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1.
The F–H···YZ2 (Y = C, Si, BH, A1H;Z = H, PH3) systems were examined using density functional theory calculations. The main focus of this work is to demonstrate that the chemistry of Y(PH3)2 exhibits a novel feature which is a central Y atom with unexpected high basicity. Further, the hydrogen bond strength can be adjusted by the substitution of H atoms of YH2 by PH3 groups. The FH···C(PH3)2 system has the strongest hydrogen bond interaction, which is larger than a conventional hydrogen bond. In addition to electrostatic interaction, donor‐acceptor interaction also plays an important role in determining the hydrogen bond strength. Therefore, a carbon atom can not only be the hydrogen bond acceptor but also can create an unusual stabilized hydrogen bond complex. Also, X3B–YZ2 (X = H, F; Y = C, Si, BH, A1H;Z = PH3, NH3) systems were examined, and it was found that the bond strength is controlled predominately by the HOMO‐LUMO gap (ΔIP). The smaller the ΔIP, the larger the bond dissociation energy of the B–Y bond. In addition, NH3 is a better electron‐donating group than PH3, and thus forms the strongest donor‐acceptor interaction between X3B and Y(NH3)2.  相似文献   

2.
Density-functional theory studies were applied to investigate the structural, electronic, and optical properties of 9-heterofluorenes achieved by substituting the carbon at 9 position of fluorene with silicon, germanium, nitrogen, phosphor, oxygen, sulfur, selenium, or boron. These heterofluorenes and their oligomers up to pentamers are highly aromatic and electrooptically active. The alkyl and aryl substituents of the heteroatom have limited influence, but the oxidation of the atom has significant influence on their molecular structures and properties. The highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) interaction theory was successfully applied to analyze the energy levels and the frontier wave functions of these heterofluorenes. Most heterofluorenes belong to type B of interaction with low-lying LUMO and have the second kind of wave function. Carbazole and selenafluorene have type C of interaction with high-lying HOMO and the third kind of wave function. Types C and D of heterofluorenes, such as carbazole, oxygafluorene, sulfurafluorene, and selenafluorene also have high triplet state energies. The extrapolated HOMO and LUMO for polyheterofluorenes indicate that polyselenonafluorene has the lowest LUMO; polycarbazole has the highest HOMO; polyselenafluorene has the highest bandgap (E(g)); and polyborafluorene has the lowest E(g). Heterofluorenes and their oligomers and polymers are of great experimental interests, especially those having extraordinary properties revealed in this study.  相似文献   

3.
A systematic density functional theory investigation has been carried out to explore the possible structures of Sc2C80 at the BMK/6‐31G(d) level. The results clearly show that Sc2@C80Ih, Sc2@C80D5h, and Sc2C2@C78C2v can be identified as three isomers of Sc2C80 metallofullerene with the lowest energy. Frontier molecular orbital analysis indicates that the two Sc2@C80 isomers have a charge state as (Sc3+)2@C806?and the Sc2C2@C78 has a charge state of (Sc3+)2C22?@C784?. Moreover, the metal‐cage covalent interactions have been studied to reveal the dynamics of endohedral moiety. The vertical electron affinity, vertical ionization potential, infrared spectra and 13C nuclear magnetic resonance spectra have been also computed to further disclose the molecular structures and properties.  相似文献   

4.
The effect of the composition ratio between arsenic and silicon atoms on the structures and properties of AsxSi6?x (x = 0–6) have been systematically investigated using the density functional theory at the B3LYP/6‐311+G* level. The AsxSi6?x clusters prefer substitutional rather than attaching structures; the Si‐rich clusters favor Si6‐like structures, whereas the As‐rich clusters prefer As6‐like structures. The As atoms locating at the framework may explain the difficulty of removal of arsenic impurities from polycrystalline silicon. In general, the average binding energies gradually decrease, implying the AsxSi6?x clusters become increasingly unstable as x increases. Both the HOMO‐LUMO gaps and the As‐dissociation energies present a strong even–odd alternation, implying alternating chemical stability, with the even x members being more stable than the odd ones. The dissociation energies of an As atom from AsxSi6?x are: 3.07, 2.84, 1.84, 2.52, 1.86, and 2.85 eV, for x = 1–6, respectively, and 3.80, 3.08, 2.64, 3.01, 2.93, 3.16 eV for Si (x = 0–5). These dissociation energy results should provide a useful reference for further experimental investigations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012  相似文献   

5.
6.
Hong Wang  Lin Wu 《中国化学》2011,29(4):735-740
Density functional calculations have been carried out on a series of fluorinated empty cages XnFn(n=2–20) with X?Si, Ge, and Sn. It indicates that the fullerene‐like cage structure with pentagons turns out to be the most stable with n increasing, and the stability of the XnFn isomers increases with the number of five‐membered rings. The HOMO‐LUMO gap for Ge (n=6, 10) cages is found to be even larger than the values for Si cages, though in bulk Ge has a smaller band gap than Si. Moreover, calculation of the Gibbs free energy of oligomerization reaction of SiF→1/n (SiF)n showed that this reaction is exothermic even at 900 K, indicating the favorability of their formation from the SiF monomer.  相似文献   

7.
In an effort to manipulate the bond strengths of hydrogen bonds, we have studied a three-component chemical system consisting of a reaction center, a conjugated bridge, and a hydrogen-bonding site. Protonation of the reaction center triggers intramolecular charge transfer from the hydrogen-bonding site, altering its affinity to bind to an acceptor. Previously, we had found that this communication (signal transduction) between the reaction center and the hydrogen-bonding site does not necessarily die out with increasing length of the conjugated bridge. In certain cases, this signal transduction is maintained-and even amplified-over long distances (I. Chao, T.-S. Hwang, Angew. Chem. 2001, 113, 2775-2777; Angew. Chem. Int. Ed. 2001, 40, 2703-2705). In this study we report the results of an extensive theoretical investigation of this problem to provide insights into this intriguing phenomenon. In the systems we investigated it was found that the push-pull process between the hydrogen-bonding site and the protonatable reaction center was mediated with the greatest facility by conjugated bridges with low-lying pi and pi* orbitals.  相似文献   

8.
Geometries, electronic structure and electronic absorption spectra of thiophene based dye-sensitized solar cells were performed using Density Functional Theory (DFT) and time dependent density functional theory (TD-DFT). Different electron donating and electron withdrawing groups have been substituted. Geometries and electronic properties have been computed at B3LYP/6-31G7 and absorption spectra at TD-B3LYP/6-31G7 level of theory. Major change in bond lengths and bond angles occurs in the system where there is electron withdrawing or electron donating groups have been substituted. In SYSTEM-2 and SYSTEM-3 intra charge transfer has been observed. HOMO of SYSTEM-2 and SYSTEM-3 is delocalized on left side while LUMO on right side of the molecule. In SYSTEM-1, HOMO is on left side while LUMO is in the center. The designed systems show two absorption peaks for each of the system. In short, choice of appropriate electron withdrawing and donating groups is very important for improving the performance of dye-sensitized solar cells.  相似文献   

9.
Fluorine‐containing amorphous carbon films [fluoring‐containing diamond‐like carbon (F‐DLC)] were fabricated on Si wafer by direct current plasma enhanced chemical vapor deposition (dc‐PECVD) technique using CF4 and Ar as gas sources, confirmed by XPS and Raman analyses. The friction tests were carried out on a rotating ball‐on‐disk apparatus in high vacuum atmosphere (≤5.0 × 10?4 Pa) at the load of 0.5 N selecting glass (mainly containing silicon–oxygen tetrahedron structure) and Al2O3 with the same hardness and surface roughness as the counterpart balls. The results indicate that glass/F‐DLC results in lower friction coefficient of 0.14 than that of the Al2O3/F‐DLC (0.20). At the same time, no wear was occurred, and the transfer layer was not formed on the counterpart ball for glass/F‐DLC, while the wear of Al2O3/F‐DLC is slightly larger than that of glass/F‐DLC. However, just like the glass ball, there is no formation of transfer layer on the Al2O3 ball surface. Furthermore, the chemical state of fluorine in the film after friction, which mainly existed in the form of the C–CF and C–F bonds, did not change compared with the F‐DLC film, while the fluorine content has changed significantly. As a result, it is assumed that interface electrostatic interaction based on acid–base theory plays an extremely important role in the process of friction. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

10.
11.
2,4-二氯-5-硝基嘧啶的甲胺化反应是制备鸟嘌呤衍生物中间体的重要反应[1],实验发现该反应主要有三种产物:究竟哪种产物占优势?主要由反应物(Ⅰ)和甲胺分子的电子结构性质决定,本文对反应物(Ⅰ)和甲胺的电子结构性质进行了abinitio研究.采用RHF能量梯度法,在DZ基组水平上对反应物(Ⅰ)和甲胺分子进行全分子几何优化,梯度模收敛指标控制为0刀003·对得到的平衡几何进行市居数分析和静电势研究.计算采用Tulbomole程序[到在SGI图形工作站上完成.反应物(I)分子的平衡几何如图1所示.从优化后的结构可知,硝基和南吠环不在…  相似文献   

12.
The reactive behavior of acetylcholine and its agonist molecules have been investigated using B3LYP hybrid density functional method at the 6‐311++G** basis set level, in the gas phase and aqueous phase. The calculations have been performed to obtain optimized geometries, relative reactivities, net atomic charges, HOMO, and LUMO energies. The solvent effect has been analyzed by using the continuum model (IPCM) and, the obtained results have shown that the all molecules have been stabilized more by solvent dielectric constant. For Ach and its analogues, it has been very well known that esteratic site and quaternary ammonium group which have reflected the difference in biological activity have been the two of the most important active site for interactions between molecule and its receptor. The structures of these analogues have provided an essential foundation for subsequent structure‐activity analysis of ligand binding at acetylcholine receptors, neuronal uptake inhibitors and transporters. Molecular modeling predictions will be important initial steps toward the development of novel pharmaceuticals in the fight acetylcholine‐related neurological disorders. This work is therefore expected to facilitate the design and development of new biologically active Ach analogues to treat Ach‐related neurological disorders and, specially is used to qualitative understanding of the reactivity and related properties and, so on can be used to a preselection of new ligands which at the moment is taken essentially from empirical knowledge. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011  相似文献   

13.
A computationally cheap approach combining time-independent density functional theory (TIDFT) and semiempirical methods with an appropriate extrapolation procedure is proposed to accurately estimate geometrical and electronic properties of conjugated polymers using just a small set of oligomers. The highest occupied molecular orbital-lowest unoccupied molecular orbital gap (HLG) obtained at a TIDFT level (B3PW91) for two polymers, trans-polyacetylene--the simplest conjugated polymer, and a much larger poly(2-methoxy-5-(2,9-ethyl-hexyloxy)-1,4-phenylenevinylene (MEH-PPV) polymer converge to virtually the same asymptotic value than the excitation energy obtained with time-dependent DFT (TDDFT) calculations using the same functional. For TIDFT geometries, the HLG is found to converge to a value within the experimentally accepted range for the band gap of these polymers, when an exponential extrapolation is used; however if semiempirical geometries are used, a linear fit of the HLG versus 1/n is found to produce the best results. Geometrical parameters are observed to reach a saturation value in good agreement with experimental information, within the length of oligomers calculated here and no extrapolation was considered necessary. Finally, the performance of three different semiempirical methods (AM1, PM3, and MNDO) and for the TIDFT calculations, the performance of 7 different full electron basis sets (6-311+G**, 6-31+ +G**, 6-311+ +G**, 6-31+G**, 6-31G**, 6-31+G*, and 6-31G) is compared and it is determined that the choice of semiempirical method or the basis set does not significantly affect the results.  相似文献   

14.
Electronic structures of D4h square‐fused zinc porphyrin sheets of two types ( SA , SB ), where SA is a directly mesomeso‐, β‐β‐, and β‐β‐linked array and SB is a directly β‐fused array, were compared using density functional theory (DFT). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of oligomeric SA n are characteristically delocalized at the cyclooctatetraene‐like sites composed of β‐pyrrolic carbons and their nearest‐neighbor nitrogens. Those of oligomeric SB n remain solitary monomeric features, reflecting weakly interacting porphyrin units. These two‐dimensionally (2D) square‐fused sheets, especially for SA n, show effective reduction of both the HOMO–LUMO energy gaps (Eg) and the lowest Q‐like excitation energies because of LUMO's greater stabilization with increasing number of porphyrins than the corresponding one‐dimensionally (1D) linear‐fused tapes. To estimate the minimum value of Eg, the electronic band structures of the infinite‐fused SA and SB were examined in detail using modern periodic DFT. Results indicate a full metal for SA , with HOMO and LUMO bands crossing the Fermi level, and a semiconductor with Eg ≈ 0.5 eV for SB . Furthermore, the phonon modes and the electron–phonon coupling (EPC) constant of SA were calculated throughout the Brillouin zone using density functional perturbation theory (DFPT), yielding a weak EPC constant, λ = 0.35. Within the standard phonon‐mediated BCS mechanism, the superconducting transition temperature, Tc is demonstrated using the McMillan formula, predicting ≈0.5 K. Results show that SA will become a rare synthetic metal/superconductor without a metal‐insulator transition coming from Peierls lattice instability because it has no serious imaginary phonon modes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009  相似文献   

15.
Ethene and two kinds of nitrating reagents (HNO3 and N2O5) were included in respective molecular systems, which progressed through a two‐stage electrophilic and free radical nitrosubstitution, resulting in the corresponding nitroethene compounds. Subsequent halogenation (using Cl2 and Br2) and amination (using ammonia) were then performed, also by electrophilic and radical substitution, to produce the target 1,1‐diamino‐2,2‐dinitroethene (FOX‐7) derivatives. All transition state species were identified using a two‐ or three‐structure Synchronous Transit‐Guided Quasi‐Newton between the Cartesian coordinates of the related molecular systems at specific reaction stages. The modeling results suggest that N2O5 is the better agent for nitration and bromine is suitable for use in halogenation. The comparable activation energies throughout the reaction stages were considered to imply the most feasible pathways of FOX‐7 synthesis. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010  相似文献   

16.
The difference between the excitation energies and corresponding orbital energy gaps, the exciton binding energy, is investigated based on time‐dependent (TD) density functional theory (DFT) for long‐chain systems: all‐trans polyacetylenes and linear oligoacenes. The optimized geometries of these systems indicate that bond length alternations significantly depend on long‐range exchange interactions. In TDDFT formalism, the exciton binding energy comes from the two‐electron interactions between occupied and unoccupied orbitals through the Coulomb‐exchange‐correlation integral kernels. TDDFT calculations show that the exciton binding energy is significant when long‐range exchange interactions are involved. Spin‐flip (SF) TDDFT calculations are then carried out to clarify double‐excitation effects in these excitation energies. The calculated SF‐TDDFT results indicate that double‐excitation effects significantly contribute to the excitations of long‐chain systems. The discrepancies between the vertical ionization potential minus electron affinity (IP–EA) values and the HOMO–LUMO excitation energies are also evaluated for the infinitely long polyacetylene and oligoacene using the least‐square fits to estimate the exciton binding energy of infinitely long systems. It is found that long‐range exchange interactions are required to give the exciton binding energy of the infinitely long systems. Consequently, it is concluded that long‐range exchange interactions neglected in many DFT calculations play a crucial role in the exciton binding energies of long‐chain systems, while double‐excitation correlation effects are also significant to hold the energy balance of the excitations. © 2016 Wiley Periodicals, Inc.  相似文献   

17.
有机/聚合物材料体系能带结构的表征电化学方法研究   总被引:12,自引:0,他引:12  
以8-羟基喹啉铝和聚(N-乙烯基)趾唑为例,采用多种电化学方法确立了有机聚合物半导体材料的最高占有轨道和最低空轨道能级的位置,所得数据与光电子发射法表征结果一致。用电化学方法测得的带隙Eg^el与吸收光谱法得到的Eg^opt吻合。用测得的能带结构参数为依据研制的电致发光器件的发光性能很高。  相似文献   

18.
The interaction of formamide and the two transition states of its amide group rotation with one, two, or three water molecules was studied in vacuum. Great differences between the electronic structure of formamide in its most stable form and the electronic structure of the transition states were noticed. Intermolecular interactions were intense, especially in the cases where the solvent interacted with the amide and the carbonyl groups simultaneously. In the transition states, the interaction between the lone pair of nitrogen and the water molecule becomes important. With the aid of the natural bond orbitals, natural resonance theory, and electron localization function (ELF) analyses an increase in the resonance of planar formamide with the addition of successive water molecules was observed. Such observation suggests that the hydrogen bonds in the formamide–water complexes may have some covalent character. These results are also supported by the quantitative ELF analyses. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012  相似文献   

19.
20.
The novel compounds (E)‐2‐(((4‐hydroxyphenyl)imino)methyl)phenol, Tetraphenyl (hydroxyl) imidazole and their corresponding Boron difluoride complexes were synthesized and characterized by spectroscopic techniques. Density functional theory calculations at B3LYP‐D3/6–311++G (d, p) level of theory were performed for the geometric parameters. The MEP surface studies were used to understand the behavior of molecules in terms of charge transfer and to determine how these molecules interact. We used the GIAO and the B3LYP‐D3 with a 6–311++ G (d, p) basis set to simulate the (1H‐NMR and 19F‐NMR) and the IR spectra, respectively. The corresponding calculated results are in good agreement with the experimental data. The stability of the molecule arising from hyperconjugation interaction and charge delocalization were analyzed using NBO analysis. FMOs revealed the occurrence of charge transfer within the molecule. The complexation using BF3.Et2O was also found to have remarkable effects on the electrochemical properties of the studied molecules, where (b) and (d) present lower chemical stability, higher reactivity and higher polarizability than (a) and (c), respectively. Moreover, the energy gap of (a) and (c) decreased after complexation using BF3.Et2O, indicating the reliability of the electrochemical evaluation of LUMO and HOMO energy levels. These values are the factors explaining the possible charge transfer interaction within the molecule. The absorption and emission spectra of the model compound were also simulated and compared to experimental observations in the DMF solvent. The results of DFT calculations supported the structural and spectroscopic data and confirmed the structure modification of frontier molecular orbitals for BF2 complexes as well as tunable potentials and energy levels.  相似文献   

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