Nitroimidazoles: Part X. Spectral studies on isojmeric-1 -substituted 4-& 5-nitroimidazoles and some 2-nitromidazoles

Abstract

Chemical shifts of C-4 in series - a of 1 -substituted 5-nitroimidazoles fall within a narrow range of 130-133 ppm and those of C-5 in isomeric 4-nitro compounds (series-b) at 120-124 ppm, offering a method for distinguishing between isomeric pairs. An additional diagnostic method utilises the observation that for series-b, the signal due to C-5 has extra multiplicity due to three-bond coupling with protons of the group on N-1, which C-4 in series-a does not exhibit. DMSO induced chemical shifts for C-5 H in series-b (δ5 DMSO-d₆-δ5 CDC1₃) are much larger than those for C-4H in series-a and are useful aids for structure assignment. Mass spectra of the l-alkyl-5-nitroimidazoles (series-a) generally show fragments due to loss of OH, which are mostly absent in series-b; the loss of N0₂ is also more intense in the former than in the latter. The phenomena are traced to participation by the alkyl group at position-1. Acid and alkali induced shifts in water and EtOH UV spectra of a variety of nitroimidazoles are described and discussed l-A.lkyl-5-nitroimidazoles undergo hypsochromic shifts in 0.1/V H₂S0₄ more readily than the 4-nitro-isomers. On silica gel plates, compounds of series-a generally move faster than those of series-b. The melting points of the 5-nitroimidazoles are as a rule lower than those of the 4-nitro counterparts.