MgF_2表面结构稳定性及电子特性的密度泛函理论研究

用密度泛函理论(DFT)研究了MgF_2(010)、MgF_2(001)、MgF_2(011)及MgF_2(110)四种表面10种构型的稳定性和电子特性.结果表明:四种表面的邻近表面几层原子均出现了明显的驰豫现象,终止于单层F原子的表面相对稳定;进一步对比分析四种表面(终止于单层F原子的稳定构型)的表面能发现,稳定性依次减弱排列为MgF_2(110)、MgF_2(011)、MgF_2(010)、MgF_2(001);最稳定的MgF_2(110)表面的态密度显示在费米能级以下较多的成键电子处于低能级区,同时由于表面的影响,导致表面F原子电荷聚集显负电性,促使表面活性增加.

D3-C32F2的稳定性和振动光谱的理论研究

应用密度泛函理论在B3LYP/6-31G(d)水平上对D3-C32的二元氟化衍生异构体进行几何优化,对优化结构的反应热和前线轨道能级差的分析表明,D3-C32F2在热力学上是稳定的,最稳定的异构体是1-8加成的D3-C32F2-1-32,振动频率计算发现,对称性会影响结构的最大振动频率及强度,衍生化位置对称性越高,振动强度越大。

Quantum Chemical Studies on Structure and Detonation Performance of Bis（2,2-dinitropropyl ethylene）formal

Based on the full optimized molecular geometric structures via B3LYP/6-311＋G（2d,p） method, a new gem-dinitro energetic plasticizer, bis（2,2-dinitropropyl ethylene）formal was investigated in order to search for high-performance energetic material. IR spectrum, heat of formation, and detonation performances were predicted. The bond dissociation energies and bond orders for the weakest bonds were analyzed to investigate the thermal stability of the title compound. The results show that the four N-NO2 BDEs are nearly equal to the values of 164.38 kJ/mol, which shows that the title compound is a stable compound. The detonation velocity and pressure were evaluated by using Kamlet-Jacobs equations based on the theoretical density and condensed HOF. The crystal structure obtained by molec-ular mechanics belongs to P21 space group, with lattice parameters Z=2, a=13.8017 A, b=13.4072 A, c=5.5635 A.

Density Functional Study on Structures and Relative Stability of Gd（H2O）n^3＋ （n=8,9）

Density functional theory calculations were performed to study the structures and relative stability of the gadolinium complexes, Gd（H2O）n^3＋ （n=8,9）, in vacuo and in aqueous solution. The polarizable continuum model with various radii for the solute cavity was used to study the relative stability in aqueous solution. The calculated molecular geometries for n=8 and 9 obtained in vacuo are consistent with those observed in experiments. It was found that while the nona-aqua complex is favored in the gas phase, in aqueous solution the octa-aqua conformation is preferred. This result, independent of the types of cavities employed, is in agreement with the experimental observation. The reliability of the present calculation was also addressed by comparing the calculated and experimental free energy of hydration, which revealed that the UA0, UAHF, and UAKS cavities are most appropriate when only the first solvation shell is treated explicitly.

First-principles investigation of cohesive energy and electronic structure in vanadium phosphides

First-principles calculations based on the density-functional theory were employed to study the crystal structure of vanadium phosphide compounds,such as V3P,V2P,VP,VP2 and VP4. Cohesive energy of five types of vanadium phosphide compounds was calculated to assess their structural stability. The charge density distribution and densities of states of vanadium phosphides were discussed to study further their electronic structures. The results show that the structure of metal-rich compounds is considerably more stable than the phosphorus-rich compositions,and covalent bond exists between the V and P atoms of V3P,V2P,VP,VP2 and VP4.

Density Functional Theory Study for Adsorption of Oxygen and Water Molecules on 6H-SiC(0001) Surface

6H-SiC is an important semiconductor material. The 6H-SiC wafer is always exposed to a high-humidity environment and the effect from the absorbed water molecule and some relative adsorbates is not negligible. Here, the oxygen and water molecules absorbed on the 6H-SiC(0001) surface and the dissociation process were studied with density functional theory. On the 6H-SiC(0001) surface, absorbed O2 is spontaneously dissociated into O*, which is absorbed on a hollow site, and further transforms the 6H-SiC(0001) surface into SiO2. The absorbed H2O is spontaneously broken into OH*and H*, which are both absorbed on the top of the Si atom, and OH* is further reversibly transformed into O* and H*. The H* could saturate the dangling Si bond and change the absorption type of O*, which could stabilize the 6H-SiC(0001) surface and prevent it from transforming into SiO2.

Structural, thermodynamics and elastic properties of Mg_(17)Al_(12), Al_2Y and Al_4Ba phases by first-principles calculations

Structural stabilities, thermodynamics stabilities, elastic properties and electronic structures of Mgl7Al12, Al2Y and AlaBa phases were analyzed by first-principles calculations with Castep and Drool3 program based on the density functional theory. The calculated results of heat of formation indicate that AI2Y phase has the strongest alloying ability. The calculated thermodynamic properties show that the thermal stability of these compounds gradually increases in the order ofMgl7Al12, A12Y and Al4Ba phases. Y or Ba addition to the Mg-Al alloys could improve the heat resistance. The calculated bulk modulus B, shear modulus G, elastic modulus E and Poisson ratio v show that the adding Y or Ba to Mg-Al alloys could promote the brittleness and stiffness, and reduce tenacity and plasticity by forming Al4Ba and Al2Y phases. The calculated cohesive energy and density of state （DOS） show that Al2Y has the strongest structural stability, then AlaBa and finally Mg17Al12. The calculated electronic structures show that Al2Y has the strongest structure stability because of the strong ionic bonds and covalent bonds combined action.

Theoretical Investigation on Interaction between Guanine and Luteolin

The interacting patterns of the luteolin and guanine have been investigated by using the density functional theory B3LYP method with 6-31＋G＊ basis set. Eighteen stable structures for the luteolin-guanine complexes have been found respectively. The results indicate that the complexes are mainly stabilized by the hydrogen bonding interactions. Meanwhile, both the number and strength of hydrogen bond play important roles in determining the stability of the complexes which can form two or more hydrogen bonds. Theories of atoms in molecules and natural bond orbital have also been utilized to investigate the hydrogen bonds involved in all the systems. The interaction energies of all the complexes which were corrected by basis set superposition error are 6.04-56.94 kJ/mol. The calculation results indicate that there are strong hydrogen bonding interactions in the luteolin-guanine complexes. We compared the interaction between luteolin and four bases of DNA, and found luteolin-thymine was the strongest and luteolin-adenine was the weakest. The interaction between luteolin and DNA bases are all stronger than luteolin-water.

Structures and Stability of Metal Amidoboranes (MAB): Density Functional Calculations

Molecule geometry structures, frequencies, and energetic stabilities of ammonia borane （AB, NH3BH3 ） and metal amidoboranes （MAB, MNH2BH3）, formed by substituting H atom in AB with one of main group metal atoms, have been investigated by density-functional theory and optimized at the B3LYP levels with 6-311G＋＋ （3dr, 3pd） basic set. Their structural parameters and infrared spectrum characteristic peaks have been predicted, which should be the criterion of a successfully synthesized material. Several parameters such as binding energies, vibrational frequencies, and the energy gaps between the HOMO and the LUMO have been adopted to characterize and evaluate their structure stabilities. It is also found that the binding energies and HOMO-LUMO energy gaps of the MAB obviously change with the substitution of the atoms. MgAB has the lowest binding energy and is easier to decompose than any other substitutional structures under same conditions, while CaAB has the highest chemical activity.

Facile formation of tetragonal-Nb2O5 microspheres for high-rate and stable lithium storage with high areal capacity

Niobium pentoxide;Ion and electron transport;Mass loading;Areal capacity;Lithium-ion batteryNiobium pentoxide(Nb2 O5) has attracted great attention as an anode for lithium-ion battery, which is attributed to the high-rate and good stability performances. In this work, TT-, T-, M-, and H-Nb2 O5 microspheres were synthesized by a facile one-step thermal oxidation method. Ion and electron transport properties of Nb2 O5 with different phases were investigated by both electrochemical analyses and density functional theoretical calculations. Without nanostructuring and carbon modification, the tetragonal Nb2 O5(M-Nb2 O5) displays preferable rate capability(121 m Ah g^-1 at 5 A g^-1), enhanced reversible capacity(163 m Ah g^-1 at 0.2 A g^-1) and better cycling stability(82.3% capacity retention after 1000 cycles)when compared with TT-, T-, and H-Nb2 O5. Electrochemical analyses further reveal the diffusioncontrolled Li+intercalation kinetics and in-situ X-ray diffraction analysis indicates superior structural stability upon Li+intercalation/deintercalation. Benefiting from the intrinsic fast ion/electron transport, a high areal capacity of 2.24 m Ah cm^-2 is obtained even at an ultrahigh mass loading of 22.51 mg cm^-2.This work can promote the development of Nb2 O5 materials for high areal capacity and stable lithium storage towards practical applications.

Modulating composite polymer electrolyte by lithium closo-borohydride achieves highly stable solid-state battery at 25℃

Rational composite design is highly important for the development of high-performance composite polymer electrolytes(CPEs)for solid-state lithium(Li)metal batteries.In this work,Li closo-borohydride,Li_(2)B_(12)H_(12),is introduced to poly(vinylidene fluoride)-Li-bis-(trifluoromethanesulfonyl)imide(PVDF-LiTFSI)with a bound N-methyl pyrrolidone plasticizer to form a novel CPE.This CPE shows superb Li^(+)conduction properties,as evidenced by its conductivity of 1.43×10^(-4) S cm^(-1) and Li^(+)transference number of 0.34 at 25℃.Density functional theory calculations reveal that Li_(2)B_(12)H_(12),which features electron-deficient multicenter bonds,can facilitate the dissociation of LiTFSI and enhance the immobilization of TFSI to improve the Li^(+)conduction properties of the CPE.Moreover,the fabricated CPE exhibits excellent electrochemical,thermal,and mechanical stability.The addition of Li_(2)B_(12)H_(12) can help form a protective layer at the anode/electrolyte interface,thereby preventing unwanted reactions.The above benefits of the fabricated CPE contribute to the high compatibility of the electrode.Symmetric Li cells can be stably cycled at 0.2mA cm^(-2) for over 1200 h,and Li||LiFePO_(4) cells can deliver a reversible specific capacity of 140mAh g^(-1) after 200 cycles at 1C at 25℃ with a capacity retention of 98%.

Structures,stabilities,and IR and ^(13)C-NMR spectra of dihedral fullerenes:A density functional theory study

Dihedral fullerenes are thermodynamically stable molecules with Dnd or Dnh symmetry. Based on experimental findings, two series of dihedral fullerenes with five-fold （C5） and six-fold （C6） symmetry have been studied using density functional theory （DFT）. The DFT calculations showed that for both series the stabilities increased with increasing fullerene size. Structural analyses indicated that the stabilities are related to specific local geometries. In the case of the more abundant C5 series, the presence of approximately planar pentagons and hexagons on the top bowl favors their formation. That is to say, those fuller- enes with small dihedral angles within the polygons are readily formed, because planar hexagons lead to strengthened conjuga- tion which lowers average bonding energies （ABE） and increases thermodynamic stabilities. Non-planar hexagons at equatorial positions in tube-shaped fullerenes have an adverse effect on the conjugation and inhibit their formation. Calculations also demonstrated that fullerenes in the two series, including C50（D5h）, C60（O6h）, C80（O5d）, C96（D6d）, Cllo（D5h）, and Cl20（D5d）, have thermodynamically stable triplet structures with strong conjugation. The calculated IR and 13C NMR spectra of the fullerenes show some similarities and regular trends due to their homogenous structures. The electronic structures indicate that short dou- ble bonds in hexagons with high electron occupancies are readily attacked by electrophilic agents and can also be coordinated by transition metals. Mechanistic discussions suggested that C2 additions and C2 losses constitute reversible processes at high temperature and C2 additions in pentagonal fusions are crucial to the kinetics of the curvature of structures. C3 additions lead to the formation of large fullerenes of other types.