Keto - enol, imine - enamine, and nitro - aci-nitro tautomerism and their interrelationship in substituted nitroethylenes. Keto, imine, nitro, and vinyl substituent effects, and the importance of H-bonding

Tautomeric isomers and conformers of 2-nitrovinyl alcohol (1), 2-nitrovinylamine (2), and 1-nitropropene (3) are reported at the MP2 and B3LYP levels of theory, using the 6-31G* basis set, with energy evaluation at B3LYP/6-311+G** and G2MP2. The nitroalkenes are the global minima on their respective potential energy surfaces. The barriers for the concerted 1,5-H transfer to the corresponding nitronic acids amount to only 5.0 kcal/mol for 1, 13.2 kcal/mol for 2, and a sizable 37.8 kcal/mol for 3. Whereas the aci-nitro tautomer of 2-nitrovinyl alcohol is easily accessible, β-iminonitronic acid has little kinetic stability. H-bonding is a strong stabilizing factor in these nitroalkenes, estimated at 7.0 and 3.7 kcal/mol for the OH and NH