Theoretical approach to the solvation of nitroenamines and the influence of solvation on isomeric equilibria

Abstract

The influence of the solvent on the isomeric equilibria and on the rotational barriers around the CC double bond of some simple nitroenamines [H₂N(R¹)CC(R²)NO₂, R¹H or Me and R²H, COMe or CO₂Me] has been investigated by means of quantum-chemical calculations at the AM1 level using the continuum approach for the solvation. The solute–solvent electrostatic interactions appear to make an important contribution to the solvation energy. Comparison with experiment is more satisfactory for the rotational barriers than for the isomeric equilibria. 3–21G Ab initio calculations have been carried out to show that the partial failure in the description of the isomeric equilibria in solution is due to the AM1 energies in the gas phase not being sufficiently accurate. For 2-nitroethenamine (R¹R²H) a discrete and a mixed discrete–continuum model have also been employed to analyse the influence of solvation on the isomeric equilibria. The relative advantages of these models have been discussed.