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Ab initio molecular orbital theory including full geometry optimization at the 4-31G level is used to examine the interactions between substitutents X(X = Li, BeH, BH2, CH3, NH2, OH and F) and substrates Y(Y = NH3+, CH3, BH3?) in the isoelectronic series XNH3+, XCH3 and XBH3?. The results indicate that the interaction energies are dominated by σ-effects. NH3+ is found to interact favorably with the σ-donors (e.g. Li, BeH and BH2) and unfavorably with the σ-acceptors (e.g. F, OH, NH2). The reverse pattern a observed for XBH3?. The range of interaction energies for XCH3 is considerably smaller than for XNH3+ and XBH3?. 相似文献
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Gary Kemister Addy Pross Leo Radom Robert W. Taft 《Journal of computational chemistry》1981,2(4):470-477
Ab initio molecular orbital theory with the STO -3G basis set is used to examine both charge and energy interactions in a series of meta- and para-substituted phenylborate anions and toluenes. Comparison of the results is made with data for substituted anilinium cations. It is concluded that whereas NH is a powerful σ acceptor, with essentially no π interaction, BH is primarily a π donor, and, to a slight extent only, a π donor. CH3 is indicated to be both a weak σ and π donor. Energies of interaction of BH and NH with a series of substituents are an order of magnitude larger than corresponding values for CH3. Interaction energies for BH are of opposite sign to those for NH. The results may be understood qualitatively using perturbation molecular orbital (PMO ) theory. 相似文献
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Ab initio molecular orbital theory is used to study substituent effects in a series of β-substituted Et radicals XCH2CH2. For X = BH2 (plan.), CH3, NH2, OH and F, only slight conformational preferences and weak stabilizations are indicated. Such behaviour may be rationalized, using a PMO model, in terms of opposing changes, accompanying variation in X, in positive and negative hyperconjugation between the XCH2 group and the CH2 centre. On the other hand, for groups containing an appropriately oriented, low-lying vacant orbital, viz. X = Li, BeH and BH2 (perp.), there is a pronounced preference for the perpendicular conformation of the radical. This is attributed to 1,3-interaction between the singly-occupied 2p(C) orbital and the vacant 2p(X) orbital. 相似文献
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Application of the configuration mixing model to the SN2 reaction illustrates that charge development in an SN2 reaction is not linearly related to the position of the transition state along the reaction coordinate. 相似文献
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Molecular analysis of the kirromycin biosynthetic gene cluster revealed beta-alanine as precursor of the pyridone moiety 总被引:1,自引:0,他引:1
Weber T Laiple KJ Pross EK Textor A Grond S Welzel K Pelzer S Vente A Wohlleben W 《Chemistry & biology》2008,15(2):175-188
Kirromycin is a complex linear polyketide that acts as a protein biosynthesis inhibitor by binding to the bacterial elongation factor Tu. The kirromycin biosynthetic gene cluster was isolated from the producer, Streptomyces collinus Tü 365, and confirmed by targeted disruption of essential biosynthesis genes. Kirromycin is synthesized by a large hybrid polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) encoded by the genes kirAI-kirAVI. This complex involves some very unusual features, including the absence of internal acyltransferase (AT) domains in KirAI-KirAV, multiple split-ups of PKS modules on separate genes, and swapping in the domain organization. Interestingly, one PKS enzyme, KirAVI, contains internal AT domains. Based on in silico analysis, a route to pyridone formation involving PKS and NRPS steps was postulated. This hypothesis was experimentally proven by feeding studies with [U-13C3(15)N]beta-alanine and NMR and MS analyses of the isolated pure kirromycin. 相似文献
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W. Pfeiffer M. Deicher R. Keller R. Magerle P. Pross H. Skudlik
Th. Wichert
H. WolfD. Forkel
N. Moriya
R. Kalish 《Applied Surface Science》1991,50(1-4):154-158Investigations of 111mCd implanted GaAs and InP crystals using the microscopically sensitive perturbed angular correlation technique show that the implanted Cd is incorporated on unperturbed substitutional lattice sites during rapid thermal annealing at significant lower temperatures than for electrical activation is required. In GaAs the higher implantation temperature at 473 K did not show any influence on this annealing stage, whereas as higher implantation dose hinders the annealing. We conclude that not only the local environment of the implant but also the long-range lattice perfection has to be restored for the electrical activation of implants in III-V compound semiconductors. 相似文献
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[structure: see text] High level ab initio molecular orbital calculations confirm experimental indications that the effect of alkyl substituents (R = Me, Et, i-Pr, t-Bu) on R-X bond dissociation energies varies considerably according to the nature of X. A simple qualitative explanation in terms of valence-bond theory is presented, highlighting the increasing importance of the stabilization of R-X by the ionic R(+)X(-) configuration for electronegative X substituents (such as F, OH, and OCH(3)). 相似文献