Synthesis, X-ray and spectroscopic analysis of 2-chloro-4-(methoxymethyl)- 6-methyl-5-nitropyridine-3-carbonitrile

Article abstract of DOI:10.1016/j.molstruc.2010.06.012

Compound 2-chloro-4-(methoxymethyl)-6-methyl-5-nitropyridine-3-carbonitrile (2) has been obtained from 4-(methoxymethyl)-6-methyl-5-nitro-2-oxo-1,2- dihydropyridine-3-carbonitrile (1) by novel protocol using Vilsmeier-Haack chlorination and its solid stat

Full text of DOI:10.1016/j.molstruc.2010.06.012

Journal of Molecular Structure 979 (2010) 108–114
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, X-ray and spectroscopic analysis of
2-chloro-4-(methoxymethyl)-6-methyl-5-nitropyridine-3-carbonitrile
a,
b,
a
a
*
**
´
´
, Jasna Halambek , Ivana Ugarkovic
Marijana Jukic , Mario Cetina
a Department of Chemistry and Biochemistry, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia
b Department of Applied Chemistry, Faculty of Textile Technology, University of Zagreb, Prilaz baruna Filipovi´ca 28a, HR-10000 Zagreb, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2010
Received in revised form 4 June 2010
Accepted 4 June 2010
Available online 12 June 2010
Compound 2-chloro-4-(methoxymethyl)-6-methyl-5-nitropyridine-3-carbonitrile (2) has been obtained
from 4-(methoxymethyl)-6-methyl-5-nitro-2-oxo-1,2-dihydropyridine-3-carbonitrile (1) by novel proto-
col using Vilsmeier–Haack chlorination and its solid state structure was analyzed using X-ray analysis.
Structural features have been also studied by IR, NMR and electronic spectroscopy, and the optical prop-
erties were investigated by UV–vis absorption and fluorescence spectroscopy. Compound 2 crystallizes
with two independent molecules in the asymmetric unit with almost identical geometric parameters.
One CAHÁ Á ÁO hydrogen bond self-assembles one type of independent molecule into chains, while second
Keywords:
Substituted pyridine
X-ray diffraction
IR
type is linked by one weak aromatic
p p stacking interaction. The absorption and fluorescence maxi-
Á Á Á
mum of compound 2 were observed at 290 nm and 480 nm, respectively. The effects of solvents were
investigated and interpreted on the emission spectra in protic and aprotic solvents in the range 200–
600 nm.
NMR
UV–vis spectroscopy
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
proflavine (antiseptics) and therefore is considered a privileged
structure in the pharmaceutical industry [7,21–24].
Heterocyclic compounds and their derivatives constitute a
number of natural substances and are widely used in the synthesis
of various biologically active compounds [1–3]. Among them, be-
cause of their chemical properties and biological activity, pyridine
and their derivatives have attracted interest of many researches
[4–7].
Different approaches in the construction of the pyridine skele-
ton with different substituents at various ring positions were
examined, as well as relationship between the structure of these
compounds and their biological activities [8–12]. In particular, cer-
tain derivatives of pyrimidines, being an integral part of DNA and
RNA, exhibit diverse pharmacological activities such as antimicro-
bial, antifungal, antibacterial, pesticidal [13–17]. Pyridine deriva-
tives are widely used as antiallergic [18], antihypertensive [19],
bronchodilators [18], antitumor [8,18], and analgesic and muscle
relaxant (interneuronal blocking) agents [20]. The pyridine ring
forms the basis of a large number of pharmacological products
such as vitamin B5, vitamin B6, pyridoxal and pyridoxamine; and
drugs such as nifedipine (cardiovascular), nichetamide (analge-
sics), sulphapyridine (sulfonamide), lacidipine (antihypertensive),
Substituted pyridine compounds such as 2-halo-substituted
pyridines are useful as intermediates for preparing insecticides,
herbicides and pharmaceuticals. Preliminary examination demon-
strates in vitro antitubercular and antifungal activity of chloropyr-
idine which are finding as a new class of antimicrobial agents [25].
Halogen-substituted 2-pyridones are also key intermediates for
metalcatalyzed coupling reactions, because the chlorine atom at
position 2 is fairly reactive and may easily be replaced by various
groups under nucleophilic attack [26].
The prominent role played by pyridines and related compounds
as privileged structures in many natural products and bioactive
compounds, led us to consider the extension of our studies to the
pyridine nucleus. The present work has been undertaken as a part
of our systematic research on the synthesis and analysis of hetero-
cyclic molecules [27–30], and continues our investigation of struc-
tures these bioactive compounds and their metal complexes. The
goal of this work was to carry out further characterization of vari-
ous derivatives of pyridines. Particularly, we wanted to determine
how the position and number of substituent modulate their spec-
troscopic properties. To evaluate the relationship between biolog-
ical activity and molecular structure, the knowledge of their
electronic structure and spectral properties are particularly impor-
tant. Therefore, in the present work, we have reported the synthe-
sis, structure and spectroscopic properties of 2-chloro-4-
(methoxymethyl)-6-methyl-5-nitropyridine-3-carbonitrile.
* Corresponding author. Tel.: +385 1 4605 282; fax: +385 1 4836 083.
** Corresponding author. Tel.: +385 1 3712 590; fax: +385 1 3712 599.
E-mail addresses: mjukic@pbf.hr (M. Jukic´), mario.cetina@ttf.hr (M. Cetina).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.06.012

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M. Jukic et al. / Journal of Molecular Structure 979 (2010) 108–114
cooled to 0–5 °C, phosphorus oxychloride (2.8 mL, 30 mmol) was
added dropwise and then allowed to stand until no further heat
of reaction was noticeable. A thick brown suspension remained
at the end of the exothermal reaction. The reaction mixture was
than heated to 55 °C for 2 h with stirring, while the reaction mon-
itoring was carried out by TLC. After completion, the reaction mix-
ture was added to cold, poured on crushed ice (40 g), slowly with
constant stirring, and then brought to pH ꢀ 7 using saturated solu-
tion of sodium hydrogen carbonate and extracted with dichloro-
methane. The organic layer was washed with water, dried over
anhydrous Na2SO4, and evaporated under reduced pressure to af-
ford the crude product. The crude residue was purified on silica
gel column chromatography over silica gel using dichlorometh-
ane/ethyl acetate (4:3) as eluent to give the title compound as a
crystalline solid (yield 89%, m.p. 75–78 °C). Elemental analysis, cal-
culated for C9H8N3O3Cl (241.6323); % C 44.74; % H 3.34; % N 17.39.
2. Experimental
2.1. General methods
All reactions were performed with commercially available re-
agents and they were used without further purification, and the
solvents were distilled and dried by standard procedures.
The reaction course and purity of the products were checked by
thin-layer chromatography (TLC) on Merck, DC-Alufolien Kieselgel
60 F254, using dichloromethane–ethyl acetate as the eluent. Inves-
tigated compound was purified by preparative thin-layer chroma-
tography on silica gel (Merck, Kieselgel 60 HF254) using the
mixtures CH2Cl2:EtOAc (3:1 v/v) and by recrystallization from
(aqueous) EtOH. The solvents were of spectroscopic grade.
The chemical structure and the purity of the compound were
confirmed by melting points, FTIR, 1H and 13C NMR spectra. The
melting points were determined using an electrothermal Buechi
apparatus and are not corrected. The IR spectra were recorded for
KBr pellets with a Bomem MB 100 mid FTIR spectrophotometer.
The 1H and 13C NMR spectra were recorded on on a Bruker Avance
300 or Bruker Avance 600 MHz spectrometer in DMSO-d6 solution
with Me4Si as internal standard. Ultraviolet absorption spectra of
synthesized compound have been recorded in the region 200–
600 nm using an Ocean optics USB 4000 spectrophotometer and
fluorescence spectra were recorded with Varian Cary fluorescence
spectrophotometer at wavelength of maximum absorption (kmax).
The effects of solvents were investigated on the emission spectra in
six different solvents in methanol, ethanol, tetrahydrofuran, N,N-
dimethylformamide, 1,2-dichloroethane and heptane. The UV–vis
spectra were obtained at room temperature and fluorescence spectra
at 25 0.1 °C, both using 1 cm optical path-length quartz cells.
Found: % C 44.69; % H 3.39; % N 17.31; IR (KBr) m
: 3083–3052 cmÀ1
(aromatic CAH), 3004, 2822 (aliphatic CAH), 2236 (C„N), 1614
(C@C), 1585 (C@N), 1545, 1359 (CANO2), 1266 (CAN), 1110
(ArACl), 781 (CACl) cmÀ1 1H NMR (DMSO-d6) d: 4.69 (2H, s,
;
CH2O), 3.29 (3H, s, OCH3), 2.50 (3H, s, CH3, C-6) ppm; 13C NMR,
APT (DMSO- d6) d: 153.65 (C-5), 152.48 (C-4), 147.09 (C-2);
144.20 (C-3); 113.12 (C-6), 109.33 (C„N); 68.83 (OCH2); 59.54
(OCH3); 21.22 (CH3) ppm.
2.3. Crystal structure determination
Single crystal of 2 suitable for X-ray single crystal analysis was
obtained at room temperature by partial evaporation from ethanol
solution. The intensities were collected at 295 K on a Oxford Dif-
fraction Xcalibur2 diffractometer with Sapphire3 detector using
graphite-monochromated Mo K radiation (k = 0.71073 Å). CrysAlis
[31] programs were used for data collection and reduction. The
crystal structure was solved by direct methods [32]. All non-hydro-
gen atoms were refined anisotropically by full-matrix least-
squares calculations based on F2 [32]. All hydrogen atoms were in-
cluded in calculated positions as riding atoms, with SHELXL97 [32]
defaults. Details of crystal data, data collection and refinement
parameters are given in Table 1. PLATON [33] program was used
for structure analysis and drawings preparation. CCDC 771895
contains the Supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
2.2. Synthesis
2.2.1. Preparation of 2-chloro-4-(methoxymethyl)-6-methyl-5-
nitropyridine-3-carbonitrile (2)
2.23 g (10 mmol) of 4-(methoxymethyl)-6-methyl-5-nitro-2-
oxo-1,2-dihydropyridine-3-carbonitrile (1) in 15 mL dry DMF were
Table 1
X-ray crystallographic data for 2.
Formula
Formula weight
Crystal size (mm)
Crystal colour, shape
Crystal system
Space group
Unit cell dimensions
C9H8ClN3O3
241.63
0.36 Â 0.38 Â 0.65
Colourless, block
Monoclinic
P21/c
3. Results and discussion
a ()
b ()
c ()
b (°)
V (3)
Z
8.1865(4)
30.8169(8)
9.9032(4)
118.291(5)
2199.97(15)
8
1.459
0.343
x
3.97–28.00
À10 6 h 6 10
À40 6 k 6 40
À13 6 l 6 12
27,829
3.1. Synthesis
2-chloro-4-(methoxymethyl)-6-methyl-5-nitropyridine-3-car-
bonitrile (2), was synthesised by treatment of starting compound,
4-(methoxymethyl)-6-methyl-5-nitro-2-oxo-1,2-dihydropyridine-
3-carbonitrile (1), with a strong chlorinating agent, such as the
Vilsmeier–Haack mixture of phosphorous oxychloride and N,N-
dimethylformamide, using three equivalents of POCl3 in excess
(Scheme 1). The reaction was conducted under anhydrous condi-
tions, because if the water was present there would be the need
to use additional amounts of phosphorus oxychloride which is de-
pleted in the presence of water.
Dcalc. (g cmÀ3
)
Absorption coefficient,
Scan mode
h range (°)
l
(mmÀ1
)
Index ranges
Collected reflections No.
Independent reflections No./Rint.
Reflections No. I P 2 (I)
5297/0.0348
3098
r
Under conventional heating conditions, this substitution reac-
tion is typically carried out at elevated temperatures using POCl3
in excess. This reagent has also proven successful for chlorination
of hydroxy substituted 2-pyridones [34,35]. Glasnov et al. showed
that this transformation could be effectively performed with
Data/restraints/parameters
5297/0/293
1.037
0.0506/0.0922
0.1378/0.1642
0.338/À0.280
Goodness-of-fit on F2, S
R [I P 2
wR [I P 2
Max./min. electron density (e À3
r(I)]/R [all data]
r
(I)]/wR [all data]
)

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M. Jukic et al. / Journal of Molecular Structure 979 (2010) 108–114
Table 2
CH2OCH3
NO2
CH2OCH3
Selected bond lengths (Å) and bond angles (°) for compound 2.
NC
O
NC
Cl
NO2
POCl3
DMF
N1AC2
1.313(3)
1.316(3)
1.720(2)
1.720(2)
C4AC5
1.375(3)
1.390(3)
1.476(3)
1.465(3)
C2ACl1
C5AN3
N
H
CH3
N
CH3
1
2
C2AC3
1.393(3)
N3AO2
1.212(2)
1.398(3)
1.433(3)
1.211(3)
1.213(2)
Scheme 1. Synthesis of the pyridine derivative 2.
C3AC7
N3AO3
1.439(3)
1.214(2)
N2AC7
1.135(3)
1.130(3)
1.398(3)
C5AC6
1.389(3)
1.387(3)
1.332(3)
microwave irradiation using dioxane as solvent and only two
equivalents of POCl3 [36].
C3AC4
C6AN1
Transformation of the compound 1 by boiling in POCl3 gave 2-
chloro-4-(methoxymethyl)-6-methyl-5-nitropyridine-3-carboni-
trile (2) which is a key compound for various transformations be-
cause this compound reacts smoothly with pyridine with the
formation of a pyridylpyridinium salt, which is a promising start-
ing material for the synthesis of various pyridine derivatives,
including condensed pyridines [37]. The improvement in product
yield shows the advantage of this reaction compared to the con-
ventional Harris–Folkers [38] procedure.
1.391(3)
1.513(3)
1.504(3)
124.6(2)
1.337(3)
1.499(3)
1.503(3)
C4ÀC8
C6AC10
N1AC2AC3
N1AC2ACl1
Cl1AC2AC3
C2AC3AC7
C2AC3AC4
C7AC3AC4
C3AC4AC5
C3AC4AC8
C8AC4AC5
C4AC5AC6
C4AC5AN3
C5AN3AO2
C5AN3AO3
O2AN3AO3
N3ÀC5ÀC6
C5AC6AC10
C5AC6AN1
C10AC6AN1
C6AN1AC2
120.17(19)
119.98(17)
116.5(2)
117.05(18)
118.1(2)
118.44(19)
125.3(2)
124.5(2)
117.14(18)
117.33(17)
123.6(2)
123.17(19)
120.40(19)
120.32(18)
115.9(2)
116.48(19)
118.26(19)
118.31(17)
124.38(19)
116.22(17)
116.27(15)
119.16(17)
119.33(15)
120.7(2)
120.09(19)
118.22(19)
118.67(17)
121.0(2)
121.23(18)
115.89(19)
115.59(17)
120.78(19)
120.88(18)
123.3(2)
3.2. X-ray crystal structure analysis
Compound 2 crystallized in monoclinic space group P21/c with
two independent molecules in the asymmetric unit (Fig. 1).
All equivalent bond lengths in two independent molecules are
within 3
lengths which differ for 4
r
values, with the exception of N3AC5 and C4AC5 bond
and 5 values, respectively (Table 2).
r
r
123.52(18)
122.6(2)
122.63(18)
We compared this structure with 2-chloro-3-cyano-pyridines in
which carbon atom substituent is bonded to C4 atom of the pyri-
dine ring. An analysis revealed that there are only two major differ-
ences in bond lengths between 2 and five closely related structures
[39–43]. Thus, C2ACl1 bond in 2-chloro-3-cyano-6-phenyl-4-p-
tolylpyridine [40] is ca. 0.035 Å longer [1.754(3) Å] than equivalent
bond in 2. The same bond in 2-chloro-3-cyano-6-methyl-5-nitro-
pyridine derivative [42], which has identical substituents at the
C2, C3, C5 and C6 atoms of the pyridine ring, is ca. 0.040 Å shorter
[1.681(6) Å] compared to 2. Furthermore, two NAO bond lengths of
nitro group differ for almost 0.05 Å [1.189(8); 1.236(8) Å] in this
very similar structure [42], and the group is almost perpendicular
with respect to the pyridine ring with dihedral angle of 77.6(7)°. In
2, two NAO bond lengths are within standard uncertainties, and
two almost identical dihedral angles are approximately 8° smaller
[69.9(3)°; 69.8(3)°] compared to the angle in former structure [42].
There is only one intermolecular hydrogen bond in 2, linking
only B independent molecules (Figs. 2 and 3). The C8BÁ Á ÁO3B
hydrogen bond self-assembles the molecules into C(6) chains
[44] parallel to the c axis [C8BÁ Á ÁO3B = 3.157(3) Å; HÁ Á ÁO3B =
2.51 Å; C8BAHÁ Á ÁO3B = 124°. It is well known that strength of
CAHÁ Á ÁO hydrogen bond generally depends on the carbon hybrid-
isation and acidity. In general, methylene group is not consider
very acidic. However, carbon acidity increases and hydrogen bond
is shorter if carbon atom is surrounded with electron-withdrawing
atoms or groups [45]. This effect is often called ‘activation’ of CAH,
although non-activated CAH groups may also act as donors [46].
One weak aromatic
p p stacking interaction [47] between
Á Á Á
pyridine rings of the neighboring A independent molecules partic-
ipates also in supramolecular aggregation (Fig. 3). An interplanar
angle between the rings is 0°, interplanar spacing is 3.8738(11) Å,
a centroid separation 4.3306(16) Å, and corresponding centroid–
centroid offset ca 1.94 Å. This
molecules into dimers. Thus, the molecules of 2 are self-assembled
p p interaction links A independent
Á Á Á
by two different types of intermolecular interactions, CAHÁÁÁO
hydrogen bond and
ular structure in which A and B molecules are not mutually con-
p p interaction, so forming such supramolec-
Á Á Á
nected by any other type of intermolecular interaction.
3.3. Spectral analysis
Fig. 1. A molecular structure of 2, with the atom-numbering scheme. For clarity,
only one of two independent molecules in the asymmetric unit is shown.
Displacement ellipsoids for non-hydrogen atoms are drawn at the 30% probability
level.
3.3.1. IR absorption spectra
The spectroscopic characterization of compound 1 is published
[30]. In the infrared spectra (in KBr) of compound 2 the band cor-

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M. Jukic et al. / Journal of Molecular Structure 979 (2010) 108–114
The IR spectra of compound 2 showed medium bands at 3083–
3052 cmÀ1 and 3004–2839 cmÀ1, which are attributed to the
vibrations of the aromatic (CAH) and aliphatic (CAH) bonds.
Strong band observed in the region at 2236 cmÀ1 in the IR spectra
is assignable to the carbonitrile
The presence of the band at 1658 cmÀ1 which is assigned to
stretching vibrations (C@O) of the ring, as well as the band at
1578 cmÀ1 assigned to
(NAH) bending vibration of ring which
m(C„N) stretching vibrations.
m
m
where observed for compound 1 in the spectra for compound 2
is also absent [30].
The band due to the skeleton deformation of stretching modes
pyridine
m(C@C) and m(C@N) bands in the region 1614–
1585 cmÀ1 were recorded. In the region 1545–1359 cmÀ1 interest-
ing features are observed for the (–NO2) symmetric stretch. The –(–
NO2) symmetric stretching mode is split into a doublet due to the
existence of non-equivalent molecules in the unit cell. The (–NO2)
band should be split into three components and these values are in
accordance with the literature [48,49].
The IR spectrum of the derivative 2 contains a medium absorp-
tion band at 1110 cmÀ1 which belongs to (ArACl), indication for
the halogen on aromatic structure. The CACl stretching vibrations
give rise to strong absorptions in the range 781 cmÀ1
.
3.3.2. NMR spectra
The 1H NMR spectrum of investigated compounds measured in
DMSO-d6 at 25 °C show a signals from the starting pyridone 1, and
the absence of the broad singlet signal approximately at 13 ppm
corresponding to NH protons. Two singlets at 4.69 ppm are attrib-
uted to two methylene protons of –CH2–O–CH3 group.
Similarly, multiple’s appearing at 3.29 ppm account for protons
due to methoxy groups –O–CH3 (C-4) downfield resonating D2O
exchangeable signal integrating for three protons, respectively.
The singlet at 2.50 ppm is corresponding for methyl groups (C-6)
three protons.
Fig. 2. A crystal packing diagram of 2, viewed along the b axis, showing parallel
arrangement of molecules and CAHÁ Á ÁO hydrogen bond which links B independent
molecules into infinite chains. Hydrogen bonds are indicated by dashed lines.
The structures of the title compound are further substantiated
by the 13C APT NMR, spectroscopic data measured in DMSO-d6.
The spectroscopic data of compound 2 indicate that the carbons
atoms give rise to different peaks within the frequency range of
21.22–153.65 ppm. Compared with the starting material 1 [30],
as expected, significant changes are observed for chemical shift
values of quaternary carbons signals assignable to CO of the
COANH amide group which disappeared approximately from
160.93 ppm.
Quaternary carbons signals is observed in the 13C APT NMR
spectrum of compound 2 at 153.65 ppm (C-5), 152.48 ppm (C-4),
147.09 ppm (C-2), 144.20 ppm (C-3), 113.12 ppm (C-6) and at
109.33 ppm (f„N), respectively.
Singlet is observed for secondary methylene carbons of –CH2–
O–CH3 group of compound 2 at 68.83 ppm. Similarly, two singlets
at 59.54 and 21.22 ppm, respectively are attributed to primary car-
bons of methyl group.
The results inferred from the IR and NMR spectral data of com-
pound 2 are found to be in agreement with the proposed structure
and supported also by X-ray diffraction data.
3.3.3. UV–vis and fluorescence spectra
The UV–vis electronic spectra of 2-chloro-4-(methoxymethyl)-
6-methyl-5-nitropyridine-3-carbonitrile (2) have been recorded
in protic and aprotic organic solvents within 200–600 nm range.
The effects of the electron-withdrawing properties of halogen sub-
stituent and solvents on the absorption and fluorescence spectra
was investigated and interpreted.
Fig. 3. A crystal packing diagram of 2, viewed along the a axis, showing CAHÁ Á ÁO
hydrogen bond and pyridine rings’ stacking of A molecules. Hydrogen bonds are
indicated by dashed lines.
A
dilute ethanolic solution of the compound
2
(c
responding to the imino group
m
(NAH) vibrations, which was re-
(2) = 4.0144 Â 10À4 mol dmÀ3) at 25 0.1 °C was prepared and
corded in the region 3312 cmÀ1 for the compound 1 [30], is absent.
pipetted into a standard 1 cm square fluorescence cell. Prescan

´
M. Jukic et al. / Journal of Molecular Structure 979 (2010) 108–114
112
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
120
100
80
60
40
20
0
fluorescence
absorbance
A
I/a.u.
300
350
400
450
500
λ/nm
550
600
650
700
Fig. 4. Absorption and emission spectrum of 2 in 95% ethanolic solution (c (2) = 4.0 Â 10À4 mol dmÀ3 at 25 0.1 °C (prescan kex = 390 nm)).
p
–
p
* transition of the conjugated system within the aromatic pyri-
dine ring, and are in agreement with literature data. Pyridine shows
an electronic transition at 257 nm because of the
* excitation for
Table 3
Fluorescence data of 2 recorded in protic and aprotic solvents at the concentration of c
(2) = 40 Â 10À4 mol dmÀ3 at 25 0.1 °C (kex = 390 nm); ks is wavelength of ‘shoulder’.
p–p
Solvent
kmax (nm) emax (106 dm3 molÀ1 cmÀ1
)
ks (nm)
non-conjugated derivatives, and shifted to longer wavelengths
when substituents are conjugated to the aromatic system [50].
The kmax of the UV absorption spectrum of compound 2 is
hypsochromically shifted by about 40 nm to 290 nm from that of
the starting 2(1H)-pyridone (1) spectrum (kmax (1) = 331 nm [30].
Electron-withdrawing substituent (–Cl) have significant effect
on the position of emission band of compound 2, bathochromically
Tetrahydrofuran
1,2-dichloroethane
N,N-dimethylformamide 487
Ethanol
Heptane
Methanol
467
460
1.11
–
442
441
445
464
444
0.814
0.661
0.285
0.161
0.101
480
442
484
shifting the maximum of the UV absorption bands for
Dkmax =
82 nm, from that of the compound 1, and producing a hypochromic
was used to determine the optimal excitations wavelength. The
wavelength kex = 390 nm was then selected for study.
effect which results in decreased emission intensity (Demax =
À0.226 Â 106 dm3 molÀ1 cmÀ1). This is in agreement with litera-
ture data that an auxochrome containing unshared electrons which
when attached to the chromophore changes both the intensity as
wells the wavelength of the absorption and emission maximum
As can be seen from Fig. 4, in the UV spectra of the 2-chloro-4-
(methoxymethyl)-6-methyl-5-nitropyridine-3-carbonitrile (2) the
absorption maxima is obtained at 290 nm (kmax (2) = 290.1 nm),
while the fluorescence emission spectrum peak is recorded
at 480 nm (kmax (2) = 480 nm) and is red-shifted with respect to
the absorption spectrum. That absorption maxima correspond to a
[51]. The decrease in the molar absorptivity (e) observed here
can be related to the effect of the overlap of the orbitals involved
in the electronic excitation. Excitation of one electron from the
500
400
tetrahydrofuran
1,2-dichloroethane
N,N-dimethylformamide
ethanol
I/a.u.
300
heptane
methanol
200
100
0
400
450
500
550
600
650
700
λ/nm
Fig. 5. Fluorescence spectra of 2 in protic and aprotic solvents at 25 0.1 °C(c (2) = 4.0 Â 10À4 mol dmÀ3; kex = 390 nm).

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M. Jukic et al. / Journal of Molecular Structure 979 (2010) 108–114
lone par present on the heteroatom to an antibonding
p
* orbital (n
*), so called ‘‘forbidden” transition is lower in energy than the
p* ‘‘allowed” transition, but the molar absorptivity of the ‘‘for-
of the compound showed bathocromic and hypochromic shifts, as
the solvent polarity increases which results in decreased emission
intensity, and the fluorescence efficiencies are reduced. Reductions
of the fluorescence efficiencies, accompanied by bathochromic
shifts of the emission spectra of compound 2 was depending on
the protonation of the ground state and/or the excited state.
?
?
p
p
bidden” transition is a thousand times smaller than ‘‘allowed” tran-
sition [51]. The effects of halogen substituent which contain
unshared electrons with the electron-withdrawing properties
attached to the aromatic ring change both the intensity as wells
the wavelength of the absorption and emission maximum even
can diminish or destroy fluorescence [52]. This effect can be from
‘‘forbidden” transitions which are related to the effect of the over-
lap of the orbital involved in the electronic excitation.
Because the substituent effects appear to exert a more pro-
nounced effect on emission than on absorption spectra [53], we
examined the fluorescence spectra of compound 2, which have
been measured in six solvents of different polarity and hydrogen
bonding donor ability such as methanol, ethanol, tetrahydrofuran,
N,N-dimethylformamide, 1,2-dichloroethane and heptane. The
concentrations were the same and the measurements were taken
at room temperature. The ultraviolet emission wavelength of the
electronic transitions of the examined compounds are given in
Table 3, and representative spectra are shown in Fig. 5.
Acknowledgements
Support for this study by the Ministry of Science and
Technology of Croatia (Project Nos. 058-0582261-2253 and
119-1193079-3069) is gratefully acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.06.012.
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