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S. Gowri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 28–35
FT-IR Spectral studies
750 and 669 cmꢃ1. The anti bonding electrons form the carbonyl
group molecular orbital was moved toward the newly formed
interaction between the systems, thus the electron density around
the carbonyl group increases consequently the stretching fre-
quency of the C@O group also increased while compared to the
picolinic acid.
The characteristic bands of PHCL crystal is shown in FT-IR spec-
trum (Fig. 3). The characteristic vibrational bands in the spectrum
were compared with the corresponding bands of picolinic acid [8].
It is observed that most of the referred bands were split or have
changes in their respective band position/intensity thus indicating
the mixed nature of picolinic acid and hydrochloric acid. The major
vibrational modes of PHCL were derived from the pyridine ring,
NAH group and C@O group. The determination of the crystal struc-
ture of mixed PHCL may help to establish definitive vibrational
assignments and thus potentially able to deliver a deeper insight
into mechanisms of dynamical interactions. The measured infrared
band positions and their assignments are given in Table 1.
The bands between 1400 and 1650 cmꢃ1 in pyridine derivatives
are assigned to CAC stretching modes. The bands corresponding to
a strong band at 1536 cmꢃ1 in IR is attributed to the Kekule CAC
stretching mode. The FT-IR activation of pyridine ring modes pro-
vides evidence for the charge transfer interaction between the do-
nor and the acceptor group through the
enhancement responsible for NLO activity.
p-system leading to b
The hydroxyl group vibrations, which are hydrogen bonded to
UV–Vis–NIR spectral studies
the aromatic ring
p-electron systems arising bands in the range
3300–3350 cmꢃ1, in the present study, it is found that a broad
band appears at 3322 cmꢃ1, also the intensity of the band was in-
creased due to conjugation/formation of hydrogen bonds [9]. The
COOH symmetric and asymmetric stretching vibrations were ob-
served at 3092 cmꢃ1 at 3300 cmꢃ1 respectively. The NAH stretch-
ing vibrations were lowered with a red shift which serves as a sign
for the ACOOH group participation in the intermolecular interac-
tion. Therefore, it could be arrived that the intermolecular hydro-
gen bonding is responsible for high b value and this mechanism
plays an important role in NLO activity of the PHCL crystal.
The aromatic stretching exhibits multiple weak bands in the re-
gion between 3100 and 3000 cmꢃ1 and the CAH vibrations of the
PHCL crystal were observed at 3092, 2857 cmꢃ1. The out-plane
deformation of CAH molecules was observed at 960 cmꢃ1 which
could be attributed to strong coupled vibrations [9]. The observed
band positions were suppressed in frequency as it was shared its
bonding electrons to the hydrogen bonding.
The UV–Vis transmittance spectrum was recorded with a Cary
5E UV–Vis–NIR spectrophotometer in the range of 200–1000 nm.
Fig. 4 shows the optical transmission spectrum of PHCL crystal
crystallized using water as solvent. It was found that PHCL crystal-
lized using water solvent has a transmittance of 74% with a lower
UV cut-off at 194 nm. A UV cut-off below 300 nm is sufficiently
useful for SHG laser generation (k = 1064 nm) or other similar
applications in the blue region [11]. Further the material was found
to be transparent in the wavelength range of 300–1000 nm for all
radiations. Just beyond 300 nm, there is an absorption illustrated
by a decrease in transmittance. This may be due to n–pꢄ transition
in the azo-methyne group. The steep decrease in transmittance at
around 250 nm may be assigned to electronic excitation in COO
group. As there is no change in the transmittance in the entire
range 1000 nm to 300 nm, infers that the materials can find appli-
cation as window in spectral instruments in those regions.
The unconjugated C@N linkage gives weak to medium bands
near 1250–1020 cmꢃ1 because of C@N vibrations. In the present
study C@N symmetric stretching was observed at 1220 and
1088 cmꢃ1 and the bending vibrations of C@N group at 547 cmꢃ1
[10]. Since, the electrons from bonding molecular orbitals were do-
nated to the hydrogen ion, the electron density around the C@N
bond was diminished and hence the stretching frequency.
The presence of picolinate ions in the PHCL crystal structures is
reflected in the highly mixed bands of the C@O stretching, AOH in-
plane deformation and together with OH out-of-plane bending
vibrations. The intensity of carboxyl band increases due to conju-
gation or formation of hydrogen bonds. The most important contri-
bution of C@O stretching vibrations were found at 1609 and
1455 cmꢃ1 and the in-plane C@O deformation could be traced at
NMR spectral study
The 1H NMR spectrum of the PHCL crystal is shown in Fig. 5a.
Four distinct proton signals appeared in the spectrum, confirming
the presence of four different proton environments in the crystal.
The doublet centred at d 8.76 ppm corresponds to the C3 aromatic
proton of picolinic acid moiety, and appearing as a doublet due to
its coupling with the neighboring C4 aromatic proton. A triplet sig-
nal centred at d 8.66 ppm is attributed solely to the C4 aromatic
proton. A doublet signal centred at d 8.43 ppm is assigned to the
C6 aromatic proton of the picolinic acid moiety. The C5 aromatic
Table 1
Measured infrared band positions (cmꢃ1) Assignment for.
Experimental
Assignment
mIR (cmꢃ1) Picolinic acid
mIR (cmꢃ1
)
hydrochloride
Picolinic acid
3410br
3464w,br
OAH Stretch + NAH
stretch
3092m
1725m
1609m
1455m
3118m
1719s
1600vs
1443m
Arom. CAH Stretch
C@O stretch
Ring stretch
Ring stretch + nAh i.p
bend
1311br
1309s
Ring stretch
(kekuley) + CAN stretch
Arom. CAH i.p bend
Arom.CAH o.p
bend + NAH i.p bend
CACAC i.p bend
CACAC o.p bend
1088w
750s
1088s
767s
669s
511s
676vs
539s
Fig. 4. UV–Vis–NIR spectrum of PHCL.