Theoretical study of low-spin, high-spin, and intermediate-spin states of [Fe(III)(pap)2](+) (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of light-induced excited spin state trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.     

Quantum mechanical/molecular mechanical investigation of the mechanism of C-H hydroxylation of camphor by cytochrome P450cam: theory supports a two-state rebound mechanism

The stereospecific cytochrome P450-catalyzed hydroxylation of the C(5)-H((5-exo)) bond in camphor has been studied theoretically by a combined quantum mechanical/molecular mechanical (QM/MM) approach. Density functional theory is employed to treat the electronic structure of the active site (40-100 atoms), while the protein and solvent environment (ca. 24,000 atoms) is described by the CHARMM force field. The calculated energy profile of the hydrogen-abstraction oxygen-rebound mechanism indicates that the reaction takes place in two spin states (doublet and quartet), as has been suggested earlier on the basis of calculations on simpler models ("two-state reactivity"). While the reaction on the doublet potential energy surface is nonsynchronous, yet effectively concerted, the quartet pathway is truly stepwise, including formation of a distinct intermediate substrate radical and a hydroxo-iron complex. Comparative calculations in the gas phase demonstrate the effect of the protein environment on the geometry and relative stability of intermediates (in terms of spin states and redox electromers) through steric constraints and electronic polarization.     

Excited states of the benzyl radical

The CI method is used in the -electron approximation with orbitals for closed and open shells to calculate the properties of excited doublet states with allowance for all singly excited configurations and some doubly excited ones, and also for the first quartet and sextet states, which are calculated in the one-configuration approximation via the open-shell theory. The energies and transition moments agree satisfactorily with the available experimental evidence. A classification and assignment is given for the excited terms. Truncation of the complete set of singly excited configurations greatly distorts the calculated spectrum. Inclusion of doubly excited configurations in the CI also produces a substantial change in the spectrum; in some cases it alters the order of adjacent terms. Conversion in CI from basis closed-shell orbitals to open-shell ones produces a considerable lowering of all terms in the spectrum. As in the case of triplet terms for molecules, weakening of electron interaction brings the lowest excited term of the radical closer to the ground-state term. The electron-density and spin-density distributions are calculated for the excited states.     

COS Activation by Zr~＋ in the Gas Phase：A Case of Two-state Reactivity Process

To elucidate the mechanisms of Zr + reacting with COS,both the quartet and doublet potential energy surfaces (PESs) for reactions of Zr + (4 F,2 D) with COS in the gas phase have been investigated in detail by means of density functional method (B3LYP).To obtain more accurate results,the coupled cluster single-point calculations (CCSD(T)) using B3LYP optimized geometries were performed.For the C-O bond activation,the calculated results indicate that both the quartet and doublet states proceed via an insertion-elimination mechanism.For the C-S bond activation,the quartet reaction has an insertion-elimination mechanism,but the doublet reaction is a direct abstraction of the sulfur atom by Zr +.The C-S bond activation is found to be energetically more favorable than the C-O bond activation.It is found that the reaction of the 4 F gound state of Zr + to yield ZrO + is spin-forbidden (Zr + (4 F) + COS (1 Σ) → ZrO + (2) + CS (1 Σ)) and the crossing points were approximately determined.All the results have been compared with the existing experimental and theoretical data.     

Electron-spin multiplicities and molecular structures of neutral and ionic scandium-benzene complexes

Scandium-benzene complexes, Sc-(C6H6)1,2 are produced by interactions between the laser-vaporized scandium atoms and benzene vapor in pulsed molecular beams, and identified by photoionization time-of-flight mass spectrometry and photoionization efficiency spectroscopy. The electron-spin multiplicities and geometries of these complexes and their ions are determined by combining pulsed field-ionization zero electron kinetic-energy spectroscopy and density-functional theory calculations. For scandium-monobenzene, a short-range quartet ground state is determined for the neutral complex, and a low-energy triplet state is probed for the ion. For the dibenzene complex, the neutral ground state is a doublet, and two low-energy ion states are singlet and triplet. The quartet and triplet states of scandium-monobenzene and the triplet state of scandium-dibenzene possess sixfold symmetry, whereas the doublet and singlet of the dibenzene complex have twofold symmetry. Moreover, ionization energies and metal-ring stretching wavenumbers are measured for both complexes.     

A Monte Carlo-quantum mechanics study of the lowest n-pi* and pi-pi* states of uracil in water

The solvatochromic shifts of the n-pi(*) and pi-pi(*) states of uracil in water are analyzed using a combined and sequential Monte Carlo/quantum mechanics (MC/QM) approach. The role of the solute polarization and electronic delocalization into the solvent region are investigated. Electronic polarization of the solute is obtained using the HF/6-31G(d), the polarizable continuum model (PCM) and an iterative procedure using MP2/aug-cc-pVDZ in the MC/QM. The in-water dipole moment of uracil is obtained, respectively, as 5.12 D, 6.12 D and 7.01 +/- 0.05 D. This latter result, corresponding to an increase of 60% with respect to the gas phase value, is used in the classical potential of the MC simulation to obtain statistically uncorrelated configurations for subsequent QM calculations of the ultraviolet-visible absorption spectrum of uracil in water. QM calculations are performed at the time-dependent density-functional theory (TD-DFT) combined with the B3LYP and B3PW91 functionals, multiconfigurational (CASSCF) and the semi-empirical all-valence electron INDO/CIS methods. Using 60 solute-solvent configurations with the explicit inclusion of 200 water molecules the solvatochromic shift is obtained as a blue shift of 0.50 eV for the n-pi(*) state and a red shift of 0.19 eV for the pi-pi(*) state, in good agreement with experimentally-inferred values. These results are compared with TD-DFT results in conjunction with PCM approaches and the importance of solute polarization and wave function delocalization over the solvent region is discussed. Our results suggest that the elusive n-pi(*) state of uracil in water lies around 255 nm hidden by the intense and broad pi-pi(*) transition with a maximum at 260 nm, inverting the relative locations of these states compared to the gas phase. This is further supported by considering the in-water dipole moment changes upon excitation, as obtained from CASSCF calculations.     

U+与CO2气相反应的密度泛函理论研究

运用密度泛函理论(DFT)中的B3LYP方法,U原子用含相对论有效原子实势(ECP)校正的基组(SDD),C、O原子采用6-311+G(d)基组,对气相中U+和CO2的反应进行了理论研究.通过研究二重和四重自旋态的反应势能面(PESs),优化得到了两条反应路径的反应物、中间体、过渡态和产物的结构.用"两态反应"(TSR)分析反应机理,结果表明体系的优先选择路径为高自旋态进入和低自旋态离开反应,发生在四重态和二重态的自旋多重度的改变使得整个反应系统能以一个低能反应途径进行.     

Theoretical study of the mechanism of valence tautomerism in cobalt complexes

Valence tautomerism is studied in the [Co(II-HS)(sq)(2)(bpy)]/[Co(III-LS)(sq)(cat)(bpy)] mononuclear cobalt complex by using DFT methods (HS, high spin; LS, low spin; cat, catecholate; sq, semiquinone; bpy, 2,2'-bipyridine). Calculations at the B3LYP* level of theory reproduce well the energy gap between the Co(II-HS) and Co(III-LS) forms giving an energy gap of 4.4 kcal/mol, which is comparable to the experimental value of 8.9 kcal/mol. Potential energy surfaces and crossing seams of the electronic states of the doublet, quartet, and sextet spin states are calculated along minimum energy paths connecting the energy minima corresponding to the different spin states. The calculated minimum energy crossing points (MECPs) are located at 8.8 kcal/mol in the doublet/sextet surfaces, at 10.2 kcal/mol in the doublet/quartet surfaces, and at 8.4 kcal/mol in the quartet/sextet surfaces relative to the doublet ground state. Considering the energy of the three spin states and the crossing points, the one-step relaxation mechanism between the Co(II-HS) and Co(III-LS) forms is the most probable. This research shows that mapping MECPs can be a useful strategy to analyze the potential energy surfaces of systems with complex deformation modes.     

Theoretical study of dynamic electron-spin-polarization via the doublet-quartet quantum-mixed state and time-resolved ESR spectra of the quartet high-spin state

The mechanism of the unique dynamic electron polarization of the quartet (S = 3/2) high-spin state via a doublet-quartet quantum-mixed state and detail theoretical calculations of the population transfer are reported. By the photo-induced electron transfer, the quantum-mixed charge-separate state is generated in acceptor-donor-radical triad (A-D-R). This mechanism explains well the unique dynamic electron polarization of the quartet state of A-D-R. The generation of the selectively populated quantum-mixed state and its transfer to the strongly coupled pure quartet and doublet states have been treated both by a perturbation approach and by exact numerical calculations. The analytical solutions show that generation of the quantum-mixed states with the selective populations after de-coherence and/or accompanying the (complete) dephasing during the charge-recombination are essential for the unique dynamic electron polarization. Thus, the elimination of the quantum coherence (loss of the quantum information) is the key process for the population transfer from the quantum-mixed state to the quartet state. The generation of high-field polarization on the strongly coupled quartet state by the charge-recombination process can be explained by a polarization transfer from the quantum-mixed charge-separate state. Typical time-resolved ESR patterns of the quantum-mixed state and of the strongly coupled quartet state are simulated based on the generation mechanism of the dynamic electron polarization. The dependence of the spectral pattern of the quartet high-spin state has been clarified for the fine-structure tensor and the exchange interaction of the quantum-mixed state. The spectral pattern of the quartet state is not sensitive towards the fine-structure tensor of the quantum-mixed state, because this tensor contributes only as a perturbation in the population transfer to the spin-sublevels of the quartet state. Based on the stochastic Liouville equation, it is also discussed why the selective population in the quantum-mixed state is generated for the "finite field" spin-sublevels. The numerical calculations of the elimination of the quantum coherence (de-coherence and/or dephasing) are demonstrated. A new possibility of the enhanced intersystem crossing pathway in solution is also proposed.     

Calculated properties of the active site of oxidized rubredoxins

For a molecular model of the Fe-S active site complex in oxidized rubredoxin, we have calculated the spin-orbit coupling between the ground sextet state and excited quartet and doublet states which gives rise to the observed zero field splitting of the sextet ground state into three spin-mixed Kramers doublets. Additionally, we have used the six spin-mixed sextet state components to calculate effective magnetic moments, magnetic field energies and nine g values corresponding to transitions between the three pairs of Kramers doublets in applied magnetic fields along three perpendicular axes. We have calculated these properties for eight conformational variations of the ligands around the Fe at the active site. The results of these calculations clearly show the origin of the observed g=4.3 signal previously described only in terms of the phenomenological spin-Hamiltonian formalism. For the eight conformations considered, five have this characteristic signal. Zero field splitting comparable to the observed values could be obtained for all symmetries studied. In addition, the calculated values of magnetic moment in all symmetries correspond to that of high spin ferric ion and do not vary appreciably with temperature above 77° K, in agreement with experimental results. From comparison of all our calculated results with experiment, it appears that the active site in oxidized rubredoxins could have small conformational variations in different rubredoxins and under the various experimental conditions used.     

Multi-state epoxidation of ethene by cytochrome P450: a quantum chemical study.

The epoxidation of ethene by a model for Compound I of cytochrome P450, studied by the use of density functional B3LYP calculations, involves two-state reactivity (TSR) with multiple electromer species, hence "multi-state epoxidation". The reaction is found to proceed in stepwise and effectively concerted manners. Several reactive states are involved; the reactant is an (oxo)iron(IV) porphyrin cation radical complex with two closely lying spin states (quartet and doublet), both of which react with ethene to form intermediate complexes with a covalent C-O bond and a carbon-centered radical (radical intermediates). The radical intermediates exist in two electromers that differ in the oxidation state of iron; Por(+)(*)Fe(III)OCH(2)CH(2)(*) and PorFe(IV)OCH(2)CH(2)(*) (Por = porphyrin). These radical intermediates exist in both the doublet- and quartet spin states. The quartet spin intermediates have substantial barriers for transformation to the quartet spin PorFe(III)-epoxide complex (2.3 kcal mol(-)(1) for PorFe(IV)OCH(2)CH(2)(*) and 7.2 kcal mol(-)(1) for Por(+)(*)Fe(III)OCH(2)CH(2)(*)). In contrast, the doublet spin radicals collapse to the corresponding PorFe(III)-epoxide complex with virtually no barriers. Consequently, the lifetimes of the radical intermediates are much longer on the quartet- than on the doublet spin surface. The loss of isomeric identity in the epoxide and rearrangements to other products arise therefore mostly, if not only, from the quartet process, while the doublet state epoxidation is effectively concerted (Scheme 7). Experimental trends are discussed in the light of the computed mechanistic scheme, and a comparison is made with closely related mechanistic schemes deduced from experiment.     

Electron spin polarization of the excited quartet state of strongly coupled triplet-doublet spin systems

The electron spin polarization associated with electronic relaxation in molecules with trip-quartet and trip-doublet excited states is calculated. Such molecules typically relax to the lowest trip-quartet state via intersystem crossing from the trip doublet, and it is shown that when spin-orbit coupling provides the main mechanism for this relaxation pathway it leads to spin polarization of the trip quartet. Analytical expressions for this polarization are derived using first- and second-order perturbation theory and are used to calculate powder spectra for typical sets of magnetic parameters. It is shown that both net and multiplet contributions to the polarization occur and that these can be separated in the spectrum as a result of the different orientation dependences of the +/-1/2<-->+/-3/2 and +1/2<-->-1/2 transitions. The net polarization is found to be localized primarily in the center of the spectrum, while the multiplet contribution dominates in the outer wings. Despite the fact that the multiplet polarization is much stronger than the net polarization for individual orientations of the spin system, the difference in orientation dependence of the transitions leads to comparable amplitudes for the two contributions in the powder spectrum. The influence of this difference on the line shape is investigated in simulations of partially ordered samples. Because the initial nonpolarized state of the spin system is not conserved for the proposed mechanism, the net polarization can survive in the doublet ground state following electronic relaxation of the triplet part of the system.     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

The performance of time-dependent density functional theory based on a noncollinear exchange-correlation potential in the calculations of excitation energies

In the present work we have studied the accuracy of excitation energies calculated from spin-flip transitions with a formulation of time-dependent density functional theory based on a noncollinear exchange-correlation potential proposed in a previous study. We compared the doublet-doublet excitation energies from spin-flip transitions and ordinary transitions, calculated the multiplets splitting of some atoms, the singlet-triplet gaps of some diradicals, the energies of excited quartet states with a doublet ground state. In addition, we attempted to calculate transition energies with excited states as reference. We compared the triplet excitation energies and singlet-triplet separations of the excited state from spin-flip and ordinary transitions. As an application, we show that using excited quartet state as reference can help us fully resolve excited states spin multiplets. In total the obtained excitation energies calculated from spin-flip transitions agree quite well with other theoretical results or experimental data.     

Extensive theoretical studies on the low-lying electronic states of indium monochloride cation, InCl+

The global potential energy curves for the 14 low-lying doublet and quartet Lambda-S states of InCl+ are calculated at the scalar relativistic MR-CISD+Q (multireference configuration interaction with single and double excitations, and Davidson's correction) level of theory. Spin-orbit coupling is accounted for via the state interaction approach with the full Breit-Pauli Hamiltonian, which leads to 30 Omega states. The computed spectroscopic constants of nine bound Lambda-S states and 17 bound Omega states are in good agreement with the available experimental data. The transition dipole moments and Franck-Condon factors of selected transitions are also calculated, from which the corresponding radiative lifetimes are derived.     

Electron paramagnetic resonance of the excited states of the three-spins-1/2 ZnTPP-3-NOPy complex

EPR spectra of the excited quartet and doublet molecular states of (tetraphenylporphinato)zinc(II) covalently bounded to 3-(N-nitronyl-notroxide) pyridine stable radical are modeled in terms of the spin-Hamiltonian given by the sum of the contributions from the radical and triplet moieties, and the interaction between them. The later is represented by anisotropic point dipolar and isotropic exchange electron spin-spin interactions. It is shown that the high field (W-band) EPR spectra depend on energy separation between the electronic doublet (D) and quartet (Q) states. This dependence was utilized to estimate the upper limit of the intensity of exchange interaction between the radical and porphyrin moieties.     

Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

Triamino-s-triazine triradical trications. An experimental study of triazine as a magnetic coupling unit

[structure: see text] Triamino-s-triazine derivatives 3a, 4, and 5 have been prepared, and their cationic states have been analyzed electrochemically. At 298 K, 3a+ has a limited lifetime in CH2Cl2 solution. However, 4+ and 5+ are long-lived under such conditions, and quartet states of 4(3+) and 5(3+) are observed by ESR spectroscopy. Variable-temperature ESR analysis and NMR shift susceptibility measurements indicate that 5(3+) is a doublet ground state with a populated quartet state.     

Theoretical study of the reaction of Ni with OCS

The reactivity of Ni⁺ with OCS on both doublet and quartet potential energy surfaces (PES) has been investigated at the B3LYP/6-311+G(d) level. The object of this investigation was the elucidation of the reaction mechanism. The calculated results indicated that both the CS and CO bond activations proceed via an insertion–elimination mechanism. Intersystem crossing between the doublet and quartet surfaces may occur along both the CS and CO bond activation branches. The ground states of NiS⁺ and NiO⁺ were found to be quartets, whereas NiCO⁺ and NiCS⁺ have doublet ground states. The CS bond activation is energetically much more favorable than the CO bond activation. All theoretical results are in line with early experiments.