Objective:
Two novel Schiff bases named, 2-((2-Hydroxybenzylidene)amino)-4,5,6,7-
tetrahydrobenzo[b] thiophene-3-carbonitrile (BESB1) and 2-((Furan-2-ylmethylene)amino)-4,5,6,
7-tetrahydro-benzo[b]thiophene-3-carbonitrile (BESB2) were synthesized.
Methods:
The structures were characterized based on CHN elemental analysis, mid-infrared (400–
4000 cm-1), Raman (100-4000 cm-1), 1H NMR, mass and UV-Vis spectroscopic measurements. In
addition, quantum mechanical calculations using DFT-B3LYP method at 6-31G(d) basis set were
carried out for both Schiff bases. Initially, we have carried out complete geometry optimizations
followed by frequency calculations for the proposed conformational isomers; BESB1 (A–E) and
BESB2 (F–J) based on the orientations of both CN and OH groups against the azomethine lonepair
(NLP) in addition to the 3D assumption.
Results:
The computational outcomes favor conformer A for BESB1 in which the C≡N and OH
moieties are cis towards the NLP while conformer G is preferred for BESB2 (the C≡N/furan-O are
cis/trans towards the NLP) which was found consistent with the results of relaxed potential energy
surface scan. Aided by normal coordinate analysis of the Cartesian coordinate displacements, we
have suggested reliable vibrational assignments for all observed IR and Raman bands. Moreover,
the electronic absorption spectra for the favored conformers were predicted in DMSO solution
using TD-B3LYP/6-31G(d) calculations. Similarly, the 1H NMR chemical shifts were also
estimated using GIAO approach implementing PCM including solvent effects (DMSO-d6).
Conclusion:
Proper interpretations of the observed electronic transition, chemical shifts, IR and
Raman bands were presented in this study.