Conformational studies of 3-aminomethylene-2,4-pentanedione using vibrational and NMR spectra, and ab initio calculations

Article abstract of DOI:10.1016/j.molstruc.2006.12.023

The IR, Raman and NMR spectra of 3-aminomethylene-2,4-pentanedione (AMP) H₂N{single bond}CH{double bond, long}C(COCH₃)₂ were measured. According to the NMR spectra in chloroform and more polar DMSO at room temperature, the sample exists as single entity. On the other hand vibrational spectra revealed that in less polar solutions AMP exists as two conformers with EZ or ZZ orientation of acetyl groups whereas in more polar solvent only one EZ conformer is observed. Such interpretation was confirmed also by the temperature-dependent measurements of IR spectra in chloroform. The observed IR and Raman bands were compared with harmonic vibrational frequencies, calculated using ab initio MP2 and B3LYP density functional methods in 6-31G^** basis set, and assigned on the basis of potential energy distribution. In addition, the geometries and relative energies of possible conformers of AMP were also evaluated at the same levels of theory and compared with the data from X-ray analysis which revealed that AMP exists in solid state as EZ conformer. The influence of environment polarity on this conformational equilibrium is discussed with respect to the SCRF solvent effect calculations using PCM, IPCM and ONSAGER models.

Full text of DOI:10.1016/j.molstruc.2006.12.023

Journal of Molecular Structure 843 (2007) 1–13
www.elsevier.com/locate/molstruc
Conformational studies of 3-aminomethylene-2,4-pentanedione
using vibrational and NMR spectra, and ab initio calculations
M. Gróf a, A. Gatial a,¤, V. Milata b, N. Prónayová c, L. Sümmchen d, R. Salzer d
a Department of Physical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
b Department of Organic Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
c Central Laboratories, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-81237 Bratislava, Slovakia
d Institute of Analytical Chemistry, Technical University Dresden, D-01062 Dresden, Germany
Received 7 November 2006; received in revised form 18 December 2006; accepted 22 December 2006
Available online 3 January 2007
Abstract
The IR, Raman and NMR spectra of 3-aminomethylene-2,4-pentanedione (AMP) H2NACHBC(COCH3)2 were measured. Accord-
ing to the NMR spectra in chloroform and more polar DMSO at room temperature, the sample exists as single entity. On the other hand
vibrational spectra revealed that in less polar solutions AMP exists as two conformers with EZ or ZZ orientation of acetyl groups
whereas in more polar solvent only one EZ conformer is observed. Such interpretation was conWrmed also by the temperature-dependent
measurements of IR spectra in chloroform. The observed IR and Raman bands were compared with harmonic vibrational frequencies,
calculated using ab initio MP2 and B3LYP density functional methods in 6-31G¤¤ basis set, and assigned on the basis of potential energy
distribution.
In addition, the geometries and relative energies of possible conformers of AMP were also evaluated at the same levels of theory and
compared with the data from X-ray analysis which revealed that AMP exists in solid state as EZ conformer. The inXuence of environ-
ment polarity on this conformational equilibrium is discussed with respect to the SCRF solvent eVect calculations using PCM, IPCM and
ONSAGER models.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Vibrational and NMR spectra; Conformational analysis; Push–pull enamines; Ab initio calculations; Solvent eVect calculations
1. Introduction
between substituents and the double bond are responsible
for their non-linear optical properties and their use as new
electro-optics materials [3–6].
Push–pull
ethylenes
with
general
formula
R1XACR2BCR3R4 are highly reactive organic com-
pounds with electron donor group at one side and strong
electron acceptor groups at the other side of ethylenic
CBC double bond. For XDNH or NR1 and X DO the R1
and R2 can be hydrogen, alkyl or hetero(aryl) group and
R3, R4 are groups such as ACN, ACOR, ACOOR,
SO2CH3, NO2. Mainly enamines (XD NH, NR1) are fre-
quently used as reactants or intermediates in chemical syn-
theses of drugs, polymers and dyes [1,2]. The polar
character of push–pull ethylenes, electronic interactions
Despite the large interest, there is a lack of interpretation
of their vibrational spectra or the interpretation was
restricted only to some vibrational bands. Theoretical and
experimental study and interpretation of vibrational spec-
tra of aminomethylene-propanedinitrile [H2NACHB
C(CN)2] (AM) and 1-aminoethylidene-propanedinitrile [H2
NAC(CH3)BC(CN)2] (AE) and their N-methyl and
phenyl derivatives has been performed [7–9]. Also, confor-
mational analysis of methoxymethylene-propanedinitrile
[H3CAOACHBC(CN)2] (MM) and 1-methoxyethylid-
ene-propanedinitrile [H3CAOAC(CH3)BC(CN)2] (ME)
has been done [10,11]. Conformational study of the similar
compound with the two acetyl groups 3-methoxymethylene-2,
*
Corresponding author. Tel.: +421 2 59325 460; fax: +421 2 52926 032.
E-mail address: anton.gatial@stuba.sk (A. Gatial).
0022-2860/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.12.023

2
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
4-pentandione [MeOACHBC(COCH3)2] (MMP) showed
that this compound exists in CCl4, CHCl3, CH2Cl2 and
CH3CN solutions at least in two conformational forms and
that conformational equilibria are changed with solvent
polarity [12]. According to the results from semi-empirical
and ab initio calculations and vibrational spectra, MMP
exists as anti-ZE and anti-EZ conformers (anti represents
the orientation of the methoxy group to the CBC double
bond and E, Z express the orientation of the carbonyl oxy-
gen to the CBC double bond consecutively for trans and
cis acetyl group). This work is the extension of such study
on amino analogue of MMP and presents the conforma-
tional study and theoretical structure investigation of
3-aminomethylene-2,4-pentanedione (AMP) H2NACHB
C(COCH3)2. This is a parent compound for large number
of 3-alkylaminomethylene-2,4-pentanediones (or 2,2-diacy-
lethenamines) whose structures were studied by vibrational
and NMR spectroscopy and X-ray analysis [13–15]. Unlike
MMP, the cis acetyl group of AMP can be Wxed by
intramolecular hydrogen bond and so the conformational
possibilities of the second trans acetyl group may be
studied. According to our knowledge, the vibrational spec-
tra of AMP were not presented yet. 15N NMR spectra of
AMP and other enamines were studied in [16] and only
recently 1H and 13C NMR spectra were mentioned together
with the 17O NMR studies of the enamines series [17,18].
The simplest demonstration of the conformational pos-
sibilities of the vinyl and acetyl groups is in methyl vinyl
ketone H2CBCHACOCH3 [19–21] where all heavy atoms
are in plane and are denoted as the s-trans (E) and s-cis (Z)
ones with respect to the orientation of the two CBC and
CBO double bonds. The s-trans conformer is more stable
by 2.4 §0.2, 3.4 and 4.5 kJ mol¡1 in the gas, 1.9 §
0.1 kJmol¡1 in the CS2 solution and 2.1kJ mol¡1 in the
liquid phase, respectively.
4000–400 cm¡1 were recorded with a Nicolet model
NEXUS 470 FTIR spectrometer at room temperature. The
solid phase measurements were performed after mixing the
powdered samples with KBr and pressing into a pellet. The
far infrared spectra in the region 600–50 cm¡1 were
recorded with a Nicolet model MAGNA 750 FTIR spec-
trometer in polyethylene pellet at room temperature. The
IR spectra of AMP in acetonitrile, acetone, dichlorometh-
ane and chloroform were measured in a cell equipped with
KBr windows. Temperature-dependent infrared spectra
were recorded in chloroform solution using variable
temperature cell Specac in temperature range 258–328K.
Raman measurements were performed using a Bruker RFS
100 Raman spectrometer with Nd3+:YAG laser at the
wavelength of 1064nm.
NMR spectroscopy was performed in the solutions of
dimethylsulfoxide (DMSO) and deuterated chloroform
(CDCl3). NMR spectroscopy was an additional method
used for the identiWcation of AMP and for the conWrmation
of its purity. 1H NMR and 13C NMR spectra were recorded
with a Varian VXR-300 spectrometer.
Semiempirical calculations using AM1 method have
been done with the MOPAC program [25]. Ab initio MP2
computations and the DFT ones employing B3LYP func-
tional were performed using GAUSSIAN98 [26] and
GAUSSIAN03 [27] programs. The 6-31G¤¤ basis set was
used in both MP2 and DFT calculations. The vibrational
analysis has been also performed to convince the stability
of calculated structures (all vibrational modes are posi-
tive). The normal coordinate calculations with geome-
tries optimized at MP2 and DFT levels were also
performed. The calculated frequencies were scaled with
scaling factors developed by Wtting the experimental and
theoretical frequencies of model compounds. The
description of normal modes was done according to the
potential energy distribution (PED). The SCRF theory
via Onsager, IPCM and PCM models including the eVect
of environment [27–29] was applied to correct the relative
ground state energies obtained for isolated molecules in
gas phase.
Previous studies of structurally similar aminodiesters
RNHACHBC(COOCH3)2 [22,23] show that these com-
pounds exist in solid state and in polar solutions as EZ con-
formers (the Wrst and second letter again denotes the
orientation of the CBO bonds towards the CBC bond
consecutively for trans and cis ester group, respectively).
However, in non-polar solution ZZ conformer was
observed, too. The same conformational equilibrium has
been observed also for 3-(dialkylamino)crotonates [24].
3. Results and discussion
3.1. NMR, IR and Raman spectra
1
2. Experimental and computational details
The H and 13C NMR spectra of AMP in DMSO and
CDCl3 solutions are presented in Figs. 1 and 2. Spectra in
chloroform are weaker due to lower solubility of AMP in
the less polar solvents. Hydrogen and carbon chemical
shifts of AMP are given in Table 1. In 1H NMR spectrum
we can observe signals of the methyl groups (at 2.0–
2.5 ppm), signals of the oleWnic hydrogen (at 7.8–8.0 ppm)
and signals of the two aminohydrogens (at 6.5–10.5 ppm).
The signal of the oleWnic hydrogen is split into a doublet
of doublets due to the interaction with the two non-equiv-
alent nitrogen protons. Thus, it is obvious that the rota-
tion of the aminogroup is restricted due to the inclusion
AMP has been prepared by the reaction of MMP with
the aqueous solution of ammonia. Slightly warmed mixture
was stirred overnight at room temperature. After that the
reaction mixture was shortly heated to reXux. After nega-
tive control to the reactants, the reaction mixture was evap-
orated in vacuum evaporator and chromatographed on
silicagel (eluent dichloromethane:methanol 10:1). Accord-
ing to the diVerential scanning calorimetry on Perkin-Elmer
DSC-7 the purity of AMP was better than 98% and its
melting point was 142°C. The infrared spectra in the region

M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
3
/ /
H2N-CH=C(COCH3)2
/ /
11
10
9
8
7
6
3.0
2.5
2.0
1.5
/ /
H2N-CH=C(COCH3)2
/ / 3.0
11
10
9
8
7
6
2.5
2.0
1.5
δ
/ ppm
Fig. 1. 1H NMR spectra of AMP in DMSO (top) and in CDCl3 (bottom) at room temperature.
H2N-CH=C(COCH3)2
H2N-CH=C(COCH3)2
δ / ppm
Fig. 2. 13C NMR spectra of AMP in DMSO (top) and in CDCl3 (bottom) at room temperature.

4
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
Table 1
The mid- and far-infrared and Raman solid phase spec-
tra of AMP are depicted in Fig. 3. Since the sample has
small vapour pressure, only the measurements in solid
phase and in solvents with diVerent polarity combined with
temperature-dependent IR measurements have been per-
formed. The comparison between the solid phase IR spec-
tra of AMP with those in acetonitrile, acetone,
dichloromethane and chloroform solutions is shown in
Figs. 4 and 5. The experimental infrared and Raman band
frequencies are given in Table 2. The comparison of infra-
red and Raman spectra measured in solvents of diVerent
polarities and in the solid phase has revealed the possible
NMR spectral data of 3-aminomethylene-2,4-pentanedione
Chemical shifts (ppm)
DMSO
CDCl3
Carbon
C8
C9
C1
C2
C4
C5
27.14
31.83
110.96
159.63
194.38
198.55
27.17
32.25
112.80
158.21
195.21
201.28
Hydrogen
CH3 trans
CH3 cis
2.16
2.29
2.28
2.49
BCAH
7.91, 7.94
7.88, 7.91
7.96, 7.99
8.58
9.94
7.93, 7.96
6.37
10.31
NAH anti
NAH syn
of the nitrogen lone electron pair into conjugation and
intramolecular hydrogen bond with the oxygen of the cis
acetyl group and because the room temperature is below
the coalescence temperature for this rotation. On the
other side all NMR spectra contain signals only for one
trans acetyl group suggesting that in the DMSO and
CDCl3 solutions only single conformer with respect to
this group is present or that the room temperature is
above the coalescence temperature for the rotation of the
trans acetyl group. We can make similar conclusions also
from the 13C NMR spectrum. The great diVerence in the
chemical shift of anti aminohydrogen is very probably
due to the higher interaction of this hydrogen with
DMSO than with CDCl3.
3
r
2
2
r
3
1680
3
r
1660
1640
1620
3
1600
r
1580
1560
Fig. 4. Details of infrared spectra of AMP in the solvents of diVerent
polarity at room temperature.
H2N-CH=C(COCH3)2
4000
3500
3000
2500
2000
1800
1600
1400
1200
1000
800
600
400
200
4000
3500
3000
2500
2000
1800
1600
1400
1200
1000
800
600
400
200
Wavenumbers (cm-1)
Fig. 3. Infrared (top) and Raman (bottom) spectrum of solid AMP at room temperature.

M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
5
rotation of both acetyl groups and the stable conformers
can have the orientation of the carbonyl oxygen towards or
away from the CBC double bond marked as Z or E orien-
tation. Therefore, AMP can theoretically exist as EZ, ZZ,
ZE and EE conformer where the Wrst letter expresses the
orientation of trans acetyl group and the second one the
orientation of cis acetyl group. In the Wrst step, we have
tried to Wnd all stable conformers at semiempirical level
using AM1 method. But in spite of several diVerent starting
geometries we were not able to Wnd any stable EE con-
former. Therefore, we also studied the dependence of the
energy for the transition between the conformers EZ!EE
by the rotation of the cis acetyl group and ZE ! EE by the
rotation of the trans acetyl group. This dependence for the
Wrst transition is depicted in Fig. 7 for several Wxed dihedral
angles of the trans acetyl group. For the second transition
we have obtained very similar results and so we can con-
clude that EE conformer with the trans orientation of both
acetyl group is not stable probably due to repulsion interac-
tion of both negatively charged oxygens.
H2N-CH=C(COCH3)2
a
b
c
*
d
*
e
Table 3 summarizes the MP2 and DFT total and relative
energies of the found AMP conformers as well as their
dipole moments. Calculated energies conWrm that the most
stable conformers are the EZ and ZZ ones with intramolec-
ular hydrogen bond. The obtained structural parameters of
these AMP conformers together with X-ray data are
included in Table 4.
The relative energies of single conformers obtained in
vacuum were corrected by including solvent eVects into the
calculations. For this purpose, Onsager model [28] and
Isodensity Surface Polarizable Continuum Model (IPCM)
[29] were used to evaluate the energy diVerences between
the conformers of AMP. The results are depicted in Fig. 8.
650
600
550
500
450
Wavenumber (cm-1)
Fig. 5. Comparison of infrared spectra of AMP in acetonitrile (b), chloro-
form (d) and solid state (e) at room temperature. Dashed line represents
infrared spectrum of pure acetonitrile (a) and chloroform (c) ¤ denotes
bands vanishing in solid phase.
existence of a second conformer in less polar solvents. The
bands marked with asterisk in Table 2 vanish in the vibra-
tional spectrum in solid phase. The intensity of these bands
decreases in a more polar solvent and they practically van-
ish in a very polar solvent. In Fig. 4 we can see such an
event in the region 1700–1550 cm¡1. The spectrum of AMP
in acetonitrile displays only one band, while the spectrum in
chloroform displays two bands. Because chloroform is less
polar than acetonitrile the other band could be attributed
to the existence of the less polar conformer. This trend is
conWrmed in Fig. 5 in the region 650–450 cm¡1. Similar
3.3. Conformational analysis
It is possible to expect the nearly planar structure of
amino group due to strong electron withdrawing acetyl
groups and the chance of the inclusion of the lone electron
pair on nitrogen into the conjugation. The planar structure
can be expected also for the heavy atoms of vinyl and both
acetyl groups due to the conjugation of the double bonds.
Therefore, the conformational possibilities of the studied
compound are evidently restricted only to the electron-
acceptor acetyl groups. Fortunately, the experimental struc-
tural data for the studied molecule are available [30] and
some comparison of the calculated and X-ray structure is
possible. Nevertheless, we must keep in mind the structure
diVerences due to crystal packing in solids that are missing
in the calculated structure corresponding to the isolated
molecule (in vacuum or in gas phase). Calculated MP2 and
DFT geometry parameters for the EZ conformer are very
similar except the double bonds where MP2 method gives
CBO bond lengths slightly longer and the CBC one
shorter. Therefore, it is diYcult to decide, which of these
two theoretical methods gives better structure. Both meth-
ods suppose planar structure for this conformer which is in
diVerences are visible also in the region 1100–900 cm¡1
.
These experimental results conWrm that the conformational
conversion between the two conformers of AMP occurs
mainly in less polar environment. In more polar environ-
ment there is virtually only one (polar) conformer which is
also present in the solid phase.
3.2. Quantum-chemical calculations
To support the experimental results we have extended
our study also by theoretical calculations. The numbering
and structure of the studied compound is depicted in Fig. 6.
The conformational possibilities of AMP are given by the

6
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
Table 2
Infrared and Raman spectral dataa of 3-aminomethylene-2,4-pentanedione
IR
Liquidb
Raman
Liquidb
CHCl3
Interpretation
Solid
Solid
CHCl3
CH3CN
KBr pellet
CH3CN
Acetone
EZ
ZZ
3482 mc
3352 w
3538 w
3412 m
ꢀ1
3337 s
3220 s
3341 vw
3218 w
3245 vw
3194 vw
3268 vw,b
ꢀ2
3160 w,sh
3115 vw,sh
3055 vw
2999 w
2977 vw
2924 s
ꢀ3
2996 m
2980 w,sh
2923 w
ꢀ4, ꢀ5
ꢀ6, ꢀ7
ꢀ8, ꢀ9
2929 w
2928 m
2847 vvw
1922 vw
1891 w
1838 w
1659 vvw
1652 w
1650 w,sh
1633 s
1660 m,sh
1639 s,sh
1623 vs
¤1608 w,sh
1655 vw
1655 vvw
1654 m,sh
1635 m,sh
1621 vs
ꢀ10
ꢀ11
ꢀ12
1633 s
1633 w
1624 vvw
1606 vw
1633 w
1621 vvw
1633 vvw
1621 vw
¤
1605 m
ꢀ12
1491 vw
1506 s
1508 m
ꢀ13
1440 vw
1426 w
1416 m
1404 w
ꢀ14
ꢀ15
ꢀ16
ꢀ17
ꢀ18
ꢀ19
ꢀ20
ꢀ21
ꢀ22
ꢀ23
1411 w,sh
1393 s
1409 w,sh
1395 vvw
1417 vw
1414 vw
1396 s
1388 s
1354 w
1346 m
1333 s
1295 s
1383 vw
1365 vw
1345 w
1335 m
1294 m
1198 m
1359 w
1351 w
1320 m
1295 m
1360 sh
1350 sh
1328 w
1293 m
1200 m
1354 w
1350 w
1324 w
1293 w
1199 w
1347 vvw
1327 vvw
1292 vvw
1199 sh
1196 s
1200 sh
¤
1125 vw
1114 w
1078 w,sh
1053 vvw
1117 w
1118 m
1121 vw
ꢀ24
¤
¤
ꢀ24
ꢀ25
1037 m
1032 m
1022 s
1014 s
ꢀ25
ꢀ26
ꢀ27
ꢀ28
ꢀ29
1032 w,sh
1024 m
1012 vw,sh
969 w
1029 w
993 vvw
969 w
955 vvw
920 w,sh
890 vvw
1026 w
920 w
1025 w
1028 vw
1000 vvw
969 w
1003 sh
968 w
968 m
970 vvw
¤
ꢀ30
920 m
914 w
921 m,sh
ꢀ30
819 w
770 vs
700 w
637 m
790 w,sh
689 vw
660 vw
620 m
ꢀ31
ꢀ32
ꢀ33
ꢀ34
700 w,sh
652 vw
622 m
699 w
652 w,sh
621 w
699 m
532 w
698 vvw
619 vvw
700 s
638 vvw
625 m
621 s
¤
577 w
571 vvw
ꢀ35
571 vw
540 s
ꢀ35
ꢀ36
535 m
533 w
540 s
503 w
ꢀ36
476 vvw
464 vvw
476 w
465 m
420 vw
409 vw
367 w
345 vw
295 m
470 m
456 m
ꢀ37
ꢀ38
465 vw
466 w
465 w
466 vw
412 vvw
409 vvw
413 vw
339 vvw
412 m
366 w
344 vw
297 m
ꢀ39
ꢀ40
ꢀ41

M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
7
Table 2 (continued)
IR
Liquidb
Raman
Liquidb
Interpretation
Solid
Solid
CHCl3
CH3CN
KBr pellet
CHCl3
CH3CN
205 vvw
Acetone
EZ
ZZ
295 m
249 w
242 vw,sh
223 vw
212 vw
103 m
297 m
252 w
246 vvw
223 m
212 w
ꢀ41
ꢀ42
ꢀ43
ꢀ44
ꢀ45, ꢀ46
87 s
65 m
ꢀ47
ꢀ48
a
Weak bands in the regions 4000–3200 cm¡1 and 2800–2000 cm¡1 have been omitted.
Solvents used.
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.
Denotes bands vanishing in solid phase.
b
c
¤
agreement with the structure in solid phase where both ace-
tyl groups are in the plane with vinyl group and for amino
group only very small deviation from the planarity has
been found. Regarding the CAH and NAH bond lengths
it is well known that X-ray underestimates these bonds by
about 0.1Å.
DiVerent situation is in the case of next two ZZ and ZE
conformers. DFT method supposes the planar structure
whereas MP2 method non-planar structure for the both
ones. For ZZ conformer there is signiWcant deviation of
both acetyl groups from planarity (more than 20° for both)
whereas for the ZE one only 6° and 15° for trans and cis
acetyl group, respectively. On the other side the amino
group of ZZ conformer is practically planar and amino
group of the ZE one is 5–7° tilted from the plane. Generally
the bond lengths as well as the dihedral angles imply that
for AMP the DFT method predicts more planar and more
conjugated structures than MP2. This can be attributed to
the artifact of the DFT method which underestimates non-
bonding interactions.
H11
H12
H10
C8
C1
H16
C4
O6
C2
H13
H14
H18
N3
C5
C9
H17
O7
H15
Fig. 6. The structure and numbering of atoms in AMP.
-250
-260
-270
-280
-290
-300
-310
fixed dihedral angle
H2N-CH=C(COCH3)
O6=C4-C1=C2 :
180
150
130
EE
The extent of conjugation in such type of compounds is
reXected in the lengths of double CBC and BCAN bonds.
With increasing conjugation the former bond becomes
longer and the later shorter. Therefore, BCAN bond order
increases due to the inclusion of the electron pair on nitro-
gen into the conjugation with CBC bond. The calculated
CBC bond length agrees quit well with the experimental
value of 1.397Å and the comparison with the same bond
length in ethylene 1.332Å [31], vinylamine 1.335 Å [32] and
1.336 Å in methyl vinyl ketone [21] implies that such elon-
gation of CBC bond is under the common inXuence of
both electron donating and withdrawing groups. On the
other side the calculated BCAN bond length of 1.336 Å is
EZ
EZ
0
30 60 90 120 150 180 210 240 270 300 330 360
dihedral angle O7=C5-C1=C2
Fig. 7. Dependence of AM1 energy of AMP on the dihedral angle of cis
acetyl group for Wxed dihedral angle of trans acetyl group.
Table 3
Calculated MP2 and DFT total, E, and relative, ꢀE, energies and dipole moments of AMP conformers
Conformer
E(MP2) (Hartree)
ꢀE(MP2) (kJ mol¡1
)
ꢁMP2 (D)
E(B3LYP) (Hartree)
ꢀE(B3LYP) (kJ mol¡1
)
ꢁB3LYP (D)
ZZ
EZ
ZE
¡437.823421
¡437.826757
¡437.813907
8.76
0.00
33.73
1.48
4.80
5.53
¡439.125738
¡439.128644
¡439.115434
7.63
0.00
34.68
1.37
4.90
5.85

8
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
Table 4
IPCM
35
Optimized geometric parameters of AMP (with 6-31G¤¤ basis set)
ONSAGER
Conformer
Method
EZ
ZZ
EZ
ZZ-EZ
ZE-EZ
30
25
20
15
10
5
MP2
B3LYP MP2
B3LYP X-raya
Bonds (Å)b
C1BC2
C2AN3
C1AC4
C1AC5
C4BO6
C4AC8
C5BO7
C5AC9
C8AH10
C8AH11
C8AH12
C9AH13
C9AH14
C9AH15
C2AH16
N3AH17
N3AH18
ZE-EZ
1.384
1.390
1.333
1.476
1.477
1.228
1.532
1.244
1.511
1.095
1.095
1.090
1.093
1.093
1.091
1.086
1.023
1.006
1.387
1.396
1.327
1.490
1.466
1.231
1.521
1.246
1.521
1.094
1.094
1.090
1.094
1.094
1.090
1.087
1.022
1.007
1.397 (2)
1.336
1.474
1.473
1.237
1.524
1.250
1.507
1.090
1.090
1.086
1.087
1.087
1.087
1.082
1.016
1.004
1.331
1.485
1.461
1.238
1.516
1.251
1.515
1.088
1.088
1.086
1.088
1.088
1.086
1.084
1.015
1.004
1.305 (2)
1.456 (2)
1.464 (2)
1.233 (2)
1.511 (2)
1.236 (2)
1.506 (2)
0.96 (–)
0.96 (–)
0.96 (–)
0.96 (–)
0.96 (–)
0.96 (–)
0.93 (–)
0.86 (–)
0.86 (–)
ZZ-EZ
0
20
40
60
80
εr
Fig. 8. The comparison of Onsager and IPCM models in the evaluation of
relative energy diVerences between the conformers of AMP depending on
relative permittivity of environment: ᭛IPCM/MP2/6-31G¤¤, ᭿Onsager/
MP2/6-31G¤¤
.
Angles (°)
C1BC2AN3
C1BC2AH16
C2BC1AC4
C2BC1AC5
C1AC4BO6
C1AC4AC8
C1AC5BO7
C1AC5AC9
C4AC8AH10
C4AC8AH11
C4AC8AH12
C5AC9AH13
C5AC9AH14
C5AC9AH15
C2AN3AH17
C2AN3AH18
126.0
125.8
126.4
126.5
128.0 (1)
value of 1.397 Å for vinylamine [32]. Comparison of the
AMP calculated CBC and BCAN bond lengths with the
same lengths for the dicyano analogue AM of 1.368/1.376 Å
(MP2/DFT CBC bond length) and 1.346/1.342 Å (MP2/
DFT BCAN bond length) imply that the extent of conju-
gation inside AMP is higher and that according to the cal-
culations two acetyl groups have higher electron
withdrawing eVect than two cyano groups.
Due to the conjugation eVect the BCAC(O) bond
lengths also become somewhat shorter than in non-conju-
gated molecules or in comparison with the experimental
length of this bond of 1.494 Å in parent methyl vinyl ketone
[21]. This conjugation apparently stabilizes the Z or E in-
plane arrangement of CBC and CBO bonds as was found
for the series of ꢂ,ꢃ-unsaturated aldehydes and ketones
[35,36]. As it was mentioned above, DFT method gives
entirely planar structure for all AMP conformers whereas
MP2 gives for ZZ and EZ ones the deviation from the pla-
nar arrangement. It is interesting to note that for AM the
BCAC(CN)2 bond lengths were calculated much shorter
(1.425–1.428 Å) implying much higher inXuence of cyano
group on these neighbouring bonds.
119.5
118.3
118.9
122.5
119.3
120.5
121.3
111.6
111.6
107.1
110.9
110.9
107.4
116.5
120.7
119.3
118.6
118.6
122.6
119.3
120.5
121.4
111.9
111.9
107.3
111.0
111.0
107.8
115.5
121.2
117.0
113.1
118.7
120.6
121.2
120.8
122.1
111.4
111.6
107.2
111.5
111.5
107.2
117.0
120.5
116.2
112.6
118.2
120.2
122.3
120.6
123.3
111.9
111.9
107.2
111.9
111.9
107.2
116.0
120.8
116.0 (–)
117.6 (1)
119.1 (1)
122.0 (2)
120.5 (2)
120.0 (1)
122.2 (1)
109.5 (–)
109.5 (–)
109.5 (–)
109.5 (–)
109.5 (–)
109.5 (–)
120.0 (–)
120.0 (–)
Dihedral angles (°)
N3AC2BC1AC4
N3AC2BC1AC5
180.0
¡0.0
180.0 ¡177.4 ¡179.9
178.0 (2)
¡1.3 (2)
180.0 (1)
¡0.2 (2)
¡0.3 (2)
179.9 (2)
¡0.0
3.8
¡15.0
¡7.0
0.1
¡0.3
¡0.2
179.6
179.7
O6BC4AC1AC2 ¡180.0 ¡180.0
O7BC5AC1AC2
C8AC4AC1BC2
0.0
0.0
0.0
0.0
161.4
169.7
C9AC5AC1BC2 ¡180.0 ¡180.0
H10AC8AC4BO6 ¡119.4 ¡119.4 ¡107.0 ¡119.4 ¡118.9 (–)
H11AC8AC4BO6 119.4
H12AC8-C4BO6
¡0.0
119.4
¡0.0
131.9
11.7
119.8
0.2
121.1 (–)
1.1 (–)
H13AC9AC5BO7 ¡121.2 ¡121.5 ¡111.3 ¡119.5 ¡120.8 (–)
The above mentioned high conjugation expanding
through the whole molecule can signiWcantly inXuence the
rotational barriers of single and double bonds as well as the
H14AC9AC5BO7 121.1
H15AC9AC5BO7 ¡0.0
H16AC2BC1AC4 ¡0.0
121.5
¡0.0
¡0.0
0.0
127.7
7.7
2.6
119.7
0.1
0.1
119.2 (–)
0.8 (–)
¡2.1 (–)
C1BC2AN3AH17
0.0
0.2
0.0
¡0.0 (–)
C1BC2AN3AH18 ¡180.0 ¡180.0 ¡179.6 ¡180.0 ¡180.0 (–)
Table 5
a
Scaling factors for MP2 and B3LYP force Welds
Experimental data from Ref. [30].
Numbering of the atoms according to Fig. 6.
b
Internal coordinate
Scale factor
MP2
B3LYP
about 0.03Å higher than the experimental value. The
shorter experimental value can reXect the higher extent of
conjugation due to the inXuence of the polarity of solid
state on the charge distribution inside this molecule. The
length of this bond is dramatically shortened from the
value of 1.474–1.465Å for methylamine [33,34] and over the
CAH stretch
HACAH deformation
CBC stretch
CBO stretch
BCAH wagging
All other
0.863
0.872
0.898
0.943
0.957
0.931
0.916
0.931
0.906
0.903
0.928
0.964

M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
9
ground and excited states energies of single conformers. So
the conformational behaviour of such types of compounds
is not easy to predict. Particularly the polarity of solvent
has a great inXuence on the charge distribution inside these
molecules [5,37].
solvents, the relative energy diVerences between the con-
formers in polar environment solutions should be taken
into account. As seen from Fig. 8, in polar environment EZ
conformer remains as the most stable one. The relative
energy of the third ZE conformer in polar environment
decreases but still remains relatively high. On the other side
relative energy of the second ZZ conformer signiWcantly
increases in polar environment and therefore its population
in such environment decreases. This can explain the disap-
pearing of some bands in acetonitrile solution in compari-
son with the chloroform solution.
NMR spectroscopy, except hindered rotation, is not very
suitable method to detect the diVerent conformations of
molecules at the room temperature due to fast exchange
between them. Nevertheless, for enamines the rotation
around the BCAN bond at room temperature is usually
slow enough and two separate signals for hydrogen or
methyl in amino group are visible in NMR spectra [38].
This was the case of AM [7] and AE [9] where separate sig-
nals for both aminohydrogens were measured. Additionally
we have successfully used 1H NMR spectroscopy in confor-
mational study of their methylamino derivatives and for
both the anti and syn conformers arising by the rotation
around the BCAN bond which could be then distin-
guished also by NMR. The same behaviour was observed
also now for AMP. We have calculated the transition state
for the rotation of ANH2 group for EZ conformer and we
obtained the value of 88.9 kJ mol¡1 using ab initio at MP2
level with 6-31G¤¤ basis set in the gas phase (er D1.0). The
inXuence of the polar environment of DMSO (er D 46.7) on
this barrier was estimated by the PCM solvation model and
the barrier reduction to 82.3 kJ mol¡1 was calculated.
On the contrary, the acetyl group rotation is much faster
However, the most positive evidence for the assignment
of some vibrational bands to other conformers is the tem-
perature dependence of their intensities. The ratio of indi-
vidual conformers changes with temperature according to
Boltzmann law. At a higher temperature, the higher
amount of the less stable conformer is present in the mix-
ture. This fact is reXected in the band intensities ratio. How-
ever, for such spectral study it is necessary to have
conformationally pure bands belonging to only one con-
former. We have performed the temperature measurements
of the infrared spectrum of AMP in chloroform in the tem-
perature range 223–323 K. We have studied temperature
dependence of the bands at 1633 and 1605cm¡1. The ratio
of bands areas at every temperature was achieved by the
deconvolution of the overlapped bands into two Lorentz
functions. The value of 1.0§0.3 kJ mol¡1 was obtained
from the slope of the linear dependence of the logarithm of
band areas ratio on 1/T which results from van’t HoV equa-
tion. Such value seems very low and can be understood as
the lowest energy diVerence between EZ and ZZ conform-
ers because we don’t know if some from the three bands of
the less stable ZZ conformer does not overlap with the
main 1633cm¡1 band of EZ conformer. The opposite mix-
ing is not possible because in very polar solvent we have
only one broad band around 1633cm¡1 for EZ conformer.
Comparison of the enthalpy diVerence between AMP con-
formers calculated by Onsager and IPCM models shows
great diVerence between them and it seems that Onsager
model overestimates too much the inXuence of polar envi-
ronment.
1
and results only in averaged unresolved signals in both H
and 13C NMR spectra at ambient temperature. The calcu-
lated barrier as the transition state for the rotation of the
trans acetyl group for the EZ ! ZZ conformers transition
is only 31.9kJ mol¡1 in the gas phase and practically the
same value was calculated by the PCM model for DMSO
using ab initio at MP2 level with 6-31G¤¤ basis set. This
value is signiWcantly lower in the comparison with the cal-
culated barrier for the ANH2 rotation and it means that
the coalescence temperature for this conformational transi-
tion can be below the room temperature. Therefore, we
have used both IR and Raman spectroscopy as the more
suitable techniques for conformational analysis. Carbonyl
group and CBC double bond in AMP molecule give very
strong well separated bands in IR and Raman spectra in
the region 1690–1550 cm¡1 which belong to the so-called
group frequencies allowing to observe the conformational
transitions (Figs. 2 and 3). We can expect three bands in
this region. They are visible in the IR and Raman solid
state spectra but in solution spectra they are overlapped
into one broad band in very polar solvents.
3.4. Normal coordinate analysis
The calculations of vibrational frequencies at MP2 or
DFT level of theory include electron correlation and
improve their prediction substantially. Nevertheless, they
are still several percent higher than the experimental values.
Properly scaled ab initio force Weld represents a good
approximation of the real force Weld and therefore the
scaled frequencies constitute a good diagnostic tool for
conformational analysis. In the scaling procedure we trans-
form the calculated harmonic force Weld from Cartesian
coordinates to a set of suitable internal coordinates and
introduce a set of scaling factors according to the diVerent
types of internal coordinates using the scheme
Fij(scaled) DFij (ab initio)£ (xi £xj)1/2, where xi and xj are
Theoretical calculations show that the most stable con-
former should be the EZ one with dipole moment of almost
5 D, while the calculated dipole moment for the second
most stable ZZ conformer is only about 1.4 D. The calcu-
lated relative energy of the third and most polar ZE con-
former is very high according to both MP2 and DFT
methods and can be excluded from the discussion about
conformational equilibrium at room temperature. How-
ever, for the conformational equilibrium in diVerent

10
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
Table 6
Comparison of calculated and observed IR vibrational frequencies for EZ and ZZ conformers of AMP
No.
EZ
ZZ
Fundamental
PED
MP2
B3LYP
observed
MP2
B3LYP
observed
ꢀ1
ꢀ2
ꢀ3
ꢀ4
ꢀ5
ꢀ6
ꢀ7
ꢀ8
3529
3280
3044
3029
3028
3016
2990
2923
2906
1690
1676
1643
1500
1443
1439
1429
1411
1408
1367
1350
1344
1301
1210
1115
1031
1024
1020
974
973
911
754
696
610
592
548
527
450
435
393
353
295
270
242
218
3544
3241
3064
3031
3029
3004
2975
2936
2914
1671
1649
1617
1469
1448
1435
1427
1404
1392
1362
1347
1334
1287
1205
1115
1031
1022
1016
975
962
899
810
693
634
612
561
535
460
459
395
357
302
295
239
201
3538
3268
3055
2996
2996
2980
2980
2924
2924
1660
1639
1623
1506
1426
1416
1404
1396
1388
1354
1346
1333
1295
1196
1118
1032
1024
1022
1014
968
920
770
700
637
621
571
540
476
456
412
343
297
252
252
223
211
3526
3295
3037
3027
3025
3012
3004
2921
2915
1694
1646
1637
1491
1451
1446
1430
1417
1409
1367
1356
1326
1301
1203
1104
1038
1025
1014
1003
970
949
771
693
610
583
566
515
465
453
414
332
298
286
255
252
190
3536
3256
3060
3031
3028
3001
2990
2934
2925
1670
1613
1606
1460
1456
1438
1434
1410
1397
1361
1353
1320
1287
1196
1098
1044
1027
1008
1008
964
936
822
690
621
616
566
536
490
461
427
323
321
293
277
262
211
NAH stretch
NAH stretch
88 NAH s(anti), 10 NAH s(syn)
88 NAH s(syn), 11 NAH s(anti)
98 CAH s
100 CH3 as(c)
95 CH3 as(t)
100 CH3 as(c)
100 CH3 as(t)
99 CH3 ss(c)
97 CH3 ss(t)
BCAH stretch
CH3 asym stretch
CH3 asym stretch
CH3 asym stretch
CH3 asym stretch
CH3 sym stretch
CH3 sym stretch
CBO stretch
ꢀ9
ꢀ10
ꢀ11
ꢀ12
ꢀ13
ꢀ14
ꢀ15
ꢀ16
ꢀ17
ꢀ18
ꢀ19
ꢀ20
ꢀ21
ꢀ22
ꢀ23
ꢀ24
ꢀ25
ꢀ26
ꢀ27
ꢀ28
ꢀ29
ꢀ30
ꢀ31
ꢀ32
ꢀ33
ꢀ34
ꢀ35
ꢀ36
ꢀ37
ꢀ38
ꢀ39
ꢀ40
ꢀ41
ꢀ42
ꢀ43
ꢀ44
ꢀ45
ꢀ46
ꢀ47
ꢀ48
91 CBO s(t)
CBC stretch
CBO stretch
ANH2 bend
29 CBC s, 27 CAN s, 24 CBO s(c)
52 CBO s(c), 26 NH2 ꢁ
57 NH2 ꢁ, 17 CBC s
94 CH3 aꢁ(t)
89 CH3 aꢁ (t)
92 CH3 aꢁ(c)
32 BCC2 as, 13 CAH ro
83 CH3 aꢁ(c)
62 CH3 sꢁ(c)
1605
CH3 asym bend
CH3 asym bend
CH3 asym bend
BCC2 asym stretch
CH3 asym bend
CH3 sym bend
CH3 sym bend
BCAH rocking
BCAN stretch
CACH3 stretch
ANH2 rocking
CH3 rocking
59 CH3 sꢁ(t)
32 CAH ro, 23 CAN s
13 CAN s, 22 BCC2 as., 17 BCC2 ss
20 CACH3 s(c)
39 NH2 ro, 10 CACH3 s(t)
61 CH3 ro(c)
1078
1053
CH3 rocking
CH3 rocking
CH3 rocking
41 CH3 ro(t)
63 CH3 ro(t)
47 CH3 ro(c), 25 CACH3 s(c)
61 CAH wa
45 CACH3 s(t), 19 CH3 ro(t)
43 NH2ꢂ, 35 CAH wa
20 CC2 ss, 27 CACH3 s(c)
45 CBO ro(t)
42 CBO wa(c), 29 CBO wa(t)
46 CBO wa(t), 25 CBO wa(c)
43 CBO ro(c), 23 CACAC ꢁ (t)
54 CBCAN ꢁ, 25 CC2 ro
75 NH2 wa
25 CACAC ꢁ(t), 25 CACAC ꢁ (c)
67 CACAC ꢁ(c), 25 CACAC ꢁ (t)
39 CC2 ro, 10 CACAC ꢁ (c)
82 CBC ꢂ
BCAH wagging
CACH3 stretch
ANH2 torsion
BCC2 sym stretch
CBO rocking
CBO wagging
CBO wagging
CBO rocking
CBCAN bend
ANH2 wagging
CACAC bend
CACAC bend
BCC2 rocking
CBC torsion
955
577
503
BCC2 bend
61 CC2 ꢁ
107 CH3ꢂ (t)
39 CC2 wa, 39 CBC ꢂ
106 CH3ꢂ (c)
190 BCACOCH3ꢂ (c)
161 BCACOCH3ꢂ (t)
CACH3 torsion
BCC2 wagging
CACH3 torsion
BCACO torsion
BCACO torsion
190
184
73
35
179
165
84
48
211
87
65
148
78
48
114
83
13
s, symmetric; a, asymmetric; s, stretch; ꢁ, deformation; ro, rocking; wa, wagging; ꢂ, torsion; (t), trans acetyl; (c), cis acetyl. PED for EZ conformer.
the scale factors for internal coordinates i and j, respectively
[39]. At this level, one scale factor is usually used for all
internal coordinates. However, if there are some modes in
vibrational spectra which are clearly separated from the
other modes, it is possible to use the individual scale factors
for the internal coordinates connected with these modes.
Therefore, in the studied molecule we have used the individ-
ual scale factors for C–H stretching, CBO stretching, CBC
stretching, H–C–H bending and BCH out of plane bend-
ing. All other internal coordinates have been scaled by one
common scale factor. The values of the mentioned six scale
factors for MP2 and DFT B3LYP frequencies in 6-31G¤¤
basis set have been obtained by the least square Wtting pro-
cedure of 171 experimental and calculated vibrational fre-
quencies at the same level of the theory for parent
compounds of our molecule: methyl vinyl ketone
[19–21,36], acraldehyde [35,40], methacrolein [35,41] and
trans-crotonaldehyde [35,42] containing carbonyl group in

M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
11
E and Z conformations towards the CBC double bond, N-
methylvinylamine [43], and dimethylamine [44,45]. The
obtained scale factors are collected in Table 5. The calcu-
lated scaled frequencies and the experimental ones are com-
pared in Table 6. The assignment of harmonic vibrational
modes was done on the basis of potential energy distribu-
tion (PED).
CBO stretching, one for CBC stretching and NH2 defor-
mation vibrations. We may ascribe their basic position to
the bands taken from methyl vinyl ketone [20] 1704cm¡1
for Z and 1676 cm¡1 for E orientation of the carbonyl bond
towards the CBC bond and 1620cm¡1 for the CBC bond
(all in solid state). From vinylamine [48] we have the bands
at 1668cm¡1 for CBC stretching and 1625 cm¡1 for NH2
deformation vibration (both in gas phase). For ꢃ-aminovi-
nyl methyl ketone [H2NACHBCHACOCH3], the bands
at 1664, 1622, 1543cm¡1 and 1640, 1603, 1501cm¡1 for E
and Z isomers, respectively, were found in the solid state
[47]. The similar three bands were found also in solutions
IR spectra. The middle bands were marked as complex
bending vibration of the NH2 group and the next two
bands as the stretching vibrations of the both double bonds
system. For cis acetyl group all three bands are downshif-
ted. In the mentioned study of 3-alkylaminomethylene-2,4-
pentanediones (or 2,2-diacalethenamines) [13] the separate
bands were found for all tree double bonds vibrations in
solid state as well as in solutions. Two highest of them were
interpreted as CBO symmetric and asymmetric vibrations
and the last one as CBC vibration with NAH rocking par-
ticipation. According to our calculation the ANH2 defor-
mation is quite clearly separated from other vibrations and
downshifted probably due to intramolecular hydrogen
bonding at 1506cm¡1. Analysis of PED shows that both
CBO vibrations are less coupled and it is better to interpret
them separately as symmetric and antisymmetric vibra-
tions. Thus, the highest band in this region 1660cm¡1
belongs to CBO stretching vibration of the trans acetyl and
Assignment of the NAH stretching modes is quite
straightforward as the highest modes in the spectra above
the 3100 cm¡1. Nevertheless it is necessary to take in
account the state of the sample and the possibilities to
create hydrogen bonds. For free NAH stretching vibration
it is possible to expect the band around 3500 cm¡1 and
usually in very dilute solutions in non-polar solvents. For
AMP we have two diVerent NAH bonds with free trans
hydrogen and intramolecularly bonded cis hydrogen.
Therefore, for the assignment of the NH2 group stretching
vibrations it is better to consider two separate NAH vibra-
tions as NH2 symmetric and asymmetric vibrations. Thus,
the highest bands at 3538 and 3482 cm¡1 in acetonitrile and
chloroform solution, respectively, can be assigned as the
free NAH stretching vibration. The intermolecularly asso-
ciated trans NAH group with the next molecule of AMP or
solvent can be visible at higher concentrations or in more
polar solvents and such vibration is shifted down to
3400 cm¡1 [46]. For intramolecularly bonded cis NAH
group the stretching vibration is shifted even more down
and is around 3340–3300 cm¡1 for the mono or dimethy-
lester [H2NACHBC(COOCH3)2] analogue of our com-
pound [47,22]. For AMP the cis NAH stretching vibration
can be assigned to the band at 3268 and 3245cm¡1 in aceto-
nitrile and chloroform solution, respectively. It is at slightly
lower frequency implying the stronger intramolecular
hydrogen bond than for the dimethylester analogue. The
same can be stated from the frequencies in the solid state
where intermolecular bond can be signiWcant. Dimethy-
lester compound [22] has NAH frequencies at 3360 and
3325 cm¡1 and we have found them for AMP at 3337 and
3220 cm¡1, respectively. Especially the second one corre-
sponding to cis NAH bond is at markedly lower value.
This fact can be explained by the structure in the solid state
where both amino hydrogens of AMP are involved into
intramolecular hydrogen bond [30]. The trans hydrogen is
bonded with the carbonyl oxygen of the trans acetyl group
(bond length of 1.953 Å) and cis hydrogen besides intramo-
lecular bond with cis acetyl (2.021 Å) also to the next inter-
molecular hydrogen bond with the next cis acetyl group
(2.262 Å).
the same vibration of the cis acetyl is the band at 1623cm¡1
.
The band between them at 1639 cm¡1 can be assigned to
CBC vibration. This frequency is about 30 cm¡1 lower
than for AM (dicyano analogue of AMP) indicating higher
conjugation in AMP than in AM. The strong coupling was
found for CBC stretching mode and according to PED cal-
culations there is high contribution of BCAN stretching
mode. As a consequence of particularly multiple character
of BCAN bond in such conjugated molecules, the appro-
priate stretching vibration can be supposed to be shifted to
higher frequencies. We have found this band about
1300 cm¡1 mixed with the CAH rocking vibration and they
are both practically at the same values as for AM.
From the next bands it is possible to comment some of
them which characterize the chemical groups in molecule.
Amino group has according to the PED calculations its
rocking, torsion and wagging vibrations at 1118, 770 and
456cm¡1, respectively. Whereas the rocking vibration is
practically at the same value as for AM, next two vibrations
are at higher frequencies. It is interesting to note that for
The highest vibrational frequencies in the CAH stretching
region belong to the BCAH stretching vibration of ethylene
group and are usually positioned above 3000cm¡1. The asym-
metric and symmetric stretching vibrations of both methyl
groups are in the expected regions below 3000cm¡1and it is
diYcult to make more accurate assignment without isotopic
studies due to the overlap of some of them.
AMP as well as for AM the bands at 770 and 654 cm¡1
,
respectively, were calculated as the torsion vibrations of the
amino group and have the same character in the vibrational
spectrum: very strong and broad in IR and missing in
Raman spectrum. From the vibrations of acetyl groups we
see that the rocking and wagging ones of carbonyl groups
are in the interval 640–540 cm¡1 and for trans acetyl group
In the double bond stretching vibration region above
1500 cm¡1 we can expect four bands for AMP: two for

12
M. Gróf et al. / Journal of Molecular Structure 843 (2007) 1–13
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are slightly higher than for the cis one. Also next bands of
the both acetyl groups (vibrations of methyl groups) are in
the expected regions. Similarly, BCAH rocking and wag-
ging vibrations are in the expected region. The former one
at 1333cm¡1 and the latter one at 968cm¡1, respectively,
indicate no dramatic inXuence of the acetyl groups on these
vibrations.
The assignment of other deformation bands of AMP is
diYcult and not very unambiguous due to large coupling
between single deformation modes and their relatively
small contributions to PED. In the assignment of other
bands the relatively high uncertainty can exist since the
experimental frequencies for similar compounds had not
been reported and described yet.
4. Conclusions
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The assignment of normal vibrational modes of AMP
was performed. The vibrational analyses supported by
theoretical calculations in vacuo and in the environments
of various polarities revealed the existence of two con-
formers of AMP originating in the rotation of acetyl
group. Whereas in the solid state and in more polar sol-
vents AMP exists as single EZ conformer, in less polar
solvent the presence of the less polar ZZ conformer has
been conWrmed. Such behaviour has been supported also
by solvent eVect calculations at ab initio MP2 level using
Onsager and IPCM models. The estimation of the experi-
mental enthalpy diVerence in chloroform solution from
the infrared measurements at various temperatures failed
due to overlapping bands of diVerent conformers.
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Acknowledgements
This work has been supported by Slovak Grant Agency
(Projects VEGA Nos. 1/3566/06 and 1/2448/05) and by Sci-
ence and Technology 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.
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