Effect of intermolecular = C-H···O interaction on the crystal structure and vibrational properties of 2,6-dimethyl-4-nitropyridine N-oxide

Article abstract of DOI:10.1016/S0022-2860(98)00429-3

The crystal structure of 2,6-dimethyl-4-nitropyridine N-oxide (DMNPO) has been determined at ambient temperature. The compound crystallizes as a monoclinic structure, space group P2/n, with 12 molecules per unit cell. The unit cell contains three non-equivalent formula units. The nitro group is not coplanar with the pyridine ring. Through a system of =C-H···O hydrogen bonds the molecules are arranged into a two-dimensional network of layers parallel to the axc plane. The IR and Raman spectra, measured in the 3500-100 cm^-1 region at ambient temperature, are correlated with X-ray structural data. The assignment of IR and Raman bands is given. The appearance of characteristic vibrational features in the spectra of this compound and the observed shifts of the =C-H and N-O IR active stretching modes, when the sample is dissolved in CCl₄, is discussed in terms of the relatively strong =C-H···O hydrogen bonds present in this crystal.

Full text of DOI:10.1016/S0022-2860(98)00429-3

MOLSTR 10326
Journal of Molecular Structure 450 (1998) 201–212
…
Effect of intermolecular yC–H O interaction on the crystal structure
and vibrational properties of 2,6-dimethyl-4-nitropyridine N-oxide
J. Hanuzaa, M. Ma˛czkaa, A. Was´kowskaa, W. Oganowskia, H. Ban-Oganowskab, E.T.G. Lutzc,
J.H. van der Maasc
aInstitute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław, Poland
bDepartment of Organic Chemistry, Academy of Economics, Wrocław, Poland
cDepartment of Analytical Molecular Spectrometry, Faculty of Chemistry, Utrecht University, Utrecht, The Netherlands
Received 14 November 1997; revised 22 December 1997; accepted 22 December 1997
Abstract
The crystal structure of 2,6-dimethyl-4-nitropyridine N-oxide (DMNPO) has been determined at ambient temperature. The
compound crystallizes as a monoclinic structure, space group P2/n, with 12 molecules per unit cell. The unit cell contains three
non-equivalent formula units. The nitro group is not coplanar with the pyridine ring. Through a system of yC–H…O hydrogen
bonds the molecules are arranged into a two-dimensional network of layers parallel to the axc plane.
The IR and Raman spectra, measured in the 3500–100 cm−1 region at ambient temperature, are correlated with X-ray
structural data. The assignment of IR and Raman bands is given. The appearance of characteristic vibrational features in the
spectra of this compound and the observed shifts of the yC–H and N–O IR active stretching modes, when the sample is
dissolved in CCl4, is discussed in terms of the relatively strong yC–H…O hydrogen bonds present in this crystal. ᭧ 1998
Elsevier Science B.V.
Keywords: X-ray crystallography; Infrared spectroscopy; Raman spectroscopy; CHO hydrogen bonding; 2,6-dimethyl-4-nitro-
pyridine N-oxide
1. Introduction
yC–H…O [6,7]. The importance of yC–H…O inter-
actions in various areas of chemistry has frequently
been reported [8–10]. These relatively weak but
specific interactions may play an important role and
be essential for the activity of therapeutic drugs.
Vibrational spectroscopy is an excellent tool for
studying this type of interaction since the formation
of yC–H…O bonding leads to a significant shift of
some vibrational bands as well as intensity changes.
In the present work, the X-ray crystal structure of
the title compound has been determined and the
results obtained have been analysed in respect of
4-nitropyridine N-oxide derivatives belong to a
class of compounds which are very extensively studied
because of their interesting biological and physical
properties. These nitro derivatives are, for example,
prospective crystals for second-harmonic generation
[1–5].
In this paper, part of a systematic investigation of
substituted pyridine N-oxide derivatives is reported.
In many of these crystals the molecules are connected
through intermolecular hydrogen bonds of the type
0022-2860/98/$19.00 ᭧ 1998 Elsevier Science B.V. All rights reserved
PII S0022-2860(98)00429-3

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J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
precise assignments of the observed vibrational
modes. Special attention has been focused on inter-
molecular interactions in the crystalline phase. The
observed modes have been assigned with reference
to previous vibrational studies and normal coordinate
analysis of 2,6-dimethylpyridine and 4-nitropyridine
N-oxide (NPO) [11–24].
Table 1
Experimental details
Crystal data:
Chemical formula
Molecular weight
Crystal system
Space group
C7H8N2O3
168.15
monoclinic
P2/n
Lattice parameters
a = 17.922(4), b = 3.933(1),
˚
c = 34.624(7) A, b = 92.58(3)Њ
3
˚
V
2438.(1) A
Z
12
2. Experimental
˚
Radiation wavelength
No. of reflections for
cell parameters
v range for cell parameters
m
CuKa, 1.541 78 A
39
2.1. Crystal growth
22, 40Њ
0.91 mm−1
In order to obtain 2,6-dimethyl-4-nitropyridine
N-oxide, 0.5 mole of 2,6-dimethylpyridine was dis-
solved in 100 cm3 of (CH3CO)2O. Then 100 cm3 of
30% H2O2 was added and the mixture was heated for
3 h at 70–80ЊC. The acetic acid and water thus formed
was distilled off under reduced pressure, then 50 cm3
of water was added and distilled off. The reaction
mixture was neutralized with KHCO3 and the reaction
product was extracted with chloroform. The 2,6-
dimethylpyridine N-oxide thus obtained was added
to 100 cm3 of 96% H2SO4. The mixture of 2,6-
dimethylpyridine N-oxide with H2SO4 was added to
a mixture of 120 cm3 H2SO4 (96%) with 180 cm3 of
concentrated HNO3. The reaction mixture was then
heated for 2 h, cooled, poured into 500 g of ice, neu-
tralized with NH4HCO3 and a concentrated NH3 solu-
tion (to pH Ϸ 9). The crystals obtained (melting point
152–153ЊC, readily soluble in chloroform, hardly
soluble in ethanol and acetone) were filtered off and
recrystallized from water.
Temperature
Crystal colour and form
Crystal size
294 K
yellow, plate
0.25 × 0.15 × 0.3 mm
1.374 Mg m−3
Dc
Data collection:
Diffractometer
No. of measured reflections
No. of independent reflections 2930
No. of observed reflections
Observation criterion
Rint
vmax (Њ)
Index range
Kuma KM-4
4509
2081
lF0l2 Ͼ 2j(F0)2
6.4% for all reflections
58.1
h: − 19, 17; k: − 4, 0; l: − 36, 36
2, measured every 50 reflections
0.062%
Standard reflections
Intensity variation
Refinement:
Refinement on
No. of reflections used in
refinement
Number of parameters varied
Final R1
F2
2081
350
0.061
0.092
Final wR
Goodness of fit
1.088
0.07, 2.1
− 0.27, 0.26
2.2. X-ray diffraction
Weight parameters c1 and c2
Min., max. residual (eA−3
)
A single crystal was tested by the usual rotation and
Weissenberg technique. The symmetry and systema-
tic absences (h0l: h + l = 2n + 1) indicated one of the
monoclinic space groups Pn or P2/n. The quality of
the crystal allowed the data collection to be restricted
to v = 58.1Њ (CuKa radiation), beyond which the
intensity faded rapidly. The intensities were corrected
for Lorentz polarization effects. Crystal data and
experimental details are given in Table 1. The statis-
tical distribution of lEl values, compared with theore-
tical values expected for the centrosymmetric and
acentric structures, gave an indication of the centric
Rint = lF20 − F2m0eanl/SlF20l.
R1 = SllF0l − lFcll/SlF0l.
wR = {S‰wꢀF02 − F2c)2]/S[w(F20)2]}1/2
.
w = ‰j2ꢀF02† + (c1P)2 + c2P]−1, where c1 and c2 are as in the table,
and P = [max(F20) + 2F2c)/3].
Goodness of fit S = SwlF0l − lFcl=(n − m)1/2 (n = no. of reflections,
m = no. of parameters).
structure. The structure was solved by a combination
of direct methods and difference Fourier synthesis.
The atomic positions were refined by the full-matrix
least-squares technique (on F2) with the SHELXL 93

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J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
atoms bound to the pyridine ring were located from a
difference Fourier map and refined isotropically,
whereas the H atoms belonging to the methyl groups
were placed in geometrically calculated positions.
The atomic scattering factors for neutral atoms were
used as in [26]. The final atomic parameters are
reported in Table 2.
Table 2
Atomic coordinates ( × 10−4) and equivalent isotropic displacement
coefficients (A × 10−3). Equivalent isotropic displacement Ueq is
defined as one-third of the trace of the orthogonalized Uij tensor
2
˚
x
y
z
Ueq
Unit A:
N(1)
N(2)
O(1)
O(2)
O(3)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
4541(2)
3869(3)
4751(2)
3310(3)
4247(2)
4985(2)
4768(2)
4094(2)
3650(2)
3871(2)
3447(2)
5689(2)
790(7)
1281(12)
649(8)
2864(13)
− 176(12)
− 605(10)
− 456(10)
1115(10)
2520(10)
2351(9)
6995(1)
5830(1)
7353(1)
5739(1)
5592(1)
6724(1)
6345(1)
6237(1)
6505(1)
6894(1)
7213(1)
6871(1)
34(1)
65(1)
48(1)
102(2)
90(1)
37(1)
43(1)
43(1)
42(1)
36(1)
52(1)
50(1)
2.3. Raman and infrared measurements
The details of IR and Raman measurements have
been described previously [7]. Polarized IR reflection
spectra of a single crystal, oriented by the X-ray
method, were measured with a Perkin–Elmer Auto-
IMAGE system microscope connected to a Perkin–
Elmer 2000 FTIR spectrometer. The spectra were
measured with the incident angle close to 0Њ and the
aperture 100 × 100 mm. Using standard spectra for
Windows software, Kramers–Kronig calculations
were carried out on a PC to transform reflection spec-
tra into transmission spectra.
3811(11)
− 2207(11)
Unit B:
N(1)
N(2)
O(1)
O(2)
O(3)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
5232(2)
3292(2)
5829(2)
3167(2)
2893(2)
5091(2)
4452(2)
3969(2)
4107(2)
4750(2)
4972(2)
5656(2)
4817(7)
− 594(8)
6506(7)
− 1190(10)
− 1569(9)
4017(9)
2245(10)
1362(9)
2161(9)
3927(9)
4892(11)
5039(11)
8748(1)
8424(1)
8845(1)
8085(1)
8678(1)
8364(1)
8262(1)
8539(1)
8919(1)
9028(1)
9427(1)
8090(1)
33(1)
45(1)
54(1)
79(1)
68(1)
36(1)
37(1)
34(1)
35(1)
33(1)
51(1)
51(1)
3. Crystal structure determination
The asymmetric unit comprises three formula units
A, B and C, shown in Figs. 1–3. Details of the
molecular structure geometry are given in Tables 3
and 4. The three nitro groups possess considerable
double bond character (average N–O distance =
Unit C:
N(1)
N(2)
O(1)
O(2)
O(3)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
1734(2)
3233(2)
1264(2)
3112(2)
3788(2)
2397(2)
2901(2)
2727(2)
2054(2)
1560(2)
826(2)
− 3140(8)
− 1127(10)
− 3686(8)
− 2279(12)
599(10)
10794(1) 35(1)
9937(1) 50(1)
11056(1) 58(1)
9622(1) 86(1)
10027(1) 79(1)
10895(1) 35(1)
10617(1) 36(1)
10239(1) 34(1)
10142(1) 39(1)
10422(1) 36(1)
10345(1) 56(1)
11307(1) 57(1)
˚
1.213(4) A), even though somewhat higher anisotro-
pic displacement amplitudes of these O atoms indicate
1536(9)
Table 3
Bond lengths (A)
− 890(10)
− 1851(9)
− 3478(10)
− 4098(10)
− 5735(12)
− 603(12)
˚
Unit A
Unit B
Unit C
N(1)–O(1)
N(2)–O(2)
N(2)–O(3)
C(2)–N(1)
C(2)–C(3)
C(2)–C(8)
C(3)–C(4)
C(4)–N(2)
C(5)–C(4)
C(5)–C(6)
C(6)–N(1)
C(6)–C(7)
1.278(4)
1.209(5)
1.231(5)
1.373(5)
1.351(5)
1.480(6)
1.392(6)
1.450(6)
1.366(6)
1.388(5)
1.380(5)
1.486(6)
1.291(4)
1.208(4)
1.222(4)
1.377(5)
1.373(5)
1.475(5)
1.368(5)
1.476(5)
1.364(5)
1.384(5)
1.373(5)
1.470(5)
1.285(4)
1.190(5)
1.233(5)
1.376(5)
1.374(5)
1.480(5)
1.383(5)
1.444(6)
1.392(6)
1.365(5)
1.364(5)
1.478(5)
2524(3)
program [25]. The check calculations in the space
group Pn resulted in numerous strong correlations
between the parameters; therefore the space group
P2/n has been chosen. The choice was later confirmed
by the satisfactory refinement results. All non-
hydrogen atoms were refined anisotropically. The H

204
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
Fig. 1. Molecular structure showing the labelling of the atoms. Displacement ellipsoids are drawn at the 50% probability level.
˚
the presence of librations perpendicular to the ring
plane, causing the N–O distance to be shortened.
The double bond character is less evident for the
and 1.373 (5) A in A, B and C, respectively) are
considerably shorter than the N(2)–C distance
(1.459(7) A). Each oxygen atom of the three
N-oxide groups is involved in intermolecular
hydrogen bond interactions of the type yC–H…O
(Table 5 and Fig. 3). This bond system causes the
molecules to lie in layers perpendicular to the b axis
˚
˚
N-oxide group (average (N–O) = 1.285(4) A). The
nitro ligand is not coplanar with the pyridine ring; it
has pyramidal character. In each of the three subunits
the two O atoms lie below and above the least-square
plane passing through the pyridine ring. In units A and
C the effect is twice as strong as in B. The correspond-
ing distances of the O(2) and O(3) atoms from the
˚
with an interplanar distance ϳ3.93 A (Fig. 3). How-
ever, in contrast to the previously described 3-chloro-
and 3-bromo-2,6-dimethyl-4-nitropyridine N-oxide
[6], where the molecules are ordered in anti-parallel
stacks of pairs suggesting some kind of dipole
interaction between the pairs, in the present
structure the hydrogen bonded layers are arranged in
parallel.
˚
plane are: 0.101(3) and − 0.084(3) A in the A subunit,
˚
− 0.0735(20) and 0.043(2) A in subunit B, and finally
the two oxygen atoms in C are distant from the ring by
˚
− 0.099(2) and 0.171(3) A, respectively.
The N(1)–C bonds (average: 1.377(5), 1.370(5)
Fig. 2. Mutual orientation of the subunits A, B and C. For clarity, the H atoms are omitted.

205
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
4. Vibrational assignment and discussion
Table 4
Bond angles (Њ)
The IR and Raman spectra (Figs. 4–8) consist of
fewer bands than expected because no factor group
splitting is observed. Moreover, although in the unit
cell three crystallographically non-equivalent mole-
cules are present and therefore each vibrational band
should be split, in principle, into three components,
for the majority of bands no splitting is observed.
It is therefore convenient to take into consideration
all the vibrations of a single molecule and to explain
the observed splitting of some bands as a result of
molecular interactions in the crystal. There are 54
internal modes for a free molecule consisting of 20
atoms. Eighteen of them describe the methyl groups,
six the nitro group and the remaining 30 the pyridine
ring vibrations. The proposed assignment of the vibra-
tional modes is presented in Table 6.
Unit A
Unit B
Unit C
O(1)–N(1)–C(2)
C(6)–N(1)–C(2)
O(1)–N(1)–C(6)
O(2)–N(2)–O(3)
O(2)–N(2)–C(4)
O(3)–N(2)–C(4)
N(1)–C(2)–C(3)
N(1)–C(2)–C(8)
C(3)–C(2)–C(8)
C(2)–C(3)–C(4)
C(5)–C(4)–C(3)
C(3)–C(4)–N(2)
C(5)–C(4)–N(2)
C(6)–C(5)–C(4)
C(5)–C(6)–N(1)
C(5)–C(6)–C(7)
N(1)–C(6)–C(7)
119.5(3)
121.8(3)
118.7(3)
122.8(5)
117.8(5)
119.4(5)
119.8(4)
116.3(3)
123.8(4)
119.3(4)
121.3(4)
118.8(4)
119.8(4)
119.4(5)
118.4(3)
124.8(4)
116.8(3)
119.1(3)
118.9(3)
121.9(3)
123.4(5)
118.7(5)
118.0(4)
118.7(3)
117.2(3)
124.1(3)
119.6(4)
121.8(4)
118.7(4)
119.5(4)
119.4(4)
118.6(3)
124.9(3)
116.5(3)
118.9(3)
121.7(3)
119.5(3)
123.5(3)
119.6(4)
116.8(3)
119.6(3)
116.9(3)
123.5(4)
119.0(4)
120.7(3)
120.6(4)
118.7(3)
119.6(4)
119.5(3)
123.5(4)
117.0(3)
Fig. 3. Packing diagram of the structure. Intermolecular hydrogen bonds are marked by dashed lines.

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J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
Fig. 4. (a) Polycrystalline and (b) CCl4 solution mid-IR spectra of DMNPO.
The assignment of most of the observed bands is
observed at 3091 and 3058 cm−1, whereas the IR spec-
trum shows one strong band at 3075 cm−1 with
shoulders at 3091 and 3061 cm−1. The IR spectrum
in CCl4 solution (Table 6) shows bands at 3096,
3087 and 3067 cm−1. The 3096 cm−1 mode (in solu-
tion) originates from the free yC–H group and, as a
result of yC–H…O interaction in the crystal, it shifts
to 3075 cm−1. It has been shown previously that inter-
action between a yC–H group and an oxygen atom
results in a shift towards smaller wavenumbers for the
C–H stretching vibration and intensity increase if
straightforward and we shall therefore discuss in this
paper only some particular points concerning the
influence of intermolecular interactions, especially
yC–H…O hydrogen bonding, and crystal packing on
vibrational spectra.
4.1. The 3200–2800 cm−1 region
In the present study, the yC–H stretch band is split
into two weak components in the Raman spectrum,
Table 5
Proposed hydrogen bonding system
…
…
…
D–H
A
D–H
H
A
D–A
Ͻ ) D–H
A
…
C(5c)–H(17) O(2a)i
0.930(4)
0.947(4)
0.890(5)
0.981(4)
0.870(5)
2.490(4)
2.538(3)
2.440(5)
2.524(3)
2.590(5)
3.411(5)
3.447(5)
3.276(5)
3.694(5)
3.354(5)
166.0(3)
160.9(3)
155.0(4)
153.1(3)
147.0(4)
…
C(8a)–H(7) O(1c)ii
…
C(3b)–H(9) O(1a)
…
C(8b)–H(14) O(1a)iii
…
C(3c)–H(18) O(1b)iv
Symmetry codes:
i: 1/2 − x, y − 1, 3/2 − z
ii: − x + 1/2, − 1 − y, z − 1/2
iii: x, y + 1, z
iv: 1 − x, 1 − y, 2 + z

207
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
Fig. 5. Far-IR spectrum of DMNPO.
Fig. 6. Raman spectrum of DMNPO.

208
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
Fig. 7. Polarized IR spectra of DMNPO: (a) Ekb; (b) EЌb.
Fig. 8. Polarized Raman spectra of DMNPO with (a) both incident and scattered light polarized along the b axis and (b) both incident and
scattered light polarized perpendicular to the b axis.

209
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
compared with the free yC–H group [27,28]. The
appearance of the 3091 cm−1 band in Raman and IR
spectra of the polycrystalline sample, which can be
assigned to the free yC–H group vibration, indicates
that not all pyridine ring hydrogen atoms in the crystal
form yC–H…O bonding. The results of the X-ray
analysis confirm this conclusion: only in two of
three crystallographically non-equivalent molecules
is this type of hydrogen bond formed, with the
O(1a)–H(9) and O(1b)–H(18) distances 2.440 and
ac plane and very weak ones along the b axis. The
opposite behaviour is expected for A, oriented almost
perpendicular to the B and C units (Fig. 2). The IR
spectrum, recorded with light polarized in the ac
plane, shows a strong band at 1341 and a weak one
at 1334 cm−1, whereas for the Ekb polarization the
1334 band is more intense than the 1341 cm−1 one.
Similar behaviour is observed in the Raman spectra:
when both incident and scattered light are polarized
along the b axis the strongest line is observed at
1336 cm−1, whereas for polarization perpendicular to
the b axis the 1344 cm−1 line becomes the most
intense. The measured spectra clearly show that the
1344–1341 cm−1 lines may be assigned to the ns(NO2)
modes of units B and C and that the second com-
ponent at 1336–1334 cm−1, attributed to molecule
A, is moved towards lower wavenumbers due to the
hydrogen-type interaction of the NO2 group with the
hydrogen atom of the pyridine ring. No ns(NO2) split-
ting was previously observed for NPO [18], 3-methyl-
4-nitropyridine N-oxide (POM) [24] and 3-halo-2,6-
dimethyl-4-nitropyridine N-oxides [6], where these
modes are located in the 1335–1350 cm−1 region.
The 1108 cm−1 band is substituent dependent
and was observed at 1180–1173 cm−1 for 3-halo-
2,6-dimethyl-4-nitropyridine N-oxides [6] and
1125 cm−1 for NPO [18]. Former studies of pyridine
and pyridine N-oxide derivatives showed that the only
vibration above 1000 cm−1, which can be observed in
a broad wavenumber range, is in-plane ring deforma-
tion (n12 in Wilson notation [29]). This mode was
observed at 1025 cm−1 for pyridine N-oxide [30],
1271 cm−1 for 2,6-dimethylpyridine [31] and
1010 cm−1 for POM [24]. Joyeux and Dao [18]
showed by means of normal coordinate analysis that
this band is not a pure n12 ring deformation but should
rather be assigned as a coupled n12 ring deformation
and n(C–NO2) stretching mode.
2.590 A, respectively. The 3067 cm−1 IR band in solu-
˚
tion can be assigned to the second free yC–H group
vibration. Because the hydrogen atoms H2 and H9 do
not interact with the N–O groups, the respective
modes in the polycrystalline sample are only slightly
moved and are observed at 3058 (RS) and 3061 cm−1
(IR). The remaining bands at 3010–3011 and 2962–
2965 cm−1 are assigned to the nas(CH3) vibrations.
The ns(CH3) mode is split into a doublet. The compo-
nent at 2919 cm−1 is clearly visible in the IR and as a
shoulder in the Raman spectrum, whereas the second
component at 2930 cm−1 is Raman active only. The
observed splitting can be explained as a result of
intermolecular interaction of the methyl groups in
6-position with the N–O groups. According to the
˚
X-ray data, the O…H distances are 2.538 and 2.524 A.
4.2. The 1650–1100 cm−1 region
In this region interesting features are observed for
the NO2 symmetric stretch, the NO stretch and the
band around 1100 cm−1. The NO2 symmetric stretch-
ing mode is split into a doublet (1344 and 1335 cm−1)
due to the existence of non-equivalent molecules in
the unit cell. In principle, the ns(NO2) band should be
split into three components. However, in only one
molecular unit (A) does the NO2 group interact with
the neighbouring H13 atom, forming hydrogen bond-
ing. The two other nitro groups (in units B and C), for
which no NO…2 H interaction exists, may have very
similar properties and therefore the ns(NO2) modes
of these groups are observed as only one band. The
polarization measurements enable us to distinguish
which band originates from the NO2 vibration of
molecule A and which of molecules B and C. Since
the molecules B and C are almost parallel to the ac
plane, the ns(NO2) vibrations of these molecules
should lead to strong dipole moment changes in the
The N–O stretch is the most interesting mode in
this region and gives much information about the
nature of intra- and intermolecular interactions. The
aromatic N-oxides show moderate double bond
character in their N–O bond and the stretching fre-
quency varies as its p-bond order. The two main
mesomeric forms of DMNPO are presented in Fig. 9.
In the present study the n(NO) mode was observed at
1281 cm−1 with a shoulder at 1275 cm−1 (IR) and at
1276 cm−1 with a shoulder at 1281 cm−1 in the Raman

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J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
Table 6
Vibrational wavenumbers for DMNPO
IR
Raman
Assignment
CCl4
kb
Ќb
Polycrystalline
bb
Ќbb
Polycrystalline
3091sh
—
3096w
3087vw
—
3067vw
3005w
2962w
—
3091w
—
—
3058w
3010w
2962w
2930w
2919sh
1616m
—
3093w
3088w
n(C–H)
overtone or combination band
n(C–H)
n(C–H)
nas(CH3)
nas(CH3)
ns(CH3)
ns(CH3)
n8a ring stretch
n8b ring stretch
3075s
3060sh
3011w
2965vw
—
2919vw
1615m
1591w
1538sh
1521s
1463m
1437w
1420w
1399w
1381w
1363vw
1343s
3064w
3008w
2966w
2930m
3059w
3009w
2960w
2931w
2921w
1618w
1616m
1616m
1537w
1524s
1456m
—
1519s
1519s
1524w
1524vw
1456vw
1439vw
1524vw
1467vw
1438vw
nas(NO2)
n19a ring stretch
n19b ring stretch
das(CH3)
ds(CH3)
ds(CH3)
ds(CH3)
ns(NO2)
ns(NO2)
1435w
1418w
1438vw
1418w
1401m
1377w
1365w
1344s
1401m
1377m
1365vw
1346m
1336s
1402m
1377w
1365w
1344vs
1336s
1379w
1360w
1335vs
1381w
1340m
1336s
1341s
1334w
1335s
1307w
1281sh
1276m
1308w
1282sh
1276m
1306w
1281sh
1276w
n14 ring in-plane deformation
n(N–O)
n(N–O)
1281s
1293vs
1279m
1273s
1281s
1274m
1285w
1218w
1099m
1035w
997w
overtone or combination band
d(C–H)
1211m
1108m
1039m
1004w
959vw
944m
924vw
912m
820vw
749s
—
559m
543s
444w
377w
364vw
302w
1218w
1109w
1039vw
1208w
1108w
1039vw
1210vw
1108w
1041vw
1003w
959vw
944w
1109w
1003vw
944w
1108w
1002vw
945w
n12*n(C–NO2)
CH3 rock
CH3 rock
n(C–CH3)
n1
945m
896m
943w
921w
910w
943w
748w
—
g(C–H)
g(C–H)
d(NO2)
g(NO2)
n6b
n6a
819vw
749m
620vw
558vw
545m
444w
381w
364w
300w
251w
749s
749m
749m
620vw
560vw
545w
444vw
381vw
363w
545w
447vw
r(NO2)
n16a
d(N–O)
d(C–CH3)
g(NO)
g(C–CH3), d(C–NO2)
t(CH3)
libration or translation
365vw
248w
296w
250w
201w
128m

211
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
The former is more intense for polarization along
the b axis and the latter for polarization in the ac
plane. In a similar way as was presented above for
the ns(NO2) mode, the 1274 cm−1 band can be
attributed to the n(NO) vibration of molecule A and
the 1281 cm−1 mode to molecules B and C. This
assignment is consistent with the X-ray analysis
since stronger C–H…O hydrogen bonding is observed
for the N–O group of molecule A than for those of
molecule B or C.
4.3. The 1100–100 cm−1 region
Fig. 9. The two main mesomeric forms of the DMNPO molecule.
The two strongest IR lines in this region, at 749 and
543 cm−1, can be unambiguously assigned to the
g(NO2) and r(NO2) modes. The d(NO2) vibration
gives rise to a very weak band at 820–819 cm−1 and
the twist t(NO2) mode was not detected in the present
studies since it is situated below 100 cm−1 [24]. The
medium intensity IR band at 912 cm−1 and the weak
one at 924 cm−1 (absent in the Raman spectrum) show
considerable shift towards smaller wavenumbers, i.e.
to 896 cm−1 in CCl4 solution. Since a shift towards
higher wavenumbers is expected for the out-of-plane
C–H bending [27] forming the yC–H…O bond in a
crystal these bands can be assigned to the g(C3b–H9)
and g(C3c–H18) modes. The line at 944 cm−1 does
not show any shift in CCl4 solution and therefore
originates from the n1 ring-stretching mode. The
polarized IR spectra support our conclusion. The
in-plane n1 vibration should be more intense for polar-
ization in the ac plane than for light polarized along
the b axis since the two molecules (B and C) give rise
to a transition moment in the ac plane but only one (A)
does along the b axis. This type of behaviour is
observed for the 944 cm−1 band. For the out-of-
plane vibrations, higher intensity is expected for Ekb
than for EЌb polarization. The only bands increasing
their intensity for polarization along the b axis are
those at 749 (g(NO2)), 910 and 921 cm−1. The two
latter bands are therefore assigned to the g(C–H)
modes.
spectrum. The respective IR band in non-polar CCl4
solution was located at 1293 cm−1. The observed shift
towards smaller wavenumbers of the n(NO) band in a
crystal, compared with CCl4 solution, is due to the
yC–H…O interactions. These interactions diminish
the intramolecular charge transfer and the p-bond
order of the N-oxide moiety, and finally lead to low-
ering of the n(NO) mode wavenumber. A similar shift
was observed for the NPO: in non-polar solvents the
N–O group absorbs at about 1300 cm−1 [32] and in the
crystal at 1269 cm−1 [18]; i.e., in the solvent the mole-
cule is best described by the form II and in the crystal
by the form I. In the case of DMNPO the situation is
similar; i.e., the main resonance form in the crystal is
form I. However, for this crystal the observed shift
is only 12 cm−1. This conclusion is consistent with the
results of X-ray analysis which show that the nitro
groups are twisted. The lowering of the n(N–O) wave-
number in CCl4 from 1300 cm−1 in the case of NPO to
1293 cm−1 in the present study of DMNPO can be
explained as a result of lower p-bond order of a free
molecule due to the presence of two electron-
releasing CH3 groups. The higher N–O p-bond
order for pyridine N-oxide derivatives was predicted
by Kulkarni et al. [33] for compounds with electron-
withdrawing substituents rather than for compounds
with electron-releasing substituents. This effect
explains also why, in the case of 4-chloro-2,6-
dimethyl-3-iodopyridine N-oxide (without the elec-
tron-withdrawing NO2 group), the n(NO) band
observed by us is moved to 1231 cm−1 [7].
5. Conclusions
The polarized IR spectra show two components of
Our study of the infrared and Raman spectra of 2,6-
dimethyl-4-nitropyridine N-oxide allows us to make a
the n(NO) stretching mode at 1274 and 1281 cm−1.

212
J. Hanuza et al./Journal of Molecular Structure 450 (1998) 201–212
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first vibrational assignment of this compound. A com-
parison of the polycrystalline and polarized spectra
with that in CCl4 solution shows that:
•
•
at least two non-equivalent molecules are pre-
sent in the unit cell;
relatively strong yC–H…O–N hydrogen
bonds are present in this crystal.There are
the following spectroscopic proofs of exis-
tence of the yC–H…O hydrogen bonds:
a shift towards smaller wavenumbers of the
yC–H and N–O stretch vibrations;
•
•
•
a shift towards higher wavenumbers of the
yC–H out-of-plane vibrations;
the relative intensity increase of the yC–H
stretch in the polycrystalline sample as com-
pared with the CCl4 solution.X-ray analysis
confirmed the presence of both nearly free
yC–H groups and strong intermolecular
yC–H…O bonds.The crystal studied is an
interesting example of application of polar-
ized light in the determination of the origin
of observed bands even if the symmetry of a
crystal is low. The polarized measurements
enabled us to:
•
•
discriminate between in-plane and out-of-
plane vibrations, especially in locating unam-
biguously n1 and g(C–H) modes situated close
to each other;
attribute the two components of the observed
ns(NO2) and n(NO) doublets to the two groups
of molecules (A and B + C) and explain the
observed differences in their frequencies as a
result of intermolecular interactions.
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