Ab initio study of the cyclooctatetraenyl radical

Ab initio calculations have been carried out on the 1,3,5,7- and 1,2,4,7-tetraene configurations of the cyclooctatetraenyl radical at UHF, ROHF, MCSCF, ROCISD, QCISD, and CCSD(T) levels of theory with 6-311G(d,p) and cc-pVDZ basis sets. Although spin contamination is present, the ROCISD calculations support the energies obtained from less intensive, UHF-based coupled cluster calculations over the energies obtained from MCSCF analysis of the pi-electron orbitals. The 1,3,5,7-form is a local minimum at the coupled cluster levels, higher in energy than the resonance-stabilized 1,2,4,7-form by 10-13 kJ/mol, but bounded by a barrier of less than 0.5 kJ/mol. The isomerization surface connecting these two structures is described and results reported from integration of the vibrational Schr?dinger equation on that surface. Excited vibrational states at energies just above the isomerization barrier are dominated by the character of the 1,3,5,7-tetraenyl radical, which suggests that chemistry involving this intermediate at typical combustion temperatures may branch at this juncture.     

Ab initio direct dynamics study of cyclopropyl radical ring-opening

Quasiclassical direct dynamics simulations, at the CASSCF(3,3)/6-31G(d) level of theory, are used to study the stereochemistry of the electrocyclic ring-opening reaction of the cyclopropyl radical. The trajectories are initiated at the reaction's transition state (TS), with their initial conditions sampled from the TS's 174 degrees C Boltzmann distribution. Intrinsic reaction coordinate calculations predict the overall reaction to have disrotatory stereochemistry. Though this is the preferred initial reaction stereochemistry in the trajectories, 43% of the trajectories follow the conrotatory path. Four unique trajectory types are observed during 200 fs dynamics of the product allyl radical. Intramolecular vibrational energy redistribution and internal rotation are incomplete on this time scale, and a statistical distribution of the allyl isomers is not observed.     

Ab initio study on the photochemical behavior of styrene

Ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations have been performed to examine the photochemical behavior of styrene upon the strong S₀‐S₂ electronic excitation in the low‐lying excited states. The optimized structure at the S₂/S₁ conical intersection (CIX) is characterized by a quinoid structure. The transition state (TS) in S₁ is in the vicinity of the S₂/S₁‐CIX. At the S₁‐TS, two reaction paths branch. One is the relaxation into the stable structure in S₁ and then emission into S₀. The other is the radiationless decay through the S₁/S₀‐CIX. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 950–956, 2002     

Ab initio study of magnetic interactions of manganese-oxide clusters

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.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

第一激发态HOOO裂解反应的理论研究

利用量子化学方法对第一激发态HOOO的裂解反应进行了理论研究.所有驻点(反应物、产物和过渡态)的几何结构优化和振动分析都是在CASSCF/6-31 G(d,p)理论水平下进行的.反应路径上的选择点及驻点都在CASPT2/6-31 G(d,p)//CASSCF(19,13)/6-31 G(d,p)和MRCI/6-31 G(d,p)//CASSCF(19,13)/6-31 G(d,p)理论水平下进行单点能量校正.CASSCF,CASPT2和MRCI水平的理论计算结果显示,第一激发态HOOO的裂解反应包含一个对称性变化的过程.它首先通过了一个Cs对称性的过渡态,然后逐渐变化为线性结构,最终生成产物O2(3Σg-)和OH(2Π).     

Ab initio study of the thermodynamic and kinetic stability of the ammonium radical

An increasing interest in the possible existence of the NH₄ radical has emerged in recent years. In this paper we report an ab initio UHF CI study of the ammonium radical, an investigation of parts of the energy surface around NH₄ and a theoretical prediction of the kinetic parameters of the radical formation and dissociation reactions within the framework of the TST theory. The ground state of the ammonium radical appears to be of the Rydberg type. Its ionization potential is found to be 4.29 eV. The NH₄ formation reaction from NH₂ + H₂ is very slightly exothermic whereas the reaction from NH₃ + H is slightly endothermic. We find a transition state of C_3v symmetry for the dissociation of NH₄ into NH₃ + H. The insertion of H₂ into NH₂ occurs according to a two-step mechanism whose determining step corresponds to the crossing of a saddle point with C_s symmetry previously obtained in the study of the reaction NH₂ + H₂ → NH₃ + H. Finally, we predict for NH₄ and ND₄ lifetimes of 0.1 and 1.4 μs respectively.     

Ab initio investigations in magnetic oxides

It is argued that band theory can be used as a valuable tool to investigate and model properties of several classes of oxide systems. This theoretical framework is presented in the first part of the article with details on the density functional theory (DFT) and its approximations leading to the local spin density functional LSDF as well as on the common methods built around it (mainly the augmented spherial wave ASW method used throughout this work). The major part of the article is devoted to case studies meant to illustrate the wealth and limits of the LSDF in addressing the electronic and magnetic structures of a selected variety of transition metal oxides of technological importance. Such systems are iron monoxide “FeO” (mainly the high pressure form), the archetype of half metallic ferro magnets CrO₂, used in magnetic recording media, the simple and double perovskite derived systems: SrFeO₃ and La₂TIrO₆ (T=Mn, Fe, Co) as well as manganese oxides (CaMnO₃, Ca₄Mn₄O₁₀ and Tl₂Mn₂O₇) for which a giant magnetoresistive GMR behaviour was identified in recent years. The computed ground state magnetic configurations and electronic structure results are discussed and confrontations with experimental data are carried out when available. The chemical bonding properties are analysed, and a new conceptual approach is provided.     

Ab initio study of water clustering in the presence of a methyl radical

The geometrical structure and binding energy of small clusters of methyl radical and water molecules (up to five water molecules) in gas phase and water media have been investigated at the MP2 level of theory using 6-311++G(2df,2p) basis set. The complexes characterized contain OH···O, CH···O, and OH···C attractive interactions with stabilization energies in the range 6–143 kJ mol^?1. The solvent has an enhancing influence on the stabilities of studied clusters. The atoms in molecules theory were also applied to explain the nature of the complexes. The interaction energies have been partitioned with the natural energy decomposition analysis showing that the most important attractive term corresponds to the charge transfer one.     

Ab initio periodic study of the conformational behavior of glycine helical homopeptides

Representative helicoidal conformations of polyglycine infinite chains have been investigated by using periodic boundary conditions, the B3LYP hybrid functional, and large basis sets, by means of the CRYSTAL code. The exploitation of the helix roto‐translational symmetry permits to optimize at a relatively low cost the structure of systems whose unit cell contains more than 300 atoms, much larger than the one investigated till now. In the present calculations, the helix symmetry is exploited at three levels. First, for the automatic generation of the structure. Second, for the calculation of the one‐ and two‐electron integrals that enter into the Fock matrix definition. Only the irreducible wedge of the Fock matrix is computed. Finally, for the diagonalization of the Fock matrix, where each irreducible representation is separately treated. The efficiency and accuracy of the computational scheme are documented, by considering cells containing up to 47 glycine residues. Results are compared with previous calculations and available experimental data. The role of hydrogen bonding in stabilizing polyglycine conformers is also addressed. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ab initio scf and Ci study of the electronic structure of the trichlorine radical

Ab initio SCF and CI calculations using a double-zeta plus polarization basis set have been carried out on the trichlorine radical Cl₃ to determine its electronic structure. The minimum in energy is determined for a bent structure at a bond angle of 146° and bond lengths of 2.18 Å (SCF ) or 2.22 Å (CI ). At linear geometry a ²Π_u state is found to be lowest, approximately 7 kcal above the bent minimum, followed by a ²∑_g⁺ state, which is around 4 kcal higher. This situation suggests that already for low quantum numbers a complex vibrational pattern in the Cl₃ infrared spectrum is to be expected due to spin-orbit coupling as well as coupling of electronic, vibrational, and rotational motion.     

Ab initio study of KN

The potential energy curves for the lowest (3)Sigma(-), (3)Pi, and (5)Sigma(-) states of the KN molecule have been calculated by the multireference singles and doubles configuration interaction method, including Davidson's corrections for quadruple excitations [MRCI(+Q)]. It is shown that the former two are bound, while the last one is repulsive. The electronic ground state of KN is predicted as (3)Sigma(-) state, although the term energy of the (3)Pi state is very small, 177.3 cm(-1). The binding energy for the (3)Sigma(-) state is evaluated as 0.838 eV, the rotational constant B(0) as 0.250 63 cm(-1), and harmonic frequency as 324.4 cm(-1). The spin-orbit coupling effects between the (3)Sigma(-) and (3)Pi states of KN are evaluated and discussed. The same MRCI(+Q) computational procedures are applied to the isovalent LiN, KC, KO, and KCl to confirm the accuracy of present calculations. Theoretical spectroscopic constants presented here will inspire experimental studies of KN.     

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH₃CC₂H₅^·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.     

Ab initio molecular dynamics study of H2 formation inside POSS compounds

The mechanism and dynamics of the formation of a hydrogen molecule by incorporating two hydrogen atoms in a stepwise manner into the cavity of some POSS (polyhedral oligomeric silsesquioxanes) compounds has been investigated by ab initio molecular orbital and ab initio molecular dynamics (AIMD) methods. The host molecules in the present reactions are two types of POSS, T(8) ([HSiO(1.5)](8)) and T(12)(D(2d)) ([HSiO(1.5)](12)). AIMD simulations were performed at the CASSCF level of theory, in which two electrons and two orbitals of the colliding hydrogen atoms are included in the active space. The trajectories were started by inserting the second hydrogen atom into the hydrogen atom-encapsulated-POSS (H + H@T(n) → H(2)@T(n); n = 8 and 12). In many cases, the gradual formation of a hydrogen molecule has been observed after frequent collisions of two hydrogen atoms within the cages. The effect of the introduction of an argon atom in T(12) is discussed as well.     

Ab initio study of the low-lying electronic states of the CH2NO2 radical

Ab initio electronic structures calculations are reported for the four low-lying electronic states X ²B₁, ²B₂, ²A₂, and ²A₁ of the CH₂NO₂ radical. The geometric parameters for the ground-state X ²B₁ are predicted by MRSDCI calculations with a double zeta plus polarization basis set. The vertical excitations energies for these electronic states are determined using MRSDCI /DZ +P calculations at the ground-state equilibrium geometry and in agreement with the recent experimental data obtained via PES of the CH₂NO anion. The oscillator strenghts and the radiative lifetimes for these electronic states and the spin properties for the ground state are calculated based on the MRSDCI wave functions, predicting results in good agreement with available experimental data. © 1994 John Wiley & Sons, Inc.     

Ab initio study on structures and stabilities of OLi n hyperlithiated compounds

The geometries of hyperlithiated compounds OLi_n were optimized by means of HF, MP2 and DFT methods with 6-31G basis set. The dissociation energies of those optimized stable geometries of OLi_n were calculated, the results are in good agreement with experimental values; and moreover, the dissociation energy of OLi_6 is predicted. In addition, the fundamental vibrational frequencies were also predicted.     

Ab initio study of long-range electron transfer between biphenyl anion radical and naphthalene

After the separation of the donor, the aeceptor, and the σ-type bridge from the π-σ-π system, the geometries of biphenyl, biphenyl anion radical, naphthalene, and naphthalene anion radical are optimized, and then the reorganization energy for the intermolecular electron transfer (ET) at the levels of HF/4-31G and HF/DZP is calculated. The ET matrix elements of the self-exchange reactions of the π-σ-π systems have been calculated by means of both the direct calculation based on the variational principle, and the transition energy between the molecular orbitals at the linear coordinate R=0.5. For the cross reactions, the ET matrix element and the geometry of the transition state are determined by searching the minimum energy splitting △_(min) along the reaction coordinate. In the evaluation of the solvent reorganization energy of the ET in solution, the Marcus' two-sphere model has been invoked. A few of ET rate constants for the intramolecular ET reactions for the π-σ-π systems, which contain     

Ab initio study of long-range electron transfer between biphenyl anion radical and naphthalene

After the separation of the donor, the acceptor, and the σ-type bridge from the π-σ-π system, the geometries of biphenyl, biphenyl anion radical, naphthalene, and naphthalene anion radical are optimized, and then the reorganization energy for the intermolecular electron transfer (ET) at the levels of HF/4-31G and HF/DZP is calculated. The ET matrix elements of the self-exchange reactions of theπ-σ-π systems have been calculated by means of both the direct calculation based on the variational principle, and the transition energy between the molecular orbitals at the linear coordinateR = 0.5. For the cross reactions, the ET matrix element and the geometry of the transition state are determined by searching the minimum energy splitting Δ_min along the reaction coordinate. In the evaluation of the solvent reorganization energy of the ET in solution, the Marcus’ two- sphere model has been invoked. A few of ET rate constants for the intramolecular ET reactions for the π-σ-π systems, which contain the biphenylyl as the donor and both biphenylyl and naphthyl as the acceptor, have been obtained. Project supported by the National Natural Science Foundation of China (Grant Nos. 29706104 and 29573112), the State Key Laboratory of Theoretical and Computational Chemistry of Jilin University.