2. Experimental
2.1. Synthesis
The starting material was synthesized by taking l-Valine
(Loba Chemie-AR grade) and Hydrochloric acid (Merck) in a
1:1 stoichiometric ratio. The required amount of starting materials
for the synthesis of l-valine hydrochloride (LVHCl) salt was
calculated according to the following reaction:
C5H11NO2 +HCl → C5H12NO2Cl
The calculated amount of hydrochloric acid was first dissolved
in deionized water. l-Valine was then added to the
solution slowly by stirring. The prepared solution was allowed
to dry at room temperature and the salts were obtained by slow
evaporation technique. The purity of the synthesized salt was
further improved by successive recrystallization process.
2.2. Solubility
In solution growth techniques, the size of a crystal depends
on the amount of the material available in the solution which in
turn is decided by the solubility of the material in the solvent.
The solubility of LVHCl in deionized water was determined as
a function of temperature in the temperature range of 30–50 ◦C.
The beaker containing the solution was maintained at a constant
temperature and continuously stirred. The amount of LVHCl
required to saturate at this temperature was estimated and this
process repeated for various temperatures. On reaching saturation,
the equilibrium concentration of the solute was determined
by gravimetric method. The variation of solubility with temperature
is shown in Fig. 1.
2.3. Growth of LVHCl
The saturated solution of LVHCl was prepared at room temperature
from the recrystallized salt. The solution was then
filtered twice to remove the suspended impurities and allowed to
crystalline by the slow evaporation technique at room temperature.
A good transparent crystal of size 20mm×6mm×4mm
harvested in a growth period of 30 days is shown in Fig. 2.
3. Characterization
The single crystal X-ray diffraction analysis of LVHCl crystal
was carried out using ENRAF NONIUS CAD4 X-ray diffractometer
and its lattice parameters were determined. Fourier
transform infrared spectrum was recorded by the KBr pellet
technique using a BRUKER 66V FTIR spectrometer to confirm
the vibrational structure of the crystalline compound with
scanning range of wavenumber 400–4000 cm−1. UV–vis spectrum
was recorded in the range of 200–2000 nm using VARIAN
CARY 5E spectrometer. Thermal behavior of the grown sample
was studied by using STA 1500 thermal analyzer. The NLO
property of LVHCl was tested by Kurtz powder SHG test using
an Nd:YAG laser (1064 nm).
4. Results and discussion
4.1. Single crystal X-ray diffraction studies
The single crystal X-ray diffraction has been carried out using
Enraf Nonius-CAD4 diffractometer. From the measurements we
found that the grown specimen of LVHCl belongs to monoclinic
system having the space group P21. The determined lattice
dimensions are given in Table 1. It is in good agreement with
the reported literature values.
4.2. Fourier transform infrared studies
The infrared spectrum of the grown crystal has been taken in
the range of 400–4000 cm−1. The sample is made as a pellet by
using KBr. The Fourier transform infrared (FTIR) spectrum of
LVHCL is shown in Fig. 3. The spectrum shows the presence of
all the functional groups in LVHCL crystal and summarized in
Table 2.
A broad, strong absorption in the 2900–3300 cm−1 range,
including the absorption at 3082 cm−1 correspond to the stretching
band of the NH3
+ ion of the amino acid. This region
results from superimposed O–H and NH3
+ stretching bands.
Absorption in this region is also characterized by multiple fine
structures on the lower wavenumber side of the band and weak
absorptions due to COO− ions. Also, prominent is the relatively
strong symmetricalNH3
+ bending band around 1519 cm−1 [14].
A strong band arising from C–COO− stretching is observed
at 1216 cm−1 [15]. Further a strong carbonyl absorptions at
1743 cm−1 confirms the COOH and COO− groups of the compound.
Appearance of this band confirms the formation hydrochloride salt of l-valine [16]. This justifies the protonation
of carbonyl group in LVHCL. The absorptions of LVHCL
has been compared with those of the parent compound (l-valine)
in Table 2. The shifts in the positions of the characteristic peaks
confirm the formation of the new compound.
4.3. Optical transmission studies
Fig. 4 shows the optical transmission spectrum of LVHCl
single crystal. From the spectrum, it is evident that the LVHCl
crystal has a very low UV cutoff wavelength of 295 nm, along
with a large transmission window in the entire visible region.
Hence, it can be utilized for SHG from a laser operating at
1064 nm or other optical application in the blue region.
4.4. Thermal studies
To find the thermal characteristics ofLVHCl, differential thermal
analysis (DTA) and thermogravimetric analysis (TGA) were
carried out simultaneously in a thermal analyzer (STA 1500).
The sample was heated at a rate of 10 ◦C/min in protected nitrogen
gas flow. 1.25 mg of the sample was taken to carry out the
experiment. Fig. 5 shows the thermograms illustrating simultaneously
recorded TGA and DTA.
From DT curve, it is observed that the material undergoes
an irreversible endothermic transition at about 211 ◦C where
the decomposition starts. The material is fully decomposed at
260 ◦C. The weight loss curve is very sharp and it starts at 211 ◦C
and ends at 277 ◦C. This weight loss is due to the liberation
of volatile substances. The sharpness of the endothermic peak
shows good degree of crystallinity of the grown ingot. The peak
at 277 ◦C indicates a phase change from liquid to vapor state as
evidence from the loss of weight in the TG curve.
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