¹H and ¹³C NMR spectra of some unsymmetric N,N′-dipyridyl ureas: spectral assignments and molecular conformation

Abstract

The ¹H NMR spectra of N-(2-pyridyl), N′-(3-pyridyl)ureas and N-(2-pyridyl), N′-(4-pyridyl)ureas in CDCl₃ and (CD₃)₂CO have been assigned with the aid of COSY and NOE experiments and chemical shift and coupling constant correlations. The ¹³C NMR spectra in CDCl₃ were analysed utilizing the HETCOR and proton coupled spectra. The ¹H NMR spectra, NOE effects and MINDO/3 calculations have been utilized to show that the molecular conformation of these compounds has the 2-pyridyl ring coplanar with the urea plane with the N–H group hydrogen bonded to the nitrogen of the 2-pyridyl group on the other urea nitrogen while the 3/4-pyridyl group rotates rapidly about the N–C³/N–C⁴ bond.

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