NMR Chemical Shift and Methylation of 4-Nitroimidazole: Experiment and Theory

Nitroimidazoles and derivatives are a class of active pharmaceutical ingredients (APIs) first introduced sixty years ago. As anti-infection agents, the structure-activity relationships of nitroimidazole compounds have been particularly difficult to study due to their low reduction potentials and unique electronic structures. In this study, we combine dynamic nuclear polarization (DNP)-enhanced solid-state (100 K), solid-state (298 K), and 1H-13C heteronuclear single quantum coherence (HSQC) solution-state NMR techniques (303 K) with density functional theory (DFT) to study the 1H, 13C, and 15N chemical shifts of 4-nitroimidazole (4-NI) and 1-methyl-4-nitroimidazole (CH3-4NI). The 4-NI chemical shifts were observed at 119.4, 136.4, and 144.7 ppm for 13C, and at 181.5, 237.4, and 363.0 ppm for 15N. The measurements revealed that methylation (deprotonation) of the amino nitrogen N(1) of 4-NI had less effect (Δδ = -4.8 ppm) on the N(1) chemical shift but was compensated by shielding of the N(3) (Δδ = 11.6 ppm) in CH3-4NI. The calculated chemical shifts using DFT for 4-NI and CH3-4NI agreed well with the experimental values (within 2 %) for the imidazole carbons. However, larger discrepancies (up to 13 %) were observed between the calculated and measured 15N NMR chemical shifts for the imidazole nitrogen atoms of both molecules, which indicate that effects such as imidazole ring resonant structures and molecular dynamics may also contribute to the nitrogen chemical environment.