Tautomeric and conformational equilibrium of 2-nitrosophenol and 9,10-phenanthrenequinonemonooxime: ab initio and NMR study

Abstract

The tautomeric and conformational equilibrium of 2-nitrosophenol and 9,10-phenanthrenequinonemonooxime was studied by ab initio methods. The geometry optimizations of the structures investigated were done without any geometrical restrictions at HF/6-31G** and MP2/6-31G** levels of theory. The transition structures for tautomeric and rotameric conversions were located. To correct for electron correlation, single-point calculations were carried out up to MP4/6-311G*//MP2/6-31G* level of theory. Ab initio calculations for 2-nitrosophenol in agreement with the available experimental data define the nitroso form as more stable. It was found that the influence of the correlation energy on the relative stabilities is smaller for the rotamers of the nitroso tautomer but substantially (4–6 kcal/mol) for the oxime forms. It was found that the barrier height of tautomerization reaction is 10.24 kcal/mol. The structure of the 9,10-phenanthrenequinonemonooxime was studied by solid and liquid state NMR spectroscopy. Ab initio calculations in agreement with our experimental data predict that the compound exists as oxime tautomer and the syn-oxime is most stable. It was found that the solvent influence on the relative stabilities of both isomers: syn- and anti-oxime. While in chloroform solution the syn-oxime is preferred but in DMSO anti-oxime is more stable in energy. At the MP4/6-311G*//MP2/6-31G**+ZPE level of theory the barrier of tautomerization was predicted to be 10.96 kcal/mol and the rotational barrier around the single C–O bond in the syn-oxime was found to be 7.57 kcal/mol. The rotation is facile and this explains the absence of nitroso tautomers in solution.