H/D Isotope Effects in Keto-Enol Tautomerism of β-Dicarbonyl Compounds -Importance of Nuclear Quantum Effects of Hydrogen Nuclei-

Deuterium isotope effects in the keto-enol tautomerism of beta-dicarbonyl compounds (malonaldehyde, acetylacetone, dibenzoylmethane, and avobenzone) have been studied using a B3LYP+D functional level of multi-component density functional theory (MC_ DFT), which can directly take nuclear quantum effects (NQEs) of the hydrogen nuclei into account. We clearly show that the keto-enol energy difference becomes smaller by deuterium substitution, which is in reasonable agreement with the corresponding experimental evidence. Our MC_DFT study also reveals the hydrogen/deuterium (H/D) isotope effect in geometries and shows that the deuterium substitution weakens the intramolecular hydrogen-bonded interaction in the enol form. Direct treatment of NQEs of hydrogen nuclei via the MC_ DFT method is essential for analyzing the H/D isotope effect in keto-enol tautomerism of beta-dicarbonyl compounds. Such isotope effects cannot be reproduced in the conventional DFT scheme with harmonic zero-point vibrational corrections.

Automated summary

Deuterium isotope effects in the keto-enol tautomerism of beta-dicarbonyl compounds were studied using the B3LYP+D functional level of MC_DFT. The study showed that deuterium substitution decreases the keto-enol energy difference and weakens the intramolecular hydrogen-bonded interaction in the enol form. Direct treatment of NQEs of hydrogen nuclei via the MC_DFT method is crucial for analyzing the H/D isotope effect.