Tautomerization and Substituent Effects on the Intramolecular Hydrogen Bonding in 4-Formyl-1-methylpyrazol-5-ol A Density Functional Theory

Abstract

A density functional theory (DFT) and the topology of the atoms-inmolecules (AIM) analysis has been applied to study the tautomerization and the substituent effects on the intramolecular hydrogen bonding (IHB) in the 4-formyl-1-methyl-pyrazol-5-ol and its -methyl, -amino, -hydroxyl and -fluoro derivatives. The geometries of the C3- and C6-susbtituted of the most stable enol structures were optimized at the B3LYP/6- 31G+(d,p) level while their final energies were evaluated using a 6- 311+G-(2df,2p) basis set expansion. Good agreement between DFT results and the published experimental results has been found. In general OH6- and F6-substitutions weaken the IHB, while the opposite is found upon Me6- and Am6-of 1a and 2a species. The trends observed in the relative stabilities of the enol structures do not follow the changes observed in the strength of the IHB.