Spectroscopic and computational analysis of the (E/Z)-isomers in the synthesis of new alkyl-oxime derivatives

Abstract

Nine new alkyl-oximes were obtained from 2-methyl-4(5)-nitro-1H-imidazole with 2-bromo-1-phenylethanone derivatives in a first step, as well as hydroxylamine hydrochloride and alkyl halide derivatives in a second step. Spectroscopic characterization mainly by NMR (nuclear magnetic resonance) allowed us to assign the 4-nitro and 5-nitro position isomers in the ethanone derivatives (3a-3c), it also allowed us to assign the geometric isomers (E/Z) in oximes (4a-4c) and alkyl-oximes (5a-5i). The HRMS (high-resolution mass spectrometry) allowed us to define the molecular ion (M + 1) with its isotopic pattern, complementing the unequivocal characterization of the nine compounds. Experimental and calculated FT-IR data (Fourier transform infrared spectroscopy) indicates that the symmetric and asymmetric bands of the nitro group do not change substantially when the ethanones become oximes. The position of the nitro group in the compounds, the optimized structures of minimum energy, as well as the geometric organization of the substituents in the isomers, have been interpreted by PBE0-D3/6–311++G(d,p) calculations and the results coincided with the spectroscopic data found experimentally.