Tautomeric equilibria, H-bonding and π-electron delocalization in o-nitrosophenol. A B3LYP/6-311 + G(2df,2p) study

• Published in: Journal of physical organic chemistry Vol. 18; no. 8; pp. 892 - 897
• Main Authors: Raczyńska, Ewa D., Krygowski, Tadeusz M., Zachara, Joanna E., Ośmiałowski, Borys, Gawinecki, Ryszard
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.08.2005
• Subjects: HOMA; intramolecular H-bonding; NICS; nitrosoenol-ketoxime tautomerism; o-nitrosophenol; π-electron distribution
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.963

Tautomeric interconversions (keto–enol and nitroso–oxime combined according to the 1,5‐proton shift into nitrosoenol–ketoxime) and related variations of π‐electron delocalization were studied for o‐nitrosophenol at the B3LYP/6–311 + G(2df,2p) level. The geometry‐ and magnetism‐based indices (HOMA and NICS) were applied to estimate π‐electron delocalization. The relative electronic (ΔE) and Gibbs free energies (ΔG) between the tautomers–rotamers were calculated to estimate tautomeric equilibrium and percentage content of the tautomeric mixture. Aromaticity of the phenyl ring and intramolecular H‐bonding were found to be the main factors stabilizing tautomeric forms (two isomeric nitrosoenols and one ketoxime) via increasing π‐electron delocalization in the chelating (quasi) ring and lowering push–pull interaction between ortho substituents. Copyright © 2005 John Wiley & Sons, Ltd. Depending on the position of H+ (tautomeric form) in o‐nitrosophenol, the bonds linked to the aromatic moiety are more single (nitrosoenol form) or double (ketoxime form) in nature. This has a significant impact on π‐electron delocalization in the phenyl and quasi ring.