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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 M&dbond;O (M = Mn(II) and Fe(II)) species to C&dbond;C 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)&dbond;O cores in metalloenzymes on the basis of isolobal analogies among O, O&dbond;O, and Fe(IV)&dbond;O. 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. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Abstract: The broken-symmetry (BS) and multideterminant approaches to atomic oxygen (O), molecular oxygen (O2) and iron–oxo (Fe(IV)n P450 have elucidated electronic structures of the ground triplet and excited singlet states, which indicate isoelectronic characteristics of the species. The dissociation processes of the O–O and Fe–O double bonds are also examined to clarify the radical character, namely O-atom property responsible for radical mechanism of hydroxylations of alkanes and epoxidation of alkenes. This isolobal analogy has indeed enabled us to propose possible reaction mechanisms of oxygenation reactions by the Fe(IV) on the basis of available theoretical and experimental results for O and O2. Similarly, an isolobal analogy of the σ* bond among Fe(IV)ane, peracids, etc. indicates the common electrophilic property for the oxygenation reactions. The small energy gaps between the high- and low-spin states of the transition structures and intermediates generated in the oxygenation reactions are found to be origins for spin crossover phenomena along the reaction pathways of these reactions. [Copyright &y& Elsevier]