Synthesis, Conformational Analysis, Infrared, Raman and UV-Visible Spectra of novel Schiff bases compiled with DFT calculations

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-carbonitril e (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 lone-pair (NLP) in addition to the 3D assumption. RESULTS: The computational outcomes favors conformer A for BESB1 in which the CXiN and OH moieties are cis towards the NLP while conformer G is preferred for BESB2 (the CXiN/furan-O are cis/trans towards the NLP) which 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: Therefore, a proper interpretation of the observed electronic transition and chemical shifts were presented in this study.