Vibrational spectra, crystal structure, DFT quantum chemical calculations and conformation of the hydrazo-bond in 6-methyl-3-nitro-2-(2-phenylhydrazinyl) pyridine

Article abstract of DOI:10.1016/j.saa.2013.01.050

The crystal and molecular structures of 6-methyl-3-nitro-2-(2- phenylhydrazinyl)pyridine (6-methyl-3-nitro-2-phenylhydrazopyridine) have been determined by X-ray diffraction and quantum chemical DFT analysis. The crystal is monoclinic, space group C2/c, with Z = 8 formula units in the elementary unit cell of dimensions a = 16.791(4), b = 6.635(2), c = 21.704(7) ?, β = 100.54(3)°. The molecule consists of two nearly planar pyridine subunits. A conformation of the linking hydrazo-bridge CNHNHC is bend and the dihedral angle between the planes of the phenyl and pyridine rings is 88.2(5)°. The hydrogen bonding of the type NH...N and possibly also CH...O favors a dimer formation in the crystal structure. The dimers are further linked by a NH...O hydrogen bond, so forming a layer parallel to the ab plane. The molecular structure of the studied compound has been determined using the DFT B3LYP/6-311G(2d,2p) approach and compared to that derived from X-ray studies. The IR and Raman wavenumbers have been calculated for the optimized geometry of a possible monomer structural model but the possibility of the dimer formation through the NH...N hydrogen bond has also been considered. The structural and vibrational properties of the intra-molecular NH...O interaction are described.

Full text of DOI:10.1016/j.saa.2013.01.050

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
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Spectrochimica Acta Part A: Molecular and
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journal homepage: www.elsevier.com/locate/saa
Vibrational spectra, crystal structure, DFT quantum chemical calculations
and conformation of the hydrazo – bond in 6-methyl-3-nitro-2-
(2-phenylhydrazinyl)pyridine
a
a
a
a,b
E. Kucharska a, J. Michalski a, , W. Sa˛siadek , Z. Talik , I. Bryndal , J. Hanuza
⇑
a Department of Bioorganic Chemistry, Institute of Chemistry and Food Technology, Faculty of Engineering and Economics, Wrocław University of Economics, Wrocław, Poland
b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
The crystal structure of 6-methyl-3-
nitro-2-(2-
phenylhydrazinyl)pyridine was
studied.
The IR and Raman wavenumbers
have been calculated from the
optimized geometry of monomer
and dimer.
X-ray, IR, Raman and DFT methods
confirm the existence of an
intramolecular NAHꢀꢀꢀO bond.
a r t i c l e i n f o
a b s t r a c t
Article history:
The crystal and molecular structures of 6-methyl-3-nitro-2-(2-phenylhydrazinyl)pyridine (6-methyl-3-
nitro-2-phenylhydrazopyridine) have been determined by X-ray diffraction and quantum chemical DFT
analysis. The crystal is monoclinic, space group C2/c, with Z = 8 formula units in the elementary unit cell
of dimensions a = 16.791(4), b = 6.635(2), c = 21.704(7) Å, b = 100.54(3)°. The molecule consists of two
nearly planar pyridine subunits. A conformation of the linking hydrazo-bridge CANHANHAC is bend
and the dihedral angle between the planes of the phenyl and pyridine rings is 88.2(5)°. The hydrogen
bonding of the type NAHꢀꢀꢀN and possibly also CAHꢀꢀꢀO favors a dimer formation in the crystal structure.
The dimers are further linked by a NAHꢀꢀꢀO hydrogen bond, so forming a layer parallel to the ab plane.
The molecular structure of the studied compound has been determined using the DFT B3LYP/6-
311G(2d,2p) approach and compared to that derived from X-ray studies. The IR and Raman wavenumbers
have been calculated for the optimized geometry of a possible monomer structural model but the possi-
bility of the dimer formation through the NAHꢀꢀꢀN hydrogen bond has also been considered. The struc-
tural and vibrational properties of the intra-molecular NAHꢀꢀꢀO interaction are described.
Ó 2013 Elsevier B.V. All rights reserved.
Received 16 October 2012
Received in revised form 2 January 2013
Accepted 18 January 2013
Available online 1 February 2013
Keywords:
6-Methyl-3-nitro-2-(2-
phenylhydrazinyl)pyridine
Hydrazo bond
IR and Raman spectra
X-ray study
Quantum chemical calculations
Introduction
The basic reaction of hydrazo compounds synthesis is reduction
of nitro compounds in an alkaline medium. Their aromatic deriva-
The pyridine derivatives are widely used in the syntheses of
various compounds i.e. hydrazone Schift bases [1] and complexes
[2,3].
tives are also produced as intermediate products in the syntheses
of benzidine and its derivatives (tolidine, dianisidine, etc.). Aro-
matic hydrazo derivatives isomerize to diaminodiphenyls, impor-
tant ligands in the complex compounds synthesis. Due to their
specific electron properties hydrazo compounds are easily oxidized
to azo derivatives in the reaction RANHANHAR0 ? RN@NR0 [4].
⇑
Corresponding author. Tel.: +48 71 3680299; fax: +48 71 3680292.
E-mail address: jacek.michalski@ue.wroc.pl (J. Michalski).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.01.050

318
E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
Hydrazo compounds also can be used to syntheses of polymers
automated four-circle diffractometer with Ruby CCD camera. The
intensity data were collected at 100(2) K using graphite-mono-
(polyimides). It can be proved successful in applications where se-
vere conditions, like elevated temperatures and solvent resistance,
will be required [5]. Surprisingly, this class of compounds has not
often been examined using the X-ray method. An analysis of Cam-
bridge Structural Database (CSD version 5.33 updates (May 2012);
ConQuest Version 1.14) identified only several structures that con-
tain the 1,2-diphenylhydrazine; two metal complexes [6,7], four
hydrazino phosphines [8], two hydrazobenzene polymorphs [9–
11]. Although, the CSD search on pyridines with ArNHANHAr sub-
stitutes in the 2-position of the ring showed one structure, namely
2-(2-phenylhydrazinyl)pyridine [12].
chromatized Mo Ka radiation (k = 0.71073 Å). Data collection, cell
refinement, and data reduction and analysis were carried out with
CrysAlisCCD and CrysAlisRED, respectively [22]. The structure was
solved by direct methods using SHELXS-97 [23], and refined on F2
by a full-matrix least-squares technique using SHELXL-97 [23]
with anisotropic thermal parameters for non-H atoms. All H atoms
were found in difference Fourier maps. In the final refinement cy-
cles, the C-bonded H atoms were treated as riding atoms in geo-
metrically optimized positions, with CAH = 0.95–1.00 Å, and with
Uiso(H) = 1.2Ueq(C) for CH or 1.5Ueq(C) for CH3. The H atoms bonded
Azo dyes are important class of pyridine derivatives that are
extensively employed as color materials and used for their unusual
electronic properties as well as their wide color range [13–15].
These dyes exhibit azo-hydrazo tautomerism that has been widely
studied using IR and Raman [16], resonance Raman [17] and elec-
tron absorption and emission spectroscopies [4].
The present paper is a continuation of our earlier work on azo and
hydrazo derivatives of pyridine [18–21]. The new structural study of
6-methyl-3-nitro-2-(2-phenylhydrazinyl)pyridine (MNPHP) was
undertaken to confirm the chemical formula and to determine the
preferred configuration of the title molecule. It consists of the phenyl
ring and 6-methyl-3-nitropyridine interconnected through the
dihydrazo bonding. This information is correlated with the results
of studies of the chemical and spectroscopic properties.
to N atoms were freely refined with Uiso(H) = 1.2Ueq(N). The figures
were made using an XP program [24]. Crystal data for MNPHP:
C12H12N4O2, M = 244.26, monoclinic, space group C2/c, a = 16.791
(4), b = 6.635 (2), c = 21.704 (7) Å, b = 100.54 (3)°, V = 2377.2
(12) Å3, Z = 8, T = 100(2) K, 5302 measured reflections, 2713 inde-
pendent reflections, 2211 reflections with I > 2r(I), S = 1.04,
hmin = 3.31°, hmax = 28.78°, R[F2 > 2 (F2)] = 0.041, wR(F2) = 0.098,
r
D
q
max = 0.28 e Åꢁ3
,
D
q
min = ꢁ0.23 e Åꢁ3. CCDC-903660 contains
the supplementary crystallographic data for this article. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Quantum chemical calculations
The geometry optimisation of the molecular structure of the
studied compound was performed for both the monomeric and di-
meric units with the use of Gaussian 03 programme package [25].
All calculations were performed using density functional three-
parameters hybrid (B3LYP) methods [26–28] with the 6-
311G(d,p) [29,30] basis set starting from the X-ray geometry. The
calculated and experimental values were compared using scaling
factors to correct the evaluated wavenumbers for vibrational
anharmonicity and deficiencies inherent to the used computa-
tional level. The Potential Energy Distribution (PED) of the normal
modes among the respective internal coordinates was calculated
for MNPHP using the BALGA [31] program. The atomic coordinates
from the X-ray studies were used as the input data in the optimi-
zation procedure and the data from optimization were introduced
to the BALGA program. The IR and Raman wavenumbers were cal-
culated for a single molecule as well as for the dimer in which two
molecules are joined through the NAHꢀꢀꢀN hydrogen bond.
Experimental
Synthesis
6-Methyl-3-nitro-2-(2-phenylhydrazinyl)pyridine
(MNPHP)
was synthesized from 2-fluoro-3-nitro-6-methylpyridine. 2.5 g
(0.01 mol) of 2-fluoro-3-nitro-6-methylpyridine was dissolved in
10 cm3 of methanol and 0.02 mol of phenylhydrazine was added.
The mixture was heated at 60 °C for 1–2 min and then stored at
ambient conditions for 24 h. The solvent was distilled under
low pressure and the residue was extracted with the warm chlo-
roform (50 °C). Non-soluble phenylhydrazine fluorohydrat was fil-
tered off and washed with warm chloroform. The chloroformic
extract was vaporized to dryness and the residue was dissolved
in a small amount of pharmaceutical petrol to remove the rem-
nants of the phenylhydrazine. Non-soluble product in a form of
yellow crystals was filtered off, dried and re-crystallized twice
from ethanol.
The yield of MNPHP (C12H12N4O2) was 65.2% and its melting
point was 128 °C. The chemical analysis: theoretical content: C
59.00%, N 22.94%, H 4.95%; found: C 59.30%, N 22.80% and H 4.79%.
Raman and IR measurements
IR spectra in the 40–4000 cmꢁ1 range were recorded at room
temperature in Nujol suspension, Fluorolube and KBr pellet with
a FTIR Biorad 575C spectrometer. They were identical in the ranges
where the bands of thinning agent are absent, so the IR spectra
measured in KBr are shown in Fig. 3. The resolution was 2.0 cmꢁ1
.
Raman spectra in the 80–4000 cmꢁ1 range were measured in
back scattering geometry with a FT Bruker 110/S spectrometer.
The resolution was 2.0 cmꢁ1. The YAG:Nd3+ (excitation wavelength
1064 nm) laser was used as an excitation source.
X-ray diffraction
Fig. 1. Molecular structure of MNPHP, showing the atom numbering scheme.
Displacements ellipsoids are drawn at the 50% probability level and H atoms are
shown as small spheres of arbitrary radii. The dotted line indicates the intramo-
lecular NAHꢀꢀꢀO hydrogen bond.
The crystallographic measurement for MNPHP (C12H12N4O2;
yellow; 0.3 ꢂ 0.2 ꢂ 0.08 mm) was performed on a Xcalibur PX

E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
319
The vector displacements of the atoms from their equilibrium
positions during the vibration and the graphical pictures of these
displacements were prepared using ANIMOL program that also
visualizes particular modes in an animated way [32].
ringis slightlydistortedwith thelargest deviations from an aromatic
ring plane for C3 [ꢁ0.011(2) Å] and C6 [ꢁ0.009(2) Å] atoms, further
bonded to the NO2 and CH3 groups, respectively. The same is the case
for the C7 and N2 atoms shifted by ꢁ0.059(2) and 0.053(2) Å from
the corresponding pyridine ring plane. The intramolecular
N2AH2ꢀꢀꢀO1 hydrogen bond involves hydrazo and nitro groups as
donor and acceptor, respectively (Table 2). The nitro group is twisted
with respect to the pyridine ring plane; the corresponding dihedral
angle is 12.6(2)°.
The interesting feature is the conformation of the MNPHP mol-
ecule. The torsion angles around the N2AN20, C2AN2 and N2AC10
bonds are 77.3(2)°, 2.3(2)° and 31.3(2)°, respectively (Table 1). As a
consequence, the dihedral angle between the mean planes of the
pyridine and phenyl rings is 88.2(5)°, leading to an unexpected
bent conformation of the molecule (Fig. 1). The conformation with
the two aryl rings being approximately orthogonal has been
previously observed in the structure of 2-(2-phenylhydrazinyl)pyr-
idine; the reported corresponding angles values are 85.8(2)°,
11.0(2)° and 9.6(2)°, respectively [13]. However, the bond lengths
NAN [1.407(2) Å] and NAC(phenyl) [1.425(2) Å] are longer and
the NAC(pyridine) bond [1.354(2) Å] is shorter for the
C2AN(H)AN(H)AC10 linking unit than the corresponding bond
lengths observed in the 2-(2-phenylhydrazinyl)pyridine [13]. Envi-
ronment of both nitrogen centers is pyramidal, although the sum
of the bond angles around N2 is 359(1)°, and the N2 atom is shifted
A linear correlation (m
exp = a + 0.965xcalc, a = 2.45 for IR and
a = 4.13 for RS) was used for scaling the theoretical wavenumbers
to compare them with the experimental values [33]. The mean
square deviation between the experimental and calculated un-
scaled wavenumbers for MNPHP was 7.7 cmꢁ1 and 7.4 cmꢁ1 for
the IR and the Raman spectra, respectively. The scaling of the cal-
culated wavenumbers improves this result to 2.7 cmꢁ1 for the IR
and 2.8 cmꢁ1 for the Raman spectra. 0.965 scaling factors were
used for the whole range of spectra.
The theoretical Raman intensities were calculated using the
RAINT computer program [34,35].
Results and discussion
Crystal structure
The molecule of MNPHP and the numbering of the atoms are
shown in Fig. 1. The internal variations between the bond lengths
are considerable but do not follow any regular pattern (Table 1)
[36–38]. The phenyl moiety of the molecule is planar. The pyridine
Table 1
Selected geometric parameters (Å,°).
Exp.
Cal.
Exp.
Cal.
Distances
N2AC2
N2AN20
N20AC10
N1AC6
N1AC2
N3AO1
N3AO2
N3AC3
C2AC3
C3AC4
1.354 (2)
1.407 (2)
1.425 (2)
1.339 (2)
1.347 (2)
1.235 (2)
1.240 (2)
1.440 (2)
1.419 (2)
1.390 (2)
1.359
1.381
1.401
1.329
1.339
1.242
1.225
1.447
1.427
1.390
C4AC5
C5AC6
C6AC7
C10AC20
C10AC60
C20AC30
C30AC40
C40AC50
C50AC60
1.373 (2)
1.398 (2)
1.498 (2)
1.391 (2)
1.393 (2)
1.393 (2)
1.382 (2)
1.382 (2)
1.387 (2)
1.377
1.402
1.503
1.397
1.401
1.390
1.389
1.393
1.385
Bond angles
C2AN2AN20
N20AN2AC10
C6AN1AC2
O1AN3AO2
O1AN3AC3
O2AN3AC3
N1AC2AN2
N1AC2AC3
N2AC2AC3
C20AC10AC60
C20AC10AN20
C60AC10AN20
C4AC3AC2
119.96 (11)
115.02 (11)
119.74 (12)
122.25 (12)
119.20 (11)
118.54 (11)
116.10 (12)
120.11 (11)
123.78 (12)
120.05 (13)
121.42 (12)
118.44 (13)
119.46 (13)
121.7
118.5
120.4
122.7
118.7
118.6
117.6
120.2
122.2
119.2
118.6
122.2
118.8
C4AC5AC6
C4AC3AN3
C2AC3AN3
C5AC4AC3
N1AC6AC5
N1AC6AC7
C5AC6AC7
C10AC20AC30
C40AC30AC20
C50AC40AC30
C40AC50AC60
C50AC60AC10
118.57 (12)
118.48 (12)
122.05 (11)
119.38 (13)
122.70 (13)
115.53 (13)
121.76 (12)
119.00 (14)
121.0 (2)
118.1
117.6
123.6
119.8
122.6
116.0
121.4
119.9
121.0
119.0
120.7
120.3
119.76 (14)
120.12 (14)
120.1 (2)
Bond angles
C2AN2AN20AC10
C6AN1AC2AN2
C6AN1AC2AC3
N20AN2AC2AN1
N20AN2AC2AC3
N2AN20AC10AC20
N2AN20AC10AC60
N1AC2AC3AC4
N2AC2AC3AC4
N1AC2AC3AN3
N2AC2AC3AN3
O1AN3AC3AC4
O2AN3AC3AC4
O1AN3AC3AC2
O2AN3AC3AC2
C2AC3AC4AC5
77.3 (2)
ꢁ127.1
ꢁ179.1
ꢁ0.5
ꢁ2.0
2.5
N3AC3AC4AC5
C3AC4AC5AC6
C2AN1AC6AC5
C2AN1AC6AC7
C4AC5AC6AN1
C4AC5AC6AC7
C60AC10AC20AC30
N20AC10AC20AC30
C10AC20AC30AC40
C20AC30AC40AC50
C30AC40AC50AC6’
C40AC50AC60AC1’
C20AC10AC60AC50
N20AC10AC60AC50
H2AN2AN20AH20
ꢁ177.2 (1)
ꢁ0.3 (2)
1.2 (2)
ꢁ178.0 (1)
ꢁ1.2 (2)
177.9 (1)
ꢁ0.1 (2)
176.4 (1)
ꢁ0.1 (2)
0.3 (2)
ꢁ179.2
ꢁ0.1
0.9
ꢁ179.3
ꢁ0.5
179.7
0.5
ꢁ177.6
ꢁ0.3
0.0
0.1
0.1
ꢁ0.4
177.6
ꢁ88.6
ꢁ178.5 (1)
0.3 (2)
2.3 (2)
ꢁ176.5 (1)
31.3 (2)
ꢁ161.2
ꢁ152.2 (1)
ꢁ1.8 (2)
176.9 (1)
177.1 (1)
ꢁ4.2 (2)
166.8 (1)
ꢁ12.4 (2)
ꢁ12.1 (2)
168.7 (1)
1.7 (2)
20.8
ꢁ0.1
ꢁ1.4
ꢁ0.5
0.9
178.6
ꢁ1.4
ꢁ1.0
179.0
0.4
ꢁ0.3 (2)
0.1 (2)
0.0 (2)
ꢁ176.5 (1)
119 (2)

320
E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
Table 2
Geometrical parameters of hydrogen bonds (Å,°).
DAHꢀꢀꢀA
DAH
HꢀꢀꢀA
DꢀꢀꢀA
DAHꢀꢀꢀA
Exp.
Cal.
Exp.
Cal.
Exp.
Cal.
Exp.
Cal.
N2AH2ꢀꢀꢀO1
0.89 (2)
0.89 (2)
0.88 (2)
0.95
1.01
1.01
ꢁ
2.05 (2)
2.22 (2)
2.36 (2)
2.34
1.88
2.37
ꢁ
2.653 (1)
2.991 (2)
2.995 (2)
3.191 (2)
2.64
3.16
ꢁ
124 (1)
144 (1)
128 (1)
149
129
134
ꢁ
i
N2AH2ꢀꢀꢀN20
N20AH20ꢀꢀꢀO2ii
C4AH4ꢀꢀꢀO2iii
ꢁ
ꢁ
ꢁ
ꢁ
Symmetry codes: (i) ꢁx + 1/2, ꢁy + 3/2, ꢁz; (ii) x, y + 1, z; (iii) ꢁx, ꢁy, ꢁz.
by only 0.053(2) Å from the pyridine ring plane, which suggests
conjugation of the pyridine -systems with the lone electron pair
p
of the N2 atom. In addition, the lone pair delocalization onto nitro-
gen is not observed in the case of the N20 atom; the N20 atom is
shifted by 0.143(2) Å out of the plane defined by the pyridine ring,
with the sum of bond angles that total 334(1)° [11,12].
In the crystal structure, a pair of NAHꢀꢀꢀN hydrogen bonds,
involving N2 atom as a donor and N20 atom (ꢁx + 1/2, ꢁy + 3/2,
ꢁz) as an acceptor (Table 2), link two molecules into a dimer with
an R22(6) ring motif [39]. The crystal structure is additionally stabi-
lized by a medium strong intermolecular hydrogen bonding be-
tween the atom C4 of the pyridine ring and the nitro O2 atom of
the molecule related by center of symmetry (ꢁx,ꢁy,ꢁz). The inter-
molecular linkage by CAHꢀꢀꢀO hydrogen bonds favors also a dimer
formation in the crystal structure, with formation of an R22(10) ring
motif (Fig. 2a). The dimers are further linked by an intermolecular
NAHꢀꢀꢀO hydrogen bond between the hydrazo N20 atom and the ni-
tro O2 atom at (x,y + 1,z), thus forming a layer parallel to the ab
plane (Fig. 2b). Between the pyridine rings forming this layer also
a p–p stacking interaction can be found. The distance between the
centroids of the pyridine N1AC6 ring at (x,y,z) and (ꢁx,ꢁy + 1,ꢁz)
is 3.604(3) Å, and the corresponding interplanar spacing and the
centroid offset are 3.356(3) and 1.31 Å, respectively.
Vibrational data and DFT calculations
The IR (in KBr pallet) and Raman spectra of the studied com-
pound are shown in Figs. 3 and 4.
The assignment of the IR and Raman bands to the respective
normal modes is proposed in Table 3.
Fig. 2. Molecular packing arrangement in MNPHP: (a) viewed down the b axis and
showing the intermolecular NAHꢀꢀꢀN and CAHꢀꢀꢀO hydrogen bonds; (b) viewed
normal to the ab plane. The dashed lines indicates intermolecular interactions. The
intramolecular NAHꢀꢀꢀO hydrogen bonds have been omitted for clarity.
The MNPHP isolated molecule consists of 30 atoms and their
vibrations are described by 84 normal modes. On the other hand,
60 atoms of the dimer give rise to 174 vibrational normal modes.
The vibrational degrees of freedom of the monomeric unit can be
subdivided into 31 stretching, 28 bending and 25 torsion
modes. The 31 stretching modes can be roughly described as
6 ꢂ
m
m
(/), 6 ꢂ
m
(h), 7 ꢂ
m
(CH), 3 ꢂ
m
(CH3), 2 ꢂ
m
(NO2), 2 ꢂ
m(NH),
2 ꢂ
(CANH),
m
(CACH3),
m
(CANO2), and m
(NN). Since the symme-
try of this molecule is C1 (for dimer is Ci), all the modes should
be active both in IR and Raman spectra. The number of the ob-
served at room temperature IR (76) and Raman (76) bands is close
to that predicted for the single molecule in the gas-phase.
The studied compound is built of five basic fragments: hydrazo
group ANHANHA, nitro group ANO2, methyl group ACH3,
pyridine and phenyl rings. The energetic positions of the vibra-
tional modes corresponding to these fragments have been well
established by other authors [39,40]. Therefore, in the present
paper we focus on vibrational characteristics of the hydrazo an-
d nitro groups, which can participate in the HB formation. The
assignment of the bands presented in Table 3 shows that many ob-
served bands correspond to coupled vibration of ring and its sub-
stituents. Such
a complex vibration involves a few internal
coordinates and, therefore, is described by their combination. In
such a case the sequence of these coordinates in the combination
Fig. 3. The IR spectra of MNPHP in the range 3500–40 cmꢁ1
.

E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
321
gives information on the contribution of a respective coordinate
to the normal mode. The contribution of the first coordinate is al-
ways the greatest (30 or more %).
Pyridine ring vibrations
The assignment of the pyridine ring vibrations in the studied
compound is relatively easy due to the strong correlation with
the spectra of pyridine [41,42]. The analysis of pyridine spectra
was given by Urena et al. [42] who proposed a new approach to
this problem based on the DFT quantum chemical calculations.
On this basis and the results obtained here the following descrip-
tion of the lines observed in the spectra is proposed:
3089–3088; (/): 1608–1489, 1471–1450, 1430–1424;
d(CH): 1391; (/): 1337–1335, 1269–1262, 1249–1241;
d(CH) + (/): 1206–1205, 1162; (/): 1337, 1066, d(/): 1040;
(CH): 986–974; (/): 960–959, 861; (CH) + (/): 827–767
827–825, 769–767; d(/): 686–630; (/): 624–613; d(/): 599,
556–534; (/): 438–435, 284, 194–192, 117–112 and d(/):
m
(CH):
m
m(/) +
m
m
m
c
m
c
c
c
Fig. 4. The Raman spectra of MNPHP in the range 3500–80 cmꢁ1
.
c
Table 3
Experimental and calculated wavenumbers with PEDs and assignments of MNPHP vibrations (/ – pyridine ring, h – benzene ring).
Dimera
Monomera IR
RS
PED
Assignment
3411, 3408 3438
3398, 3393 3399
3115, 3115 3116
3093, 3093 3092
3092, 3092 3091
3082, 3082 3082
3074, 3074 3067
3063, 3063 3058
3055, 3055 3046
3021, 3021 3018
2989, 2989 2992
2934, 2934 2940
3318 w
3290 w
3152 vw
3095 vw
3088 sh
3074 vw
3052 vw
3048 vw
3027 vw
3009 w
3319 vw
3290 vw
m
m
m
m
m
m
m
m
m
m
m
m
NH-100
NH-99
m
m
m
m
m
m
m
m
m
m
m
m
(NH)h
(NH)/
(CH)/
(CH)h
(CH)/
(CH)h
(CH)h
(CH)h
(CH)h
as(CH3)
as(CH3)
s(CH3)
CH/-99
CHh-99
CH/-99
CHh-98
CHh-100
CHh-100
CHh-97
CH3-100
CH3-100
CH3-100
3096 vw
3089 vw
3082 sh 3073 sh
3057 vw
3044 vw
3027 vw
2960 s 2955 s 2976 vw
2923 vs
2841 m
1608 sh
1598 vs
1589 vs
1576 vs
1528 vw
1493 m
1479 sh
1471 vs
1455 sh
1434 vw
1421 vw
2925 vw
2856 vw
1604 vw
1598 vw
1589 vw
1577 vw
1527 w
1489 vw
1474 vw
1464 m
Combination
1591, 1587 1592
1581, 1580 1585
1576, 1575 1572
1556, 1556 1561
1517, 1514 1526
1489, 1488 1487
1480, 1479 1482
1462, 1461 1469
1443, 1443 1443
1437, 1437 1430
1430, 1430 1421
1406, 1405 1412
1379, 1375 1380
1364, 1364 1365
1315, 1315 1316
1310, 1310 1310
1283, 1283 1287
1262, 1261 1258
1254, 1248 1251
1215, 1213 1237
1192, 1190 1176
1160, 1160 1161
1143, 1143 1143
1141, 1141 1142
1123, 1120 1130
1068, 1068 1070
1052, 1052 1052
1031, 1030 1028
1022, 1022 1024
1012, 1012 1012
m
m
m
m
m
h-57 + dCHh-17 +
m
/-12
m
m
m
m
m
h + d(CH)h
/ + mh
h + m/
as(NO2) + das(HNANH)/
/ + das(HNANH)/
/-36 +
h-41 +
m
h-31 + dNH-9
m
/-30 + dCHh-8
NO2-49 + dNH-23 +
/-53 + dNH-18
m
/-11
dNH-30 +
dCHh-54 +
dNH-24 +
dasCH3-64 + dCH/-12 +
dasCH3-100
dCHh-29 + dNH-20 +
m
m
m
m
m
/-18 + dCHh-13 +
m
NO2-10 +
NO2-11 +
m
h -8
das(HNANH)h
dh + d(CH)h
ds(HNANH)
das(CH3)
das(CH3) + d(CH)/
d(CH)h + dh + d(NNH)h
+ m/ + d(C-NO2)
h-36
/-15 + dCHh-14 +
m
m
h-10
1450 vw
1437 vw
1430 vw
1424 vw
m/-9 + mNO2-8
m
h-17 +
/-32 + dNH-15 + dasCH3-15 +
/-32 + NO2-24 + dCH/-19 + dNH-18
m
/-12
m
(CAN)/-13 + dCHh-7
m
m
/ + d(NNH)/
/ + s(NO2) + d(CH)/ + d(NNH)/
1391 m
1375 w
m
m
1378 vw 1371 sh dsCH3-95
ds(CH3)
d(CH)h
dCHh-75 +
m
m
h-19
/-31 + dCH/-6 +
(CAN)h-18 + dCHh-13
/-51 + dCH/-11
1335 m
1285 sh
1269 s
1337 vs
1285 w
1262 w
1253 sh
1249 vs
1205 w
1177 vw
1162 vw
1153 v
m
m
m
m
m
NO2-34 +
m
NN-5
m
m
m
m
m
s(NO2) +
h + (CAN)h + d(CH)h
/ + d(CH)/
m/
h-56 +
m
m
1255 s
h-25 +
/-26 +
m
m
(CAN)h-20 +
/-CH3-20 +
(CA N)/-16 + dNH-12 +
m
NN-13 + dNH-11 + dCHh-9 +
m
NO2-7
h +
q(HNANH)
1241 w
1206 m
1177 vw
1162 w
1154 vw
m
NO2-19 + dCH/-10
/ +
m
(/-CH3) + ms(NO2)
dCH/-19 +
dCHh-75 +
dCH/-41 +
dCHh-81 +
m
m
m
c
c
m
m
m
m
m
m
/-13 +
m
/-CH3-7
d(CH)/
d(CH)h
d(CH)/
d(CH)h
m
m
m
+
+
m
m
(CA N)/
h-19
/-31 +
m
/-NO2-13
/
h-21
1137 vw
1078 vw
1066 vw
NN-39 + dCH/-19 +
h-45 + dCHh-35
/-31 + /-CH3-12 +
CH3-41 + d/-32 + d(C-N)/-15
CH3-80 + /-CH3-16
h-75 + dCHh-25
m
/-14 +
m
h-13
(NAN) + d(CH)/
h + d(CH)h
/ + (/-CH3)
1077 vw
1066 w
1040 w
m
m
/-NO2-11 + dCH/-11
m
d(CA CH3) + d/
(CH3)
h + d(CH)h
dh
c
q
m
1025 w
1025 vw
998 m
986 vw 974 vw
982, 982
965, 965
962, 962
978
965
958
dh-99
c
c
978 w
964 w
CH/-100
CHh-100
c
c
(CH)/
(CH)h
(continued on next page)

322
E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
Table 3 (continued)
Dimera
Monomera IR
RS
PED
Assignment
950, 950
946, 946
888, 887
854, 853
828, 825
814, 813
803, 803
757, 757
752, 751
736, 735
721, 720
693, 687
687, 681
666, 664
638, 628
614, 613
609, 608
598, 594
568, 568
945
939
863
840
820
804
802
757
738
731
722
683
679
644
616
611
598
573
556
960 sh
949 sh
893 w
861 w
844 m
827 w
812 sh
767 m
742 w
734 sh
718 m
691 m
681 w
648 w
622 vw
614 vw
599 w
578 vw
959 vw
m
c
c
/-42 +
c
CHh-100
CHh-100
CH3-39
m
c
c
/ + d(CACH3)
(CH)h
(CH)h
890 vw
861 w
dNO2-42 +
m
/-24 +
m
m
h-18
(CAN)h-17
NO2-17 + /-14
ds(NO2) + m/
844 vw
825 vw
812 vw
769 vw
746 vw
731 vw
722 vw
695 vw
686 vw
630 sh
624 vw
613 vw
m
c
c
c
c
h-40 + dNO2-22 +
CH/-60 +
CHh-91
m
c
c
c
c
d/ +
x
c
c
h + ds(NO2)
x
c
(CH)/
(CH)h
+
c
/ +
x
x
(NO2)
(NO2)
/-44 +
CHh-40 +
CHh-14 +
NO2-54 + /-39
h-97
d/-NN-29 + d/-23 + dNO2-16 +
d/-36 + dh-19 + NH-18 + /-CH3-12
c
CH/-27 +
x
NO2-21
/ +
(CH)h
(CH)h
(NO2) +
c
(CH)/
+
c
h-35
+
c
h
d/-38 +
x
c
c
m
/-CH3-10 + d/-NN-10 + dNO2-9
c
c
c/
h
c
NH-8
(HNANH) + d/
(HNANH)
c
c
c
c
d/ + dh +
dh +
dh +
c
dh-51 +
c
c
/-25 +
/-27 +
/-CH3-10
/-CH3-15
/-CH3-14 + dh-11 +
/-NN-h-11 + /-7
d/-42 + dh-18 + NH-15 + d/-NO2-11
c
c
/
dh-53 +
/ + q(CH3)
d/-41 + dNO2-15 +
NH-64 +
m
m
/-NO2-9
d/ + d(NO2)
(HNANH)
d/ + dh + (HN-NH)
575 vw
c
c
c
c
556 w 534 sh 554 vw
c
c
553, 547
492, 489
475, 475
437, 436
406, 406
378, 377
357, 356
324, 312
295, 295
267, 266
246, 238
207, 206
177, 176
168, 167
87, 85
514
494
433
419
404
371
359
296
277
250
235
209
172
156
84
500 m
467 w
504 vw
473 vw
435 vw
c
c
c
c
c
h-30 +
h-55 +
/-90 +
NH-44 + d(CAN)h-20 +
h-100
q
c
c
NO2-16 +
(C-N)h-17 +
/-CH3-10
c
(C-N)h-16 +
c
NH-12 + d/-NN-9
c
c
c
c
c
h +
h +
/ +
(HNANH)
h
q
q
(NO2)
(NO2)
qNO2-11
438 vw
409 vw
392 vw
370 vw
350 vw
296 vw
284 vw
247 vw
236 sh
206 vw
192 vw
176 vw
112 vw
c(/-CH3)
qNO2-13 + d(CAN)/-11
391 vw
372 vw
363 vw
293 vw
284 vw
251 vw
230 vw
206 vw
194 vw
177 vw
117 w
d/-CH3-35 +
d/-45 +
d/-NN-24 +
/-53 + /-NO2-24 +
d(CAN)h-80 + (CAN)/-207
(CAN)h-44 + d(CAN)/-41 + d/-NO2-7 +
d/-NO2-45 + d(CAN)h-20 + d/-16 + d/-CH3-6 +
(CAN)/-36 + dh-NN-22 + /-20 + /-CH3-8
d(CAN)/-56 + (CAN)h-33 + h-11
/-NO2-67 + /-23 + CH3-12
NO2-87 + (C-N)/-11
qNO2-18 +
c
NH-10 + d/-NO2-11 + d(C-N)h-8 + d/-6
d(/-CH3) +
d/ + /-NO2
(/-HNANH-h)
/ + (/-NO2)
q(NO2)
m
/-NO2-39 + d/-CH3-7
(C-N)h-22 + d/-CH3-15 + d/-NO2-12 +
/-CH3-14
m
m
qNO2-12 +
m
NN-5
m
c
c
c
c
c
c
T(HNANH)
(C/NNCh)
d(/-NO2) + s(C/NNCh)
c
c
/-CH3-6
NN-5
s
m
c
c
c
s(HNANH)
s(HNANH)
s(/-NO2)
s(/-NO2)
s(/-CH3)
c
c
c
c
s
s
85, 85
75, 74
67
95 s
c
69, 58
T(of whole dimer)
51, 47
42, 33
29, 28
56
42
30
85 sh
74 sh
69 sh
s
c
c
CH3-90 +
(CAN)h-87 + d/-NN-11
/-NN-80 +
c
/-NO2-5
s(/-CH3)
qh
Flapping of whole molecule
c
/-NO2-10 + dh-NN-10
27
16
c
c
(/-HNANH-h)
(/-HNANH-h)
18
c
(/-HNANH-h)-100
13, 11
Flapping of whole molecule
x
– wagging;
s
– twisting; T – translation of the whole unit =
m(C/ꢁNN-Ch)
a
scaling factor: fsc = 0.965. In-plane vibrations:
v
– stretching; d – bending; q – rocking; out-of-plane vibrations: c – torsional;
363–350, 206 cmꢁ1. The characteristic vibration of the pyridine
ring, the ring breathing (ring pulsation), was assigned to the two
bands at 992 and 1031 cmꢁ1 in the Raman spectrum [40]. The
highest intensity of these bands was in the Raman spectrum.
and 2960 (2976 RS) cmꢁ1 for
for s(CH3). The other weak bands in this range arise from a com-
bination and overtone bands.
The asymmetric bending vibrations of the methyl groups should
appear between 1410 and 1550 cmꢁ1 [43–46]. This is true for the
studied compound. das(CH3) vibration contributes to the bands ob-
served at 1455 (1450 RS), 1434 (1437 RS) and (1424 RS) cmꢁ1. The
symmetric ds(CH3) vibrations also appear in the typical range and
are observed at 1375 (1378–1371 RS) cmꢁ1 giving a great contribu-
tion 95% to the normal modes. The other bands involving the
m
as(CH3) and 2923 (2925 RS) cmꢁ1
m
Phenyl ring vibrations
The following wavenumbers observed in the spectra are pro-
posed as a phenyl ring modes:
(h) + d(CH): 1608–1604; (h): 1598–1589; d(CH) +
1464, 1430–1421, 1285; (h): 1255–1253; d(CH) +
1154–1153, 1078–1077, 1025; d(h): 998; (CH): 964–890, 964,
(CH): 812, 734–731; (CH) + (h):
(h): 504–467, 392–
m
(CH): 3096–3095, 3082–3027;
(h): 1493–
(h): 1177,
m
m
m
m
m
methyl groups are observed in the following ranges:
1241 (1249 RS); rocking (CH3): 1040; wagging (CH3): 960
(/-CH3):
m(/-CH3):
c
q
x
949–890;
746–742;
m
(h): 861–844;
c
c
c
(959);
c
(/-CH3): 438 (435); d(/-CH3) 370 (372 RS) and s
.
c
(h): 695–691; d(h): 648–613 and c
85 cmꢁ1
391 cmꢁ1. This bands correspond to literature date [43–45].
Methyl group vibrations
Nitro group vibrations
Three fundamental
m(CH3) bands corresponding to one sym-
The nitro group bonded to the pyridine ring (C/ANO2) gives rise
metric and two asymmetric vibrations are usually observed in
the range between 2950 and 2990 cmꢁ1 [43–46]. Our DFT calcula-
tions for monomer locate these modes at the following wavenum-
to the eight vibrational normal modes. They are described as
stretching
bending vibrations: d(NO2),
(NO2) and (/-NO2). Observed main IR and (RS) modes of the
C/ANO2 group in the studied compound have been identified
mas(NO2),
ms(NO2) and
m(/-NO2) vibrations and five
x
(NO2), d(/-NO2) and out-of-plane
bers:
mas(CH3): 3018, 2992 and
m
s(CH3): 2940 cmꢁ1. They agree
q
c
very well with the experimental values found at 3009 and 2955

E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
323
Fig. 5. Atom displacements in selected normal modes of CANHANHAC subunit (experimental IR and (Raman) wavenumbers are given).
and assigned to the following bands:
s(NO2) 1335 (1337); (/-NO2) 1162 (1162) and 1066 (1066); scis-
soring ds((NO2) 861 (861) and 844 (844); (NO2) 718 (722); rock-
ing (NO2) 500 (504) and 467 (473); (/-NO2) 284 (284); d(/-NO2)
206 (206);
(/-NO2) 112 (117) and (95) cmꢁ1. These modes involve
m
as(NO2) 1576 (1577);
of freedom. Five of them refer to stretching vibrations:
(NH)/, (C/AN) and (ChAN), as well as
modes to the bending vibrations: das(HNANH), ds(HNANH),
d(NNH)h, d(NNH)/, (HNANH), (NHANH) and (C/NNCh). The
m
(NH)h and
m
m
m
m
m
m(NAN); and seven
x
q
c
c
s
s
s
bands corresponding to these vibrations are observed in the IR
the most significant PED contributions of the CANO2 and NO2 coor-
dinates into the normal modes. Some modes contain lower contri-
bution of these atomic displacements and respective internal
coordinates. This situation appears for the modes observed in the
ranges 686–500, 409–350 and 296–293 cmꢁ1 (Table 3).
and (RS) spectra at the following wavenumbers:
m
m
(NH)h
(NH)/
3318 (3319) cmꢁ1
3290 (3290)
,
das(HNANH)
ds(HNANH)
d(NNH)h
1576 (1577)–1493 (1489)
1471 (1464)
1421 (1430)
Hydrazo bond vibrations
Particular attention was paid to the discussion of bridge
C/ANHANHACh moiety. Its vibrations are described by 12 degrees
(continued on next page)

324
E. Kucharska et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 317–325
(4) The splitting of the
m(NAH) IR band in the range 3318–
d(NNH)/
1391
3290 cmꢁ1 and clear broadening of one of its component is
indicative for the existence of the intra-molecular interac-
tion between the hydrazo N2AH2 bond and O1 oxygen of
the nitro group.
m
(ChAN)
(C/AN)
(NAN)
1285 (1285)
1206 (1205)
(1137)
578 (575)
236 (230)
192 (197)
m
m
c
(HNANH)
(5) The normal modes of the hydrazo bridge shown in Fig. 5
contain the characteristic vibrations of this unit. The follow-
ing wavenumbers are particularly typical for the studied
s(C/NNCh)
s(NHANH)
conformation of this bridge:
1528–1489, 1471–1464, d(NNH) 1430–1391,
1255–1253, 1206–1205,
(NAN) 1137 cmꢁ1 and torsional
vibrations (C/NANCh), (HNANH) and T(HNANH) in the
range 251–176 cmꢁ1
m
(NH) 3318–3290, d(HNANH)
m(CAN) 1285,
Fig. 5 shows selected normal modes of hydrazo ANHANHA bond.
Apart of the modes having significant contribution of hydrazo
group internal coordinates, several other modes exhibit smaller
contribution of hydrazo bond vibrations. These are: 1255
m
s
s
.
(1253) cmꢁ1 – twisting of the HNANH bond:
q(HNANH); 648
(630), 556 (554) and 681 (686) cmꢁ1 – out-of-plane bending:
c
m
(HNANH); 296 (293) cmꢁ1 – stretching of the whole molecule:
(/-HNANHAh) and 247 (251) cmꢁ1 – translation of the whole hy-
Acknowledgements
The authors wish to thank Professor Tadeusz L is from Univer-
sity of Wrocław for data collection and preliminary structure
solving.
drazo-bond: T(HNANH). Hydrazo bond, appearing between two
massive h and / units, participate in several normal modes that
couple the complex vibrations of this bond and both rings.
Intra- and intermolecular interactions
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The intramolecular N2AH2ꢀꢀꢀO1 HB is characterized by the DꢀꢀꢀA
crystallographic distance 2.653(1) Å and angle 124°. Its theoretical
value from the geometry optimisation is 2.640 Å and angle
128.94°. From the X-ray study follows that the nitrogen–oxygen
distances in the nitro group are somewhat different, being for
the N3AO2 bond slightly longer than for N3AO1 i.e.1.240(2) and
1.235(2) Å, respectively. This confirm that the oxygen atoms have
different properties, one of them is terminal and the other partici-
pates in the intramolecular HB.
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Conclusions
(1) The studied compound crystallizes in monoclinic space group
C2/c with eight formula units in the unit cell. The molecule
consists of two nearly planar pyridine and benzene rings. A
conformation of the linking units of CANHANHAC hydrazo-
bridge is unusual being intermediate between the trans and cis
forms (see Fig. 1) with the torsion angle of 77.3(2)°.
(2) Although the X-ray study basing on the shortest contacts
between adjacent molecules in the unit cell suggests the
possibility of intermolecular NAHꢀꢀꢀN and NAHꢀꢀꢀO hydrogen
bonds formation, the IR and Raman spectra do not confirm
this suggestion.
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