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The spin transition in the reactions of the derivatives of imidazo[1,2-a]pyrazin-3(7H)-one with a triplet molecular oxygen is examined by the geometry optimization and the electronic matrix elements for spin-orbit coupling (SOC) is evaluated by using the full Pauli-Breit SOC operator. An optimal state is present for the suppression of SOC in the charge-transfer-induced luminescence (CTIL) for the peroxide, which is related with the ability of a substituent to donate an electron.

We have performed hybrid density functional theory (DFT) calculations on the geometric and electronic structures of low-lying doublet and quartet ferryl–oxo [Fe(IV)O] oxidants and a doublet perferryl–oxo [Fe(V)O] oxidant in Cytochrome P450. Fully optimized structures of compound I models have been determined, and the proper symmetry of wave functions has been restored by the spin-projection technique. The results show that the perferryl–oxo species is relatively low lying, as compared with the excited state of the ferryl–oxo species, if the iron–oxo bond is properly described as the mixing of several appropriate excited electronic configurations to minimize electron repulsion. This means that the perferryl–oxo species is virtually in a mixed-valent resonance state, ↑Fe(V)O ↔ ↑Fe(IV)•↑–↓•O, containing a highly reactive pπ atomic oxygen radical. The anionic thiolate ligand acts as a Lewis σ base and functions to achieve the stability of the perferryl–oxo complex and to activate the oxo ligand trans to it by asymmetric bond distortion along the O–Fe–S axis by lengthening the Fe–O bond and shortening the Fe–S bond, prior to the hydrogen-atom abstraction from the substrate. [ABSTRACT FROM AUTHOR]

Electronic structures of the Compound I (CpdI) in P450 are investigated on the basis of spin coupling forms of iron-oxo (Fe(IV)&dbond;O) cores and radical ligand (•L) groups to generalize previous singlet and triplet diradical (TD) mechanisms for oxygenations of alkanes with Fe( IV)&dbond;O. Orbital interaction schemes for four lower-lying spin configurations of CpdI with H&bond;C bond of substrate are examined to elucidate how magnetic coupling modes correlate with radical reaction pathways for hydroxylation reactions on the basis of the broken symmetry (BS) molecular orbital (MO) model. The configuration correlation diagrams for the four configurations model are depicted on the basis of the isoelectronic analogy among O, O2, and Fe( IV)&dbond;O, in addition to Coulomb exchange energy on the iron site, which determines its local spin configuration. Important role of ligand spin (•L) of CpdI for regulation of hydroxylation mechanisms is clarified with the aid of the spin coupling forms. Transition states for one quartet and three doublet configurations under the BS MO approximation are examined on the basis of potential curve crossings along reaction pathways. The four transition structures and corresponding radical intermediates for methane and trimethyl methane with CpI are located by the BS hybrid Kohn–Sham density functional theory (DFT) (B3LYP) method to confirm the orbital interaction schemes. Spin density populations obtained by the BS B3LYP calculations are found to be consistent with the theoretical predictions based on the four configurations model. The configuration and state correlation diagrams by BS B3LYP before and after spin projection are also consistent with the BS MO interaction schemes, which provide local SD and TD mechanisms of hydroxylation with CpdI. The present BS MO-theoretical framework is useful for systematic understanding of a lot of recent BS hybrid DFT computational results for hydroxylation reactions with CpdI and configuration correlation diagrams reported by several groups. Implications of the present theoretical and computational results are discussed in relation to several experimental characteristics of hydroxylation reactions with iron-oxo species and P450. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

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]