The Experts below are selected from a list of 51 Experts worldwide ranked by ideXlab platform
Fernando P. Cossío - One of the best experts on this subject based on the ideXlab platform.
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Interplay between aromaticity and strain in double group transfer reactions to 1,2-benzyne.
Journal of computational chemistry, 2016Co-Authors: Israel Fernández, Fernando P. CossíoAbstract:Density Functional Theory calculations are used to explore the double hydrogen atom transfer from different alkanes to 1,2-benzyne. State-of-the-art calculations including the Activation Strain Model of reactivity, Energy Decomposition Analysis, and Valence Bond Methods, reveal the origins of the relatively low activation barriers computed for these processes compared to the analogous reaction involving acetylene. In addition, the interplay between the in-plane aromaticity of the corresponding transition states and the variation of the π-aromaticity associated with the benzyne moiety as well as their influence on the barrier heights of the transformations are analyzed in detail.
Israel Fernández - One of the best experts on this subject based on the ideXlab platform.
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Interplay between aromaticity and strain in double group transfer reactions to 1,2-benzyne.
Journal of computational chemistry, 2016Co-Authors: Israel Fernández, Fernando P. CossíoAbstract:Density Functional Theory calculations are used to explore the double hydrogen atom transfer from different alkanes to 1,2-benzyne. State-of-the-art calculations including the Activation Strain Model of reactivity, Energy Decomposition Analysis, and Valence Bond Methods, reveal the origins of the relatively low activation barriers computed for these processes compared to the analogous reaction involving acetylene. In addition, the interplay between the in-plane aromaticity of the corresponding transition states and the variation of the π-aromaticity associated with the benzyne moiety as well as their influence on the barrier heights of the transformations are analyzed in detail.
Ian H. Williams - One of the best experts on this subject based on the ideXlab platform.
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Ab initio Valence Bond study of the origin of barriers to hydrogen exchange reactions : application of the Valence Bond self-consistent-field method to the F + HF → FH + F reaction
The Journal of Physical Chemistry, 1992Co-Authors: Paul R. Benneyworth, Gabriel G. Balint-kurti, Michael Davis, Ian H. WilliamsAbstract:A combination of ab initio Methods, in particular the Valence Bond self-consistent field (VB-SCF) and the Valence Bond configuration interaction (VB-CI) Methods, are used to examine the origins of barriers to chemical reactions in a quantitative manner. Valence Bond Methods are well suited to this type of study as their underlying philosophy relies on constructing the molecular (or supermolecular) electronic wave functions from those of the constituent atoms or fragments.
Sason Shaik - One of the best experts on this subject based on the ideXlab platform.
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The V state of ethylene: Valence Bond theory takes up the challenge
Theoretical Chemistry Accounts: Theory Computation and Modeling, 2014Co-Authors: Huaiyu Zhang, Sason Shaik, Benoît Braïda, Philippe C HibertyAbstract:The ground state and first singlet excited state of ethylene, so-called N and V states, respectively, are studied by means of modern Valence Bond Methods. It is found that extremely compact wave functions, made of three VB structures for the N state and four structures for the V state, provide an N → V transition energy of 8.01 eV, in good agreement with experiment (7.88 eV for the N → V transition energy estimated from experiments). Further improvement to 7.96/7.93 eV is achieved at the variational and diffusion Monte Carlo (MC) levels, respectively, VMC/DMC, using a Jastrow factor coupled with the same compact VB wave function. Furthermore, the measure of the spatial extension of the V state wave function, 19.14 a02, is in the range of accepted values obtained by large-scale state-of-the-art molecular orbital-based Methods. The σ response to the fluctuations of the π electrons in the V state, known to be a crucial feature of the V state, is taken into account using the breathing orbital Valence Bond method, which allows the VB structures to have different sets of orbitals. Further Valence Bond calculations in a larger space of configurations, involving explicit participation of the σ response, with 9 VB structures for the N state and 14 for the V state, confirm the results of the minimal structure set, yielding an N → V transition energy of 7.97 eV and a spatial extension of 19.16 a02 for the V state. Both types of Valence Bond calculations show that the V state of ethylene is not fully ionic as usually assumed, but involving also a symmetry-adapted combination of VB structures each with asymmetric covalent π Bonds. The latter VB structures have cumulated weights of 18–26 % and stabilize the V state by about 0.9 eV. It is further shown that these latter VB structures, rather than the commonly considered zwitterionic ones, are the ones responsible for the spatial extension of the V state, known to be ca. 50 % larger than the V state.
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topology of electron charge density for chemical Bonds from Valence Bond theory a probe of Bonding types
Chemistry: A European Journal, 2009Co-Authors: Lixian Zhang, Fuming Ying, Philippe C Hiberty, Sason ShaikAbstract:To characterize the nature of Bonding we derive the topological properties of the electron charge density of a variety of Bonds based on ab initio Valence Bond Methods. The electron density and its associated Laplacian are partitioned into covalent, ionic, and resonance components in the Valence Bond spirit. The analysis provides a density-based signature of Bonding types and reveals, along with the classical covalent and ionic Bonds, the existence of two-electron Bonds in which most of the Bonding arises from the covalent-ionic resonance energy, so-called charge-shift Bonds. As expected, the covalent component of the Laplacian at the Bond critical point is found to be largely negative for classical covalent Bonds. In contrast, for charge-shift Bonds, the covalent part of the Laplacian is small or positive, in agreement with the weakly attractive or repulsive character of the covalent interaction in these Bonds. On the other hand, the resonance component of the Laplacian is always negative or nearly zero, and it increases in absolute value with the charge-shift character of the Bond, in agreement with the decrease of kinetic energy associated with covalent-ionic mixing. A new interpretation of the topology of the total density at the Bond critical point is proposed to characterize covalent, ionic, and charge-shift Bonding from the density point of view.
Frank Jensen - One of the best experts on this subject based on the ideXlab platform.
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Using Valence Bond Methods to estimate intramolecular basis set superposition errors
The Journal of Chemical Physics, 2017Co-Authors: Frank JensenAbstract:We show that a Valence Bond type wave function with non-orthogonal orbitals, and enforcing a restriction of which basis functions a given molecular orbital is allowed to be expanded in, can be used to estimate intramolecular basis set superposition errors. The method can be considered as a generalization of the Valence Bond strictly/extremely localized molecular orbital Methods, or as a generalization of the strictly monomer molecular orbital, block-localized wave function, and absolutely localized molecular orbital approaches for intermolecular systems. The method underestimates charge transfer interactions but can be used to estimate the magnitude of basis set superposition errors for relative conformational energies.
