硼氢化反应机理Ⅴ——HOMO-HOMO相互作用对有机腈类硼氢化反应的影响(英文)

利用量子化学从头计算法ＲＨＦ／ＳＴＯ－３Ｇ基组对甲基腈和氢氰酸硼氢化反应进行了理论研究。微扰分子轨道理论分析表明：在甲基腈硼氢化反应中,过渡态的ＨＯＭＯ是由ＨＯＭＯ－ＬＵＭＯ相互作用和ＨＯＭＯ－ＨＯＭＯ相互作用形成的。通过与氢氰酸硼氢化反应的比较,即考虑甲基对ＨＯＭＯ－ＨＯＭＯ相互作用的影响,可知供电子基团能提升ＨＯＭＯ的能级进而增加了有机腈类硼氢化反应的速度     

乙炔与氢化锂单体和二聚体加成反应中的HOMO-HOMO相互作用

用前线分子轨道分析乙炔与氢化锂单体和二聚体加成反应的过渡态.结果表明,在其反应过渡态形成过程中,反应物间的HOMO-LUMO和HOMO-HOMO相互作用均起重要作用.较强烈的HOMO-HOMO相互作用与过渡态附近反应由亲电性加成向亲核性加成转变相关,该反应特性的转变又与LiH中的氢原子在反应中的电荷转移相关.     

Frontier orbitals aromaticity: A simple approach to the analysis of pericyclic reactions

A new and simple approach to the analysis of pericyclic reactions based on combination of the interaction of HOMO-LUMO molecular orbitals and Huckel-Mobius aromaticity concept in transition state is reported. This approach seems to be more powerful than the Woodward-Hoffmann approach which appears to be limited to reactions with sufficiently high symmetry, and the Huckel-Mobius that use the non-participating lowest basis set (Ψ₁). One important impact of the current approach is its capability in addressing the photochemical concerted reactions.     

Reaction ergodography for the additions of HLi and its dimer to acetylene

The ab initio calculation has been performed with the addition pathways of HLi and its dimer to acetylene at the RHF/3-21G basis set. It shows that the reaction mechanisms of these two reactions are rather similar. In either of two reaction pathways, there is a meta-stable molecular complex near the isolated reactant state. This kind of addition can be treated approximately as the unimolecular reaction in which the molecular complex rearranges into the product. We have estimated the activation entropies and the statistical A factors of these two reactions by the use of RRKM theory. Frontier molecular orbital analysis of these two transition states reveals their HOMOS to be formed from both HOMO-LUMO and HOMO-HOMO interactions.     

Application of intermolecular SCF-perturbation theory to the regioselectivity in the Diels-Alder reaction

Intermolecular perturbation theory in the density matrix formalism is applied to investigate the directional behaviour of an electron-donating (-CH₃) or an electron-accepting (-CN) group in 1- or 2-substituted butadienes in the Diels-Alder reaction with acrylonitrile. The calculated CNDO/2 perturbation energies are analysed in three different ways by considering: a) the different perturbation energies, b) the diatomic parts of the interaction energy and c) the HOMO-LUMO contribution to the second-order energy. The regioselectivity is due to a subtle balance of charge-transfer interactions and steric effects of the substituents on the diene and the dienophile. The changes of intra- and intermolecular diatomic energy contributions are correlated with the process of bond formation and bond weakening. The intermolecular perturbation energies are dominated by pairwise interactions between the terminal C-atoms and by the secondary Woodward-Hoffmann interaction. These three localized interactions determine the endo addition and reflect the orienting power of the substituents.     

The IRC method in chemical reactions: Reaction ergodography for the addition of LiH to acetylene

The ab initio calculation have been performed on the addition of LiH to acetylene at RHF/3-21G basis set. The geometries and energies of the isolated reactant, molecular complex, transition state and product have been determined on the singlet potential energy surface of the ground state. Our results indicate that there is a meta stable molecular complex near the isolated reactant in the reaction pathway. The process from isolated reactant to molecular complex is a non-bonding-exchanging reaction process, and the process from molecular complex to product is the rate-controlling step of the reaction. We also estimate the activated entropy and the frequency factor of the rate-controlling step by using the RRKM theory. The FMO analysis for the transition state reveals the HOMO of transition state to be formed from both HOMO-LUMO and HOMO-HOMO interactions.     

Comparing a photoinduced pericyclic ring opening and closure: differences in the excited state pathways

The photochromicity of fulgimides rests on the existence of open (E) and closed ring (C) isomers. As predicted by the Woodward-Hoffmann rules both isomers can photochemically be interconverted. This interconversion has been studied by femtosecond fluorescence and transient absorption spectroscopy. For either direction (E --> C cyclization and C --> E cycloreversion) a biphasic fluorescence decay on the 0.1-1 ps time scale is observed. The longer time constants of the decays equal the formation times of the photoproducts. The time constants retrieved (0.06 and 0.4 ps for E --> C, 0.09 and 2.4 ps for C --> E) and the associated spectral signatures differ substantially. This indicates that no common excited-state pathway for the two directions exists, as one would infer from a simple Woodward-Hoffmann consideration. These findings support recent quantum dynamic calculations on the excited-state topology of fulgimides.     

The Importance of Charge Transfer between the Retinal Chromophore and the Protein Environment in Bacteriorhodopsin: A Theoretical Analysis on Reduced and Oxidized Chromophores

Abstract— The importance of charge transfer(CT) between the retinal chromophore and the protein environment in the ground state of bacteriorhodopsin(BR) has been verified by using ab initio and semiempirical molecular orbital methods. We hypothesize that the chromophore is stabilized in BR by highest occupied molecular orbital-lowest unoccupied molecular orbital(HOMO-LUMO) interaction with the protein environment. If sufficient charge is transferred between two sites due to the strong HOMO-LUMO interaction, the chromophore might be treated as a one-electron reduced species(when it behaves as an electron acceptor), or as a one-electron oxidized one (when it acts as an electron donor).In both optimized geometries, the -conjugated systems exhibit a drastic decrease in bond alternation. To estimate the rotational barrier for thermal isomerization between the al-trans and the 13,15-dicis form, the potential energy curve around these two bonds was computed. The first -_* transition energy was also calculated for an inspection of the opsin shift. The barrier height and the transition energy became much lower as a result of the chromophore reduction. The site selectivity in photo- and thermal isomerization and the opsin shift in BR can be well explained by considering CT from the protein environment to the chromophore.     

Closo-alanes (Al4H4, AlnHn+2, 4 <or= n <or= 8): a new chapter in aluminum hydride chemistry

Anion photoelectron spectroscopy and density functional theory were employed to study aluminum hydride clusters, AlnHm- (4 相似文献   

Understanding the Woodward-Hoffmann rules by using changes in electron density

The Woodward-Hoffmann rules for pericyclic reactions are explained entirely in terms of directly observable physical properties of molecules (specifically changes in electron density) without any recourse to model-dependent concepts, such as orbitals and aromaticity. This results in a fundamental explanation of how the physics of molecular interactions gives rise to the chemistry of pericyclic reactions. This construction removes one of the key outstanding problems in the qualitative density-functional theory of chemical reactivity (the so-called conceptual DFT). One innovation in this paper is that the link between molecular-orbital theory and conceptual DFT is treated very explicitly, revealing how molecular-orbital theory can be used to provide "back-of-the-envelope" approximations to the reactivity indicators of conceptual DFT.     

Perturbational analysis of the regioselectivity in the acrolein dimerization

Using intermolecular SCF-perturbation theory, the controlling factors determining the regioselectivity in the acrolein dimerization are derived by a rigorous decomposition of the interaction energy. This partitioning of the interaction energy is performed up to a level where a comparison with frontier orbital models is possible. The regioselectivity is found to be determined by orbital interactions occuring via the terminal atoms and atoms involved in secondary Woodward-Hoffmann interactions.     

A Basis for Orbital Symmetry Rules

The influence of molecular symmetry on reaction rates is examined with an approach in which reactions are viewed as electronic transitions between states of reacants and products (described, in turn by quasiadiabatic potential surface). The moleculer Hamiltonian is used to derive selection rules for these transitions. The complete Hamilatonian has no useful symmetery. Neglect of non-Born-Oppenheimer and spin-orbit terms (and of other angular momentum coupling terms) leads to an apporixmate Hamiltonian and to selection rules which from the basis of the Woodward-Hoffmann rules. This apporch provides an alternative to the adiabatic potantial surfaces, reaction coordinates, and transition state theory used in more familiar discussions of the Woodward-Hoffmann rules. Further, it provides a particulary clear method for discussing violations of these symmetry rules, and for differentiating concerted and nonconcerted reactions.     

4,5-二芳基噁唑的光化学环化消除合成菲并[9,10-d]噁唑

具有异芪生色团结构的分子是有机光化学研究的重要对象^[1]。4,5-二芳基嗯唑(1)具有部分结构与异芪类似,本文对其分子内环化消除过程和产物进行了研究。     

Au42: an alternative icosahedral golden fullerene cage

We present a new icosahedral gold fullerene, Au42, based on density functional theory calculations. The Au42 fullerene has a nanoscale hollow space that can hold up to 13 Au atoms. The Au42 fullerene also has a larger HOMO-LUMO gap compared to the compact-filling geometries. However, unlike the known gold fullerene Au32, the Au42 fullerene does not satisfy the 2(N + 1)2 aromatic rule and has a positive NICS value at the center of the cage. These two nanometer-sized gold fullerenes can be used as golden cages to accommodate other atoms or molecules for the purpose of studying fundamental chemistry because of their apparently dissimilar chemical characteristics, or they can be used as structural motifs to build highly stable core-shell nanoclusters or novel cluster-assembled materials.     

Primary and secondary orbital effects in dyotropic rearrangements

A new class of orbital symmetry controlled reactions which involve the simultaneous migration of two σ-bonds is discussed. Analysis of the stereochemical fate of the migrating groups in thermal and photochemical shifts, based on the Woodward-Hoffmann roles and supported by MINDO/2 calculations, is presented.     

Electronic structure of xDNA

xDNA is an artificial duplex made of natural and benzo-homologated bases. The latter can be seen as a fusion between benzene and a natural base. We have used two different ab initio techniques, one based on B3LYP and a Gaussian expansion of the wave functions, and the other on GGA and plane-waves, to investigate the electronic properties of an xDNA duplex and a natural one with an analogous sequence. The calculations were performed in dry conditions, i.e., H atoms were used to neutralize the charge. It is found that the HOMO-LUMO gap of xDNA is about 0.5 eV smaller than that of B-DNA, independent of the technique used. The pi-pi* gap of xDNA is 1.3 or 1.0 eV smaller than that of B-DNA, depending on whether one uses B3LYP/6-31G or GGA/plane-waves, respectively. An analysis of how saturation changes the electronic properties of the nucleotide pairs that make up these duplexes suggests that different saturation schemes significantly affect the HOMO-LUMO gap value of xDNA and B-DNA. The same is not true for the pi-pi* gap. That xDNA has a smaller pi-pi* gap than B-DNA suggests that xDNA could be a plausible candidate for molecular-wire applications.     

Capillary theory of free fluid surfaces

The traditional formulation of capillary theory does not explicitly contain the general dimension equation that is valid also for its own scalar variables. Its introduction enables the experimentally determinable physical properties to be interpreted. These properties individually characterize the bulk phases generating the layers that enter into the capillary interaction. Not only empirically known approximate relationships, such as the van der Waals and Walden equations, the Watson’s formula, and the Cailletet–Mathias rule, can be derived through them, but also new findings can be made. By extending the formulation with a new type of parameters, the relationships of temperature, density-dependence, etc. may be directly generated. The individual concept, which differs from the traditional theory in only one extra power law, is compatible with other capillarity methods based on material structure and outperforms the heuristic power of the traditional theory in terms of operability.     

Designing Hole Transport Materials with High Hole Mobility and Outstanding Interface Properties for Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have attracted much attention due to their rapid increase in power conversion efficiencies (PCEs), and many efforts are devoted to further improving the PCEs. Designing highly efficient hole transport materials (HTMs) for PSCs may be one of the effective ways. Herein we theoretically designed three new HTMs (FDT−N, FDT−O, and FDT−S) by introducing a nitrogen-phenyl group, an oxygen atom, and a sulfur atom into the spiro core of an experimentally synthesized HTM (FDT), respectively. And then we performed quantum chemical calculation to study their application potential. The results show that the devices with FDT−O and FDT−S instead of FDT may have higher open circuit voltages owing to their lower highest occupied molecular orbital (HOMO) energy levels. Moreover, FDT−S exhibits the best hole transport performance among the studied HTMs, which may be due to the significant HOMO-HOMO overlap in the hole hopping path with the largest transfer integral. Furthermore, the results on interface properties indicate that introducing oxygen and sulfur atoms can enhance the MAPbI₃/HTM interface interaction. The present work not only offers two promising HTMs (FDT−O and FDT−S) for PSCs but also provides theoretical help for subsequent research on HTMs.     

Collinear collision of an atom with a homonuclear diatomic molecule

The problem of collinear scattering of an atom from a homonuclear diatomic molecule is formulated in terms of a first-order nonlinear matrix differential equation for the variable coefficient of reflection. For a homonuclear molecule when the target Hamiltonian is invariant under the parity transformation, only transitions between even states or odd states are possible. This selection rule reduces the number of open or closed channels that contribute to the reflection and transmission coefficients. But for numerical calculation, under the conditions of the problem, one can approximate the target Hamiltonian by the Hamiltonian of a displaced harmonic oscillator. In this approximation, the reflectional symmetry of the Hamiltonian is not preserved and transitions between any two levels of the target are possible. To simplify the problem further, the interaction between the projectile and the target is assumed to be a sum of two Gaussian terms. For this combination of the potentials the many-channel interaction can be expressed analytically. By fitting the Lennard–Jones potential with a sum of two Gaussian potentials and solving the matrix differential equation, transition probabilities are obtained for the He? H₂ collision. The numerical results are compared with the results found by Secrest and Johnson, and by Clark and Dickinson.     

Core molecule dependence of energy migration in phenylacetylene nanostar dendrimers: Ab initio molecular orbital-configuration interaction based quantum master equation study

The core molecule dependence of energy (exciton) migration in phenylacetylene nanostar dendrimers is investigated using the ab initio molecular orbital (MO)-configuration interaction based quantum master equation approach. We examine three kinds of core molecular species, i.e., benzene, anthracene, and pentacene, with different highest occupied MO-lowest unoccupied MO (HOMO-LUMO) gaps, which lead to different orbital interactions between the dendron parts and the core molecule. The nanostars bearing anthracene and pentacene cores are characterized by multistep exciton states with spatially well-segmented distributions: The exciton distributions of high-lying exciton states are spatially localized well in the periphery region, whereas those of low-lying exciton states are done in the core region. On the other hand, for the nanostar bearing benzene core, which also has multistep exciton states, the spatial exciton distributions of low-lying exciton states are delocalized over the dendron and the core regions. It is found that the former nanostars exhibit nearly complete exciton migration from the periphery to the core molecule in contrast to the latter one, in which significant exciton distribution remains in the dendron parts attached to the core after the exciton relaxation, although all these dendrimers exhibit fast exciton relaxation from the initially populated states. It is predicted from the analysis based on the MO correlation diagrams and the relative relaxation factor that the complete exciton migration to the core occurs not only when the HOMO-LUMO gap of the core molecule is nearly equal to that of the dendron parts attached to the core (anthracene case) but also when fairly smaller than that (pentacene case), whereas the complete migration is not achieved when the HOMO-LUMO gap of the core is larger than that of the dendron parts (benzene case). These results suggest that the fast and complete exciton migration of real dendrimers could be realized by adjusting the HOMO-LUMO gap of the core molecule to be smaller than that of dendron parts, although there exist more complicated relaxation processes as compared to simple dendritic aggregate models studied so far.