Extended Hartree–Fock theory of chemical reactions. IX. Diradical and perepoxide mechanisms for oxygenations of ethylene with molecular oxygen and iron‐oxo species are revisited

Abstract

Symmetry and broken symmetry (BS) in molecular orbital description of transition structures and intermediates in oxygenation reactions have been revisited to elucidate states correlation diagrams and mechanisms for addition reactions of molecular oxygen and metal-oxo MAO (M ϭ Mn(II) and Fe(II)) species to CAC double bonds. Relative stabilities between diradical (DR) and perepoxide (PE) intermediates were thoroughly investigated by several BS hybrid DFT (HDFT) methods and BS CCSD(T) method with and without spin projection. It has been found that recovery of spin symmetry, namely eliminating spin contamination error from the BS solutions, is crucial for the elucidation of reasonable state correlation diagrams and energy differences of the key structures in the oxygenation reactions because the singlet-triplet energy gap for molecular oxygen is large (22 kcal/mol). The BS HDFT followed by spin correction reproduced activation barriers for transition structures along both PE and DR reaction pathways by the use of the CASPT2 method. Basis set dependence on the relative stability between PE and DR intermediates were also examined thoroughly. Solvation effect for DR and PE intermediates was further examined with self-consistent reaction field (SCRF) and SCIPCM methods. Both BS HDFT and CASPT2 have concluded that the DR mechanism is favorable for the addition reaction of singlet oxygen to ethylene, supporting our previous conclusions. The BS HDFT with spin correction was concluded to be useful enough for theoretical investigations of mechanisms of oxygenation reactions. Implications of the computational results were discussed in relation to the theoretical framework (four configuration model) for elucidation of possible mechanisms of epoxidation reactions with Fe(IV)AO cores in metalloenzymes on the basis of isolobal analogies among O, OAO, and Fe(IV)AO. Correspondence between magnetic coupling mode and radical pathway in oxygenations with these species was clarified based on the BS MO interaction diagrams, leading to local singlet and triplet diradical mechanisms for epoxidations.

Synthesis of 1,2‐Dioxetanes as Thermochemiluminescent Labels for Ultrasensitive Bioassays: Rational Prediction of Olefin Photooxygenation Outcome by Using a Chemometric Approach

“…[31] Moreover, the prediction of the mechanism of singlet oxygen reactions has provenachallenging task for many reasons, [32] including because different possible reaction pathways (Scheme 3) are often predicted depending on the level of theory employed. [31] Moreover, the prediction of the mechanism of singlet oxygen reactions has provenachallenging task for many reasons, [32] including because different possible reaction pathways (Scheme 3) are often predicted depending on the level of theory employed.…”

“…The mechanism of singlet oxygen addition to double bonds has been studied computationally severalt imes, but the rigorous approach to thisd iradical system requires expensive treatments of multireference states and it has been appliede xclu-sively on very simple systems. [31] Moreover, the prediction of the mechanism of singlet oxygen reactions has provenachallenging task for many reasons, [32] including because different possible reaction pathways (Scheme 3) are often predicted depending on the level of theory employed. [33] To rationalizea ll of our results, we decided to avoid an extremely demanding and time-consuming quantum mechanical approach, considering that both excited and radicals tates are involved in the reaction.…”

Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex

“…Since early 1980s86, we have also performed the BS MO computations of model systems of metalloenzymes. As a continuation of previous work3, we, here (in the part XV), focus attention to manganese oxides in native and artificial PSII systems to derive comprehensive guiding principles for understanding of radical mechanisms of oxygen evolution reactions.…”

“…In this series of articles1–3, we have performed theoretical studies of the nature of chemical bonds in metalloenzymes from the viewpoint of protein‐assisted confined states of magnetic inorganic complexes that have strongly correlated electron systems. Past decades structure and reactivity of manganese oxide enzymes have been accepted both experimental4–75 and theoretical interest1–3, 76–144. The nature of chemical bonds of these active sites has indeed been investigated with both experimental and theoretical methods: for example, X‐ray diffraction and various spectroscopic observations, and broken‐symmetry (BS) molecular orbitals (MO) and Kohn‐Sham (KS) density functional (DFT) calculations.…”

“…The multi‐steps photo‐induced removals of electrons from manganese clusters play crucial roles in the water oxidation process in Eq. (1b), indicating an important question for quantum chemistry: Where is the photo‐induced hole in the high‐valent (HV) manganese oxide1–3, 86–91 …”