Analysis and interpretation of metal-radical coupling in a series of square planar nickel complexes: correlated Ab initio and density functional investigation of [Ni(L(ISQ))(2)] (L(ISQ)=3,5-di-tert-butyl-o-diiminobenzosemiquinonate(1-))

This paper reports a detailed theoretical study of the interaction between a central low-spin d(8) nickel ion and two N,N-coordinating diiminobenzosemiquinonate(1-) ligands in a square planar arrangement. Such complexes have recently attracted much attention due to their unusual bonding patterns, structures, optical, and magnetic properties. Geometry optimizations using various levels of density functional theory (DFT) result in excellent agreement with the experimentally determined structure and in particular reproduce the quinoidal distortions in the aromatic rings well. A detailed analysis of the orbital structure reveals that the complex features essentially two strongly interacting ligand radicals which interact with each other via an efficient superexchange mechanism that is mediated by a back-bonding interaction to the central metal. An analysis of the broken symmetry DFT wave function is presented and a new index for the diradical character is proposed which shows that [Ni(L(ISQ))(2)] has a diradical character of approximately 77%. These results are in full agreement with elaborate multireference post-Hartree-Fock ab initio calculations for [Ni(L(ISQ))(2)] using the difference dedicated configuration interaction (DDCI) method as well as second-order multireference M?ller-Plesset (MR-MP2) theory, which give diradical characters of 65-80%. On the basis of these calculations our best estimate for the singlet-triplet gap in this system is 3096 cm(-)(1). This very large value results from an efficient mixing of the ionic configurations into the mainly singlet diradical ground state which is feasible because the semiquinonate SOMOs are delocalized and, therefore, have moderate on-site Coulomb repulsion parameters. As pointed out in the discussion, this represents an interesting difference to the case of magnetically interacting transition metal ions which typically show much smaller magnetic exchange couplings.     

Second hyperpolarizability (gamma) of singlet diradical system: dependence of gamma on the diradical character

The dependence of the second hyperpolarizability (gamma) on the diradical character (y) for singlet diradical systems is investigated using a model compound, the p-quinodimethane (PQM) molecule with different both-end carbon-carbon (C-C) bond lengths, by several ab initio molecular orbital and density functional theory methods. The diradical character based on UHF calculations indicates that at equilibrium geometry PQM is in a singlet ground state and primarily exhibits a quinoid structure, whereas the diradical character increases when increasing both-end C-C bond lengths. At the highest level of approximation, that is, using the UCCSD(T) method with the 6-31G+diffuse p (zeta = 0.0523) basis set, the longitudinal static gamma of PQM presents a maximum value for intermediate diradical character (y approximately 0.5) while the gamma values are larger for intermediate and large diradical character (y approximately 0.5-0.7) than for small diradical character (y < 0.2). This feature suggests that the gamma values of singlet diradical systems in the intermediate and somewhat strong correlation regimes are significantly enhanced as compared to those in the weak correlation regime. These results are substantiated by a complementary study of the variation in gamma upon twisted ethylene.     

Electronic structures and molecular properties of chalcogen nitrides Se2N2 and SeSN2

The electronic structures and molecular properties of S2N2 as well as the currently unknown chalcogen nitrides Se2N2 and SeSN2 have been studied using various ab initio and density functional methods. All molecules share a qualitatively similar electronic structure and can be primarily described as 2pi-electron aromatics having minor singlet diradical character of 6-8% that can be attributed solely to the nitrogen atoms. This diradical character is manifested in the prediction of their molecular properties, in which coupled cluster and multiconfigurational approaches, as well as density functional methods, show the best performance. The conventional ab initio methods RHF and MP2 completely fail to describe these systems. Predictions for the vibrational frequencies, IR intensities, Raman activities, and 14N, 15N, and 77Se chemical shifts, as well as singlet excitation energies of Se2N2 and SeSN2, have been made. The computed high-level spectroscopic data will be of considerable value in future efforts aimed at the preparation of the conducting polymers (SeN)x and (SeNSN)x.     

Theoretical evidence for the singlet diradical character of square planar nickel complexes containing two o-semiquinonato type ligands

Density functional theory and complete active space self-consistent field computations are applied to elucidate the singlet diradical character of square planar, diamagnetic nickel complexes that contain two bidentate ligands derived from o-catecholates, o-phenylenediamines, o-benzodithiolates, o-aminophenolates, and o-aminothiophenolates. In the density functional framework, the singlet diradical character is discussed within the broken symmetry formalism. The singlet-triplet energy gaps, the energy gained from symmetry breaking, the spin distribution in the lowest triplet state, and the form of the magnetic orbitals are applied as indicators for the singlet diradical character. Moreover, a new index for the diradical character is proposed that is based on symmetry breaking. All symmetry breaking criteria show that the complexes obtained from o-catecholates and o-benzodithiolates have the largest and the smallest singlet diradical character, respectively. The singlet diradical character should be intermediate for the complexes derived from o-phenylenediamines, o-aminophenolates, and o-aminothiophenolates. The diradical character of all complexes suggests the presence of Ni(II) central atoms. This is also indicated by the d-populations computed by means of the natural population analysis.     

Electronic structure of the alkyne-bridged dicobalt hexacarbonyl complex Co(2) micro-C(2)H(2) (CO)(6): evidence for singlet diradical character and implications for metal-metal bonding

A series of ab initio calculations are presented on the alkyne-bridged dicobalt hexacarbonyl cluster Co2 micro-C2H2 (CO)6, indicating that this compound has substantial multireference character, which we interpret as evidence of singlet diradical behavior. As a result, standard theoretical methods such as restricted Hartree-Fock (RHF) or Kohn-Sham (RKS) density functional theory cannot properly describe this compound. We have therefore used complete active space (CAS) methods to explore the bonding in and spectroscopic properties of Co2 micro-C2H2 (CO)6. CAS methods identify significant population of a Co-Co antibonding orbital, along with Co-pi* back-bonding, and a relatively large singlet-triplet energy splitting. Analysis of the electron density and related quantities, such as energy densities and atomic overlaps, indicates a small but significant amount of covalent bonding between cobalt centers.     

8-羟基喹啉铍及其衍生物电子光谱性质的含时密度泛函理论研究

运用密度泛函理论(DFT)B3LYP方法和abinitioHF单激发组态相互作用(CIS)法分别优化了有机金属配合物8-羟基喹啉铍(BeQ₂)及其3种衍生物分子的基态及最低激发单重态几何结构.系统分析了分子结构、前线分子轨道特征和能级分布规律以探索电子跃迁机理.应用含时密度泛函理论(TD-DFT)计算分子的电子光谱,揭示了BeQ₂及其衍生物的发光源于配体中π→π^*电子跃迁,指出通过配体修饰可以有效地影响配合物前线分子轨道分布,调整发光波段,并有效提高电荷转移量.     

Theoretical study of some bis-verdazyl diradicals: singlet–triplet energy gap

The bis-verdazyl diradical (BVD) system is closely examined by using the multiconfiguration wavefunctions as well as the density functional theory (DFT). The totally symmetric singlet ground state turns out to have strong multiconfiguration character at all levels of theory. The singlet ground state takes on the planar structure while the most stable triplet state corresponds to the twisted form. The MCSCF+MCQDPT2 calculations are shown to be sufficient to predict the singlet–triplet energy gap which is insensitive to the electronic characters of the ring substituents.     

Copper corroles: the question of noninnocence

In this paper, the results are presented from a comparative study of the electronic and geometric structure of copper corroles by means of either density functional theory (DFT, using both pure and hybrid functionals) and multiconfigurational ab initio methods, starting from either a complete active space (CASSCF) or restricted active space (RASSCF) reference wave function and including dynamic correlation by means of second-order perturbation theory (CASPT2/RASPT2). DFT geometry optimizations were performed for the lowest singlet and triplet states of copper corrole, both unsubstituted and meso-substituted with three phenyl groups. The effect of saddling on the electronic structure was investigated by comparing the results obtained for planar (C(2v)) and saddled (C(2)) structures. With DFT, the origin of the saddling distortion is found to be dependent on the applied functional: covalent Cu 3d-corrole π interactions with pure functionals (BP86, OLYP), antiferromagnetic exchange coupling between an electron in the corrolate (C(2)) b type π orbital, and an unpaired Cu(II) 3d electron with hybrid functionals (B3LYP, PBE0). The CASPT2 results essentially confirm the suggestion from the hybrid functionals that copper corroles are noninnocent, although the contribution of diradical character to the copper-corrole bond is found to be limited to 50% or less. The lowest triplet state is calculated at 0-10 kcal/mol and conform with the experimental observation (variable temperature NMR) that this state should be thermally accessible.     

光诱导分步分态的苯型脱芳构化反应的理论研究

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)研究了一类重要的含有杂原子的光催化苯型脱芳反应. 研究结果表明, 该脱芳反应是一个分步分态的[4+2]环加成反应, 即前一步脱芳发生在三重态, 后一步脱芳发生在单重态基态. 其中, 苯乙酮基团可被看作为光的接收装置, 吸光后使得体系从单重态经内转换和系间窜越无能垒到达三重态, 并在三重态完成质子转移生成具有反应活性的双烯, 进而引发后续的脱芳反应. 更重要的是, 通过构建势能面发现该反应具有高度的立体选择性, 与实验结果完全相符.     

Electronic and photophysical properties of selected organic boron-containing molecules: Insight into effects of heteroatom substitution and aggregation

The density functional theory (DFT) and time-dependent DFT methods were used to investigate the electronic and optoelectronic properties of several main group atom-doped polycyclic aromatic hydrocarbons, such as oxygen-substituted PHO1 and PHO2, and sulfur-substituted PHS1 and PHS2. The ground-state structures of these molecules generally have an open-shell singlet configuration with a certain diradical character. In comparison with PHO1 and PHO2, PHS1 and PHS2 own larger diradical character indices due to their increased anti-aromaticity. Although the substitution of sulfur for the peripheral oxygen has a significant effect on the molecular geometry, the adiabatic excitation energy levels of the corresponding low-lying excited states of these molecules are less changed. Calculations reveal that here the intersystem crossing (ISC) and reverse intersystem crossing processes in CH₂Cl₂ mainly occur between the S₁ and T₂ states, and the cis molecules PHO2 and PHS2 have better charge transportation performance. Furthermore, the electronic and photophysical properties of these B-containing molecules are predicted to be tuned by the peripheral atom substitution and the structural and aggregation changes.     

Structural, electronic, and optical properties of phosphole-containing pi-conjugated oligomers for light-emitting diodes

The purpose of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of phosphole derivatives, including 2,5-diphenylthiooxophosphole (2a), 2-phenyl-5-biphenylthiooxophosphole (3a), 2-phenyl-5-stilbenylthiooxophosphole (4a), 2,5-dithienylthiooxophosphole (2b), 2-thienyl-5-biphenylthiooxophosphole (3b), 2-thienyl-5-stilbenylthiooxophosphole (4b), and dibenzophosphole 1. These thiooxophospholes show great potential for application in OLEDs as efficient red emitters due to the tuning of the optical and electronic properties by the use of various substituents at the 2,5-positions of the phosphole ring. The geometric and electronic structures of the oligomers in the ground state were investigated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states were optimized with ab initio CIS. To assign the absorption and emission peaks observed in the experiment, we computed the energies of the lowest singlet excited states with time-dependent DFT (TD-DFT). All DFT calculations were performed using the B3LYP functional and the 6-31G (d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, and electron affinities for the phosphole derivatives are significantly affected by varying the phosphole ring substituents at the 2,5-positions, which favor the hole and electron injection into OLEDs. The absorption and emission spectra exhibit red shifts to some extent [the absorption spectra: 339.63 (1)<358.65 (2a)<373.77 (3a)<443.89 nm (4a) and 403.03 (3b)<449.11 (2b)<460.19 nm (4b); the emission spectra: 418.42 (1)<513.62 (2a)<556.51 (3a)<642.59 nm (4a) and 568.31 (2b)<631.11 (3b)<647.35 nm (4b)] and the Stokes shifts are unexpectedly large ranging from 78 to 228 nm resulting from a more planar conformation of the excited state for the phosphole derivatives.     

Effects of ethynyl substituents on the electronic structure of cyclobutadiene

The effects of ethynyl substitution on the electronic structure of cyclobutadiene are investigated in this work. Ethynyl substituted cyclobutadienes may be involved in Bergman cyclization reactions and are possible intermediates in the formation of fullerenes and graphitic sheets. Prediction of the electronic structure of cyclobutadiene is challenging for single-reference ab initio methods because of Jahn-Teller distortions and the diradical character of the singlet state. The equation-of-motion spin-flip coupled-cluster with single and double excitations (EOM-SF-CCSD) method accurately describes diradical states and is used to determine vertical and adiabatic singlet-triplet energy splittings in the substituted cyclobutadienes. The adiabatic singlet-triplet gaps decrease upon substituent addition, but the singlet states remain lower in energy. However, the results are affected by spin-contamination of the reference state and deteriorate when an unrestricted HF reference is employed. Additional insights in the electronic structure of cyclobutadienes are obtained by analyzing natural charges and spin densities. The substituents pull the charge out of the cyclobutadiene ring; however, the natural charges and spin densities are found to be nearly independent of the geometry and spin state.     

B(C2H5)2q及其衍生物电子光谱性质的密度泛函理论研究

采用密度泛函理论(DFT)B3LYP、abinitioHF和单激发组态相互作用(CIS)等方法分别优化了有机配合物B(C2H5)2q及其衍生物的基态及最低激发单重态几何结构.用含时密度泛函理论(TD-DFT)对B(C2H5)2q及其衍生物的电子光谱进行了研究.发现该类物质是配体发光配合物,其发光源于8-羟基喹啉配体内π*     

Identifying of charge-transfer transitions and reactive centers in M(diimine)(dithiolate) complexes by DFT techniques

This review is focused on theoretical aspects of mixed diimine–dithiolate complexes by means of DFT and TD-DFT methods. Thus, the geometry, the character of charge-transfer transitions and excited states in a series of M(diimine)(dithiolate), where M = Ni, Pd and Pt, is examined by DFT and TD-DFT techniques combined with polarized continuum model. The theoretical calculations reveal not only the role of the ligands – namely diimine and dithiolato and their substituents – but also the role of the metal in the excited triplet and singlet states and as a consequence in the properties of these complexes (electronic and photophysics) and their potential use as photosensitizers, NLO materials, light energy conversion materials and biological agents. The calculated energies of the lowest triplet and singlet state in all these complexes are in good agreement with absorption spectra and luminescence studies—where they are available. The contribution of the metal in the chemical and photophysics properties of this class of compounds is also demonstrated by two indices derived by DFT techniques: NICS (for chemical) and Fukui functions (for chemical and photophysical properties). The former acts as a meter of the delocalization of these molecules whereas the latter identifies the reactive centres of the molecule. All the theoretical results are in accordance with the experimental ones—geometrical structures, absorption, luminescence and ¹H NMR spectra as well as products of given reactions, indicating the applicability of the DFT and TD-DFT techniques in examining the properties of metal coordinated complexes especially in a series of the same class of compounds.     

Theoretical investigation of the stereoselective stepwise cope rearrangement of a 3-vinylmethylenecyclobutane

The potential energy surface of the rearrangement of 3-vinylmethylenecyclobutane to 4-methylenecyclohexene has been studied computationally using density functional theory (B3LYP) and complete active space ab initio methods (CASSCF and CASPT2). The parent reaction is nonconcerted and occurs through several parallel diradical pathways. Transition structures and diradical intermediates are highly comparable in energy, with no deep potential energy well on the potential energy surface. In the substituted system, stereoelectronic effects of the trialkylsiloxy group regulate torquoselectivity in the bond-breaking processes and this, combined with low barriers to cyclization, leads to a stepwise Cope rearrangement that is, nevertheless, stereoselective.     

取代对N,N-二甲基苯乙烯基蒽的含时密度泛函研究

运用B3LYP方法和HF单激发态相互作用（CIS）方法分别优化了2-对N,N-二甲基苯乙烯基蒽（2-pNC）和9-对N,N-二甲基苯乙烯基蒽（9-pNC）分子的基态及最低激发单重态几何结构.系统分析了前线分子轨道特征,并探索了电子跃迁机理,应用含时密度泛函理论计算了分子的电子光谱.     

The fulvenediyls and related biradicals: Molecular and electronic structure

The structures, stabilities, and electronic properties of the nine fulvenediyls have been investigated and compared to the isomeric benzynes using density functional theory (DFT) and ab initio multireference configuration interaction methods (MRCI). Given the significant biradical character of several singlet fulvenediyls, the BLYP method reproduces the relative energies of these systems rather accurately. In contrast, some triplet states (3A'-12, 3A'-13, and 3B2-14) suffer from artifactual symmetry breaking towards a nonplanar geometry at the DFT level. The structures and properties of the title biradicals are readily rationalized within the framework of through-space and through-bond molecular orbital interactions. The degree of coupling between the formally unpaired electrons strongly depends on the number and arrangement of intervening sigma-bonds, and often parallels the trends observed for annellated arynes of similar topology. In some cases, novel structural patterns can be identified that are characteristic of five-membered-ring systems. These similarities and differences between five- and six-membered-ring arynes are discussed on the basis of molecular orbital arguments.     

Biphenalenylidene: the forgotten bistricyclic aromatic ene. A theoretical study

The Lawesson reagent and P(2)S(5) mediated reductive coupling of phenalenone (6) gives LPAH peropyrene (5) in 47% and 54% yields. The mechanism of the reaction involves the formation of phenalenethione (10), Z- and E-1,1'-biphenalenylidene (3), and 9 as intermediates. The electrocyclization reaction of Z-3 to 9, followed by aromatization, gives 5. The results of an ab initio and DFT study of 3 and 2,2'-biphenalenylidene (12) are reported. E-3 and Z-3 have a diradical character with twist angles of 44.8 degrees and 57.8 degrees (at UB3LYP/6-311G**). Delta E(++)(Tot) = 10.2 kJ/mol and Delta G(++)(298) = 10.6 kJ/mol for E-3 <==> Z-3 diastereomerization. These unusually low energy barriers are due to the ground-state diradical destabilization and the aromatic stabilization of the transition state TS-3. Triplet Z-3 is higher in energy than singlets E-3 and Z-3 by 10.4 and 3.1 kJ/mol. In the concealed non-Kekulé 12, singlet 12 is more stable than the triplet by 1.3 kJ/mol. Singlet 12 is more stable than singlet E-3 by 2.0 kJ/mol, and orthogonal singlet TS-12 is lower in energy than singlet TS-3 by 6.0 kJ/mol. The energy barriers for the hexatriene-cyclohexadiene electrocyclization Z-3 --> 9 are Delta E(++)(Tot) = 94.8 and Delta G(++)(298) = 98.3 kJ/mol (at (U)B3LYP/6-31G). The reaction occurs thermally in a conrotatory mode.