Experimental and theoretical investigation of C-nitroso rotation in 2-nitrosoimidazoles

Abstract

1-Methyl-2-nitrosoimidazole exhibits broadened 1H nuclear magnetic resonance signals at room temperature. The concentration independency of this spectrum as well as the linearity of the 700-nm absorbance with concentration rules out equilibrating nitroso monomer – nitroso dimer. It is suggested that there are equilibrating s-cis and s-trans nitroso monomer rotational isomers. At −59 °C two sets of signals in a 3:1 ratio are observed. Analysis of CH3 chemical shifts in terms of the magnetic anisotropy of the nitroso group suggests that the s-trans is the major isomer. The broadening observed at higher temperatures leads to a barrier for rotation (from the trans isomer) of 12.4 kcal mol−1. Ab initio calculations with 2-nitrosoimidazole using the 3-21G basis set show that the fully planar s-cis and s-trans forms are minima, with the s-trans more stable by 2.5 kcal mol−1, and a barrier to rotation (from the s-trans) of 13.0 kcal mol−1. These results are in good agreement with those from solution phase. Various factors suggest that the π conjugative interaction between the nitroso group and imidazole ring is relatively unimportant.