2 research outputs found
UV-visible and NMR spectroscopic studies of colorimetric thiosemicarbazide anion sensors
Four model thiosemicarbazide anion chemosensors containing three N
–
H bonds, substituted with phenyl
and/or 4-nitrophenyl units, were synthesised and studied for their anion binding abilities with hydroxide,
fl
uoride, acetate, dihydrogen phosphate and chloride. The anion binding properties were studied in
DMSO and 9 : 1 DMSO
–
H
2
O by UV-visible absorption and
1
H/
13
C/
15
N NMR spectroscopic techniques and
corroborated with DFT studies. Signi
fi
cant changes were observed in the UV-visible absorption spectra
with all anions, except for chloride, accompanied by dramatic colour changes visible to the naked eye.
These changes were determined to be due to the deprotonation of the central N
–
H proton and not due
to hydrogen bonding based on
1
H/
15
N NMR titration studies with acetate in DMSO-
d
6
–
0.5% water. Direct
evidence for deprotonation was con
fi
rmed by the disappearance of the central thiourea proton and the
formation of acetic acid. DFT and charge distribution calculations suggest that for all four compounds the
central N
–
H proton is the most acidic. Hence, the anion chemosensors operate by a deprotonation
mechanism of the central N
–
H proton rather than by hydrogen bonding as is often reported