D. Debnath et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 140 (2015) 185–197
187
Synthesis of the dyes
Ar–H), 7.36 (s, 3H, Ar–H), 3.80–3.44 (m, 6H, N–CH3); 13C NMR
(125 MHz, CDCl3): d (ppm) 27.16 (CH3), 28.17 (CH3), 114.21 (C),
115.99 (CH), 125.64 (CH), 127.01 (C), 145.22 (C), 163.31(C@O),
165.56 (C@O), 175.35 (C). Anal. Calcd. for C12H12N6O4: C, 47.37;
H, 3.98; N, 27.62, Found: C, 47.29; H, 3.92; N, 27.54.
All the dyes were prepared following the conventional diazoti-
zation process with some modification. As a typical example,
aniline (0.651 g, 7.0 mmol) was precipitated out as a salt of aniline
hydrochloride and made a solution by adding 5 mL of water. It was
then placed in an ice bath and cooled down its temperature to
0–5 °C. An aqueous solution of sodium nitrite (0.484 g, 7.0 mmol
in 10 mL of water) was added slowly into the cooled stirred ani-
line-hydrochloride solution. A light yellow solution, generally
called diazotized solution of aniline, formed and was allowed to
stir further 30 min at the same temperature. 1,3-Dimethyl-6-
aminouracil (1.0 g, 6.46 mmol) was dissolved in hot glacial acetic
acid (5 mL) and to make a clear solution 3 mL of distilled water
was added. The solution was also then placed in an ice bath to cool
it to 0–5 °C. To this solution the diazotized solution was then added
slowly with constant stirring for 30 min. A solution of sodium
hydroxide (4%) was instilled into the reaction mixture to make
the pH ꢂ 5–6. A bright yellow color precipitation was started to
appear. The stirring was continued at r.t. for additional 1 h. The
resulting precipitate was filtered, washed with cold water and
dried over air oven. The product was recrystallized from hot etha-
nol and gave brown plate crystals of 1,3-dimethyl-5-(phenylazo)-
6-aminouracil.
X-ray crystallography
The determination of the crystal class, orientation matrix and
accurate unit cell parameters of the dye, 1,3-dimethyl-5-(pheny-
lazo)-6-aminouracil (0.45 ꢃ 0.38 ꢃ 0.32) was performed using a
Bruker APEX-II CCD diffractometer with graphite monochroma-
tised Mo-Ka (k = 0.71073 Å) radiation. The crystal was kept at
295(2) K during data collection. The structure was solved using
Olex2 structure solution program by following Charge Flipping
solution method [42,43]. All the non-hydrogen atoms were refined
with anisotropic thermal parameters. All hydrogen atoms were
located by geometrical calculation and rode on the parent atom
with isotropic thermal parameters (Uiso(H) = 1.2 Ueq(C or N)).
The structure was refined with the SHELXL-2013 [44] refinement
package using Least Squares minimization. Crystallographic data
and the parameters of structure refinement are summarized in
Table 1.
Determination of pKa values by using spectrophotometry
1,3-dimethyl-5-(phenylazo)-6-aminouracil (1,3-DM-5-PA-6AU) (1)
Bright yellow crystal, Yield: 78%; M.pt. 258–260 °C; FT-IR (KBr,
For the determination of acidity constant (pKa) values of all the
investigated dyes, a stock solution (1.0 ꢃ 10ꢁ3 mol/dm3) of known
weight of a particular dye was prepared in DMSO solvent. Required
volume of dye solution from the stock solution was put into
the aqueous buffer solution to make the final concentration in
the order of 1 ꢃ 10ꢁ5 mol/dm3 in the desired buffer solution
(20:1, v/v aq. buffer/DMSO). After the preparation of a set of
solutions in different buffers, the solution mixtures were settled
for 30 min. UV–vis absorption spectra of all the dyes were recorded
thereafter. The pKa values of all the investigated dyes were
calculated from the obtained spectra following two well-known
methods [45]: (1) half-height method [45a] and (2) modified
t
cmꢁ1). 3274 (–NH2), 1707, 1628 (–C@O), 1523 (–C@C), 1454
(–N@N–), 1357, 1155 (–C–N); 1H NMR (CDCl3, d ppm). 14.12 (br,
1H, –N–NH–), 8.67 (br, 1H, –NH–), 7.72–7.57 (m, 2H, Ar–H), 7.42
(s, 3H, Ar–H), 3.73–3.36 (m, 6H, N–CH3); 13C NMR (125 MHz,
CDCl3): d (ppm) 27.2 (CH3), 27.89 (CH3), 117.27(CH), 127.26 (CH),
129.76 (CH), 144.12 (C), 151.23 (C@O), 153.45 (C@O), 163.33 (C).
Anal. Calcd. for C12H13N5O2: C, 55.59; H, 5.05; N, 27.01, Found: C,
55.47; H, 4.99; N, 26.97.
1,3-dimethyl-5-(p-methylphenylazo)-6-aminouracil (1,3-DM-5-p-
Me-PA-6AU) (2)
Yellow crystal, Yield: 83%; M.pt. 264–262 °C; FT-IR (KBr,
t
cmꢁ1). 3265 (–NH2), 1700, 1622 (–C@O), 1526 (–C@C), 1450
(–N@N–), 1350, 1166 (–C–N); 1H NMR (CDCl3, d ppm). 14.18 (br,
1H, –N–NH–), 8.87 (br, 1H, –NH–), 7.75–7.62 (m, 2H, Ar–H), 7.40
(s, 3H, Ar–H), 3.72–3.34 (m, 6H, N–CH3); 13C NMR (125 MHz,
CDCl3): d (ppm) 21.2 (CH3), 27.24 (CH3), 28.52 (CH3), 116.27(C),
117.26 (CH), 129.76 (CH), 137.59 (C), 138.46 (C), 149.23 (C@O),
151.45 (C@O), 164.33 (C). Anal. Calcd. for C13H15N5O2: C, 57.13;
H, 5.53; N, 25.63, Found: C, 57.01; H, 5.48; N, 25.55.
Table 1
Crystal data of 1,3-dimethyl-5-(phenylazo)-6-aminouracil (1).
Empirical formula
C12H13N5O2
Formula weight
Temperature
Wavelength
Crystal system
Space group
259.27
295.15 K
0.71073 Å
Monoclinic
P21/c
Unit cell dimensions
1,3-dimethyl-5-(p-chlorophenylazo)-6-aminouracil (1,3-DM-5-p-Cl-
PA-6AU) (3)
a (Å)
b (Å)
c (Å)
V (Å3)
Z
10.4315(11)
8.1867(9)
14.5684(15)
1216.5(2)
Yellow crystal, Yield: 72%; M.pt. 262–261 °C; FT-IR (KBr,
t
cmꢁ1). 3289 (–NH2), 1712, 1628 (–C@O), 1523 (–C@C), 1448
(–N@N–), 1352, 1162 (–C–N); 1H NMR (CDCl3, d ppm). 13.87 (br,
1H, –N–NH–), 9.00 (br, 1H, –NH–, 7.83–7.67 (m, 2H, Ar–H), 7.46
(s, 3H, Ar–H), 3.81–3.46 (m, 6H, N–CH3); 13C NMR (125 MHz,
CDCl3): d (ppm) 26.94 (CH3), 28.12 (CH3), 115.97(C), 118.02 (CH),
129.34 (CH), 131.57 (C), 139.14 (C), 158.41(C@O), 165.35(C@O),
172.39 (C). Anal. Calcd. for C12H12ClN5O2: C, 49.07; H, 4.12; N,
23.84, Found: C, 49.02; H, 4.04; N, 23.78.
4
Dcal (Mg mꢁ3
)
1.416
Absorption coefficient (mmꢁ1
F(000)
)
0.102
544
Crystal size
0.45 ꢃ 0.38 ꢃ 0.32
3.994–56.652
ꢁ13 6 h 6 12, ꢁ10 6 k 6 10,
ꢁ19 6 l 6 19
2H range (°)
Index ranges
Reflection observed
3022 [R(int) = 0.0575]
Full-matrix least-squares on F2
3022/0/173
R1 = 0.0466, wR2 = 0.1167
R1 = 0.1013, wR2 = 0.1539
0.859
Refinement method
Data/restraints/parameters
Final R indices [I > 2
1,3-dimethyl-5-(p-nitrophenylazo)-6-aminouracil (1,3-DM-5-p-NO2-
PA-6AU) (4)
r
(I)]
R indices (all data)
Yellow crystal, Yield: 64%; M.pt. 288–286 °C; FT-IR (KBr,
Goodness-of-fit on F2
t
cmꢁ1). 3389, 3256 (–NH2), 1680, 1612 (–C@O), 1511 (–C@C),
Largest difference peak and hole
0.20 and ꢁ0.17
1434 (–N@N–), 1344, 1153 (–C–N); 1H NMR (CDCl3, d ppm).
(e Åꢁ3)
14.49 (br, 1H, –N–NH–), 9.33 (br, 1H, –NH–), 7.63–7.47 (m, 2H,