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.     

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.     

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.     

Electronic states and spectroscopic properties of MgH in absence and presence of spin–orbit coupling − a configuration interaction study

Electronic spectrum of astrophysically important molecule magnesium hydride (MgH) has been studied using configuration interaction methodology excluding and including spin–orbit coupling. Potential energy curves of several spin-independent (Λ?S) electronic states have been constructed and spectroscopic constants of low-lying bound Λ?S states within 8.2 eV of term energy are reported in the first stage of calculations. The X²Σ⁺ is identified as the ground state in the Λ?S level. In the subsequent stage, the spin–orbit interaction has been incorporated and its effects on the potential energy curves and spectroscopic features of different electronic states of the species have been investigated. The X²Σ⁺_1/2 is identified as the spin–orbit (Ω) ground state of the species. Transition moments of several dipole-allowed transitions are computed in both the stages and radiative lifetimes of the corresponding excited states are computed. Electric dipole moments (µ) for a number of low-lying bound Λ?S states as well as several low-lying Ω-states are also calculated in the present study.     

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.     

First principles study of the electronic structure and phonon dispersion of naphthalene under pressure

The structural, electronic and vibrational properties of crystalline naphthalene has been investigated within the framework of density functional theory including van der Waals interactions. The computed lattice parameters and cohesive energy have good agreement with experimental data. We study on the structural and electronic properties of the naphthalene under the hydrostatic pressure of 0–20 GPa. The isothermal equations of state calculated from the results show good agreement with experiment in the pressure intervals studied. The phonon dispersion curves have been computed at ambient and hydrostatic pressure of 10 and 20 GPa. We have also calculated the quasiparticle band structure of naphthalene with the G₀W₀ approximation.     

Multireference configuration interaction study of ground and low-lying excited states of the AlC radical

This work explored the spectroscopic parameters and vibrational properties of the 21 Λ–S and 42 Ω states of the AlC radical. The PECs were calculated with the CASSCF method, which was followed by the icMRCI+Q approach. The A⁴Π, a²Π, 5²Π, 2²Δ, and 1²Φ states as well as the first well of B⁴Σ^? state were inverted with the spin–orbit coupling (SOC) effect included; the 1⁴Δ, 1⁴Σ⁺, and 2²Σ^? states as well as the second wells of the B⁴Σ^?, 2²Σ⁺, 3²Σ⁺, 4²Π and 5²Π states were weakly bound, which well depths were less than 650 cm^?1; the B⁴Σ^?, 2²Σ⁺, 3²Σ⁺, 4²Π, 5²Π, and 2²Δ states had double wells and the second wells of these states except for B⁴Σ^? had only several vibrational states; the avoided crossings existed between the 2²Σ⁺ and 3²Σ⁺ states, the 3²Σ⁺ and 4²Σ⁺ states, the B⁴Σ^? and 3⁴Σ^? states, the 2²Δ and 3²Δ states, the 4²Π and 5²Π states, the 5²Π and 6²Π states, as well as the 2⁴Π and 3⁴Π states. The extrapolation scheme, core–valence correlation and scalar relativistic corrections were included. The spectroscopic parameters and vibrational properties were determined. The TDM curves between two different Λ–S states were calculated and Franck–Condon factors of some transitions were evaluated. The SOC effect on the spectroscopic and vibrational properties was evaluated.     

Experimental and theoretical study of the electronic states and spectra of PbLi

Gas phase emission spectra of the hitherto unknown free radical PbLi have been measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast flow system in which lead vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Five electronic transitions have been found in the wavenumber range 3800-10 000 cm⁻¹. Bands from two excited states to the ground state were measured at high spectral resolution such that rotational analyses could be performed and accurate molecular parameters derived. In order to aid in the analysis of the experimental data, a series of relativistic configuration interaction calculations has been carried out to obtain potential curves for the low-lying states of PbLi and also electric dipole transition moments connecting them. As in the lighter molecules of this group, CLi and SiLi, the ground state of PbLi is found to be 3/2) with a spin splitting of about 2000 cm⁻¹. The first excited state is (A 1/2), and two observed band systems are assigned to the transitions A→X₁ and A→X₂. Two more excited states, (B 3/2) and C 1/2, are identified from the observed spectra with the help of the computed data, and their spectroscopic constants are determined. In contrast to PbH and PbF, the ab initio results indicate a very complicated low-energy electronic structure for the PbLi radical, with 19 bound electronic states calculated to lie below 3 eV.     

Quantum computing methods for electronic states of the water molecule

ABSTRACT

We compare recently proposed methods to compute the electronic state energies of the water molecule on a quantum computer. The methods include the phase estimation algorithm based on Trotter decomposition, the phase estimation algorithm based on the direct implementation of the Hamiltonian, direct measurement based on the implementation of the Hamiltonian and a specific variational quantum eigensolver, Pairwise VQE. After deriving the Hamiltonian using STO-3G basis, we first explain how each method works and then compare the simulation results in terms of gate complexity and the number of measurements for the ground state of the water molecule with different O–H bond lengths. Moreover, we present the analytical analyses of the error and the gate-complexity for each method. While the required number of qubits for each method is almost the same, the number of gates and the error vary a lot. In conclusion, among methods based on the phase estimation algorithm, the second-order direct method provides the most efficient circuit implementations in terms of the gate complexity. Moreover, Pairwise VQE serves the most practical method for near-term applications on the current available quantum computers. Finally the possibility of extending the calculation to excited states and resonances is discussed.     

Ab initio MRCI＋Q study on potential energy curves and spectroscopic parameters of low-lying electronic states of CS＋

Carbon monosulfide molecular ion （CS＋）, which plays an important role in various research fields, has long been attracting much interest. Because of the unstable and transient nature of CS＋, its electronic states have not been well investigated. In this paper, the electronic states of CS＋ are studied by employing the internally contracted multireference configuration interaction method, and taking into account relativistic effects （scalar plus spin–orbit coupling）. The spin–orbit coupling effects are considered via the state-interacting method with the full Breit–Pauli Hamiltonian. The potential energy curves of 18 Λ–S states correlated with the two lowest dissociation limits of CS＋ molecular ion are calculated, and those of 10 lowest Ω states generated from the 6 lowest Λ–S states are also worked out. The spectroscopic constants of the bound states are evaluated, and they are in good agreement with available experimental results and theoretical values. With the aid of analysis of Λ–S composition of Ω states at different bond lengths, the avoided crossing phenomena in the electronic states of CS＋ are illuminated. Finally, the single ionization spectra of CS （X1Σ＋） populating the CS＋（X2Σ1/2＋, A2Π3/2, A2Π1/2, and B2Σ1/2＋） states are simulated. The vertical ionization potentials for X2Σ1/2＋, A2Π3/2, A2Π1/2, and B2Σ1/2＋ states are calculated to be 11.257, 12.787, 12.827, and 15.860 eV, respectively, which are accurate compared with previous experimental results, within an error margin of 0.08 eV~0.2 eV.     

Structure of excited electronic states of Mg-chlorin and Mg-bacteriochlorin dimers

Within the framework of a procedure proposed previously for fragment-by-fragment quantum-chemical calculation of aggregates of molecules with π-electronic chromophore groups, account is taken of hyperconjugation, which allows practically exact reproduction of results obtained by the all-valence CNDO/S method. Calculations of excited electronic states of sandwichlike Mg-chlorin (Mg-Ch)₂ and Mg-bacteriochlorin (Mg-BCh)₂ dimers with variation of the interplanar distance between monomers are made. It is shown that on passing from a Mg-porphin dimer to the (Mg-Ch)₂ and (Mg-BCh)₂ dimers there is a considerable decrease in the energies of charge-transfer states, which are resonant in nature (CR-states). Moreover, (Mg-BCh)₂ has a considerably reduced energy interval between the lower CR-state and the forbidden component of the lower Q-state, which is indicative of easier charge separation in the lower excited electronic singlet states under the action of the field of the environment of tetrahydroporphyrin dimers than of dimers of porphyrins and hydroporphyrins, which can be of biological importance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 166–175, March–April, 1998.     

Long-lived nuclear spin states in the solution NMR of four-spin systems

The existence of long-lived nuclear spin states in four-spin systems is explored by solution-state NMR experiments. Long-lived states are proved to exist in three different natural product molecules, each containing either a AA'BB' or a AA'XX' proton spin system. The measured state lifetimes are between four and eight times the spin-lattice relaxation time constants.     

Electronic and magnetic structures of V-doped zinc blende Zn1-xVxNyO1-y and Zn1-xVxPyO1-y

Electronic and magnetic structures of zinc blende ZnO doped with V impurities are studied by first-principles calculations based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA).Calculations for the substitution of O by N or P are performed and the magnetic moment is found to be sensitive to the N or P content.Furthermore,the system exhibits a half-metallic band structure accompanied by the broadening of vanadium bands.The mechanism responsible for ferromagnetism is also discussed and the stability of the ferromagnetic state compared with that of the paramagnetic state is systematically investigated by calculating the total energy difference between them by using supercell method.