A New Group Theoretical Approach to the Nonorthogonal Problem in Valence Bond Theory

Introduction There has been a very significarnt resurgence of interest in ab initio valence bond calculations recently. This is because the VB calculation based on nonorthogonal basis can provide intuitive understanding about many very important phenomena in chemistry. However, practical calculation based on nonorthogonal basis is still a great challenge even to deal with a quite small system due to the well-known N! (or

Role of hydrogen bonding in solubility of poly(N-isopropylacrylamide) brushes in sodium halide solutions

By employing molecular theory, we systematically investigate the shift of solubility of poly（N-isopropylacrylamide）（PNIPAM） brushes in sodium halide solutions. After considering PNIPAM–water hydrogen bonds, water–anion hydrogen bonds, and PNIPAM–anion bonds and their explicit coupling to the PNIPAM conformations, we find that increasing temperature lowers the solubility of PNIPAM, and results in a collapse of the layer at high enough temperatures. The combination of the three types of bonds would yield a decrease in the solubility of PNIPAM following the Hofmeister series： Na Cl＆gt;Na Br＆gt;Na I. PNIPAM–water hydrogen bonds are affected by water–anion hydrogen bonds and PNIPAM–anion bonds. The coupling of polymer conformations and the competition among the three types of bonds are essential for describing correctly a decrease in the solubility of PNIPAM brushes, which is determined by the free energy associated with the formation of the three types of bonds. Our results agree well with the experimental observations, and would be very important for understanding the shift of the lower critical solution temperature of PNIPAM brushes following the Hofmeister series.

Theoretical Study on the Excited-state Intramolecular Hydrogen Abstraction Reactions of Butanal

The excited-state intramolecular hydrogen abstraction reactions of butanal have been investigated using the CAS-MP2/6-311＋G^＊//CASSCF/6-31G^＊ methods. Calculated results show that the hydrogen transfer induced fluorescence quenching of the n,π^＊-excited state of covalent butanal with three paths： （1） The first path corresponds to direct S0-react reconstitution, which involves the first S1 decay by partial hydrogen atom transfer. （2） The second stepwise mechanism can be viewed as a full hydrogen atom transfer followed by a partial hydrogen atom back transfer, electron transfer （near S1/S0 or S0-TS） and finally a proton transfer to S0-react. （3） On the triplet surface, the surface crossing to the singlet state would be clearly much efficient at the T1/S0 region due to the large SOC value of 8.3 cm^-1. The S0-react decay route from T1/S0 was studied with an intrinsic reaction coordinate （IRC） calculation at the CASSCF level, resulting in the S0-React minimum.

Solution-based Chemical Strategies to Purposely Control the Microstructure of Functional Materials

Micro/nanostructured crystals with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy conversions. Low-temperature, aqueous chemical routes have been widely investigated for the synthesis of particles, and arrays of oriented nanorods and nanotubes. In this paper, based on the ideal crystal shapes predicted by the chemical bonding theory, we have developed some potential chemical strategies to tune the microstructure of functional materials, ZnS and Nb205 nanotube arrays, MgO wiskers and nestlike spheres, and cubic phase Cu2O microcrystals were synthesized here to elucidate these strategies. We describe their controlled crystallization processes and illustrate the detailed key factors controlling their growth by examining various reaction parameters. Current results demonstrate that our designed chemical strategies for tuning microstructure of functional materials are applicable to several technologically important materials, and therefore may be used as a versatile and effective route to the controllable synthesis of other inorganic functional materials.

THE CORRELATION EFFECT ON THE CHAIN MOTION CONTRIBUTION TO IONIC TRANSPORT IN POLYMER ELECTROLYTES

The ionic transport process in polymer electrolytes (such as polyethylene oxide) wassimulated numerically on a two dimensional square lattice where charge carriers areaccommodated by the lattice sites connected randomly with available bonds to represent theamorphous chain configuration. Following the dynamic bond percolation theory(DBPT),the chainmotion contribution to the ionic conduction was incorporated via periodical renewal of the randombond configuration. To check and extend the prediction made by DBPT employing global abruptbond renewal,spatial correlation of the bond reassignment was introduced to the system by: 1)regional bond renewal and 2) organized bond motion. It is found that the difference between thediffusivities simulated involving regional bond renewal and those of DBPT becomes negligiblewhen the bond renewal rate approaches the carrier hopping rate.

THEORETICAL ANALYSIS OF HYDROGENATION MECHANISM CATALYZED BY Pd-Fe_2O_3/D_(3520)

The mechanism of catalytic hydrogenation of styrene is analyzed by catalysis theory and frontier molecular orbital theory.

Paired-permanent approach for VB theory (Ⅱ)——An ab initio spin-free VB program

Paired-permanent approach for VB theory is extensively developed. Canonical expansion of a paired-permanent is deduced. Furthermore, it is shown that a paired-permanent may be expressed in terms of the products of sub-paired-permanents of any given order and their corresponding minors. An ab initio spin-free valence bond program, called Xiamen, is implemented by using paired-permanent approach. Test calculation shows that Xiamen package is more efficient than some other programs based on the traditional VB algorithm, and it provides a new practical tool for quantum chemistry.

Theory and algorithms for the excited states of large molecules and molecular aggregates

This project aims to attack the frontiers of electronic structure calculations on the excited states of large molecules and molecular aggregates by developing novel theoretical and computational methods. The methodology development is especially based on the time-dependent density functional theory (TDDFT) and valence bond (VB) theory, and is expected to be computationally effective and accurate as well. Research works on the following related subjects will be performed: (1) The analytical energy-derivative approaches for electronically excited state within TDDFT will be developed to reduce bypass the computational costs in the calculation of molecular excited-state properties. (2) The ab initio methods for electronically excited state based on VB theory and hybrid TDDFT-VB method will be developed to overcome the limitations of current TDDFT in simulating photophysics and photochemistry. (3) For larger aggregates, neither ab initio methods nor TDDFT is applicable. We intend to build the effective model Hamiltonian by developing novel theoretical and computational methods to calculate the involved microscopic physical parameters from the first-principles methods. The constructed effective Hamiltonian is then used to describe the excitonic states and excitonic dynamics of the natural or artificial photosynthesized systems, organic or inorganic photovoltaic cell. (4) The condensed phase environment is taken into account by combining the developed theories and algorithms based on TDDFT and VB with the polarizable continuum solvent models (PCM), molecular mechanism (MM), classical electrodynamics (ED) or molecular dynamics (MD) theory. (5) Highly efficient software packages will be designed and developed.

Unusually high electron density in an intermolecular non-bonding region:Role of metal substrate

It has been demonstrated that intermolecular interaction,crucial in a plenty of chemical and physical processes,may vary in the presence of metal surface.However,such modification is yet to be quantitatively revealed.Here,we present a systematical density functional theory study on adsorbed bis（para-pyridyl）acetylene（BPPA） tetramer on Au（111） surface.We observed unusually high electron density between two head-to-head N atoms,an intermolecular ＂non-bonded＂ region,in adsorbed BPPA tetramer.This exceptional electron density originates from the wavefunction hybridization of the two compressed N lone-electron-pair states of two BPPA,as squeezed by a newly revealed N-Au-N threecenter bonding.This bond,together with the minor contribution from N...H-C intermolecular hydrogen bonding,shortens the N-N distance from over 4 A to 3.30 A and offers an attractive lateral interacting energy of 0.60 eV,effectively to a surface-confined in-plane pressure.The overlapped non-bonding vvavefunction hybridization arising from the effective pressure induced by the N-Au-N three-center bonding,as not been fully recognized in earlier studies,was manifested in non-contact Atomic Force Microscopy.

Bond, vibration and microwave dielectric characteristics of Zn_(1- x)(Li_(0.5)Bi_(0.5))_(x)WO_(4) ceramics with low temperature sintering

The bond,vibration and microwave dielectric characteristics of Zn_(1- x)(Li_(0.5)Bi_(0.5))_(x)WO_(4)(x=0-0.12)ce-ramics were investigated by XRD refinement,Raman and FT-IR spectroscopy and complex bond valence theory.The results showed that proper substitution of(Li_(0.5)Bi_(0.5))2þcan improve the sintering charac-teristics and microwave dielectric properties of ZnWO_(4).The increase of Q×f value was mainly attributed to dense and uniform microstructure,and the subsequent decrease resulted from the deterioration of structural stability and relative density.According to the complex bond valence theory,the chemical bond characteristics of ZnWO_(4) and Bi_(2)WO_(6) played an important role in the dielectric properties of the samples.Additionally,the samples(x=0.02)sintered at 900℃ showed satisfactory properties:ε_(r)=15.332,Q×f=35,762 GHz,and t_(f)=-65 ppm/℃,making it a potential candidate material for LTCC applications.

SrGa_(12)O_(19):The first low-ε_(r) Ga-based microwave dielectric ceramic with anomalous positiveτ_(f)

Dielectric ceramics with low permittivity(ε_(r)),high quality factor(Q×f),and near-zero resonant frequency(τ_(f))in the microwave bands are key materials used in fifth/sixth-generation(5G/6G)telecommunication,whileτ_(f) of most low-εr microwave dielectric ceramics is relatively negative.In this work,the first low-εr Ga-based ceramic SrGa_(12_O_(19) with an anomalous positiveτ_(f) was reported,and the causes of the positiveτ_(f),intrinsic polarization,and loss mechanism were systematically studied.X-ray diffraction(XRD)and transmission electron microscopy(TEM)revealed that the SrGa_(12_O_(19) ceramic formed a pure hexagonal magnetoplumbite structure with spinel blocks and rock-salt blocks stacked along the crystallographic c-axis.When sintered at 1430℃,it possessed the optimal microwave dielectric properties of a lowεr of 14.46,high Q×f of 64,705 GHz,and exceptional positiveτ_(f) of+55.7 ppm/℃,along with a low linear thermal expansion coefficient(αL)of 11.617 ppm/℃.The large positive deviation betweenεr andεr(C–M)of 45.31%resulted from the rattling effect of atoms in the rock-salt block.The unique positiveτ_(f)(+55.7 ppm/℃)was governed by the rattling effect,resulting in a positiveταm(the temperature coefficient of ion polarizability)of 8.489 ppm/℃ and a large negative temperature coefficient of permittivity(τε)of−132.864 ppm/℃.Phillips–Vechten–Levine(P–V–L)chemical bond theory revealed greater contributions of the spinel block to bond ionicity(fi,52.95%),permittivity(ε,55.15%),bond energy(E,56.87%),and lattice energy(U,74.88%)than those of the rock-salt block.The intrinsic dielectric properties were analyzed using infrared(IR)reflectivity spectra.The favorable performance of the SrGa_(12_O_(19) ceramic indicated that it is a novelτ_(f) compensator.This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.

Metastability of π-π stacking between the closed-shell ions of like charges

Planar cations or anions can form stacks in crystals or solutions,where the surrounding or environment plays a decisive role as demonstrated in previous studies.However,it remains unclear whether these counterintuitive interactions possess any inherent stability or are thoroughly repulsive if the constraint of environment is removed.In this work,we explored the inherent stability ofπ-πstacking between closed-shell ions of like charges with prototypes derived from experimental studies.The inherent metastability was identified by the characteristic local minima and the transition states preventing their dissociation and verified by ab initio molecular dynamics(AIMD)simulations.The nature of involved interactions was deciphered with the energy decomposition approach based on the block-localized wavefunction method(BLW-ED).Like the conventional neutralπ-πstacking interactions,electron correlation is the most attractive energy component.But it is overturned by the Coulombic repulsion between net charges for all modes of dimerization,resulting in the overall repulsive inter-cation or anion in-teractions.Contributions from van der Waals interactions were also observed in the reduced density gradient analysis.The origin of the metastability was elucidated by examining the contributions of individual physical factors to the well-depths.The inherent metastability originates from the electron correlation,which dramatically increases due to the enhanced overlap between ions from a transition state to its corresponding minimum.

Theoretical Investigation on Mechanical and Thermal Properties of a Promising Thermal Barrier Material:Yb_3Al_5O_(12)

An investigation on the mechanical and thermal properties of Yb3Al5O12 is conducted by a combination of first- principles calculations and chemical bond theory calculation. Density functional theory （DFT） computations were performed for the structural, mechanical, and thermal properties, and the results are confirmed by chemical bond theory. Based on the calculated equilibrium crystal structure, heterogeneous bonding nature is revealed. The full set of elastic constants and mechanical properties of Yb3Al5O12 are presented for the first time. The thermal expansion coefficient of Yb3Al5O12 is calculated to be 7.5 × 10^-6 K-1 by chemical bond theory. In addition, the minimum thermal conductivity of Yb3Al5O12 is estimated to be 1.22 W m-t K-1, and the origin of such low thermal conductivity is discussed. Our theoretical results highlight the potential of Yb3Al5O12 as a prospective thermal barrier coating material.

Ab initio VB Studies of the Ground and Low-lying Excited States of BeH and BH

A scheme has been proposed to classify valence bond (VB) wave functions for the calculations of ground and excited states, according to the symmetry properties of one electron orbitals which are involved in the construction of VB wave functions. This scheme is illustrated by the examples of BeH and BH. Ab initio VB computations of these two test molecules in combination with the present classification scheme give reliable results. For example, calculation results show that the state C 2Σ +of BeH is stable, with the bonding energy 0 87 eV and bond length 0 238 nm, which are in good agreement with those obtained by Gerratt et al . The bonding features of ground and low lying excited states of BeH and BH are discussed.

Pulling growth technique towards rare earth single crystals

Pulling growth technique serves as a popular method to grow congruent melting single crystals with multiscale sizes ranging from micrometers to centimeters.In order to obtain high quality single crystals,the crystal constituents would be arranged at the lattice sites by precisely controlling the crystal growth process.Growing interface is the position where the phase transition of crystal constituents occurs during pulling growth process.The precise control of energy at the growing interface becomes a key technique in pulling growth.In this work,we review some recent advances of pulling technique towards rare earth single crystal growth.In Czochralski pulling growth,the optimized growth parameters were designed for rare earth ions doped Y_3Al_5O_(12)and Ce:(Lu_(1-x)Y_x)_2Si O_5on the basis of anisotropic chemical bonding and isotropic mass transfer calculations at the growing interface.The fast growth of high quality rare earth single crystals is realized by controlling crystallization thermodynamics and kinetics in different size zones.On the other hand,the micro pulling down technique can be used for high throughput screening novel rare earth optical crystals.The growth interface control is realized by improving the crucible bottom and temperature field,which favors the growth of rare earth crystal fibers.The rare earth laser crystal fiber can serve as another kind of laser gain medium between conventional bulk single crystal and glass fiber.The future work on pulling technique might focus on the mass production of rare earth single crystals with extreme size and with the size near that of devices.

Fast growth of cerium-doped lutetium yttrium orthosilicate single crystals and their scintillation properties

The growth of high-quality and large-size cerium-doped lutetium yttrium orthosilicate(Ce:(LuY)2 SiO5,Ce:LYSO)crystals with lower cost has great influence on their applications in various physical devices.According to the chemical bonding theory of single crystal growth,the fast thermodynamic growth direction of Ce:LYSO is[010]direction.In this work,the maximum pulling rate of 3.5 mm/h is obtained along[010]direction of Ce:LYSO via the Czochralski(Cz)method.Finally,Ce:LYSO bulk single crystals with diameter of 64 mm and length of 220 mm and mass of 4.2 kg are grown.The luminescence performance of low-doped Ce:LYSO for nuclear irradiation(^(22)Na)detector device was studied.Light yield of26193 ph/MeV and decay time of 38 ns are obtained.The present work provides a promising fast growth approach to achieving large size functional bulk crystal via both thermodynamic and kinetic controls.

Interface adhesion properties characterization of sulfide electrode materials by the combination of BOLS and XPS

Although sulfide electrode materials in lithium battery systems have been intensively investigated due to their low-cost, high theoretical specific capacity, and energy density, there are few studies fousing on the adhesion properties, including the physical origin of hetero-coordination resolved interface relaxation, binding energy and the energetic behavior, and even the accurate quantitative information. In this paper, we present an approach for quantifying the interface adhesion properties of sulfide electrode materials resolved by the combination of bond order-length-strength theory(BOLS) and X-ray photoelectron spectroscopy(XPS), which has enabled clarification of the interface adhesion nature. The results show that the Cu 2p, Fe 2p, and S 2p electrons of Cu S and FeS_(2) compounds shift negatively due to the charge polarization of the conduction electrons of the heteroatoms, while Mo 3d, Sn 3d electrons of Mo S2 and Sn S2 and the C 1 s and S 2p electrons of CS compound shift positively due to the quantum trapping. It is noted that the exact interface adhesion energies of Cu S is 3.42 J m^(-2), which is consistent with the calculation result. The approach can not only clarify the origin of the interface adhesion properties of sulfide electrode materials,but also derive their quantification information from atomistic sites.