Spectroscopic studies of aryl substituted 1-phenyl-2-pyrazolines: Steric and electronic substitution effects

Abstract

A series of aryl substituted 1-phenyl-2-pyrazolines containing electron-donating and electron-withdrawing substituents on different positions of the C3– or C5-aryl groups were synthesized and their steric and electronic effects on characteristic spectral data were investigated by experimental spectroscopic methods (UV–Vis, IR and NMR) and DFT computations. Whereas the C5-aryl group of the heterocyclic ring behaves as σ-donor/acceptor substituent, the π-donor/acceptor ability of the C3-aryl group depends on the co-planarity of the substituent with the aryl ring and also the extent of the orientation of this C3-aryl ring towards the C[dbnd]N double bond of the heterocyclic ring. A significant through conjugation of the lone pair on the N1-atom towards the C3-aryl ring is observed when the electron-withdrawing nitro group is located on the para-position of this aryl ring. The experimental spectroscopic results for the substitution effects are also supported by (TD)DFT/6-311++G(d,p) computed UV–Vis spectra. Experimental and theoretical NMR chemical shifts and spin-spin coupling constants obtained for these 2-pyrazolines validated by Karplus diagram approves the structures predicted for these compounds. © 2018 Elsevier B.V.