to assign appearing bands to stretching vibrations
of C H groups in particular, differently substituted aromatic
rings.
Simultaneously, in the case of A2 and K2 some changes in
positions and intensities of the bands characteristic of CH3 group
stretching vibrations are also observed. The band appearing at
2941 cm−1 (see Table 3) slightly shifts to lower frequency but its
intensity notably increases, in opposite to deformation vibration
in which cases the bands corresponding to them shift to higher
wavenumbers.
Substitution of hydrogen atoms by phenyl groups in C N
of the compound, similar as by CH3 groups in 2,6-position
of aromatic ring does not influence on position of the band
corresponding to stretching vibrations of phenyl ring in the
region of 1600–1400 cm−1 (see Table 3). In ketimine K1 spec-
trum additional band appearing at 1578 cm−1 and as a shoulder
in azomethine A1 spectrum can be interpreted as characteris-
tic for stretching vibration of mono substituted aromatic ring.
This band in the case of K2 appears at 1490 cm−1 and is not
observed in azomethine A2 spectrum. It is obvious that the bands
assigned to stretching and deformation vibrations of C H group
in azomethines disappear in ketimines spectra due to substitution
of hydrogen atom by phenyl ring.
It can be concluded that described above differences in FTIR
spectra of azomethines and corresponding to them ketimines
are the result of changing of charge distribution in the com-
pound chains after substitution of hydrogen atom by phenyl
ring. The presence of CH3 groups in 2,6-positions of aromatic
ring increases these changing. It can strongly influence on their
chemical and physical properties.
in the benzene ring and in the imine group, respectively. The
band of ␲–␲ transition, at about 274–300 nm was observed.
*
The second absorption band characteristic to the imine group
in the ketimine and azomethine compounds is located in the
spectral range of 340–360 nm and its position and intensity are
dependent on the chemical constitution of the compounds and
kind of solvent (see Table 4).
2.2.4. UV–vis spectra of compounds in DMA solution
For the ketimine the presence of methyl groups in the amine
(K2) effects the bathochromic shift of the absorption band char-
acteristic for C N group in comparison with the unsubstituted
ketimine (K1) in DMA solution (360 and 349 nm, respectively).
Different behaviour for azomethine compounds was detected.
The UV–vis spectra of azomethines A1 and A2 in DMA solution
show the absorption band responsible for transition in azome-
thine moiety at 350 nm being independent of the amine structure
(see Table 4).
2.2.5. UV–vis spectra of compounds in m-cresol (MC)
solution
The compounds studied exhibit solvatochromic behaviour
what is a common phenomenon in conjugated compounds and
its origin is molecular [24–26]. It arises from conformational
changes of the compounds as a result of their interactions with
solvent molecules.
A shift is observed for both bands when an aprotic sol-
vent such as DMA is replaced by protic m-cresol (MC). With
an increase of polarity of the solvent and consequently with
increase in the solvent refractive index (n2D0 = 1.5400 for MC,
n2D0 = 1.4380 for DMA) the UV–vis spectra of ketimines show
a red shift which can be additionally attributed to correspon-
dence between OH group in MC and the ketimine group (C N)
in K1 and K2. The polar hydroxylic solvent interacts with the
compound via H-bond interactions resulting in bathochromi-
cally shifted absorption bands (see Table 4). However, it should
be noted that for the ketimine the presence of methyl groups in
the amine (K2) effects the smaller shift of the absorption band
characteristic for C N group in comparison with the unsubsti-
tuted ketimine (K1) in MC solution in relation to the values in
DMA solution (shift for K1 is equal to 51 nm, while for K2 is
10 nm). The explanation of this behaviour may be a hindered
rotation caused by the presence of the substituents which effects
the compound K2 to be more stiff and more restricted for H-
bond formation. Additionally for the compounds from different
diamines and p-dibenzoylbenzene, i.e. K1 and K2, the intensity
of the band responsible for transition in benzene ring is compara-
ble. When aniline was replaced by 2,6-dimethylaniline intensity
of the band due to transition in ketimine group is much lower and
a long tail toward longer wavelength is observed. Fig. 4 shows
the electronic absorption spectra of the ketimines K1 and K2 in
m-cresol solution.
2.2.3. UV–vis spectroscopy analysis
The UV–vis spectra of the four compounds also provided
additional evidence confirming their molecular structures. In the
following discussion, the effects of structural modifications will
be explored in terms of the effects of backbone variation. The
UV–vis spectra of compounds in DMA and MC solution were
investigated (see Table 4).
Consistent with the chemical nature of the compounds, the
UV–visspectraofketiminesandazomethinesmodelcompounds
show the presence of two bands ascribed to electronic transition
Table 4
Solution UV–vis data of ketimines and azomethines studied
Code
Solution UV–vis spectra
λmax (nm) in DMA
λmax (nm) in MC
Differentbehaviourforazomethinecompoundswasdetected.
First, we would like to pronounce that the UV–vis data of azome-
thines are independent of the diamine structure. In MC solutions
they exhibit the imine absorption band at the same value at
340 nm. However, the intensity of this band is also lower for
K1
K2
A1
A2
282, 349
274, 360
300, 350
282, 349
340, 400
300, 370
300, 340
300, 340

A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
1035
interesting to add that different behaviour for compounds from
2,6-dimethylaniline was observed. In this case the higher melt-
ing point for azomethine compound (A2) was detected (226 ◦C)
in comparison to K2 (202 ◦C). Also it should be emphasized
that compounds from 2,6-dimethylaniline (K2, A2) exhibited
significantly higher Tm than the proper compounds synthesized
from aniline (see Fig. 5). The presence of hindering four methyl
groups probably restricts rotation at phenyl ring–nitrogen link-
age and influences on increase of melting temperature.
As it was said previously K1 may exist in cis and trans form,
what was confirmed by NMR, theoretical calculation and addi-
tionally by DSC method. Dependly on the heating rate different
spectra were detected. Melting point of the K1 in the heating rate
20 ◦C/min under N2 atmosphere at 180 ◦C was observed, while
when the rate was lower (5 ◦C/min) two endotermic peaks on
DSC-gram (melting points) of the K1 at 169 and 180 ◦C were
detected, being probably characteristic for two isomers being
present (see Fig. 6). However, we were not able to separate the
isomers.
Fig. 4. Solution UV–vis spectra of the compounds: (—) K1, (- - -) K2 in m-cresol
solution.
The compounds studied readily crystallise which is facili-
tated by their symmetry and rigid structure. Their X-ray diffrac-
tograms reveal a set of well-defined Bragg reflections [27]. The
X-ray diffractograms of the ketimines and azomethines com-
pounds showed different patterns (see Fig. 7).
More detailed work on the crystallinity of compounds is in
progress.
Fig. 5. DSC results obtained for ketimines and azomethines.
the compound from 2,6-dimethylaniline, as it was observed for
the ketimines. A major feature of the absorption spectra of the
azomethines A1 and A2 in m-cresol solution is little blue shift of
the band responsible for transition in imine group in comparison
to this band in DMA solution (see Table 4).
The observed absorption shifts when going from DMA solu-
tionto MC solutionclearly indicatethatthispropertyisgoverned
by interaction between the compound and environment.
3. Modelling of possible isomeric structures of the
compounds
Alkyl- or aryl-substituted azomethines and ketimines are
known to exist as stereolabile E, Z isomers that interconvert with
a free energy of activation covering the range of 12–29 kcal/mol,
depending on the nature of the substituents. This process might
be due, in principle, either to the rotation at the C N bond or to
the inversion of the syn-anti [28].
2.2.6. Melting point of the imine compounds
Presence of double bonded carbon–nitrogen atoms (C N)
in ketimines and azomethines compounds creates possibility of
different isomers formation. In the case of compounds having in
each molecule two double bonds one can expect three isomeric
structures, i.e. trans–trans (E–E), trans–cis (E–Z) and cis–cis
(Z–Z) even though the trans structure is usually supposed to be
the most thermodynamically stable.
All the compounds synthesized were crystalline and their
melting points (Tm) were detected by differential scanning
calorimetry (DSC). Melting points of the compounds from ani-
line (A1, K1) were found at the range 159–180 ◦C (see Fig. 5).
However, it is necessary to mention that azomethine com-
pound (A1) exhibited lower Tm than ketimines one (K1). It is
Fig. 6. Differential scanning calorimetry (DSC) curves of K1: (a) heating rate 20 ◦C/min, (b) heating rate 5 ◦C/min.

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A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
Fig. 7. X-ray spectra of ketimines and azomethines.
The reaction of amine with diketone leads to Z–Z, Z–E and
E–E isomers, while in the case of azomethine oligomers only
one isomeric form is dominated.
the rotation of (C6H5)N C(C6H5) group with respect to the
(C6H5)N C(C6H5) moiety is observed.
Taking into account the symmetry of K1 it could be conspicu-
ous ketimine K1 at symmetry C2h (form V), C2 (form II) and CS
(form I). Energetical differences between conformers (I, II, IV, V
and VII) are very small, and calculations by semiempiric method
AM1 suggest that these conformers could coexist with K1.
In Supplementary materials the structural parameters (calcu-
lated) of K1 are presented. In Fig. 9 labeling of atoms in the
conformers of K1 are showed. Analysis of the bond lengths in
K1 shows, that the values are similar in conformers I–VII (see
Supplementary materials, especially see R12 and R38).
Differences we see in valence angles (see Supplementary
materials). It appears that the C2–C13–N7 (A5), N7–C12–C15
(A21) and N36–C35–C37 (A61) angles in I conformer (123.3◦,
127.3◦, 127.3◦, respectively) are larger than in the another con-
formers of K1. Analysis of the valence angles in ketimine group
of K1 shows, that the values are similar in conformers I–VII (see
A16 (C3–N7–C12) and A62 (C35–N36–C38) in Supplementary
materials).
Additionally, formation of conformers cannot be excluded.
The calculations were performed by semiempiric method AM1
[29] by using the Gaussian 03 [30] program and the main goal
of the calculations was to found the most stable conformers of
the ketimine K1 and azomethine A1.
The approximate estimations indicate that K1 may exist in
about 36 × 22 = 2916 possible conformers forms. Energy was
calculated for 36 conformers for each Z and E isomer of K1, and
the most stable forms whereas chosen. Fig. 8 presents the most
stable forms of K1, while in Table 5 the E and ꢀE (calculated)
are presented for the compound K1.
The difference of ꢀE of the ketimine K1 conformers I–VII
were in the range of 0.0–1.9 kcal/mol. The calculated vibrational
frequency are positive what confirmed the found conformers of
K1 to correspond with minimum of potential energy hypersur-
face. The most stable conformer of K1 is V and exhibits C2h
symmetry (see Table 5 and Fig. 8).
Ketimine model compound K1 could exist in E–E (V–VII)
and Z–Z (I–IV) isomeric form (see Fig. 8). Addition-
ally, the isomers could exist in different conformers form,
because the rotation of (C6H5)N C(C6H5) group in relation
to (C6H5)N C(C6H5), (C6H5)N C(C6H5)–C and the rota-
tion of aromatic rings about C6H5–N and C6H5–C bonds
are possible. Our calculation showed that the most stable
conformers are the Z (I–IV) and E (V–VII) forms because
On the other hand C6–C3–N7 (A6), N7–C12–C14 (A20) or
C31–C35–N36 (A59) angles (117.9◦, 118.9◦, 118.9◦, respec-
tively) in I conformer are smaller than in another conformers
(see Supplementary materials). Differences we see also in tor-
sion angles (see Supplementary materials).
The calculations showed that A1 may exists in about
34 × 3 = 243 possible conformers forms. Energy was calculated
for all conformers of A1, and the most stable forms were chosen.
Fig. 8 presents the most stable forms of the A1, while in Table 5
the E and ꢀE (calculated) are presented for the compound A1.
The difference of ꢀE of the azomethine A1 conformers
I–III were in the range of 0.0–0.31 kcal/mol. The calculated
vibrationalfrequencyarepositivewhat confirmedthefoundcon-
formers of A1 to correspond with minimum of potential energy
hypersurface. The most stable conformer of A1 is I and exhibits
the Ci symmetry.
Azomethine model compound A1 could exists in E–E (I),
Z–Z (II) and Z–E (III) isomeric form (see Fig. 8). Al1 could
existsindifferentconformerforms, becausetherotationofgroup
around the single bonds are possible. As for K1 energetical dif-
ferences between the most stable conformers (I–III) are very
small, and calculations by semiempiric method AM1 suggest
that this conformers could coexist with A1.
Table 5
Total energy (E) and relative energy (ꢀE) of conformers I–VII of K1 and I–III of
A1 performed by semiempiric method AM1 by using the Gaussian 03 program
Conformers
E (kcal/mol)
ꢀE (kcal/mol)
I (K1)
186.0950
186.0092
186.7385
186.0092
185.9556
187.8293
186.0165
124.9151
125.2303
125.0603
0.14
0.05
0.78
0.05
0.00
1.87
0.06
0.00
0.32
0.15
II (K1)
III (K1)
IV (K1)
V (K1)
VI (K1)
VII (K1)
I (A1)
II (A1)
III (A1)

A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
1037
Fig. 8. Molecular conformers of K1 and A1 performed by semiempiric method AM1 by using the Gaussian 98 program.
In Supplementary materials the structural parameters (calcu-
lated) are presented of the A1. In Fig. 10 labeling of atoms in
the conformers of A1 are shown.
Analysis of the bond lengths in A1 shows, that the bond
lengths are of the similar values in conformers I–III (see
Supplementary materials). Similar result for K1 was observed.
For example, bond lengths of imine group in K1 and A1 com-
pounds appears at about 1.29 A (see Supplementary materials,
especially R12, R38 and R13, R27, respectively).
As in the case of K1, differences we see in valence
angles (see Supplementary materials). It appears that the
C3–C1–N12 (A2), C1–N12–C13 (A19), N12–C13–C14 (A20),
C13–C14–C17 (A24), C19–C15–C24 (A28) angles in I con-
former are larger than in the another conformers of A1.
On the other hand C4–C1–N12 (A3), N12–C13–C38 (A21),
C13–C14–C16 (A23), C18–C15–C24 (A27) or N25–C24–C37
(A43) angles in I conformer are smaller than in another con-
formers (see Supplementary materials).
Analysis of the valence angles in azomethine bond of A1
shows, that the valence angles are not of the similar values in
conformers I–III (see Supplementary materials). It appears that
the C1–N12–C13 angles in I conformer are larger than in the
conformers II and III of A1 (125.7◦, 121.2◦, 121.2◦, respec-
tively).
Comparison of the valence angles in imine group of A1 and
K1 shows, that valence angle in C1–N12–C13 of A1 is larger
than in the C3–N7–C12 of K1 (125.7◦, 124.5◦, respectively).
Differences we see also in torsion angles (see Supplementary
materials).
Summary this part of our work we would like to empha-
sized that the presented theoretical results show that ketimine
and azomethine model compounds could exist in isomeric form.

1038
A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
Fig. 9. Labeling of atoms in the conformers of the K1 compound.

A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
1039
compounds was obtained. On the other hand DMA as a solvent
was with successful used in the synthesis of the azomethine
compounds. The structure of the compounds were confirmed
by NMR, FTIR, and UV–vis spectra and the differences and
similarities in spectral characteristics between ketimines and
azomethines compounds were presented. The following differ-
ences were observed:
• Up-field shifting of the imine band of ketimines in the 13C
NMR spectra in comparison with azomethines (167, 159 ppm,
respectively).
• Coexistence of the isomers of ketimine compounds what were
observed as a splitting of the signals in 13C NMR spectrum,
while azomethines exist only in the one isomeric form.
• Coexistence of the conformers of ketimine compound K2 was
1
observed by H NMR method, what was not observed for
azomethine A2.
• Imine (C N) group stretching vibration band of azomethines
is shifted towards higher wavenumbers in comparison with
ketimine compounds.
• Hypsochromic shift of the absorption band characteristic for
C N group in azomethines in MC solution in comparison
with DMA, while for ketimines the reverse behaviour was
detected.
• Ketimine K1 may exist in about 2916 possible conformers
formswhileinthecaseofazomethineA1only243conformers
forms are possible. The most stable conformer of K1 exhibits
C2h symmetry while for A1 the Ci symmetry was found. The
energy of ketimine K1 is higher than the energy of azomethine
A1 (186 kcal/mol, 125 kcal/mol, respectively).
5. Experimental section
5.1. Materials
Acetone was dried and distilled in the usual manner.
Methanol (POCh), ethanol (POCh), hexane (POCh), N,N-
dimethylacetamide (DMA) (Aldrich) and terephthalic aldehyde
(TA) (Aldrich) were used without any purification. Aniline
(Aldrich) and 2,6-dimethylaniline (Aldrich) were distilled prior
to their use. p-Dibenzoylbenzene (DB) was synthesized using
Friedel–Crafts reaction as described in Ref. [25].
Fig. 10. Labeling of atoms in the conformers of the A1 compound.
Additionally, the approximate estimations indicate that K1 may
exist in about 2916 possible conformers forms while in the
case of azomethine A1 only 243 conformers forms are pos-
sible. The most stable conformer of K1 exhibits C2h symme-
try while for A1 the Ci symmetry was found. Additionally it
should be stressed that the energy of ketimine K1 is higher
than the energy of azomethine A1 (186 kcal/mol, 125 kcal/mol,
respectively).
5.2. Characterisation techniques
The obtained models were characterised by the following
techniques: elemental analysis (240C Perkin-Elmer analyser),
1H NMR and 13C NMR (Varian Inova 300 Spectrometer, CDCl3
and DMSO-d6 solvent against TMS as an internal reference),
and FTIR. Infrared spectra were acquired on a DIGILAB
FTS-40A Fourier transform infrared spectrometer in the range
of 4000–400 cm−1 at a resolution of 2 cm−1 and for an accu-
mulated 32 scans. Samples were analyzed in a form of pellets in
potassium bromide. Molecular weights of the synthesised com-
pounds were determined using ESI–MS experiments with an
electrospray ionization (ESI). ESI–MS experiments were per-
formed using a Finnigan MAT TSQ 700 triple stage quadrupole
4. Conclusion
Novel aromatic azomethines and ketimines were synthesized
with high yields via the condensation of amine with dialdehyde
or diketone in DMA or amine solution, respectively. In the case
of using amine as a solvent, the highest yields of the ketimine

1040
A. Iwan et al. / Spectrochimica Acta Part A 66 (2007) 1030–1041
mass spectrometer. Melting point of the synthesized compounds
were determined by differential scanning calorimetry (DSC) on
a TA-DSC 2010 apparatus using sealed aluminium pans under
nitrogen atmosphere (flow rate of ca. 30 ml min−1). UV–vis
solution absorption spectra were recorded using a Hewlett-
Packard 8452A spectrophotometer. Wide angle X-ray diffrac-
tion patterns were recorded using powder samples on a HZG-4
diffractometer working in typical Bragg geometry (Cu K␣
radiation).
5.6. Synthesis of the azomethine A2 using DMA as a solvent
A solution of dialdehyde TA (1 mmol) in 5 ml of DMA was
added to a solution of 2,6-dimethylaniline (2.5 mmol) in DMA
(5 ml) with the presence of p-toluenesulfonic acid (0.06 g). The
mixture was refluxed with stirring for 6 h. Then the model com-
pound was filtered, washed and dried at 60 ◦C under vacuum
for 12 h, and finally crystallised from an ethanol. Elemental
analysis—calcd for C24H24N2: C, 84.67%; H, 7.11%; N, 8.23%.
Found: C, 84.15%; H, 6.90%; N, 8.13%. ESI–MS calcd for
C24H24N2 340.5; found 341.0. UV–vis (DMA, nm): 282 and
349; ε (l/mol cm) = 8860, 1675. Tm = 226 ◦C.
5.3. Synthesis of the ketimine K1 using aniline as a solvent
A mixture of diketone DB (1 mmol, 0.2863 g) and ani-
line (10 ml) with the presence of p-toluenesulfonic acid (PTS)
(0.06 g) was refluxed with stirring for 10 h. The reaction was
conducted in a nitrogen atmosphere and the condenser was fitted
with a Dean-stark trap. After cooling, the mixture was precipi-
tated with 100 ml of methanol. Then the model compound was
filtered, washed and dried at 60 ◦C under vacuum for 12 h. The
yield was 90% after recrystallization from acetone–hexane mix-
ture (1:1, v/v). Elemental analysis—calcd for C32H24N2: C,
88.04%; H, 5.54%; N, 6.42%. Found: C, 88.48%; H, 5.25%;
N, 6.49%. ESI–MS calcd for C32H24N2 436.5; found 437.5.
UV–vis (DMA, nm): 282 and 349; ε (l/mol cm) = 36,351, 6956.
Tm = 180 ◦C.
Acknowledgements
Authors thank Prof. Jan Cz. Dobrowolski from Industrial
Chemistry Research Institute in Warsow, Poland for fruitfull
discussions and helpful suggestions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2006.05.016.
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