H2NSi radical: structures,isomerization pathways and electronic states characterization

Using state-of-the-art theoretical methods, the stable isomers of H₂NSi which are relevant for astrophysics, astrochemistry and ammonia silicon surface chemistry, were investigated. These computations are performed using configuration interaction ab initio methods and the aug-cc-pVQZ and cc-pVQZ basis sets. Calculations confirm the existence of three stable isomers: H₂NSi, trans-HSiNH and H₂SiN. The intramolecular isomerization and the H-abstraction reaction pathways on the lowest doublet potential energy surfaces are given. Insight into the pattern of the lowest doublet and quartet electronic states of these molecular species are also presented.      

C20分子异构体的电子态

使用量子化学从头计算方法优化了C₂₀分子三种异构体（环形、碗形和笼形）的单重和三重电子态几何结构。从优化的几何结构出发，由二阶微扰（MP2）方法得到的单点能给出了一个不符合Hund法则的结果，也就是三种异构体的能量都是单重电子态的最低。使用UHF波函数给出了三种异构体的总电荷密度和静电势，结果显示在笼形和碗形结构的中心区域存在负电荷。最后，本文分析了环形结构的成键特征。     

A theoretical investigation of the oxidation states of palladium complexes and their role in the carbonylation reaction

Two different, yet related, topics are discussed: (i) the reduction of palladium (II) in Pd(OAc)₂ complexes reacting with phenyl phosphines and leading to Pd(0) phosphine complexes, and (ii) the carbonylation reaction of allyl chlorides catalyzed by these Pd(0) species. The results show that the overall reduction is an exothermic process that can be accomplished along two different reaction paths, one being clearly favoured over the other. Similarly, three different channels have been determined for the carbonylation reaction that primarily differ in the timing and the way in which the reacting species bind the metal. In the first path (the σ-path), the allyl fragment interacts very weakly with the metal, whereas the CO molecule strongly binds it and reacts with the allyl. The second channel (the π–η³ pathway) is characterized by a π–η³ interaction between the allyl fragment and the palladium, to which the CO molecule binds, before the two units react affording the product. In both cases, two consecutive migrations of the chlorine ‘assist’ the course of the reaction. In the third case (the η² pathway) the allyl fragment initially enters the palladium coordination sphere, and the CO molecule then simultaneously binds it and the phosphorous atom of one phosphine ligand. The first two paths are favoured.     

二价金属元素掺杂对LiCoO2体系电子输运性质的影响

为了解释Ca掺杂与Mg掺杂在影响锂离子二次电池正极材料LiCoO₂体系电子输运性质方面的不同效应，采用基于密度泛函理论的第一性原理方法研究了该体系的电子结构.计算结果表明，虽然在LiCoO₂体系中用Ca或Mg替代Co都会在费米能级附近产生部分占据的受主带，但两者对应的电子态都具有明显的局域化特征；此外，与Mg掺杂体系明显不同的是，Ca掺杂体系的受主带与价带之间存在清晰的带隙.这一带隙的存在正是Ca掺杂不能明显提高LiCoO₂体系电导率的主要原因.此外，Ca²⁺与Mg²⁺离子半径的较大差别也是造成这两个掺杂体系的电导率存在明显差异的一个重要因素.     

An ab initio study of the structural,elastic, electronic and optical properties of the newly synthesized nitridoaluminate LiCaAlN2

The structural parameters, elastic constants, electronic structure and optical properties of the recently reported monoclinic quaternary nitridoaluminate LiCaAlN₂ are investigated in detail using the ab initio plane-wave pseudopotential method within the generalized gradient approximation. The calculated equilibrium structural parameters are in excellent agreement with the experimental data, which validate the reliability of the applied theoretical method. The chemical and structural stabilities of LiCaAlN₂ are confirmed by calculating the cohesion energy and enthalpy of formation. Chemical band stiffness is calculated to explain the pressure dependence of the lattice parameters. Through the band structure calculation, LiCaAlN₂ is predicted to be an indirect band gap of 2.725 eV. The charge-carrier effective masses are estimated from the band structure dispersions. The frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for polarized incident light in a wide energy range. Optical spectra exhibit a noticeable anisotropy. Single-crystal and polycrystalline elastic constants and related properties, including isotropic sound velocities and Debye temperatures, are numerically estimated. The calculated elastic constants and elastic compliances are used to analyse and visualize the elastic anisotropy of LiCaAlN₂. The calculated elastic constants demonstrate the mechanical stability and brittle behaviour of the considered material.     

Rydberg states of the HeH+ ion calculated by ab initio methods: potential energies and effective principal quantum numbers

More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm^?1. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H⁺₂.     

An MRCI investigation of the electronically excited states of difluorocarbene and its monovalent ions

The MRCI/CBS computational protocol was employed to describe the low-lying electronic states of CF₂ in excellent agreement with experiment. Also of interest for modeling the fluorocarbon plasmas are the difluorocarbene ions, and the geometries, adiabatic excitation energies and orbital characteristics of their electronically excited states were established. Most of the transitions leading to the excited states were found to be monoelectronic and, for the dipole-allowed ones, the oscillator strengths and radiative lifetimes were predicted to aid in the spectroscopic studies.     

Structural,elastic, electronic and thermal properties of M2SbP (M = Ti,Zr and Hf)

The structural, elastic, electronic and thermal properties of M₂SbP (M = Ti, Zr and Hf) were studied by means of a pseudo-potential plane-wave method based on the density functional theory within both the local density approximation and the generalised gradient approximation. The optimised zero-pressure geometrical parameters, i.e. the two unit cell lengths (a, c) and the internal coordinate (z), were in good agreement with available experimental and theoretical data. The effect of high pressure, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than along c-axis. The anisotropic independent elastic constants were calculated using the static finite strain technique. Numerical estimations of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature for ideal polycrystalline M₂SbP aggregates were performed in the framework of the Voigt–Reuss–Hill approximation. The calculated band structures show that all studied materials are electrical conductors. Analysis of the atomic site projected densities showed that the bonding is of covalent–ionic nature with the presence of metallic character. The density of states at the Fermi level is dictated by the transition metal d–d bands; the Sb element has little effect. Thermal effects on some macroscopic properties of M₂SbP were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the volume expansion coefficient, heat capacity and Debye temperature with pressure and temperature in the ranges 0–50 GPa and 0–2000 K were obtained successfully.     

Ab initio Study of Electronic Structure and Magnetic Properties of Two Novel Neptunium （V） Sulfates： iaK3（ipO2）4（SO4）4（H2O）2 and iaipO2SO4H2O

Ab initio within the full potential linearized augmented plane wave （FP-LAPW） method with the GGA＋U approach is applied to study the electronic structures of two compounds NaK3（NpO2）4（SO4）4（H2O）2 and NaNpO2SO4H2O. The present calculations show that the major part of the spin magnetic moment in these two compounds is from Np（V） ions, and the origin of the cation-cation interactions between Np comes from the spin polarization effect within Np-ONv-Np bonds.     

Ab initio study of PN electronic states – a qualitative interpretation of the perturbation and predissociation effects on observed transitions

Valence and high electronic states of PN have been calculated with accurate quantum chemistry methods. The variety of theoretical methods used includes complete active space self-consistent field, multireference configuration interaction and the newly developed explicitly correlated coupled cluster methods. The large correlation-consistent atomic orbitals basis sets AVQZ, AV5Z and AV(5+d)Z are used for the potential energy curves calculations in the bonding and long-range regions. The spectroscopic constants (R_e, B_e, ω_e, ω_ex_e, α_e, D_e, T_e) and the vibrational levels of the bound valence states (X¹Σ⁺, A¹Π, a³Σ⁺, d ³Δ, e³Σ^?, C¹Σ^?, b³Π, D ¹Δ and E¹Σ⁺ and some higher bound states) are determined and compared with experimental findings when available. Significant spin–orbit interactions between triplet states and A¹Π and E¹Σ⁺ excited states are found near the crossing points of the potential energy curves and could explain predissociation phenomena and the perturbations of the vibrational levels experimentally observed for PN in their A¹Π and E¹Σ⁺ states.     

Explorative computational study of the singlet fission process

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the ¹TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the ¹TT state are such that they can be approximated with those of the ⁵TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the ¹TT and S₁ states are close in energy with a large non-adiabatic coupling matrix element at the S₁ geometry, and the energy of the S₀ state is well below that of the ¹TT state at the ¹TT geometry.

The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The ¹TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and ¹TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and ¹TT[2] excited states.     

Modification of the structural and electronic properties of graphene by the benzene molecule adsorption

A survey of the literature data on the adsorption of benzene on graphene or carbon nanotubes indicates that the distance between the graphene sheet and benzene molecule is determined from weak van der Waals forces (∼3.40 Å). In our theoretical study, it was found that the benzene/graphene structure (in a specific configuration with carbon atoms located at the atop positions, stacked directly on the top of each other) forms strong covalent bonds, if the distance between the graphene and benzene is about 1.60 Å. Such a short distance corresponds to about a half of the usual separation between the graphite layers. It was also shown that at such a short distance the carbon atoms of the benzene molecule move towards the graphene sheet, whereas the hydrogen atoms move in a different direction, thus breaking the benzene planar structure.     

NaCCH自由基低能电子态的理论研究

采用CASSCF/CASPT2方法研究了NaCCH自由基的低能电子态,在C2v对称性下给出了NaCCH自由基9个低能激发态的几何、组态、CI系数、振子强度、垂直激发能和绝热激发能.研究结果表明,NaCCH自由基的基态X1Σ+几何和转动系数与实验值是一致的;谐振频率与其他理论方法得到的值也符合的很好;HOMO和LUMO轨道的能量间隔为3.80eV,对应着13Σ+的绝热激发能;除13Π外,其它低能激发态均是稳定状态;基态到三重态是自旋禁阻的.     

Experimental and theoretical studies of the vibrational spectra of CHD2Br

The dideuterated form of methyl bromide, CHD₂Br, has been synthesized and the gas-phase infrared spectra investigated in the range 400–10,000?cm^–1 using a medium-resolution FTIR spectrometer. The nine fundamental bands have been characterized in detail. Six of them, i.e. ν ₁, ν ₄, ν ₅, ν ₇, ν ₈ and ν ₉, have been rotationally analysed through the assignment of the partially resolved structure of the ^PQ_K and ^RQ_K cluster of lines and the spectroscopic parameters have been derived in the symmetric top limit approximation. Among the fundamental levels, anharmonic resonance occurs between ν ₇/ν ₄?+?ν ₈ and ν ₈/ν ₆?+?ν ₉. An isotopic ^79/81Br shift was found for ν ₆ and in the more complex region of the ν ₈ fundamental. High-quality ab initio calculations – carried out at coupled cluster level [CCSD(T)] employing the correlation-consistent basis set of Dunning (cc-pVTZ) – were performed to determine quadratic, cubic and quartic (semidiagonal) force constants. Using these constants and applying second-order vibrational perturbation theory (VPT2), with allowance for resonances (when necessary), permitted us to identify and assign, in addition to the fundamentals, about 70 overtones and combination bands up to three quanta.     

用CCSD(T)方法计算基态SH和SD的光谱常数(英文)

利用CCSD(T)方法和系列相关一致基cc-pVXZ及aug-cc-pVXZ(X=D,T,Q,5)对SH和SD分子的基态平衡几何,谐振频率和离解能进行了优化计算.利用优选出的CCSD(T)/aug-cc-pV5Z方法对SH和SD的基态进行单点能计算,并将计算结果拟合成了Murrell-Sorbie函数.利用得到的解析势能函数,计算了SH和SD的其余3个光谱常数(ω_ex_e,α_e和B_c),结果表明:除SD的ω_ex_e值外,其余结果均与实验值符合得相当好,但计算得出的ω_ex_e值与推导出的值25.134 cm~(-1)符合得很好.     

High-pressure theoretical and experimental study of HgWO4

HgWO₄ at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30 GPa is proposed.