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The electronic structures of a cyanide bridged Fe–Co molecular square, [Co2Fe2(CN)6(tp*)2(dtbbpy)4](PF6)2·2MeOH (1) (tp* = hydrotris (3,5-dimethylpyrazol-1-yl) borate, dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), which exhibits thermal and photo-induced two-step charge-transfer induced spin transitions (CTIST), are investigated in detail by density functional theory (DFT) and time-dependent (TD) DFT calculations. For the three phases observed in the experiment, three different model structures are constructed based on the geometries of X-ray crystallography analysis measured at low (100 K), middle (298 K) and high (330 K) temperatures. The calculated results elucidate that the ground states at the low and the high temperatures are diamagnetic [(FeIILS)2(CoIIILS)2] and ferromagnetic [(FeIIILS)2(CoIIHS)2], respectively. On the other hand, the one-electron transferred [FeIILSFeIIILSCoIIHSCoIIILS] state becomes the ground state at the intermediate temperature phase. A magnetic interaction between FeIII and CoII in the [(FeIIILS)2(CoIIHS)2] state is ferromagnetic and the most stable spin-coupling state is the all-ferromagnetic state. The TD-DFT calculation shows the two significant peaks of FeII t2g → CoIII eg around 800 nm. The results support that the experimental broad absorption peak at 770 nm is an inter-valence charge transfer (IVCT) band.

The buckyferrocenes, synthesized through face-to-face fusion of ferrocene and fullerene fragments (C60Me10), are expected to enjoy the rich scientific heritage of ferrocene and fullerene with an extensively large π-conjugation network between the two Fe atoms [Y. Matsuo, K. Tahara and E. Nakamura, J. Am. Chem. Soc., 2006, 128, 7154]. However, the addition of pentamethyl groups at each end of the fullerene fragment breaks the π-conjugation path as well as metal-metal spin correlation between the two-ferrocene faces in a buckyferrocene. We found that the unblocking of π-conjugation from different positions in fullerene fragments have substantial effects on their topologies, spin densities, diradical characters as well as nonlinear optical (NLO) properties of these buckyferrocenes. We study the topological dependence of open-shell diradical character and second hyperpolarizability (γ), the third-order NLO properties at the molecular scale, in several buckyferrocenes. On the basis of their different diradical characters (yi), which are defined by the occupation number of the lowest unoccupied natural orbital (LUNO) + i (i = 0, 1,…), these buckyferrocenes are categorized into three groups, i.e., closed-shell (yi = 0), intermediate open-shell singlet (0 < yi < 1), and almost pure open-shell singlet (yi = 1) compounds. For example, we found that buckyferrocenes including (CpFe)2η(5)C60Me10 and (CpFe)2η(5)C70Me10 have closed-shell configurations. The buckyferrocenes (CpFe)2η(5)C60, (CpFe)2η(5)C70, (CpFe)2η(5)C70Me8, (CpFe)2η(5)C70Me4 and (CpFe)2η(5)C30 are intermediate open-shell singlet, while (CpFe)2η(5)C60Me4 and (CpFe)2η(5)C60Me8 are pure open-shell singlet complexes. Interestingly, the γzzzz amplitude of (CpFe)2η(5)C60, an open-shell intermediate diradical complex, is about 41 times and 13 times as large as those of its closed-shell ((CpFe)2η(5)C60Me10) and pure diradical ((CpFe)2η(5)C60Me8) counterparts, respectively. Similarly, the γzzzz amplitudes of (CpFe)2η(5)C70, (CpFe)2η(5)C70Me4, and (CpFe)2η(5)C70Me8 with intermediate diradical character are about 36, 17, and 9 times as large as that of their closed-shell (CpFe)2η(5)C70Me10 counterpart. The fact that larger γzzzz values are obtained for buckyferrocenes with intermediate diradical characters is in line with the "y-γ correlation" obtained from the valence configuration interaction (VCI) results for a two-site diradical model [M. Nakano, et al., Phys. Rev. Lett., 2007, 99, 033001] as well as for fullerene and graphene systems. The γzzzz density analysis shows that the large positive contributions originate from the large γzzzz density distributions on the upper- and lower-extended edges of the buckyferrocenes, between which significant spin polarizations appear within the spin-unrestricted DFT level of theory. These results demonstrate that such buckyferrocenes are potential candidates for a novel class of open-shell singlet NLO systems, where γzzzz values are modulated by tuning their diradical character through the use of suitably modified fullerene fragments.

One-dimensional (1D) coordination polymers (CPs) experiences limitations in exfoliation into individual strands, which hamper their utility as functional 1D nanomaterials. Here we synthesize chiral 1D-CPs that feature the bis(dipyrrinato)zinc(II) complex motif. They can be exfoliated into single strands upon sonication in organic media, retaining lengths of up to 3.19 μm (ca. 2600 monomer units). Their chiroptical structure allows the exfoliated wires to show circularly polarized luminescence at an intensity 5.9 times that of reference monomer complexes.

The titled complex exhibits multi-functional luminochromism in the solid state, despite its simple chemical structure. The complex shows solid-state vapochromism and mechanochromism. The polymer analogue of the complex undergoes solvatochromism and thermochromism.

Hydroxyl groups on an unsaturated imine, which may be readily obtained from the corresponding unsaturated aldehyde and the 1,2-ethanolamine derivative, were found to efficiently activate a [4+4] dimerization reaction to produce the eight-membered 1,5-diazacyclooctane. A novel OH–π interaction between the two imines, in addition to the stabilization of the eight-membered diacetals, was proposed based on density functional theory calculations.

An estimation scheme is established for the spin contamination error (SCE) in spin-polarized density functional theory calculations with the plane-wave basis for crystals of one-dimensional materials. The size consistency of the scheme was confirmed via H-chain models, and the accuracy was confirmed by the comparison with the reported experimental values. The scheme was applied to real systems, specifically, tribromo-trioxotriangulene and [Ni(cyclohexanediamine)2Br]Br2 crystals. It clarified the SCEs on the energies of calculated models (0.049–3.093 eV). The presented scheme will enable the appropriate investigation and design of functional materials of nano (molecular) wire, superconductors, catalysts, quantum computing, and so on.

A series of tetragold(I) complexes supported by tetraphosphine ligands, meso- and rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso- and rac-dpmppm) were synthesized and characterized to show that the tetranuclear Au(I) alignment varies depending on syn- and anti-arrangements of the two dpmppm ligands with respect to the metal chain. The structures of syn-[Au4 (meso-dpmppm)2X]X'3 (X = Cl; X' = Cl (4 a), PF6 (4 b), BF4 (4 c)) and syn-[Au4 (meso-dpmppm)2]X4 (X = PF6 (4 d), BF4 (4 e), TfO (4 f); TfO = triflate) involved a bent tetragold(I) core with a counter anion X incorporated into the bent pocket. Complexes anti-[Au4 (meso-dpmppm)2]X4 (X = PF6 (5 d), BF4 (5 e), TfO (5 f)) contain a linearly ordered Au4 string and complexes syn-[Au4 (rac-dpmppm)2X2]X'2 (X = Cl, X' = Cl (6 a), PF6 (6 b), BF4 (6 c)) and syn-[Au4 (rac-dpmppm)2]X4 (X = PF6 (6 d), BF4 (6 e), TfO (6 f)) consist of a zigzag tetragold(I) chain supported by the two syn-arranged rac-dpmppm ligands. Complexes 4 d-f, 5 d-f, and 6 d-f with non-coordinative large anions are strongly luminescent in the solid state (λmax = 475-515 nm, Φ = 0.67-0.85) and in acetonitrile (λmax = 491-520 nm, Φ = 0.33-0.97); the emission was assigned to phosphorescence from (3) [dσ*σ*σ* pσσσ] excited state of the Au4 centers on the basis of DFT calculations as well as the long lifetime (a few μs). The emission energy is predominantly determined by the HOMO and LUMO characters of the Au4 centers, which depend on the bent (4), linear (5), and zigzag (6) alignments. The strong emissions in acetonitrile were quenched by chloride anions through simultaneous dynamic and static quenching processes, in which static binding of chloride ions to the Au4 excited species should be the most effective. The present study demonstrates that the structures of linear tetranuclear gold(I) chains can be modified by utilizing the stereoisomeric tetraphosphines, meso- and rac-dpmppm, which may lead to fine tuning of the strongly luminescent properties intrinsic to the Au(I) 4 cluster centers.

The cyanide-bridged [CoFe] one-dimensional chain, [Co(II)((R)-pabn)][Fe(III)(Tp)(CN)3](BF4)·MeOH·2H2O, where (R)-pabn = (R)-N2,N(2')-bis(pyridin-2-ylmethyl)-1,1'-binaphthyl-2,2'-diamine and Tp = hydrotris(pyrazolyl)borate, exhibits magnetic and electric bistabilities originating from an electron transfer coupled spin transition between Fe-CN-Co pairs. The use of L-edge X-ray absorption spectroscopy (XAS) in combination with L-edge X-ray magnetic circular dichroism (XMCD) is explored for the investigation of the electronic structure and magnetization of Co and Fe ions separately, in both diamagnetic and paramagnetic states. It has been established from susceptibility results that the switching between diamagnetic and paramagnetic phases emanates from electron transfer between low spin Fe(II) and Co(III), resulting in low spin Fe(III) (S = 1/2) and high spin Co(II) (S = 3/2). The XAS and XMCD results are consistent with the bulk susceptibility measurements, where greater detail regarding the charge transfer process is determined. The Fe-CN-Co electron transfer pathway is highlighted by a strongly XMCD dependent transition to a cyanide back bonding orbital, giving evidence for strong hybridization with Fe(III) t2g orbitals. In addition to thermally induced and photoinduced switching, [CoFe] is found to exhibit a switching by grinding induced dehydration. Analysis of XAS shows that on grinding diamagnetic [CoFe], 75% of metal ions lock into the magnetic Co(II)Fe(III) phase. Density functional theory calculations based on the [CoFe] crystal structure in the magnetic and nonmagnetic phases aid the spectroscopic results and provide a complementary insight into the electronic configuration of the [CoFe] 3d shells, quantifying the change in ligand field around Co and Fe centers on charge transfer.

Activation mechanisms of oxygen dianion and hydroxide by high-valent transition metal ions for the oxygen–oxygen (O–O) bond formation of water splitting reaction have been investigated on the theoretical grounds, together with experimental results. First of all, broken-symmetry MO formulations are revisited to elucidate the instability of the dπ–pπ bond in high-valent metal–oxo M(X) O (M = Mn, Fe, Ru, etc.; X = IV, V) systems that react with hydroxide anion (or radical) or water to afford hydroperoxide anion (or peroxide). The triplet instability of these bonds entails strong or intermediate diradical characters: •M(X−1) O• and ••M(X−2) O••; the broken-symmetry (BS) molecular orbitals (MO) resulted from strong electron correlation, leading to the concept of electron localizations and local spins. As a continuation of these theoretical results, the BS MO interaction diagrams, namely orbital and spin correlation diagrams, for one-electron and electron-pair transfer mechanisms for the O–O bond formation have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of mechanisms of oxygenation reactions by p450, methane mono- oxygenase (MMO) and homolytic radical coupling mechanisms by oxygen evolving complex (OEC) of PSII. The spin alignments in high-valent M(X) O systems are found directly corresponding to possible mechanisms of the O–O bond formation between Mn(X) O and hydroxide (OH) anion via one-electron, electron-pair transfer and their superposed (chameleon) processes. The broken-symmetry (BS) UB3LYP calculations of the model systems have been performed to confirm these mechanisms for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms and orbital and spin correlation diagrams. Implications of the theoretical results are discussed in relation to three scenarios of the O–O bond formation for water oxidation: (a) HO–OH, (b) OOH and (c) O–O, and water oxidation of OEC of PSII.

The luminescent cyclometalated platinum(II) complex [Pt(HL)Cl] ( 1 ) (where HL = 2-phenyl-6-(1 H -pyrazol-3-yl)-pyridine anion) and its deprotonated dimer complex [Pt 2 (L) 2 ] ( 2 ) were theoretically studied via density functional theory (DFT) and time-dependent DFT (TDDFT) in conjunction with the polarizable continuum model (PCM) solvent effect to investigate their molecular geometries (at the S 0 , S 1 , and T 1 states), electronic structures, and photophysical properties (absorption and phosphorescence spectra) and to elucidate the reason for the significantly higher phosphorescence quantum yield of 2 (59%) compared to that of the 1 (14%).

We show the significant role of the Rydberg-like unoccupied molecular orbital in the photoexcitation process at the rubrene/graphite interface. Two-photon photoemission (2PPE) spectroscopy revealed a prominently strong resonant excitation of an unoccupied molecular orbital (MO) at 3.26 eV above the Fermi level (J. Phys. Chem. C 2012, 116, 5821). The excitation was attributed to be mediated by the image potential state (IPS) on graphite. We investigate in this paper the nature of the MO by 2PPE spectroscopy and density functional theory (DFT) calculations. The DFT calculation involving diffuse atomic orbitals predicts a nearly cylindrical unoccupied MO extending over four phenyl rings at the energy of the interest. Polarization dependence of 2PPE spectroscopy shows that the unoccupied MO is nearly symmetric to the plane of the light incidence in accordance with the calculated MO. The two-dimensional free electron wave function of the IPS can interact with the MO that has no node within the molecular framew...

Lately, it was found that the Au nanoclusters stabilised by poly(N-vinyl-2-pyrrolidone) [PVP; (C6H9ON)n], abbreviated to Au:PVP, can oxidise p-hydroxybenzyl alcohol selectively into the corresponding aldehyde in water without degradation. This observation indicates that Au cluster can exhibit high catalytic activity without any metal oxide supports. From previous works, it was found that the anionic Au clusters played an important role for the activation of oxygen molecule on the Au clusters. However, the catalytic activity of neutral Au clusters for the aerobic oxidation reaction of alcohol is not still investigated in detail. In order to examine the catalytic activity of neutral Au clusters, the aerobic oxidation of p-hydroxybenzyl alcohol to the corresponding aldehyde catalysed by Au6 has been investigated quantum chemically using density functional theory with the PBE0 functional. Possible reaction pathways are investigated taking account of full structure relaxation of the model systems. From the cal...

Residual chlorines, which originate from HAuCl4, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO2 (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA–PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO2 (110) surface, abbreviated as rTiO2 (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO2 surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO2 (110) surface. The calculation result...

A molecular structure of a double-decker phthalocyaninato-terbium(III) anion (TbPc2−) is optimised by density functional theory (DFT) and DFT with a dispersion interaction (DFT-D) method. The DFT-D method such as B3LYP-D and B97-D functional sets estimates phthalocyaninato (Pc) ions to be planar although B3LYP and PBE1PBE make them concave. In addition, the planar structure corresponds to an experimental X-ray crystallographic result. On the other hand, an optimised geometry of Pc2− dimer by B3LYP-D estimates the convex Pc2− structure due to the dispersion interaction between outer aromatic rings. These results suggest the importance of the dispersion interaction for the structure of TbPc2−.

Because chlorine residues, which originate from HAuCl 4 , enhance the aggregation of gold particles during the preparation processes of gold-supported catalysts, chlorine removal via hydrogen is used to prepare highly dispersed Au catalysts by the impregnation method. However, details of this phenomenon have yet to be investigated. Herein DFT calculations are performed for HCl elimination reactions from HAu n Cl model clusters ( n = 1–6). The calculation results reveal that the activation energies of these model reactions have odd–even oscillations and increase as the Au cluster size increases.

We propose an accurate scheme to evaluate the redox potential of a wide variety of transition metal complexes by adding a charge-dependent correction term for a counterion around the charged complexes, which is based on Generalized Born theory, to the solvation energy. The mean absolute error (MAE) toward experimental redox potentials of charged complexes is considerably reduced from 0.81 V (maximum error 1.22 V) to 0.22 V (maximum error 0.50 V). We found a remarkable exchange-correlation functional dependence on the results rather than the basis set ones. The combination of Wachters+f (for metal) and 6-31++G(d,p) (for other atoms) with the B3LYP functional gives the least MAE 0.15 V for the test complexes. This scheme is applicable to other solvents, and heavier transition metal complexes such as M1(CO)5(pycn) (M1 = Cr, Mo, W), M2(mnt)2 (M2 = Ni, Pd, Pt), and M3(bpy)3 (M3 = Fe, Ru, Os) with the same quality.

We have performed a reparameterization of PM6 (called rPM6) to compute open-shell species, specifically organic diradical molecules, within a framework of the spin-unrestricted semiempirical molecular orbital (SE-UMO) method. The parameters for the basic elements (hydrogen, carbon, nitrogen, and oxygen) have been optimized simultaneously using the training set consisting of 740 reference data. On the basis of the GMTKN30 database, the mean absolute error of rPM6 is decreased from 16.1 to 14.1 kcal/mol, which reassures its accuracy for ground-state properties. Applications of the spin-unrestricted rPM6 (UrPM6) method to small diradicals and relatively large polycyclic aromatic hydrocarbons have provided substantial improvement over the standard SE-UMO methods like UAM1, UPM3, and the original UPM6. The UrPM6 calculation is much less susceptible to spin contamination and, therefore, reproduces geometric parameters and adiabatic singlet-triplet energy gaps obtained by UDFT (UB3LYP and/or UBHandHLYP) at much lower computational cost.

To explore the prospect of molecules involving silicon-silicon multiple bonds as nonlinear optical molecular systems, the relationship between the structure and the second hyperpolarizabilities γ of the oligomeric systems composed of carbon and silicon π-structures is investigated using the density functional theory method. It is found that these compounds indicate intramolecular charge transfer (ICT) from the silicon units to the carbon units together with nonzero diradical characters. The γ values of these compounds are shown to be 2-13 times as large as those of the carbon analogs. Although asymmetric carbon and silicon π-systems exhibit comparable enhancement to the corresponding symmetric systems, donor-π-donor structures exhibit remarkable enhancement of γ despite of their both-end short silicon π-chain moieties (donor units). Further analysis using the odd electron and γ densities clarifies that the intermediate diradical character also contributes to the enhancement of γ. These results predict that even short π-conjugated silicone moieties can cause remarkable enhancement of γ by introducing them into π-conjugated hydrocarbon structures.

The spin-unrestricted coupled-cluster method was employed to investigate the origin of the second hyperpolarizabilities (I3) in model systems involving metal-carbon bonds with various bond lengths as a function of their diradical character (y) and charge transfer (CT). These systems exhibit unique features: (i) σ electrons give the dominant contribution to I3, (ii) the Ï€ electrons contribution to I3 is negative, (iii) when the bond length increases, I3 exhibits two positive extrema, which are associated with the CT nature and the intermediate diradical character, respectively, (iv) and one negative extremum corresponding to intermediate CT and diradical character, and (v) in the bond stretching process, the maximum I3 amplitude per σ bond is about 7 times larger than that per Ï€ bond. These features are significantly different from those observed in pure organic systems.

Atypically anisotropic and large changes in magnetic susceptibility, along with a change in crystalline shape, were observed in a CoII complex at near room temperature. This was achieved by combining oxalate molecules, acting as rotor, and a CoII ion with unquenched orbital angular momentum. A thermally controlled 90° rotation of the oxalate counter anion triggered a symmetry-breaking ferroelastic phase transition, accompanied by contraction-expansion behavior (ca. 4.5 %) along the long axis of a rod-like single crystal. The molecular rotation induced a minute variation in the coordination geometry around the CoII ion, resulting in an abrupt decrease and a remarkable increase in magnetic susceptibility along the direction perpendicular and parallel to the long axis of the crystal, respectively. Theoretical calculations suggested that such an unusual anisotropic change in magnetic susceptibility was due to a substantial reorientation of magnetic anisotropy induced by slight disruption in the ideal D3 coordination environment of the complex cation.

Bis(dipyrrinato)metal(II) and tris(dipyrrinato)metal(III) complexes have been regarded as much less useful luminophores than their boron difluoride counterparts (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, BODIPYs), especially in polar solvent. We proposed previously that dissymmetry in such metal complexes (i.e., two different dipyrrinato ligands in one molecule) improves their fluorescence quantum efficiencies. In this work, we demonstrate the universality and utility of our methodology by synthesizing eight new dissymmetric bis(dipyrrinato)zinc(II) complexes and comparing them with corresponding symmetric complexes. Single-crystal X-ray diffraction analysis, (1)H and (13)C NMR spectroscopy, and high-resolution mass spectrometry confirm the retention of dissymmetry in both solution and solid states. The dissymmetric complexes all show greater photoluminescence (PL) quantum yields (ϕPL) than the corresponding symmetric complexes, allowing red to near-infrared emissions with large pseudo-Stokes shifts. The best performance achieves a maximum PL wavelength of 671 nm, a pseudo-Stokes shift of 5400 cm(-1), and ϕPL of 0.62-0.72 in toluene (dielectric constant es = 2.4), dichloromethane (es = 9.1), acetone (es = 21.4), and ethanol (es = 24.3). The large pseudo-Stokes shift is distinctive considering BODIPYs with small Stokes shifts (∼500 cm(-1)), and the ϕPL values are higher than or comparable to those of BODIPYs fluorescing at similar wavelengths. Electrochemistry and density functional theory calculations illustrate that frontier orbital ordering in the dissymmetric complexes meets the condition for efficient PL proposed in our theory.

Formal [4+4] reaction of the unsaturated benzyl- or allylimines, which is efficiently mediated by primary amine, provides the 2,6,9-triazabicyclo[3.3.1]nonane derivatives. Variously substituted homo- and hetero-coupling products are obtained in good to excellent yields by quite a simple procedure under mild conditions: by mixing the unsaturated aldehydes with the amines at room temperature.

Recently, Hirao and coworkers have reported the synthesis of cyclopentadienyl iron complexes of sumanene. From their experimental works, it was found that the concave-bound cyclopentadienyl iron complex of sumanene was only obtained in the crystal prepared. Conversely, theoretical calculations showed the convex-bound cyclopentadienyl iron complex of sumanene was relatively stable than the concave-bound one. Thus, there is a confliction between the experimental and theoretical research results. To solve this problem, the density functional theory calculations were carried out for the several types of cyclopentadienyl iron complexes with/without AlCl ion. These calculations suggest that AlCl ion plays an important role for the stabilization of the concave-bound cyclopentadienyl iron complex of sumanene. © 2012 Wiley Periodicals, Inc.

In this article, we combine the approximate spin projection (AP) and the spin-restricted (R) calculations based on ONIOM method for a geometry optimization of large biradical systems such as polynuclear metal complexes. This two-layer quantum mechanics/quantum mechanics′ (QM/QM′) (AP/R) method treats atoms of an inner layer around the spin sources with the AP method, whereas the whole atoms including the peripheral ligands are calculated by the R method. The AP/R method is applied for the geometry optimizations of chromium(II) ions in Cr2(O2CCH3)4(OH2)2 (1) and Cr2{O2CPh(CHMe2)3}4 (2) complexes that involve quadruple CrCr bonds. The optimized structural parameters indicate that the error of the two-layer combination is smaller than 0.01 A in the optimized CrCr bond length on both complexes. The error is only 12.3% of the spin contamination error and 1.7% of the static correlation correction in the complex 1. The error of the two-layer combination in a total energy is also estimated to be lower than 1 kcal mol−1 in both complexes. © 2012 Wiley Periodicals, Inc.

We assessed the applicability and basis set dependency of the B3LYP functional to investigate magnetic interactions of Mn complexes. For the purpose, we constructed a test set consisting of 16 Mn complexes with various oxidation states and structural motifs. The B3LYP results correctly reproduced magnetism and magneto–redox correlation of the standard μ-oxo motifs with superexchange paths, while it does not work for weak magnetic complexes. We also showed that a modest basis set yields results similar to those of triple-zeta plus diffuse-and-polarization functions. This basis set is expected to be a standard basis set for investigating magnetism of manganese complexes.

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II)4O4 model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn4O4(OAc)6 and Mn3CaO4(OAc)6 using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV)4O4(OAc)6 although this model cluster consists of superexchange type of units, Mn(IV)2O2 that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O–O bond formation in the OEC.

Recently, graphene sheet is one of interesting systems to realize novel electronic properties. Especially, interaction between graphene and adsorbed oxygen molecule is very important to control electronic condition. In this paper, we employed some aromatic hydrocarbons as simple systems of graphene sheet and ab initio MO calculations were carried out to investigate inter-molecular interaction. It is found that not triplet but singlet O 2 molecule have potential of chemisorption onto graphene surface. From the calculated potential energy surface (PES) for distance between benzene and O 2 molecules, meta-stable structure is found at about 1.5 A with potential barrier. In the optimized structure of its meta-stable state, structural strain can be relaxed through bending of planer benzene ring. Its energy is estimated at 70.10 kcal/mol for benzene. We also estimated the strain effects for naphthalene and pyrene molecules as larger case of graphene and they were 80.85 and 72.45 kcal/mol, respectively.

First principle calculations of the effective exchange integrals (J) in the Heisenberg model for diradical species are presented for both symmetry-adapted multi-reference (MR) and single-reference broken-symmetry (BS) methods. The Mukherjee-type state-specific MR coupled cluster singles and doubles (MkCCSD) method with several different reference orbitals including BS natural orbitals is used to calculate the singlet–triplet energy gaps (S–T energy gap or 2J) and diradical characters for the antiaromatic molecules [a cyclopropenyl anion (CPA), b cyclobutadiene (CBD), and c cyclopentadienyl cation (CPC)], the cyclobutadiene derivatives with polar substitutents [d aminocyclobutadiene (ACBD), e formylcyclobutadiene (FCBD), and f 1-amino-2-formyl-cyclobutadiene (AFCBD)] and finally the cyclobutadine derivatives with radical substitutents [g 1,2-bis(methylene)cyclobutadiene (1,2-BMCBD) and h 1,3-bis(methylene)cyclobutadiene (1,3-BMCBD)]. For the BS methods, the spin-unrestricted Hartree–Fock based CCSD (UHF-CCSD), the CCD with the spin-unrestricted Brueckner determinant (UBD), and BS density functional theory (UDFT) computations are performed. Comparison between MkCCSD and the UHF-CCSD results indicates that spin-contamination of UHF-CCSD solutions still remains. In comparison with UHF-CCSD, the UBD results show that spin-contamination involved in BS solutions is greatly suppressed. To eliminate the spin contamination, an approximate spin-projection (AP) scheme is applied to the BS solutions. The AP procedure with the use of the expectation value of the total-spin operator corresponding to UHF-CCSD and UBD results yields good agreement with the MkCCSD results. As for the AP correction of the UDFT methods, three different computational schemes for predicting the expectation value of the total-spin operator are examined. Systematic comparisons between these methods are presented for the S–T energy gaps (2J). Implications of the present computational results have been discussed in relation to the design of magnetic oligomers and polymers.

In this paper, we investigate the current–voltage (I–V) characteristics between adjacent bases in two types of artificial metal–DNA (M–DNA) models, i.e., hydroxypyridone (=2-methyl-3-hydroxy-4-pyri...

We have investigated the through-bond exchange interactions in three non-Kekule hydrocarbon diradicals on the basis of single- and multireference coupled cluster and related broken-symmetry (BS) methods. The singlet–triplet energy gap (S-T gap) and diradical characters for these species are evaluated. It is found that the spin contamination involved in the BS solutions is non-negligible and the approximate spin-projection method greatly improves the usual BS solutions. As for Mukherjee’s state-specific multireference coupled cluster (MkMRCC) computations, the size-consistent correction with the UHF localized natural orbitals (ULO) is useful to obtain the qualitatively correct 2J values.

The electronic structures of ground and excited states of [Rh2(CH3COO)4(H2O)2] in aqueous solution are studied using a density functional theory (DFT) with a time-dependent (TD) method. Up to now, several theoretical assignments and explanations of its excitation characters have been reported based on the absorption spectra. In this study, we reinvestigate its absorption spectrum by the TD-DFT approach with the polarisable continuum model in order to clarify the excitation characters of the complex, especially in the aqueous solution well.

Herein, we have theoretically investigated the optical properties of the two Au8 complexes with different shapes of the Au8 cluster core. From the results of TD-DFT and DFT calculations, it was fou...

The electronic configurations and the nature of chemical bonds of the classical lantern-type dinuclear rhodium(II) tetraacetato complexes [Rh2(CH3COO)4(L)2] (L = H2O, free) have been carefully inve...

In the present study, the concerted and stepwise reaction mechanisms for 1,3-dipole cycloaddition of ozone with ethylene (1) and acrylonitrile (2) are investigated. The stationary points are optimized by using four hybrid R(U)DFT methods. A geometry optimization method based on an approximate spin projection (AP-opt method) is applied to eliminate a spin contamination from the broken-symmetry (BS) solution. The AP-opt method reveals that a diradical intermediate for the stepwise pathway is spurious due to the spin contamination. The revised reaction profile with no diradical intermediate supports the stereospecificity. On the basis of the experimental data, the RCCSD(T) method outperforms AP-UCCSD(T), AP-UBD(T), and MkMRCCSD(4e,4o) for the systems, indicating that the RCCSD(T) method can describe the diradical character of ozone within a framework of single reference wave function. The subsequent single point energy calculations show that the highly synchronous transition state is much more favorable than the asynchronous one for 1. In the case of 2, there is not much difference between two transition states because of its asymmetric structure and charge separations in the transition states.

The barrier height for automerization of cyclobutadiene (CBD) is examined by broken-symmetry (BS) HF, post-HF, and density functional methods. To remove the spin contamination error, we use an approximate spin projection (AP) method. Comparison between the results of BS methods and those of the localized UHF natural orbital (ULO) based Mukherjee’s state-specific MRCC (ULO-MkCC) method shows that spin-unrestricted Brueckner doubles including perturbative triple excitations (UBD(T)) with the AP correction performs well within a framework of single reference approaches. On the other hand, UDFT methods except for UBLYP and UM06-2X give negative barrier heights, which are not consistent with the both the experimental and MRCC results.

Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the dπ-pπ bond in high-valent (HV) Mn(X)O systems and the dπ-pπ-dπ bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: •Mn(IV)O• and •MnOMn•; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Mukherjee's type of multireference coupled-cluster (MkMRCC), approximate spin-projected spin-unrestricted CC (APUCC), and AP spin-unrestricted Brueckner's (APUBD) methods were applied to didehydronated ethylene, allyl cation, cis-butadiene, and naphthalene. The focus is on descriptions of magnetic properties for these diradical species such as S-T gaps and diradical characters. Several types of orbital sets were examined as reference orbitals for MkMRCC calculations, and it was found that the change of orbital sets do not give significant impacts on computational results for these species. Comparison of MkMRCC results with APUCC and APUBD results show that these two types of methods yield similar results. These results show that the quantum spin corrected UCC and UBD methods can effectively account for both nondynamical and dynamical correlation effects that are covered by the MkMRCC methods. It was also shown that appropriately parameterized hybrid density functional theory (DFT) with AP corrections (APU...

Broken-symmetry (BS) and approximate spin-projected (AP) BS hybrid density functional theory (DFT) calculations were performed to elucidate possible mechanisms of hydroxylation reactions of methane and alkanes with soluble methane monooxygenase (sMMO) and related metalloenzymes. The BS HDFT (UB3LYP) method was employed to elucidate electronic and spin structures of the key intermediate “Q” and to locate transition structures for hydroxylation reactions in the lowest-spin (LS) singlet and the highest-spin (HS) states of sMMO. The spin density populations and chemical indices obtained by the BS B3LYP calculations were found to be consistent with orbital interaction models for hydroxylation with MMO. However these indices in turn indicated significant spin contamination errors in the BS LS solution. The elimination of the errors with the AP procedure indeed reduced the barrier height for the recombination step of alkyl and hydroxyl radicals in the pure LS singlet state, leading to a rebound process. Then present computational results indicated that hydroxylation reactions proceed through the continuous diradical (diradicaloid) mechanism without discreet free radical fragments in the pure LS singlet state. The computational results are, respectively, compatible with local singlet (SD) and local triplet (TD) diradical mechanisms for hydroxylation in the LS and HS states; those were already applied to P450 successfully. Thus magnetic (exchange) coupling modes (LS and HS) in MMO, P450 and related metalloenzymes are directly related to local SD and TD mechanisms for hydroxylation, indicating the correspondence between the magnetic coupling mode and the radical reaction mechanism. These theoretical results enable us to examine recent BS hybrid DFT computational results for hydroxylation reactions with sMMO by several groups. Implications of the present theoretical and computational results are also discussed in relation to several experimental aspects of hydroxylation reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The chemical species of solvent molecule (WB) between two manganese ions in the active site of arginase is investigated based on the structural and magnetic property. The calculated magnetic coupling constant (J ab ) values at UBH&HLYP level of theory are − 1.9 cm− 1 for WB = H2O and − 5.7 cm− 1 for WB = OH− , respectively. In comparison with the experimental value (J ab = ∼ − 2.0 cm− 1), H2O is the most likely candidate to WB in the initial structure. Natural orbital analyses reveal that WB plays an important role for keeping the geometry of the Mn–O(WB)–Mn core rigid rather than mediating superexchange interaction. The result indicates that WB does not have to shift to one Mn ion as an initiation of the hydrolysis reaction.

We investigate for the first time the electronic structure of the biomimetic diiron complex 1 [Fe(IV)2(μ-O)2(5-Me3-TPA)2]4+ (5-Me3-TPA = tris(5-methyl-2-pyridylmethyl)amine) using broken-symmetry hybrid density functional theory method. According to previous studies, the Fe(IV)2(μ-O)2 core of intermediate Q of soluble methane monooxygenase shows antiferromagnetic coupling between local S = 2 states at each iron site. In contrast to Q, each local Fe(IV) ion of the synthetic complex does not have an S = 2 but has an S = 1 state based on the calculated geometric parameters and energetic stability. Our calculation supports the experimental result.

In this article, we summarize a recent progress in our spin-projected energy derivative method for broken-symmetry (BS) solutions of singlet biradical systems. Because, the spin projection procedure that is based on the approximate spin projection (AP) method does not involve a spin contamination error, it improves the accuracy of an optimized geometry and a calculated vibrational frequency for the singlet biradical molecule with the low cost of computation. To examine the spin contamination error in the BS energy derivatives and to demonstrate the efficiency of the AP method, we carry out the geometry optimization and the vibrational calculation of singlet CR2 (R = H, F, and CH3) molecules that are the typical singlet biradical systems. The results indicate that the error in the singlet CH2 and C(CH3)2 molecules are serious, however, the error in the singlet CF2 molecule, which has electron withdrawing fluorine atoms is negligible. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

In this article, we performed spin-unrestricted Hartree Fock (UHF) based coupled-cluster (CC) and Bruckner CC double calculations of methylene, fluorine, HHeH diradical, and ozone. Mukherjee-type (Mk) Multireference coupled-cluster (MRCC) calculations of these species were also conducted by the use of UHF natural orbital (UNO) and UHF localized natural orbitals (ULO) as reference orbitals to elucidate size-consistent error. Comparing calculated results of spin-unrestricted Hartree-Fock based coupled-cluster (UCC) and spin-unrestricted Bruckner-Double (UBD) with those of MkMRCC, it was found that spin-contamination errors of UCC and UBD solutions still remained, though this type of errors for these solutions was considered to be negligible in general. The comparison also revealed scope and applicability of approximate spin-projection (AP) scheme for UCC and UBD, indicating that the AP procedure corrected spin-contamination errors to yield good agreement with MRCC in energy. We also performed pure- and hybrid- density functional theory (DFT) calculations of the same systems, and those calculations provided reasonable singlet-triplet energy gaps, which were qualitatively good agreement with those of MRCC, AP-UCC, and AP-UBD calculations. Implications of the computational results are discussed in relation to applicability of hybrid DFT to large superexchange coupled systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010