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

Abstract

Can. J . Chem. 66, 3044 (1988). I-Methyl-2-nitrosoimidazole exhibits broadened 'H 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:l 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-'. 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-', and a barrier to rotation (from the s-trans) of 13.0 kcal mol-'. These results are in good agreement with those from solution phase. Various factors suggest that the .rr conjugative interaction between the nitroso group and imidazole ring is relatively unimportant. JUDY L. BOLTON, MICHAEL R. PETERSON et ROBERT A. MCCLELLAND. Can. J. Chem. 66, 3044 (1988). Le mtthyl-1 nitroso-2 imidazole prtsente en tnlargissement des signaux de rtsonance magnetique nucleaire du 'H a la temperature ambiante. L'absence de relation entre le spectre et la concentration aussi bien que la relation de proportionnalite existant entre la concentration et I'absorption ii 700 nm tcartent la posibilite d'un Cquilibre monomere nitroso -dimere nitroso.On suggkre qu'il y a un tquilibre entre les isomkres rotationnels s-ciset s-trans du monomkre nitroso. On observe a -59°C deux groupes de signaux dans un rapport 3:l. L'analyse des dCplacements chimiques en fonction de l'anisotropie magnktique du groupe nitroso suggkre que l'isomkre s-trans est majoritaire. L'Clargissement des signaux observe plus haute temptrature conduit a une barrikre 2 la rotation de 12.4 kcal mol-' (pour l'isomkre bans). Les calculs ab initio effectues avec le nitroso-2 imidazole en utilisant l'ensemble de base 3-2 IG montrent que les formes totalement planes s-cis et s-trans sont minimales. La forme S-bans est plus stable de 2.5 kcal mol-' avec une barribre a la rotation de 13.0 kcal mol-'. Ces resultats s'accordent parfaitement avec ceux obtenus en solution. Divers facteurs suggkrent qu l'interaction .rr conjuguee entre le groupe nitroso et le cycle imidazole joue un r61e relativement peu important.[Traduit par la revue]Extensive studies of the 2-nitroimidazole radiosensitizers as has been characterized for nitrosobenzenes (14)(15)(16)(17). This have revealed a number of biological and biochemical effects paper presents our examination of this phenomenon, at both the that are associated with reductive metabolism (1-6). The experimental and theoretical level. As will be shown, our initial implication is that biological reduction is producing some premise was incorrect. The nrnr ...

Molecular structure and relative proton and electron affinities of isomeric nitroimidazoles

“…The latter is very close to 90" in case of the 2-nitro isomer, and is about 7" off the perpendicular for the other two. One interesting feature of the data for the 2-nitroimidazole is that the barrier, 12.4 kcal/mol, is very similar to that previously obtained, 13.0 kcal/mol for the 2-nitrosoimidazole analog (16). These barriers arise through the interaction in the fully planar forms of the electron-rich IT electrons of the imidazole ring and the IT electrons of the nitro and nitroso substituents.…”

“…The initial estimate for the geometry of 2-nitroimidazole was based upon a previously optimized geometry for 2-nitrosoimidazole (16). However, the SCF failed to converge at the 3-21G basis set level, both in the planar ground state and in the torsional transition state.…”