Conformational studies of aminomethylene-malonic acid dimethylester and its N-methyl derivatives using vibrational spectroscopy, X-ray analysis and ab initio calculations

Article abstract of DOI:10.1016/j.molstruc.2008.10.051

The IR and Raman spectra of aminomethylene-malonic acid dimethylester (AMDME) [NH₂{single bond}CH{double bond, long}C(COOCH₃)₂] and its N-methyl derivatives (MAMDME and DMAMDME) were measured in solid phase and in different solvents at various temperatures. X-ray analysis revealed that AMDME exists in solid phase as EZ conformer, MAMDME as ZZa conformer and DMAMDME as ZE conformer (the first and second E or Z letters express the orientation of the carbonyl oxygen to the C{double bond, long}C double bond for trans and cis methylester group, respectively, and the third letter a denotes anti position of methylamino group with respect to the C{double bond, long}C double bond). In less polar solutions dominantly two ZZ and EZ conformational forms of AMDME and ZZa and EZa of MAMDME are observed, whereas in more polar environments a third conformational form (ZE and ZEa, respectively) also appeared. The behaviour of DMAMDME is different because there is no intramolecular hydrogen bond and in less polar solutions exists in two ZZ and ZE conformational forms. Very weak indications of a third conformational form (probably EZ conformer) were observed only in more polar surroundings. From the solution IR temperature dependent spectra the energy difference between ZE and ZZ conformers of ΔH = 1.8 ± 0.5 kJ mol^-1 in chloroform and ΔH = 4.2 ± 0.5 kJ mol^-1 in acetonitrile was estimated with the ZZ one being more stable. The geometries and relative energies of the possible conformers of all three compounds were evaluated using ab initio MP2 and DFT B3LYP methods in 6-31G^** basis set with PCM solvent effect inclusion. The influence of environment polarity on the conformational equilibrium is discussed.

Full text of DOI:10.1016/j.molstruc.2008.10.051

Journal of Molecular Structure 924–926 (2009) 54–61
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Conformational studies of aminomethylene-malonic acid dimethylester
and its N-methyl derivatives using vibrational spectroscopy,
X-ray analysis and ab initio calculations
b
a
c
d
M. Gróf a, A. Gatial a, , P. Matejka , J. Kozíšek , V. Milata , N. Prónayová
*
ˇ
ˇ
a Department of Physical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
b Department of Analytical Chemistry, Institute of Chemical Technology, CZ-16628 Prague, Czech Republic
c Department of Organic Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
d Central Laboratories, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
The IR and Raman spectra of aminomethylene-malonic acid dimethylester (AMDME)
[NH2ACH@C(COOCH3)2] and its N-methyl derivatives (MAMDME and DMAMDME) were measured in
solid phase and in different solvents at various temperatures. X-ray analysis revealed that AMDME exists
in solid phase as EZ conformer, MAMDME as ZZa conformer and DMAMDME as ZE conformer (the first
and second E or Z letters express the orientation of the carbonyl oxygen to the C@C double bond for trans
and cis methylester group, respectively, and the third letter a denotes anti position of methylamino group
with respect to the C@C double bond). In less polar solutions dominantly two ZZ and EZ conformational
forms of AMDME and ZZa and EZa of MAMDME are observed, whereas in more polar environments a third
conformational form (ZE and ZEa, respectively) also appeared.
Received 10 September 2008
Received in revised form 27 October 2008
Accepted 30 October 2008
Available online 6 November 2008
Keywords:
Vibrational spectra
Conformational analysis
Push–pull compounds
Ab initio calculations
Solvent effect calculations
The behaviour of DMAMDME is different because there is no intramolecular hydrogen bond and in less
polar solutions exists in two ZZ and ZE conformational forms. Very weak indications of a third conforma-
tional form (probably EZ conformer) were observed only in more polar surroundings. From the solution IR
temperature dependent spectra the energy difference between ZE and ZZ conformers of
D
H = 1.8 0.5 kJ mol–1 in chloroform and
ZZ one being more stable.
D
H = 4.2 0.5 kJ mol–1 in acetonitrile was estimated with the
The geometries and relative energies of the possible conformers of all three compounds were evaluated
using ab initio MP2 and DFT B3LYP methods in 6-31G** basis set with PCM solvent effect inclusion. The
influence of environment polarity on the conformational equilibrium is discussed.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
this type of compounds with the bigger donor or acceptor substit-
uents the polar character of push–pull ethylenes, the electronic
Push–pull ethylenes form a specific group of organic com-
pounds characterized by the presence of different electron nature
substituents on each side of the C@C double bond. They are highly
reactive compounds of the general formula R1XACR2@CR3R4 with
electron donor groups at one end and strong electron acceptor
groups at the other end of ethylenic C@C double bond. Very often
R2 = H and for X@NH or NR1 and X@O as the electron donor groups
R1 can be hydrogen, alkyl, cycloalkyl, aryl or hetero(aryl) groups.
On the other side as the electron acceptors R3, R4 are the groups
such as ACN, ACOR, ACOOR, ASO2CH3, ANO2. Mainly enamines
(X@NH, NR1) are frequently used as reactants or intermediates in
chemical syntheses of drugs, polymers and dyes [1–3]. Alkoxym-
ethylenes (X@O) are also often used in organic synthesis [4,5]. In
interactions between substituents and the double bond are respon-
sible for their non-linear optical properties and their use as possi-
ble novel electro-optics materials [6–12].
Despite the large interest in this type of compounds, there are
not too much works devoted to the interpretation of their vibra-
tional spectra or the interpretation was restricted only to some
vibrational bands. In the case when both R3, R4 groups are identical
the compound exists in one isomeric form and the conformational
possibilities are caused by the electron donor or acceptor R1 and R3,
R4 groups, respectively. Thus theoretical and experimental study
and interpretation of vibrational spectra of aminomethylene-prop-
anedinitrile [H2NACH@C(CN)2] (AMPDN) and its N-methyl deriva-
tives has been performed [13] and for N-methyl substituted
compound the conformers with anti and syn orientation of the
methylamino group towards C@C double bond were found. The
conformational analysis of methoxymethylene-propanedinitrile
* Corresponding author. Tel.: +421 2 59325460; fax: +421 2 52926032.
E-mail address: anton.gatial@stuba.sk (A. Gatial).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.10.051

M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
55
[H3CAOACH@C(CN)2] (MMPDN) has been also performed [14,15]
and only anti orientation of methoxy group was found.
mained, the reaction mixture was evaporated on a vacuum evapo-
rator and chromatographed on silica gel. The obtained product was
recrystallized from the minimal amount of chloroform and n-hex-
ane mixture in refrigerator.
X-ray diffraction data were recorded using Oxford Diffraction
GEMINI R diffractometer. The requirement for the collection of
these data was the preparation of appropriate single crystals. All
measurements were carried out at the temperature of 100 K. The
The conformational analysis of 3-methoxymethylene-2,4-pen-
tanedione [H3CAOACH@C(COCH3)2] (MMPDO) has shown that this
compound exists in CCl4, CHCl3, CH2Cl2 and CH3CN solutions at
least in two conformational forms and that the conformational
equilibria are changed with the solvent polarity [16]. According
to the results from semiempirical and ab initio calculations and
from vibrational spectra, MMPDO exists as ZEa and EZa conformer
(a represents the anti orientation of the methoxy group to the C@C
double bond and E, Z express the orientation of the carbonyl oxy-
gen with respect to the C@C double bond consecutively for trans
and cis acetyl group).
The conformational study of 3-aminomethylene-2,4-pentanedi-
one [H2NACH@C(COCH3)2] (AMPDO) revealed that this compound
exists in solid phase as well as in more polar surroundings as EZ
conformer (E denotes the away orientation of the carbonyl oxygen
from the C@C double bond, Z implies the towards orientation of the
carbonyl oxygen to the C@C double bond, again consecutively for
trans and cis acetyl group) [17]. However, in less polar solvents
we have observed the presence of two conformers of AMPDO, EZ
and ZZ, which are both stabilized by an intramolecular hydrogen
bond.
The previous study of some push–pull aminodiesters [18,19]
showed that these compounds exist in solid phase in EZ conforma-
tion (E and Z denote again the orientation of the carbonyl oxygen
with respect to the C@C double bond for trans and cis ester groups).
In less polar surroundings ZZ conformer was observed too. How-
ever, in more polar environments a third conformational form
(EZ conformer) also appeared. These compounds are in the case
of amino or methylamino electron donor group stabilized by the
intramolecular hydrogen bond between the amino hydrogen and
the carbonyl oxygen of the ester group in cis position.
The conformational possibilities of AMDME and DMAMDME are
the same and are given by the rotation of the both ester groups
around the single CAC bond with respect to the C@C double bond.
The orientation of the carbonyl oxygen can be towards (Z-con-
former) or away (E-conformer) from the C@C double bond and
thus, AMDME and DMAMDME can theoretically exist in four con-
formational forms ZZ, EZ, ZE and EE, where the first and second let-
ters denote the position consecutively for trans and cis ester group,
respectively. The difference between these compounds is that
AMDME can be stabilized by an intramolecular hydrogen bond.
In the case of MAMDME, which can be also stabilized by an intra-
molecular hydrogen bond, an additional conformational possibility
arises from the rotation of the methylamino group around the
NAC@ bond which can be anti (i.e. the methyl group is oriented
away from the C@C double bond) or syn (i.e. the methyl group is
oriented towards the C@C double bond) oriented. Thus, MAMDME
can theoretically exist in eight conformational forms ZZa, EZa, ZEa,
EEa, ZZs, EZs, ZEs and EEs.
MoKa type of radiation was used in all experiments.
The infrared spectra in the region 4000–400 cm–1 were re-
corded with a Nicolet model NEXUS 470 FTIR spectrometer at room
temperature. The solid phase measurements were performed after
mixing the powdered sample with KBr and pressing into a pellet.
The far infrared spectra in the region 600–50 cm–1 were recorded
with a Nicolet model 5700 FTIR spectrometer in a polyethylene
pellet at room temperature. The solution IR spectra of the three
compounds in acetonitrile and chloroform were measured in a
Specac temperature cell equipped with KBr windows. Raman mea-
surements were performed using Bruker RFS 100 Raman spectrom-
eter with Nd3+: YAG laser at the wavelength of 1064 nm. The
Raman spectra of the powdered solid as well as of the solutions
were obtained in the region 4000–50 cm–1 at room temperature.
NMR spectroscopy was performed in the solutions of dimethyl-
sulfoxide (DMSO) and deuterated chloroform (CDCl3) to confirm
the identity of the samples. The 1H NMR and 13C NMR spectra were
recorded at room temperature with both Varian VXR-300 and INO-
VA 600 spectrometers. Purity of the samples was checked using
differential scanning calorimetry with Perkin-Elmer DSC-7 calo-
rimeter and melting points were 126, 64 and 57 °C for AMDME,
MAMDME and DMAMDME, respectively.
Ab initio MP2 computations and the DFT ones employing B3LYP
functional were performed using Gaussian 03 program [20]. The
6-31G** basis set was used in the both MP2 and DFT calculations
and calculated energies were corrected with zero point energy
(ZPE). The self-consistent reaction field (SCRF) theory via polari-
zable continuum model (PCM) including the effect of environment
[21,22] was applied to correct the relative ground state energies
obtained for isolated molecules.
3. Results and discussion
3.1. X-ray analysis, IR and Raman spectra
X-ray analysis of AMDME and MAMDME revealed that these
compounds exist in solid phase with different orientation of trans
ester group. While AMDME exists in solid phase as EZ conformer
[23], MAMDME is present as ZZa conformer [24]. The single crystal
of MAMDME turned out to be a non-merohedral twin. Further-
more, there are two pairs of independent molecules of MAMDME
in the unit cell. The former X-ray study of DMAMDME [25] re-
vealed that this compound exists in solid phase in ZE conformation
with the carbonyl of cis methylester twisted by 68° to the plane
formed by the other two C@C and C@O double bonds.
The comparison of infrared and Raman spectra of AMDME mea-
sured in the solvents of different polarity and in solid phase re-
vealed the possible existence of more than one conformer of
AMDME in less polar surroundings as well as in more polar sol-
vents (Fig. 1). The temperature dependence of the conformational
equilibrium of AMDME in acetonitrile solution is depicted in
Fig. 2 together with the deconvolution of the measured spectra into
individual bands. We can expect three bands for each conformer in
the region of 1780–1580 cm–1 (two for the C@O stretching vibra-
tion and one for the C@C stretching vibration). There are more than
three bands in the solution spectra in this region. Some of them
disappear from the solid phase spectrum and their intensity
In the case of all the mentioned conformers of the three studied
compounds we consider the syn orientation of the methoxy group
in both ester groups (i.e. the methyl group is oriented towards the
carbonyl oxygen) because the anti position is energetically very
disadvantageous.
2. Experimental and computational details
AMDME, MAMDME and DMAMDME were prepared by the reac-
tions of dimethyl 3-methoxymethylenemalonate in methanol with
an aqueous solution of ammonia, methylamine and dimethyl-
amine, respectively. The slightly warmed mixture was stirred over-
night at room temperature. The reaction mixture was then briefly
heated to reflux. After ensuring that no starting derivative re-

56
M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
H2N-CH=C(COOCH3)2
a
b
?ZE
IR spectrum
c
EZ
?ZE
pure chloroform
ZZ
ZZ
pure acetonitrile
EZ
ZZ
EZ
AMDME in chloroform
AMDME in acetonitrile
AMDME solid in KBr
1780
1760
1740
1720
EZ
1700
1680
1660
1640
1620
1600
1580
Raman spectrum
pure chloroform
pure acetonitrile
AMDME in chloroform
AMDME in acetonitrile
AMDME solid
EZ
ZZ
ZZ
EZ
?ZE
ZZ
b
?ZE
a
c
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
Wavenumber(cm-1)
Fig. 1. IR (top) and Raman (bottom) spectra of AMDME in chloroform (curve a), in acetonitrile (curve b) and as solid (curve c). Dotted line is pure chloroform and dashed line
is pure acetonitrile.
EZ
H2N-CH=C(COOCH3)2 in acetonitrile
Temperature: - 38oC
EZ
ZZ
?ZE
EZ
ZZ
ZZ
?ZE
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
H2N-CH=C(COOCH3)2 in acetonitrile
Temperature: +70oC
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
Wavenumber (cm-1)
Fig. 2. IR spectra of AMDME in acetonitrile at À38 °C (top, dashed line) and at +70 °C (bottom, dashed line). Solid IR spectrum is obtained as the sum of the deconvoluted
bands.
depends on the polarity of the solvent as well as on the tempera-
ture. Their assignment is based on the theoretical calculations of
the conformer energies in polar surrounding and on the vibrational
frequencies of individual conformers.

M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
57
The infrared and Raman spectra of MAMDME measured in solid
phase as well as in solvents of different polarity have a similar nat-
ure with those of AMDME. The spectra point out the possible exis-
tence of more than one conformer in less polar solvents as well as
in more polar surroundings (Fig. 3). In the solution spectra there
are again more than three bands in the C@O and C@C double bonds
stretching vibrations region. Some of them disappear in the solid
phase spectrum and their intensity depends again on the polarity
of the solvent and temperature.
Although the infrared and Raman spectra of DMAMDME mea-
sured in solvents of different polarity do not contain so many
bands as the spectra of AMDME and MAMDME, their comparison
with the spectra measured in solid phase also implies the existence
of at least two conformers of DMAMDME in solution. The bands
that disappear from the vibrational spectrum in solid phase (at
1714, 1678 and 1612 cm–1) are less intense in the more polar ace-
tonitrile (Fig. 4). This means that they belong to a conformer that is
less polar than in the solid phase present ZE conformer. We ob-
served only some very weak indications of a third conformational
form in the spectrum measured in acetonitrile (very weak band at
1757 cm–1). In addition, the intensity of bands shows also the tem-
perature dependence in less polar chloroform as well as in more
polar acetonitrile (Fig. 5).
third and the fourth most stable conformers are ZE and EE con-
formers, whose MP2 energies are by 12 and 17 kJ mol–1 higher,
respectively. The influence of the polar environment on the con-
former energies is included by the PCM solvation model and the re-
sults for AMDME are depicted in Fig. 6. As we can see, the ZE and EE
conformers remain energetically disadvantageous in less polar sur-
roundings. However, the energy difference between EZ and ZZ con-
former in a less polar environment is only of 1–2 kJ mol–1 what is
in agreement with the trend observed in the vibrational spectra
where we suppose the presence of these two conformers in the less
polar chloroform. We suppose the presence of the third ZE con-
former in the acetonitrile solution what is also in a good agreement
with the results from the PCM solvation model. According to this
model the ZZ and EZ conformers are of equal stability and the rel-
ative energy of the ZE conformer decreases to 5–6 kJ mol–1. The
presence of the EE conformer can be excluded due to its highest
relative energy in the gas phase as well as in polar environment.
The total energies and dipole moments of individual conformers
of MAMDME are included in Table 2. The calculated relative ener-
gies are similar as for AMDME and the most stable conformer is
again the ZZa one. EZa conformer is by 3–4 kJ mol–1 less stable.
The third and the fourth most stable conformers are ZEa and EEa
conformers, whose MP2 energies are by 14 and 20 kJ mol–1 higher,
respectively. We consider only the anti orientation of the methyl
group bound on nitrogen as the syn position is energetically very
disadvantageous (MP2 energies of syn conformers are by over 30
kJ mol–1 and DFT ones over 40 kJ mol–1 higher). The results of
the solvation PCM model for MAMDME are depicted in Fig. 7. As
we can see the ZEa and EEa conformers are very disadvantageous
in less polar surroundings. However, the energy difference be-
tween EZa and ZZa conformer in a less polar environment is of
1–2 kJ mol–1 what is in agreement with the trend observed in
the vibrational spectra where we suppose the presence of these
3.2. Quantum-chemical calculations
For all calculations we consider the syn orientation of the meth-
oxy groups with respect to the carbonyl oxygen in both ester
groups because the anti position is energetically very disadvanta-
geous. The total energies and dipole moments of the individual
conformers of AMDME are included in Table 1. According to these
calculations the most stable conformer is the ZZ one. The next most
stable is EZ conformer which is by 2–4 kJ mol–1 less stable. The
H3C-NH-CH=C(COOCH3)2
a
b
?ZEa
?ZEa
ZZa
?ZEa
IR spectrum
pure chloroform
EZa
EZa
EZa
c
pure acetonitrile
ZZa
ZZa
MAMDMEinchloroform
MAMDMEinacetonitrile
MAMDMEsolidinKBr
EZa
1780
1760
1740
1720
1700
ZZa
1680
1660
1640
1620
1600
1580
Raman spectrum
pure chloroform
pure acetonitrile
MAMDMEinchloroform
MAMDMEinacetonitrile
MAMDMEsolid
EZa
ZZa
c
EZa
?ZEa
1640
ZZa
?ZEa
?ZEa
b
a
1780
1760
1740
1720
1700
1680
1660
1620
1600
1580
Wavenumber (cm-1)
Fig. 3. IR (top) and Raman (bottom) spectra of MAMDME in chloroform (curve a), in acetonitrile (curve b) and as solid (curve c). Dotted line is pure chloroform and dashed
line is pure acetonitrile.

58
M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
(CH3)2N-CH=C(COOCH3)2
a
b
?EZ
c
ZE
IR spectrum
pure chloroform
ZZ
ZE
ZZ
pure acetonitrile
DMAMDME in chloroform
DMAMDME in acetonitrile
DMAMDME solid in KBr
ZE
ZZ
1600
1780
1760
1740
1720
1700
1680
1660
1640
1620
1580
Raman spectrum
pure chloroform
pure acetonitrile
ZE
ZZ
ZZ
DMAMDME in chloroform
DMAMDME in acetonitrile
DMAMDME solid
ZZ
ZE
ZE
b
a
?EZ
c
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
Wavenumber (cm-1)
Fig. 4. IR (top) and Raman (bottom) spectra of DMAMDME in chloroform (curve a), in acetonitrile (curve b) and as solid (curve c). Dotted line is pure chloroform and dashed
line is pure acetonitrile.
(H3C)2N-CH=C(COOCH3)2 in acetonitrile
Temperature: -39oC
ZZ
ZE
ZE
ZZ
ZZ
ZE
?EZ
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
(H3C)2N-CH=C(COOCH3)2 in acetonitrile
Temperature: +74oC
1780
1760
1740
1720
1700
1680
1660
1640
1620
1600
1580
Wavenumber (cm-1)
Fig. 5. IR spectra of DMAMDME in acetonitrile at À39 °C (top, dashed line) and at +74 °C (bottom, dashed line). Solid IR spectrum is obtained as the sum of the deconvoluted
bands.

M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
59
Table 1
Calculated MP2 and DFT total E and relative
DE energies (with zero point energy correction) and dipole moments l of individual conformers of AMDME.
Conformer
E (MP2)
Hartree
D
E (MP2)
lMP2
D
E (B3LYP)
Hartree
D
E (B3LYP)
lB3LYP
D
kJ mol–1
kJ mol–1
ZZ
EZ
ZE
EE
À587.906606
À587.905685
À587.901858
À587.899967
0.00
2.42
12.46
17.42
1.50
3.36
4.17
5.85
À589.576604
À589.575107
À589.570627
À589.567504
0.00
3.93
15.69
23.89
1.48
3.58
4.37
6.44
vacuum
30
25
20
15
10
5
vacuum
EZ-ZZ (MP2)
H2N-CH=C(COOCH3)2
25
20
15
10
5
EZa-ZZa (MP2)
ZEa-ZZa (MP2)
EEa-ZZa (MP2)
ZE-ZZ (MP2)
EE-ZZ (MP2)
H3C-NH-CH=C(COOCH3)2
EZ-ZZ (B3LYP)
ZE-ZZ (B3LYP)
EE-ZZ (B3LYP)
EZa-ZZa (B3LYP)
ZEa-ZZa (B3LYP)
EEa-ZZa (B3LYP)
0
0
0
10
20
50
60
70
80
30
40
0
10
20
30
40
50
60
70
80
εr
εr
Fig. 6. Energy difference between the conformers of AMDME as the function of the
relative permittivity of environment calculated using PCM model for different
solvents at ab initio (MP2) and DFT (B3LYP) level in 6-31G** basis set.
Fig. 7. Energy difference between the conformers of MAMDME as the function of
the relative permittivity of environment calculated using PCM model for different
solvents at ab initio (MP2) and DFT (B3LYP) level in 6-31G** basis set.
two conformers in the less polar chloroform. On the other hand, we
suppose the presence of the third ZEa conformer in the acetonitrile
solution what is also in a good agreement with the results of the
PCM solvation model. According to this model the ZZa and EZa con-
formers are in more polar environment of equal stability and the
relative energy of the ZEa (as well as EEa) conformer decreases to
tween ZE and ZZ conformers in a less polar solvent is only of 2–3
kJ mol–1 what is in agreement with the trend observed in the
vibrational spectra where we suppose the presence of these two
conformers in the less polar chloroform. However, in more polar
surroundings the calculated energy differences between the men-
tioned conformers decrease below 2 kJ mol–1 and the relative
energies of the third EZ conformer or the fourth EE conformer re-
main twice as large. According to the given results we can assign
the indications of the presence of a third conformational form to
the EZ conformer.
7–9 kJ mol–1
.
The total energies and dipole moments of individual conform-
ers of DMAMDME are included in Table 3. The situation is differ-
ent from the trend calculated in the case of AMDME and
MAMDME as the carbonyl oxygen in cis position is not bound
with an intramolecular hydrogen bond. The most stable con-
former is again the ZZ conformer. However, EZ and ZE conformers
are both approximately of equal stability and their relative energy
is by 4–5 kJ mol–1 higher than the energy of ZZ conformer. The
least stable conformer is the EE conformer with relative energy
of 11–14 kJ mol–1. The results of the solvation PCM model for
DMAMDME are depicted in Fig. 8. The situation differs from that
calculated in the case of AMDME and MAMDME. In a less polar
environment the energies of the both conformers with E orienta-
tion of the trans methylester group (i.e. EZ and EE) are higher than
the energies of conformers with Z orientation of the methylester
group in trans position (i.e. ZZ and ZE). The energy difference be-
3.3. Conformational analysis
As has been shown by the theoretical calculations of AMDME in
vacuum as well as in solvents of different polarity, we can assign
the bands in less polar chloroform to the two most stable ZZ and
EZ conformers with the ZZ conformer being the most stable one.
The bands whose intensities increase in the more polar acetonitrile
can be assigned to the more polar EZ conformer. The intensity of
these bands seems also to increase with the temperature. More-
over such an assignment can be supported also by the trends of
the calculated frequencies of all three bands in this region pre-
sented in Table 4. All three bands of EZ conformer in the double
Table 2
Calculated MP2 and DFT total E and relative
DE energies (with zero point energy correction) and dipole moments l of individual conformers of MAMDME.
Conformer
E (MP2)
Hartree
D
E (MP2)
lMP2
D
E (B3LYP)
Hartree
D
E (B3LYP)
lB3LYP
D
kJ mol–1
kJ mol–1
ZZa
EZa
ZEa
EEa
À627.052932
À627.051890
À627.047450
À627.045406
0.00
2.74
14.39
19.76
2.01
3.91
4.66
6.41
À628.860371
À628.858838
À628.853826
À628.850590
0.00
4.02
17.18
25.67
1.94
4.06
4.84
6.92

60
M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
Table 3
Calculated MP2 and DFT total E and relative
DE energies (with zero point energy correction) and dipole moments l of individual conformers of DMAMDME.
Conformer
E (MP2)
Hartree
D
E (MP2)
lMP2
D
E (B3LYP)
Hartree
D
E (B3LYP)
lB3LYP
D
kJ mol–1
kJ mol–1
ZZ
EZ
ZE
EE
À666.187895
À666.186213
À666.186168
À666.183921
0.00
4.42
4.53
2.56
4.22
4.22
5.69
À668.125998
À668.123986
À668.123415
À668.120426
0.00
5.28
6.78
2.57
4.56
4.53
6.20
10.43
14.63
the EZa conformer increase their relative intensity in more polar
solvents and at higher temperatures. Their assignment can be
again supported by the calculated trends of vibrational frequencies
for ZZa and EZa conformers in Table 4. The bands present mainly in
the vibrational spectrum of the more polar acetonitrile (marked by
‘‘?”) could be assigned to the ZEa conformer (it is the third most
stable conformer and it has a higher dipole moment than ZZa or
EZa conformer). The different nature of the solid phase spectrum
of MAMDME in comparison with the solid phase spectrum of
AMDME confirms the existence of a different conformer in solid
phase (EZ and ZZa in the case of AMDME and MAMDME, respec-
tively) and the split of each vibrational mode into two bands con-
firms the existence of two pairs of independent molecules in the
unit cell.
16
14
12
10
8
vacuum
EZ-ZZ (MP2)
(H3C)2N-CH=C(COOCH3)2
ZE-ZZ (MP2)
EE-ZZ (MP2)
EZ-ZZ (B3LYP)
ZE-ZZ (B3LYP)
EE-ZZ (B3LYP)
6
4
The situation in the case of DMAMDME seems to be slightly
simpler than for two former compounds. Experimental evidences
for a third conformer are weak and in the interpretation of vibra-
tional spectra we can consider two dominant conformers. Accord-
ing to the theoretical calculations, the bands that disappear from
the solid phase spectrum can be assigned to the ZZ conformer,
which is the most stable conformer and has a lower dipole moment
than in the solid phase present ZE conformer. Also the calculated
trends in the vibrational frequencies for all three bands are in
excellent agreement with the experimental ones (Table 4). Such a
simplification gave us the possibility to estimate the energy differ-
ence between the ZE and ZZ conformers from IR spectra measured
at different temperatures. After the deconvolution of the measured
IR spectra into the bands with Lorentz profile (Fig. 5) and using the
band area for the band pairs of ZE and ZZ conformers at 1622 and
1612 cm–1 we have obtained using van’t Hoff equation the energy
2
0
40
70
80
10
50
60
0
20
30
εr
Fig. 8. Energy difference between the conformers of DMAMDME as the function of
the relative permittivity of environment calculated using PCM model for different
solvents at ab initio (MP2) and DFT (B3LYP) level in 6-31G** basis set.
bonds region are calculated at higher frequencies in comparison
with the ZZ conformer. The bands present mainly in the vibrational
spectrum of the more polar acetonitrile (marked with ‘‘?”) could be
then assigned to the ZE conformer (it is the third most stable con-
former and it has a higher dipole moment than both more stable ZZ
and EZ conformers).
The assignment of vibrational bands for MAMDME is very sim-
ilar as for AMDME. According to the theoretical calculations in vac-
uum as well as in the solvents of different polarity, we can assign
the bands in the less polar chloroform to the ZZa and EZa conform-
ers with the dominance of ZZa conformer. The bands assigned to
difference
D
H(ZE – ZZ) = 4.2 0.5 kJ mol–1 in chloroform and
1.8 0.5 kJ mol–1 in acetonitrile. Temperature dependence IR spec-
tra were measured in the interval from –46 up to +56 °C for chlo-
roform and from –39 up to +74 °C for acetonitrile with the step of
about 10 °C. These data are in very good agreement with the calcu-
Table 4
Comparison of the calculated (MP2 and B3LYP level of theory) and experimental vibrational frequencies of AMDME, MAMDME and DMAMDME conformers.
MP2
B3LYP
Experimentala
AMDME
ZZ
EZ
ZE
ZZ
EZ
ZE
ZZ
EZ
ZE
1737
1791 (1734)b
1755 (1697)
1730 (1663)
1824 (1767)
1759 (1700)
1736 (1669)
1827 (1770)
1789 (1733)
1743 (1673)
1768 (1692)
1719 (1648)
1678 (1633)
1810 (1728)
1728 (1659)
1684 (1638)
1814 (1732)
1767 (1691)
1685 (1638)
1696
1663
1624
1717
1674
1634
1646
MAMDME
ZZa
EZa
ZEa
ZZa
EZa
ZEa
ZZa
EZa
ZEa
1790 (1731)
1749 (1688)
1724 (1664)
1821 (1765)
1760 (1694)
1729 (1669)
1825 (1768)
1788 (1730)
1743 (1675)
1766 (1694)
1711 (1647)
1687 (1631)
1808 (1726)
1722 (1666)
1691 (1632)
1812 (1730)
1766 (1694)
1690 (1638)
1695
1646
1612
1712
1666
1622
1736
1689
1637
DMAMDME
ZZ
EZ
ZE
ZZ
EZ
ZE
ZZ
EZ
ZE
1798 (1741)
1781 (1726)
1734 (1660)
1806 (1750)
1782 (1727)
1735 (1660)
1811 (1754)
1795 (1740)
1738 (1663)
1780 (1702)
1754 (1675)
1662 (1609)
1795 (1713)
1767 (1689)
1668 (1616)
1806 (1723)
1775 (1697)
1671 (1616)
1714
1678
1612
1757
1727
1690
1622
a
Observed frequencies are from IR spectra in acetonitrile solution.
Values in parenthesis are the corresponding scaled vibrational frequencies using the scale factors from [17].
b

M. Gróf et al. / Journal of Molecular Structure 924–926 (2009) 54–61
61
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ꢀ AMDME exists in solid phase as EZ conformer. We can observe
the presence of EZ and ZZ conformers in less polar solvents.
Additionally, in more polar environments also ZE conformer
appears.
ꢀ MAMDME exists in solid phase as ZZa conformer. We can
observe the presence of ZZa and EZa conformers in less polar sol-
vents. Additionally, in more polar environments ZEa conformer
appears as well. Thus the behaviour of this compound is analo-
gous to that of AMDME.
ˇ
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ˇ
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ꢀ DMAMDME exists in solid phase as ZE conformer. We can
observe the presence of ZE and ZZ conformers in less polar sol-
vents. The energy difference between these conformers
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D
H(ZE – ZZ) = 4.2 0.5 kJ mol–1 in chloroform and 1.8 0.5 kJ
mol–1 in acetonitrile was estimated. Only a weak indication of
a third conformational form (probably EZ conformer) was
observed in more polar surroundings.
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J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
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Acknowledgements
This work has been supported by Slovak Grant Agency (Projects
VEGA No. 1/3566/06 and 1/0225/08) and by Science and Technol-
ogy Assistance Agency under the contract No. APVV-20-0073-04.
NMR experimental part of this work was facilitated by the support
of Slovak National Research and Development Program No.
2003SP200280203.
ˇ
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