Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap ΔEs-T= Es - ET is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shownthat when the radical centers R(R=H2C- ,H2N - or HN'- ) are twisted sufficiently far out ofconjugation with the benzene ring, ΔEs-T tends to zero or is negative, i.e. ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho-or para-phenylene coupling unit.     

Energetics of dioxygen binding into graphene patches with various sizes and shapes

Density functional theory (DFT) calculations were employed to investigate the electronic properties of an H-atom terminated graphene patch (hydrographene) smaller than a rhombic C96H26 structure with zigzag edges. Depending on shapes and sizes of hydrographenes, some hydrographenes have the triplet ground state where unpaired electrons are localized on their zigzag edges. The stability of the triplet spin state is diminished, decreasing the hydrographene sizes. The existence of the localized spin densities allows triplet dioxgen to bind into a hydrographene. According to the DFT calculations, the energetics of the dioxygen bindings is negatively influenced by downsizing hydrographenes, as well as depends on their shapes. The size-and shape-dependences of the dioxygen bindings reflect from the stability of the triplet state of a hydrographene, because its localized unpaired electrons can be utilized to be attached to an unpaired electron of triplet dioxygen.     

Quantum Chemical Study on the Structural Characteristics and Stability of AlS_n~± Clusters

The geometric configurations and electronic structures of AlSn ±(n = 1~10) clusters were studied by the B3LYP(DFT) method at the 6-311G** level. The changing rules of the ground state structure features, charge transfer and bonding characteristics of the aluminum-sulfur doped clusters were discussed in detail. The ground states of AlSn ±(n 2) are all AlcoreSshell planar or solid double ring structures formed by inserting one Al atom to the Sm and Sn-m(mn) rings at the same time. Their molecular orbitals are mainly composed of Al s- and p-states mixed with S p-states. Finally, the stabilities of AlSn ± clusters have been obtained by analyzing the energy of the ground state structures.     

Terahertz time-domain spectroscopic investigation on quinones

Well resolved far-infrared spectra of 1,4-benzoquinone, 1,4-naphthoquinone, and 9, 10-anthraquinonme in polycrystalline form have been measured with terahertz time domain spectroscopy at room tem- perature. The characterizations of power absorption and index of refraction in the frequency range 0.3 -2.0 THz are presented. Theoretical calculation is applied to assist the analysis and assignment of individual THz absorption spectra of the p-quinones with semiempirical AM1, Hartree-Fock (HF), and density functional theory (DFT) method. Observed THz responses are assigned to the translational and torsional vibrations of p-quinone dimer held together by weak hydrogen bonds.     

Investigation of the Spin Crossover Complex [Fe（bapbpy）（NCS）_2 ] in Its Low-spin and Highspin State by DFT/TD-DFT/BS-DFT Calculations

The spin crossover（SCO） compound [Fe（bapbpy）（NCS）2],where bapbpy contains two fused N,N-bis（2-pyridyl）amines,has been studied by DFT/TD-DFT/BS-DFT methods.Several density functionals and basis sets were used in the calculation to obtain optimized geometries of the compound in the low-（LS） and high-spin（HS） states.The vibrational modes and IR spectra,spin splitting energies,excited states and UV/Vis absorption spectra were obtained.The structural parameters of the calculated isolated complex are in good agreement with the X-ray data.We investigate three dimers of [Fe（bapbpy）（NCS）2] complex for their magnetic properties.It has been found that the complex（1,3） has ferromagnetic character while the others are antiferromagnetic in nature by using a broken symmetry approach in the DFT framework（BS-DFT） with support from the coupling constant values（J） and spin density plots.     

Theoretical Study on the Dark Oxidation Reaction Mechanism of Ethers

1 INTRODUCTION Ethers are a kind of organic compounds that are easily oxidized under the conditions of lacking light and any additional excitement. According to dif- ferent mechanisms, the oxidation reactions could be classified into two types: photooxidation reaction and dark oxidation reaction. The former is the reaction with excited state oxygen molecule (singlet state), while the latter is the reaction with ground state oxygen molecule (triple state) without illuminance or any exciter…     

Theoretical and experimental investigations of the enantioselective epoxidation of olefins catalyzed by manganese complexes

The enantioselective epoxidation of olefin by Mn^II（R,R-PMCP）(OTf)2, H₂O₂ and H₂SO₄ was explored by DFT calculations and experiments. Theoretical results suggest that [MnV（O）(R,R-PMCP)（SO4）]+species with a triplet ground spin state serves as the active species for the olefin epoxidation. It can be generated by the H₂SO₄ assisted O-O heterolysis of Mn^III（OOH） species. Mn^III-persulfate is also involve...     

Ab initio investigation on the structures and spectra of the firefly luciferin

The ground state (S0) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (Tv) of three low-lying states (S1, S2, and S3) were calculated by TD-DFT B3LYP//CASSCF method. The S1 geometry was optimized by CASSCF method. Its Tv and the transition energy (Te) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S1 state Tv values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated Tv values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired.     

Effect of Fluorination on Aggregate Structure of Perylene Diimide

The effect of fluorination on the aggregate structure of a novel fluorinated perylene diimide, N, N‘-diperfluorophenyl-3, 4, 9, 10-perylenetetracarboxylic diimide 1, was investigated by UV-Vis absorptions and the conformation simulations from AM1 semi-empirical quantum mechanics modeling. The results showed that in the solid film 1 molecules stacked with the perfluorinated phenyl groups straightly over or below the perylene cores of the adjacent 1 molecules.     

Synthesis and applications of photochromic fulgides in optical information storages

A pyrryl substituted photocbromic fulgide, (1-p-methoxyphenyl-2-methyl-5-phenyl)-3-pyrryl-ethylidene (isopropylidene) succinic anhydride, was synthesized. Its chemical structure was identified by IR, 1H NMR MS and elemental analysis. The title compound was used to prepare the optical disks by the spin coating method and the vacuum evaporation method. The results of the static state examination show that the optical disks prepared with this kind of molecules possess excellent thermal stability and fatigue resistance, i.e. over 450 writing-erasing recycles, the relative contrast between writing state and erasing state remains at 40%.     

构象对同位乙烯双自由基体系基态自旋多重度及其稳定性影响的理论研究

用DFT,CASSCF和QCISD(T)方法6-31G^*基组计算了构象对同位二取代乙烯双自由基体系基态自旋多重度及其稳定性的影响.结果表明,用DFT或CASSCF方法计算的单、三重态的能量差随自由基与乙烯间的二面角增加成不规则变化;用QCISD(T)方法计算的单、三重态的能量差随二面角的增加而逐渐降低,并呈规律性变化,说明QCISD(T)方法用于计算分子的磁性是可信的.对于同位二取代乙烯双自由基体系,无论双自由基旋转,还是单自由基旋转,高自旋基态稳定性随自由基与乙烯间二面角的增加而降低,只是降低的幅度不同,当二面角接近90°时,同位乙烯由具有平面或近似平面构象时强的铁磁耦合单元变成接近垂直平面构象时弱的反铁磁耦合单元或弱的铁磁耦合单元.     

Design of high-spin molecules incorporating charged plus neutral spin centers

A series of diradicals comprised of m-phenylene exchange coupling units and spin-bearing centers of the same (homo-spin) and different (hetero-spin) types were compared, using the semiempirical AM1-CI molecular orbital method. Two meta-coupled neutral (or charged) hetero-spin centers result in a high-spin ground state, while coupling of one neutral and one charged spin center gives rise to a low-spin ground state in the cases studied. The latter result is ascribed to the large splitting of partially occupied molecular orbitals by substitution, leading to dominance of purely ionic resonance structures in the singlet states of the monocharged “hetero-spin” cases. Effects of substitution and HOMO–LUMO splitting may be evaluated by computational methods to identify systems where resonance effects may override spin parity effects. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:161–167, 1998     

A semiempirical investigation of interelectronic exchange coupling in bisected poly(1,4-phenylene) polycation model systems

AM 1 SCF -MO -CI computations find the bisected biphenyl dication to have nearly degenerate triplet and singlet states, with the lowest-energy state being a quininoid singlet planar dication. The bisected perchlorobiphenyl dication favors a triplet ground state by a small amount (0.4-1.9 kcal/mol), in qualitative agreement with recent experimental findings and with theoretical expectations that such an orthogonal open-shell pi-system should exhibit ferromagnetic exchange coupling. The higher oligomeric bisected para-linked phenylenes polycations do not show an appreciable computational preference for a high-spin multiplicity ground state either with or without perchlorine substitution. Chlorine substitution para to the 1,1′-linkage may lend a unique stabilization to the biphenyl system, which is not available in higher oligomeric analogs of poly(1,4-phenylene)s. The small magnitude of ferromagnetic exchange in these systems suggests that small geometric or substituent effects may confound experimental efforts to design polymeric ferromagnetic materials by this strategy. © 1992 John Wiley & Sons, Inc.     

以杂环为铁磁耦合单元的双中心双自由基高自旋有机分子的理论设计

利用－·N－S－为自旋中心（SC），苯为端基（EG），苯、吡啶、哒嗪、嘧啶 、吡嗪、三嗪为耦合单元（FC），设计三种不同排列方式的新型稳定高自旋分子。 由于自旋密度在杂环（FC）和－·N－S－（SC）组成的体系中自由基双中心的部分 离域，导致－·N-S－自由基的特殊稳定性。三种不同的排列方式中，其三重态的 稳定性随主要SC（－N－）原子间距离的增大而降低。从三个系统八个体系三重态 的稳定性来看，FC上的杂原子位于取代基的间位能提高体系的铁磁耦合作用，而位 于邻位和对位则不利于铁磁耦合作用。     

Theoretical Design of High-spin Organic Molecules with-. N-S-as a Spin-containing Fragment and Heterocycle as End Groups

Novel stable high-spin molecules possessing three different arranging fashions were designed with -(.) N-S-as a spin-containing (SC) fragment, an aromatic group, such as benzene (1), pyridine (2), pyridazine (3), pyrimidine (4), pyrazine (5) or triazine (6) as end groups (EG), and phenyl as a ferromagnetic coupling (FC) unit.The effects of different EG on the spin multiplicities of the ground states and their stabilities were investigated by means of the AM1-CI approach. All the investigated molecules corresponded to the FC and possessed high-spin ground states. The spin on the two atoms of the SC fragment was not in agreement with the delocalization results in the specific stability of -(.) N-S-. In those molecules, the stabilities of the triplet states decreased when the distance between the atoms of central SC fragments (-N-) increased. The stabilities of the triplet states of compounds 1a-n, 1b-n and 1c-n, with heterocycles as EG were higher than those of the triplet states of those compounds with phenyl as EG. Furthermore, the stabilities of the triplet states of the compounds with pyrimidine and triazine as EG were higher than those with pyridine, pyridazine or pyrazine as EG.     

Theoretical Design of High－spin Organic Molecules with Heterocycles as Ferromagnetic Coupling Units

IntroductionThe design and the syntheses of organicmolecules with very high- spin ground states havebeen a topic of great interest[1— 5] .One of rationalapproaches to designing high- spin molecules,which has been proposed and studied by severalgroups[6,7] ,consists in conceptually dividing themolecules into two components,i.e.,a spin- con-taining( SC) fragment which provides the unpairedelectron and a ferromagnetic coupling ( FC) unitwhich is connected with radical centers ferromag-netically…     

Molecular design for high-spin molecules in view of vibronic couplings

A high-spin ground state is possible if a molecule has degenerate or pseudo-degenerate frontier orbitals. Since strong vibronic couplings, or electron-vibration interactions give rise to reduce the degeneracy or pseudo degeneracy, a lower-spin state is the ground state in such a molecule. Therefore small vibronic couplings are desirable for designing molecules with a high-spin ground state. Vibronic coupling constants of derivatives of m-phenylene diamine are evaluated. The calculated results are analyzed based on vibronic coupling density which enables us to control the vibronic coupling constants. Based on the vibronic coupling density analysis, we succeed in recovering the high-spin ground state from the closed-shell singlet ground state of a methoxy derivative of m-phenylene diamine by introducing an appropriate substituent.     

A computational study of organic polyradicals stabilized by chromium atoms

Density functional theory has been used to investigate the properties of organic high spin molecules. The M05/cc-pVDZ calculations predict a septet ground state for the 2,3,6,7,10,11-hexahydro-1,4,5,8,9,12-hexaoxocoronene-2,3,6,7,10,11-hexayl radical (coronene-6O). The computations show further that the formation of intermolecular carbon-carbon bonds yields a singlet ground state for the dimer rather than a possible tridectet state as expected from the monomer's multiplicity. A benzene molecule placed between coronene-6O molecules leads to the desired high-spin cluster, but the overall stability of the cluster is low. A chromium atom inserted between two peripheral C(6) rings of coronene-6O yields a sandwich structure with the expected tridectet ground state and a binding energy which is 15 times larger than the corresponding tridectet dimer stabilized by a benzene molecule. The presented DFT calculations suggest that a chromium atom can effectively link organic polyradicals to larger magnetic units.