Intramolecular hydrogen bonding in 2-nitromalonaldehyde: Infrared spectrum and quantum chemical calculations

Article abstract of DOI:10.1016/j.molstruc.2005.08.012

2-Nitromalonaldehyde (NO₂MA), a simple compound to study the intramolecular hydrogen bond, has been synthesized and deuterated at the enolated proton. Molecular structure and vibrational frequencies of NO ₂MA have been investigated by means of density functional theory (DFT) calculations. The geometrical parameters obtained in the B3LYP level using 6-31G**, 6-311G**, and 6-311++G** basis sets and compared with the corresponding parameters of malonaldehyde (MA). Frequencies calculated at B3LYP level using the 6-311G** and 6-311++G** basis sets are in good agreement with the corresponding experimental results for light and deuterated compounds in CCl ₄/CS₂ solution. The percentage of deviation of the bond lengths and bond angles was used to give a picture of the normal modes, and serves as a basis for the assignment of the wavenumbers. Theoretical calculations show that the hydrogen bond strength of NO₂MA is slightly stronger than that of MA, which is in agreement with the spectroscopic results. The observed νOH/νOD and γOH/γOD appears at about 2880/2100 and 911/695 cm^-1, respectively, are consistent with the calculated geometry and proton chemical shift results. To investigate the effect of NO₂ group on the hydrogen bond strength, the charge distributions, steric effects, and electron delocalization in NO₂MA and MA were studied by the Natural Bond Orbital (NBO) method for optimized model compounds at B3LYP/6-311++G** level of theory. The results of NBO analysis indicate that the electron-withdrawing effect of NO₂ group decreases the hydrogen bond strength, but the steric and resonance effects increase the strength of the bond.

Full text of DOI:10.1016/j.molstruc.2005.08.012

Journal of Molecular Structure 782 (2006) 191–199
www.elsevier.com/locate/molstruc
Intramolecular hydrogen bonding in 2-nitromalonaldehyde:
Infrared spectrum and quantum chemical calculations
a,
a
S.F. Tayyari , Z. Moosavi-Tekyeh , M. Zahedi-Tabrizi , H. Eshghi ,
b
a
*
J.S. Emampoura, H. Rahemic, M. Hassanpourd
aChemistry Department, Ferdowsi University, Mashhad 91774-1436, Iran.
bChemistry Department, Alzahra University, Tehran 1993891167, Iran.
cChemistry Department, Urmia University, Urmia57159-165, Iran.
dChemistry Department, Khayyam Higher Education Center, Mashhad 91897, Iran
Received 28 May 2005; revised 9 August 2005; accepted 15 August 2005
Available online 21 September 2005
Abstract
2-Nitromalonaldehyde (NO2MA), a simple compound to study the intramolecular hydrogen bond, has been synthesized and deuterated at
the enolated proton. Molecular structure and vibrational frequencies of NO2MA have been investigated by means of density functional theory
(DFT) calculations. The geometrical parameters obtained in the B3LYP level using 6-31G**, 6-311G**, and 6-311CCG** basis sets and
compared with the corresponding parameters of malonaldehyde (MA). Frequencies calculated at B3LYP level using the 6-311G** and 6-
311CCG** basis sets are in good agreement with the corresponding experimental results for light and deuterated compounds in CCl4/CS2
solution. The percentage of deviation of the bond lengths and bond angles was used to give a picture of the normal modes, and serves as a
basis for the assignment of the wavenumbers.
Theoretical calculations show that the hydrogen bond strength of NO2MA is slightly stronger than that of MA, which is in agreement with
the spectroscopic results. The observed nOH/nOD and gOH/gOD appears at about 2880/2100 and 911/695 cmK1, respectively, are
consistent with the calculated geometry and proton chemical shift results.
To investigate the effect of NO2 group on the hydrogen bond strength, the charge distributions, steric effects, and electron delocalization in
NO2MA and MA were studied by the Natural Bond Orbital (NBO) method for optimized model compounds at B3LYP/6-311CCG** level
of theory. The results of NBO analysis indicate that the electron-withdrawing effect of NO2 group decreases the hydrogen bond strength, but
the steric and resonance effects increase the strength of the bond.
q 2005 Elsevier B.V. All rights reserved.
Keywords: 2-Nitromalonaldehyde; Intramolecular hydrogen bond; Infrared spectra; Steric effect; Natural Bond Orbital theory.
1. Introduction
The vibrational spectra of these compounds have been
the subject of numerous investigations, which support the
existence of a strong intramolecular hydrogen bond of
chelating nature in the enol form of b-diketones [8–15].
This hydrogen bond formation leads to an enhancement of
the resonance conjugation of the p-electrons, which
causes a marked tendency for equalization of the bond
orders of the valence bonds in the resulting six-member
chelated ring. Therefore, it seems that any parameter that
affects the electron density of the chelated ring will
change the hydrogen bond strength. Alkyl or aryl
substituents at the a-position of b-diketones cause a
dramatic decrease in the enol content, which becomes
very low for bulky substituents, but enhances the
hydrogen bond strength [16–20].
The cis-enol form of b-diketones is stabilized by a strong
intramolecular hydrogen bond [1–4], which belongs to the
class of the Resonance Assisted Hydrogen Bonds (RAHB)
[5]. The strength of the bridge could be more or less
modified by the presence of substituents, which, depending
on the nature and position of the substituents, different
effects could be induced to the p-electron delocalization in
the enol ring [6,7].
*
Corresponding author. Tel.: C985118780216; fax: C985118438032.
E-mail address: sftayyari@yahoo.com (S.F. Tayyari).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.08.012

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S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
white needles in 75% yield mp 50–51 8C, 1H NMR
(CDCl3): d 9.5 (s, 1H, CH) and d 13.75 (s, 1H, OH).
H
H
6
H
O
O
O
O
O
O
5
O
O
O
O
1
A-IV
A-I
A-III
A-II
N
2
4
11
10
O
H
3
2.1.3. Deuterated NO2MA (DNO2MA)
N
7
N
N
Deuterated NO2MA was prepared with the same
procedure reported for NO2MA using D2SO4 for generation
of DCl as described for HCl, then passing gaseous DCl
through suspension of NaNO2MA in CCl4 to obtain
DNO2MA. The resulted solution, after separation of NaCl,
was directly used to obtain the IR spectrum of deuterated
NO2MA in solution.
O
O
O
9
8
O
O
0.0
12.54
8.44
1.23
Fig. 1. Schematic representation for the two stable structures of NO2MA,
A-I, and A-IV, open stucture, A-II, and rotated NO2 group, A-III and their
relative energies in kcal/mol.
NO2MA can have two types of intramolecular hydrogen
bonded conformations that are stabilized by p-delocali-
zation, one between OH group with the carbonyl group, A-I,
and the other between OH group with one of the NO2
oxygen atoms [7], A-IV in Fig. 1. On the other hand, MA
and its derivatives form intermolecular hydrogen bond in
the solid state [21–23].
2.2. Instrumentation
The mid-IR spectra of NO2MA and its related
compounds were recorded by using Bomem MB-154
Fourier Transform Spectrophotometer in the region 500–
4000 cmK1 in KBr pellet and in CCl4/CS2 solution. The
spectrum was collected with a resolution of 2 cmK1 by
coadding the results of 16 scans.
The Far-IR spectra in the region 600–50 cmK1 were
obtained using a Thermo Nicolet NEXUS 870 FT-IR
spectrometer equipped with DTGS/polyethylene detector
and a solid substrate beam splitter. The spectra were
collected with a resolution of 4 cmK1 by coadding the
results of 128 scans.
The main intention of this study was to improve the
understanding of the factors that govern the intramolecular
hydrogen bond strength in the cis enol form of b-diketones.
In the present paper: (1) we recorded the FT-IR spectra of
NO2MA and its deuterated analogue in CCl4 solution. Based
on DFT calculation at B3LYP/6-311CCG** level of
theory and frequency changes upon deuteration, a complete
assignment of vibrational frequencies was performed and
the results were compared with those of MA. (2)
Comparison of NO2MA and MA geometrical parameters,
charge distribution, steric effects, and resonance energies
give a clear understanding of substitution effects of NO2
group on the structure and hydrogen bond strength of the
system.
The NMR spectra were obtained on a FT-NMR, Brucker
Aspect 3000 spectrometer at 100 MHZ frequency using
2 mol% solutions in CDCl3 at 22 8C.
3. Theoretical methods
Geometrical calculations were performed using Gaussian
03 version B05 [25] and NBO 5.0 [26] programs. Geometry
of cis enol form of NO2MA, A-I in Fig. 1, and MA are fully
optimized with hybrid density functional B3LYP [27,28]
using 6-31G**, 6-311G**, and 6-311CCG** basis sets.
Frequencies calculations were performed at B3LYP level of
theory using 6-311G** and 6-311CCG** basis sets.
Orbital population and Wiberg bond orders [29] were
calculated with NBO 3.0 program implemented in Gaussian
03. Natural steric analysis [30,31] and second order
interaction energies (E2) [32] were performed at the
B3LYP/6-311CCG** level using NBO 5.0 program,
which applied the wavefunction information file generated
by the earlier version of NBO (3.0).
2. Experimental
2.1. Preparation
2.1.1. Sodium 2-nitromalonaldehyde monohydrate
(NaNO2MA$H2O)
This compound was prepared as reddish-brown crystals
by dropwise adding a solution of Mucobromic acid [24] in
ethanol to an aqueous solution of sodium nitrite at 54G1 8C.
As small quantities of HCN are evolved in this reaction,
appropriate precautions were taken.
2.1.2. 2-Nitromalonaldehyde
Because of unstability of NO2MA in aqueous solutions, it
was obtained by bubbling HCl to a suspension of NaNO2-
MA in dry CCl4. HCl was prepared by adding dropwise
H2SO4 to NaCl and then passing the gas over P2O5 for
further drying. By bubbling dry HCl gas through the
suspension, a clear solution with precipitated NaCl was
obtained. CCl4 solution was decanted from NaCl and
distilled by rotary evaporator. NO2MA was obtained as
To show the effect of hydrogen bond formation on the
p-electron delocalization and charge distribution, the open
structure, A-II in Fig. 1, was also fully optimized at the
B3LYP/6-311CCG** level of theory. To understand
the effect of resonance between NO2 group and enol ring
on the hydrogen bond strength and p-delocalization, the
NO2 group was rotated by 908 about the C-N bond, A-III in
Fig. 1, and its geometry was fully optimized, with CCNO

S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
193
˚
dihedral angle constrain, at the B3LYP/6-311CCG** level
of theory.
0.027–0.036 A, respectively, shorter than those in MA.
These results suggest a stronger hydrogen bond in NO2MA
than that in MA. On the other hand, by rotating the NO2
group by 908 about C–N bond, A-III, the O/O distance
The assignment of the experimental frequencies are
based on the observed band frequencies and intensity
changes in the infrared spectrum of the deuterated species
confirmed by establishing one to one correlation between
observed and theoretically calculated frequencies.
˚
increases by about 0.05 A (see Table 1). Two factors could
be considered to account for this hydrogen bond weakening,
lower p-electron conjugation between enol ring and NO2
group and less steric effects between the NO2 group and the
rest of the molecule.
Table 1 also shows that upon nitro substitution on
a-position both C–C and CaC bond lengths are increased,
whereas C–O and CaO bond lengths are decreased. The
bond length increment in CaC could be attributed to the
p-electron delocalization through NO2–CaC fragment of
the molecule, which reduces its double bond character. The
bond length increment in C–C could be attributed to reduced
electron delocalization in OaC–C fragment of the chelated
ring, which increases its single bond character. This
interpretation also accounts for shortening of CaO bond
length. Reduction in the C–O bond length could be well
described by considering increased p-electron delocaliza-
tion in the C–O side due to the effect of the NO2 group. The
effect of the nitro group on the p-delocalization in the
chelated ring could also be explained in the terms of
the Gilli’s symmetry coordinates [35] q1 (dCKCKdCaC),
q2 (dC–OKdCaO), QZq1Cq2 (see Table 1). By comparing
these coordinates for NO2MA and MA, one can deduce that
there is more bond equalization, in NO2 substituted MA
than in MA itself. These coordinates considerably increase
in A-II, which indicate the effect of hydrogen bond
formation on the bond equalization in the enol ring.
4. Results and discussion
4.1. Molecular geometry
All possible single molecular structures of NO2MA have
been considered by Buemi and Zuccurello [7], which the
geometrical parameters were determined at B3LYP/6-
31G** level of theory. We only considered the most stable
structure, A-I, and its open structure, A-II in Fig. 1.
The full-optimized geometries of cis-enol form of A-I,
A-II, A-III, and MA and the reported experimental structure
of MA [33] are compared in Table 1.
In all calculations for A-I and A-II the oxygen atoms of
NO2 group lie in the plane of enol ring and, therefore, a
significant conjugation between the NO2 group and the enol
ring is to be expected. The planarity of the molecule is in
agreement with the microwave experimental data [34].
As it is obvious from Table 1, the main effect of NO2
substitution is shortening of the O/O distance and
lengthening of the O–H bond length in comparison with
the corresponding values for MA. The calculated O/O
and O/H distances in NO2MA are 0.02–0.028 and
Table 1
Selected geometrical parameters of NO2MA and MA calculated at B3LYP levela
NO2MA
MA
Exp.[33]
A-I
B1
A-II
B1
A-III
B1
B2
B3
B1
B2
B3
˚
Bond lengths (A)
O–H
1.004
1.664
2.559
1.303
1.234
1.374
1.448
1.096
1.084
1.014
1.008
0.964
0.994
0.997
1.007
1.641
2.554
1.319
1.245
1.368
1.437
1.105
1.089
1.000
(0.969)b
1.68
O/H
O/O
C2–O1
1.614
2.534
1.304
1.241
1.378
1.445
1.098
1.086
1.635
2.543
1.301
1.233
1.371
1.443
1.094
1.089
–
1.738
2.61
1.7
1.671
2.57
2.807
1.324
1.21
2.587
1.319
1.238
1.364
1.438
1.103
1.086
2.553
1.32
1.314
1.231
1.36
1.317
1.237
1.361
1.434
1.101
1.085
C4aO5
C2aC3
C3–C4
1.234
1.348
1.454
1.089
1.091
1.356
1.48
1.442
1.086
1.101
C4–H11
C2–H10
1.101
1.085
Bond angles (8)
O1–C2–C3
122.8
120.8
121.0
0.069
122.4
122.5
122.8
123.1
126.5
124.2
124.2
123.9
124.0
123.0
124.5
119.4
0.086
0.106
0.192
C3–C4–O5
120.9
120.5
121.1
120.8
121.3
122.4
123.3
119.7
123.4
118.9
123.5
119.5
C2–C3–C4
q1
q2
Q
0.063
0.068
0.114
0.083
0.081
0.074
0.08
0.074
0.143
0.067
0.13
0.072
0.14
0.146
0.26
0.082
0.165
0.074
0.155
0.069
0.143
0.073
0.153
a
B1, B2, and B3 stands for 6-311CCG**, 6-311G**, and 6-31G**, respectively.
Assumed value.
b

194
S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
Table 2
ring. Comparison between bond orders in A-I and A-II clears
the significant effect of intramolecular hydrogen bond
formation on the p-electron delocalization in the enolated
ring.
Selected Wiberg bond order of NO2MA and MA calculated at B3LYP/6-
311CCG** level
Bond
A-I
A-II
A-III
MA
It is noteworthy that the bond order of O–H could be
correlated with the O/O distance; the O–H bond order
increases by increasing the O/O distance.
O1–H6
C2–O1
C2–C3
C3–C4
C4–O5
C3–N7
N7–O8
N7–O9
0.6255
1.2578
1.4825
1.1504
1.6756
0.9647
1.4869
1.4838
0.7491
1.1549
1.5898
1.0595
1.8239
0.9522
1.4778
1.4973
0.6481
1.2054
1.561
0.6406
1.1899
1.6027
1.2058
1.6446
–
1.161
1.6784
0.8889
1.5247
1.5247
4.2.2. Charge analysis
The charge distribution calculated by the NBO method
for optimized geometries of A-I, A-II, A-III, and MA are
tabulated in Table 3. According to this Table, the electron
withdrawing effect of the NO2 group considerably reduces
the electronic charge over the enolated ring in A-I and A-III
conformers, compared with MA. Weaker intramolecular
hydrogen bond in A-III, with least steric and resonance
effects, clearly explains the electron withdrawing effect on
weakening of hydrogen bond in these systems. Therefore, to
explore the reasons for stronger hydrogen bond in NO2MA
compared with that in MA, resonance and steric effects
should be considered.
–
–
Table 3
Selected natural charges (e) for the optimized rotamers of NO2MA and MA
at B3LYP/6-311CCG** level
Atom
A-I
A-II
A-III
MA
O1
O5
C3
C2
C4
H6
K0.6
K0.588
K0.159
0.345
0.39
K0.593
K0.508
K0.118
0.29
K0.616
K0.584
K0.144
0.311
K0.642
K0.618
K0.464
0.305
0.379
0.385
0.381
0.51
0.487
0.511
0.507
4.2.3. Electron delocalization
4.2. NBO analysis
4.2.1. Bond order
Delocalization of electron density between occupied
Lewis-type (bond or lone pair) NBO orbitals and formally
unoccupied (antibond or Rydberg) non-Lewis NBO orbitals
corresponds to a stabilizing donor–acceptor interaction. The
energy of this interaction can be estimated by the second
order perturbation theory [32].
The calculated Wiberg bond orders [29] for A-I, A-II,
A-III, and MA are collected in Table 2.
Table 2 shows that the bond orders of CaC and C–O
considerably and C–C and CaO slightly change from MA
to A-I. These changes are in line with the bond lengths
changes discussed in the last section and confirm significant
increase in the p-electron delocalization through NO2–
CaC–O fragment of A-I molecule. Slightly increase in the
CaO bond order and decrease in the C–C bond order are
also in agreement with reduction of electron delocalization
in the OaC–C fragment of the chelated ring.
Rotation of NO2 group by 908 considerably affects the
bond orders of CaC, C–O, C–N, and N–O, which also
confirms contribution of NO2 group in OaN–CaC–O
fragment of the molecule when it is coplanar with the enol
Table 4 lists the calculated second order interaction
energies (E2) between the donor–acceptor orbitals in A-I, A-
II, A-III, and MA. The most important interaction energy,
related to the resonance in the enol ring, is electron donation
from LP(2)O1 to the antibonding acceptor p*(C2–C3)
orbital in the NO2MA (54.4 kcal/mol). The contribution of
this interaction in MA is reduced by about 7 kcal/mol,
which indicates the effect of NO2 group on the resonance in
the enolated ring. An energy decrease of about 11 kcal/mol
in this interaction along with a decrease of about 24, 10, and
5 kcal/mol in the LP(2)O5/s*(O1–H6), p(C2–C3)/
p*(C4–O5), and LP(1)O1/s*(C2–C3), respectively, in
Table 4
Selected second-order perturbation energies E(2) (donor/acceptor)a
Donor
Type
Acceptor
type
A-I
A-II
A-III
MA
C2–C3
C2–C3
O5
p
C4–O5
C2–C3
C4
p*
p*
28.8
7.8
18.6
5.7
29.0
5.7
30.7
5.9
p
CR
RY*
s*
5.3
5.8
5.2
5.4
O1
LP(1)
LP(2)
LP(1)
LP(2)
LP(2)
LP(2)
p
C2–C3
C2–C3
C4
5.6
0.7
5.5
5.7
O1
p*
54.4
9.9
43.5
13.5
–
49.4
10.8
16.9
13.6
18.6
8.1
47.3
10.8
20.6
11.2
18.3
–
O5
RY*
s*
O5
O1–H6
C3–C4
C4–H11
N7–O9
24.0
12.2
16.8
28.9
O5
s*
s*
20.5
20.4
24.7
O5
C2–C3
p*
a
Energy in kcal/mol.

S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
195
Table 5
Important pairwise steric exchange energies DE (i,j) (kcal/mol) interactions between NLMOs i,j
NLMO (i)
NLMO (j)
A-I
A-II
–
A-III
MA
O1–H6
C2–C3
s
p
p
s
s
s
O5
LP (2)
p
17.43
7.95
15.1
2.39
2.27
2.36
13.09
–
16
–
N7–O9
O1
7.56
15.4
8.44
C2–C3
LP (2)
LP (1)
LP (2)
LP (2)
15.21
2.48
–
16
2.3
–
C2–H10
C4–H11
C2–H10
O1
O9
3.09
3.60
O8
–
–
A-II compared to those in A-I indicate the hydrogen bond
formation effect on the p-electron delocalization in the enol
ring. These interactions are only slightly compensated by
LP(2)O5/s*(C3–C4) with a value of about 8 kcal/mol.
The relatively high interaction energy of p(C2–C3)/
p*(N7–O9) in A-I, 28.9 kcal/mol, compared with that in
A-III, 8.1 kcal/mol, clearly indicates the increase of
p-electron delocalization through NO2–CaC fragment of
the molecule in A-I and considerably reduction in electron
delocalization due to the removing of NO2 group from the
plane of enol ring in A-III.
5. Vibrational analysis
Since it has been shown that MA derivatives exist as the
cis enol form in solutions and trans enol form in the solid
state, which in the latter the molecule stabilized through an
intermolecular hydrogen bond [21–23], therefore, we only
considered the infrared spectrum of NO2MA in solution.
The IR spectra of NO2MA and its deuterated analogous
in solution are shown in Figs. 2 and 3, respectively.
Lorentzian function has been utilized for the deconvolution
of infrared spectrum of NO2MA in the 3500–2000 region as
shown in Fig. 4. The calculated and experimental
vibrational frequencies for NO2MA and its deuterated
analogue are tabulated in Tables 6 and 7, respectively.
The Cartesian representation of the normal modes, in the
absence of the potential energy distribution (PED), permits,
in the quantum mechanic ab initio DFT method, to study the
distorted geometry of each mode [36].
4.2.4. Steric effect
The most important pairwise steric exchange energies,
DE(i,j), interactions between NLMOs are listed in Table 5.
According to this Table, the strong steric exchange energy
between p(C2–C3) and p(N7–O9) in A-I and A-II, 7.95 and
7.56 kcal/mol, respectively, disappears in A-III rotamer.
There are also some steric effects between s(C4–H11) and
s(C2–H10) with electron lone pairs of nitro oxygen atoms,
which are absent in the case of A-III conformation. These
steric interactions are responsible for pushing oxygen atoms
of the enol ring, Which results in shorter O/O distance.
Longer O/O distance in A-II compared with that in A-I
results in a strong steric interaction between s(C2–H10) and
LP(1)O1 (8.44 kcal/mol), which the corresponding inter-
action in A-I and A-III is considerably weaker (2.4–2.5 kcal/
mol).
The assignment of the experimental frequencies are
based on the observed band frequencies and intensity
changes in the infrared of the deuterated species confirmed
by establishing one to one correlation between observed and
theoretically calculated frequencies.
5.1. Vibrational assignment of NO2MA in solution
5.1.1. The CH and OH stretching region
Deconvolution of IR spectrum of NO2MA in CCl4 in
3500–2000 cmK1 region, Fig. 4, exhibits a broad band with
Fig. 2. IR spectrum of NO2MA in CCl4/CCl4.
Fig. 3. IR spectrum of D-NO2MA in CCl4.

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S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
about 440 cmK1 centered at 2880 cmK1 is assigned to OH
stretching. The weakness and broadness of this band is the
characteristic of the enol form of b-diketones and appears at
3000–2600 cmK1 [8,9]. It has been shown that increasing the
hydrogen bond strength decreases the intensity of this band
and increases its bandwidth [9]. Upon deuteration, this band
disappears and a new band with considerably reduced
1
bandwidth, n ⁄ z150 cmK1, appears at about 2100 cmK1
.
2
It has been shown for several enolated b-diketones that the
1
1
n ⁄ OH/n ⁄ OD is greater than 2. This great reduction in
2
2
bandwidth is attributed to existence of a double minimum
potential function for the proton movement in these
hydrogen-bonded systems [9]. This result is consistent with
the microwave results, which indicates a double minimum
potential for NO2MA, with a tunneling frequency of 35G
15 cmK1, [34]. nOH/nOD ratio of about 1.37 for the titled
compound suggests an intramolecular hydrogen bond with
moderate strength, with a hydrogen bond strength of about
that of trifluoro-acetylacetone [9]. The position of nOH in
MA is not very clear, but the corresponding band in
deuterated MA appears at 2140 cmK1 [13], which is in
agreement with the calculated results.
Fig. 4. The deconvoluted IR spectrum of NO2MA in CCl4 in the nOH
region.
several sharp bands in this region. The two strongest bands at
3085 and 2920 cmK1 are attributed to the vinylic and
aldehydic CH stretching modes. According to the calculation
results these two bands are somewhat coupled to the OH
stretching modes and upon deuteration considerably loose
1
their intensity. The broad bandwidth at half-height, n ⁄ , of
2
Table 6
Fundamental band assignment of NO2MAa
No.
Theoretical
A
Exp.
Assignment
B
F
F
I
I
R
F
I
1 A0
2
3207
3084
3063
1700
1629
1573
1431
1393
1381
1367
1284
1137
938
3
49
15
59
3224
3091
2937
1719
1664
1619
1450
1419
1405
1380
1294
1159
975
1
2
69
3083
3
2
nCHv(73)CnOH(13)
100
42
8
2920
2880
1657
1598
1525
nCHa(55)CnOH(22)CnCHv(10)
nOH(59)CnCHa(21)CnCHv(10)
nCaO (15)CnCaC(10)CdCH(20)
naC–CaC(18)CdOH(19)CnaNO2(15)
naNO2(31)CdOH(14)CdCHv(11)
nC–C(11)CdCH(21)CnaNO2(10)CnCaC(8)
dOH(25)C nsC–CaC–O(16)C nsCaC–CaO(19)
dCHa(27)C nC–O(10)C nsNO2(16)C nC–N(12)
nsNO2(21)C dCH(22)C nC–N(10)
dOH(21)C nC–O(15)CdCHv(27)
nC–N(15)C dCCC(12)CnC–C(10)
nsCaC–C(17)CD(36)
3
69
51
vbr
60
100
57
–
4
5
100
21
0
100
14
0
21
18
2
6
7
–
8
8
15
38
42
42
8
71
90
1
1382
1358
1348
1250
1120
927
sh
32
31
60
8
9
3
10
11
12
13
14
15
16
17
18
19
20 A00
21
22
23
24
25
26
27
78
56
9
0
6
5
8
3
2
828
8
820
8
3
808*
528
30
26
3
dNO2(47)CD(27)
D(33)CrNO2(19)
532
6
537
7
0
507
0
515
0
11
4
506
D(36)CdCH(18)CrNO2(18)
nC–NO2(20)C dCaC–O(11)
nO/O(62)
dC–NO2(30)CnO/O(20)
gCH v(38)C gOH(26)C gCHa(17)
gCHa(37)CgOH(19)
441
0
447
0
441
1
283
2
287
2
1
287
sh, br
3
252
1
254
1
0
252
1060
1029
936
13
3
1080
1038
984
19
1
0
1012
1004
911
5
5
11
14,br
15
1
12
2
5
24
1
gOH(42)CgCH v(20)
uNO2(49)
728
752
3
744*
369
369
0
378
0
1
G(51)CgCH(35)
G(32)CgCH(30)
292
1
313
1
1
[287]
200
–
196
3
206
3
1
6
gNO2(70)
tNO2(72)
55
0
70
0
0
a
I, IR relative intensity, RaRaman relative intensity; FZfrequency in cmK1, A and B are stand for calculation at B3LYP/6-311CCG** and B3LYP/6-
311G** level of theory, respectively; sh, shoulder; w, weak; s, strong; br, broad. ( ), band overlapped contributions less than 5% are omitted and percent of
contributions are given in parenthesis.

S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
197
Table 7
Fundamental band assignment of D-NO2MA. Frequencies are in cmK1
No.
Theoretical
Exp.
Assignment
A
F
B
F
I
I
R
F
I
1 A0
2
3204
3066
2254
1699
1597
1531
1428
1381
1367
1309
1172
1078
909
0
2
3222
3090
2154
1717
1640
1553
1447
1408
1383
1317
1194
1099
920
0
5
58
3078
2928
2100
1655
1551
1498
1400
1363
1348
1271
1149
1061
894
2
nCHv(84)
nCHa(90)
nOD(77)
75
10
6
4
3
41
56
66
74
w,vbr
67
100
9
4
nCaO(15)CnCaC(9)C dCHa(20)C nC-O(6)C nC-C(5)
naNO2(31)C nCaC(8)C nCaO(7)CnC–C(8)
nCaO(11)C nC-O(10)C dOD(9)C dCHv(7)
nC-C(11)C dCH(21)C naNO2(10)CnCaC(8)
nCN(12)C nC–O(8)CdCHa(22)C nsNO2(13)
nsNO2(24)C dCH(24)C nC-N(11)
dCH(44)C nC-O(14)CdO-D(6)CnCaC(5)
nC-N(12)C dOD(16)C dCCC(11)
dOD(20)C dCHv(15)CnC-C(11)CnC-O(8)
dOD(19)C nCaC(9)C nCaO(15)CnC-C(5)
dNO2(41)C dOD(9)CdCCC(10)
D(29)C rNO2(29)
5
100
9
100
31
2
6
6
43
6
7
0
2
9
3
73
47
27
6
100
0
12
60
54
8
8
78
35
3
10
11
12
13
14
15
16
17
18
19
20 A00
21
22
23
24
25
26
27
3
1
25
7
37
9
5
16
13
19
1
a
819
6
813
5
*
530
6
534
9
1
525
21
17
502
1
511
0
8
507
D(35)C rNO2(14)
nCN(19)CD(41)
b
434
0
440
1
3
b
b
276
2
278
3
1
nO/O(84)
dC–NO2(34)CnO/O(30)
gCHv(44)C gCHa(42)
250
1
252
1
0
1040
1006
734
0
1044
1014
752
1
4
–
-
1
1
2
988
(695)
7
gCHa(26)C gCHv(31)
gOD(37)C gCN(30)
4
9
0
14
(14)
724
13
0
768
22
0
1
(695)
b
gOD(40)C gCN(28)C gCH(17)
G(52)C gCH(36)
G(40)CgCH(36)
358
368
1
b
b
b
292
1
313
1
1
194
3
204
4
1
gNO2(42)
tNO2(72)
55
0
70
0
0
See footnote of Table 6.
a
Solvent overlapped.
b
Not measured.
5.1.2. Below 1800 cmK1 region
The two very strong bands at 1657 and 1598 cmK1 in the
infrared spectrum of NO2MA, based on normal mode
analysis, assigned to the asymmetric and symmetric
CaC–CaO stretching, respectively, which the latter is
mixed with the OH in-plane bending and asymmetric NO2
stretching motions. The corresponding bands in the parent
molecule, MA, appear at 1655 and 1593 cmK1. Upon
deuteration the former shifts only 2 cmK1 and the latter
shifts 100 cmK1 towards lower frequencies. This frequency
shifts are in well agreement with the results of the DFT
calculations. The coupling between symmetric CaC–CaO
stretching and OH bending is the main reason of this large
red shift of the 1598 cmK1 band. Although, a small part of
this large frequency shift caused by the coupling of this band
with the nasNO2, which in the light compound appears below
and in the deuterated case appears at above of the band. A
blue shift of about 26 cmK1 in nasNO2 upon deuteration
confirms this explanation. Another feature of this band is its
Raman intensity change upon deuteration, which is also
characteristic of the enolated b-diketones [8,9,12–15].
According to the DFT calculations, the Raman intensity of
In addition to the symmetric and asymmetric NO2
stretching and in-plane CH bending modes, five bands are
expected to be observed in this region in relation to the enol
ring motions due to the CaO, C–O, CaC, C–C stretching,
and the OH bending modes.
A relatively weak band is observed at 1710 cmK1
,
which upon deuteration neither its position nor its
intensity changes. Assigning of this band to any normal
mode corresponding to the cis enol form, A-I, is unlikely.
This band could be assigned to the CaO stretching of the
carbonyl group in another rotamer, which is engaged in an
intramolecular hydrogen bonding between the hydroxyl
group and one of the oxygen atoms of the nitro group (A-
IV, Fig. 1). Buemi and Zuccurello, according to an ab
initio calculations [7], suggested the presence of this
conformer up to 10 percent, although, the weakness of
1710 cmK1 band indicates much lower content of this
rotamer, if exists any. This is in agreement with the NMR
results, which does not show any measurable content of
other forms.

198
S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
this band greatly increases upon deuteration. This behavior
is caused by decoupling from OD bending and increasing the
CaC and CaO character of this mode.
6. Conclusion
We have tried to analyze the vibrational spectra of
nitromalonaldehyde and its deuterated analogues by
establishing one to one correlations between theoretically
calculated frequencies at fairly high level with the
experimental results. From the OD stretching and OH/OD
out-of-plane bending modes a stronger intramolecular
hydrogen bond in NO2MA than that in MA was concluded.
By applying the Natural Bond Orbital calculations, the
electron inductive, steric, and conjugation effects of NO2
group were analyzed. According to these calculations,
steric, and conjugation effects increases the hydrogen bond
strength, while the electron withdrawing effect decreases the
strength of the bond.
The strong bands at 1525 and 1358 cmK1 are assigned to
the asymmetric and symmetric NO2 stretching vibrations,
respectively. In aliphatic compounds nasNO2 and nsNO2
appear at 1556–1545 cmK1 and 1390–1355 cmK1, respect-
ively, and by conjugation with aromatic groups both of them
show red shift [37]. With increasing the electron with-
drawing ability of the substituted groups the frequency of
the asymmetric NO2 stretching increases [38,39]. Therefore,
the relatively low frequencies of these two bands in NO2MA
could be attributed to the conjugation between the NO2
group and enol ring. Therefore, it could be concluded that
the enolated ring has a character near to that of aromatic
rings and acts as electron donor to the NO2 group.
The band at 1250 cmK1, which upon deuteration
disappears, assigned to the OH in-plane bending mode.
This band is coupled with C–O stretching and C–H in-plane
bending modes and upon deuteration disappears and a new
band appears at 1061 cmK1, which is mainly dOD.
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