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

Abstract

Deuterium isotope effects in the keto-enol tautomerism of β-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 β-dicarbonyl compounds. Such isotope effects cannot be reproduced in the conventional DFT scheme with harmonic zero-point vibrational corrections.

Nuclear quantum effect and H/D isotope effect in excited state intramolecular proton transfer and electron‐induced intramolecular proton transfer reactions in 8‐hydroxyquinoline

“…Similar NQEs on geometrical parameters have been observed in typical hydrogen-bonded systems [19,29].…”

“…Thus, the intramolecular hydrogen bond strength becomes weak as the NQE of the migrating hydrogen (deuterium) atom decreases. Similar NQEs on geometrical parameters have been observed in typical hydrogen‐bonded systems [19, 29].…”

“…Although we have successfully revealed H/D isotope effects in various systems, molecules, and reactions by MC_DFT calculations with these values so far, it is not clear that these values are also appropriate for the TS structures of proton transfer reaction and the optimized structures on S1 state. In addition, as we reported in previous studies, the nuclear wavefunction for the migrating proton becomes more diffusive at the TS structure [22, 23, 26, 27, 29]. Thus, we first analyzed the optimum exponent values of protonic and deuteronic basis functions for the optimized structures at electronic excited state and TS structure.…”

“…Although the larger nuclear basis set improves the accuracy of the MC_DFT calculations, we have demonstrated that the H/D isotope effects in molecular properties and chemical reactions could be adequately analyzed by the MC_DFT calculations using single 1s -type GTF as a nuclear basis function [22][23][24][25][26][27][28][29][30]. In such MC_DFT calculations, the exponent α value in the nuclear basis function should be treated as the variational parameter to improve the quality of the nuclear wavefunction.…”

“…To adequately analyze these phenomena, the NQE must be taken into account.To this end, we have developed the multicomponent quantum mechanics (MC_QM) methods [18][19][20][21], which can include the NQE of light particles, such as proton and deuteron, beyond the BOA. We have successfully analyzed the H/D isotope effects, LBHB systems, and several chemical reactions with the aid of the MC_QM methods [22][23][24][25][26][27][28][29][30]. It should be noted that similar methods have been proposed and developed by several groups, that is, nuclear orbital plus molecular orbital methods by Nakai group [31][32][33], nuclear-electronic orbital methods by Hammes-Schiffer group [34-38], any-particle molecular orbital methods by Reyes [39,40], and so forth.…”