Substituent Effects on the Strength of the Intramolecular Hydrogen Bond of Thiomalonaldehyde

Abstract

The effect of CH3, NH2, OH, and F substituents on the intramolecular hydrogen bond (IHB) of thiomalonaldehyde (TMA) was analyzed through the use of B3LYP density functional theory calculations. The geometries of the C1-, C2-, and C3-susbtituted enol and enethiol tautomers were optimized at the B3LYP/6-31G(d) level while their final energies were evaluated using a 6-311+G(3df,2p) basis set expansion. In general C1-substitution strengthens the IHB of the enolic tautomer, while C3-substitution strengthens the IHB of the enethiolic form. These changes are related with an enhancement of the intrinsic acidity of the OH and the SH groups, respectively. Important cooperative effects are also present when the substituent can form an additional IHB with either the oxygen atom or the sulfur atom of TMA. However, the trends observed in the relative stabilities of the enol and the enethiol tautomers do not follow the changes observed in the strength of the IHB. C1-substitution specifically stabilizes the enethiol form...