Valence Bond Scheme Research Articles

Optimum valence bond scheme for its applications to the prediction of nano-structures and the study of matter properties

The optimum valence bond scheme is a new theoretical method in generating the initial geometric configurations in molecular dynamics simulations of cluster systems. We will present the application of such a new method to the prediction of nano-structures and the study of matter properties, especially for the low-dimensional nano-structures, such as clusters and nano wires. The optimum valence bond scheme uses the atomic geometry of structures and the space distribution of the valence electrons (mainly the molecular orbitals near the Fermi levels, i.e., the generalized frontier orbitals) to determine the possible stable geometric configurations of nano-structures. Silicon clusters are used to demonstrate the features of the optimum valence bond scheme. Metallic clusters such as those of lithium, sodium, beryllium and magnesium are used as examples to illustrate the application of the scheme to the prediction of structures and the studies of the evolution of the material properties with the sizes of clusters. We will use the adsorption process of lithium ion and MoS nano wire to illustrate the application of the optimum valence bond scheme in the studies of the ionic conduction mechanism of the energy storage materials. We will finish the paper by summarizing the direction for further development of the optimum valence bond scheme.

VBEFP: A Valence Bond Approach That Incorporates Effective Fragment Potential Method

An ab initio explicit solvation valence bond (VB) method, called VBEFP, is presented. The VBEFP method is one type of QM/MM approach in which the QM part of system is treated within the ab initio valence bond scheme and the solvent water molecules are accounted by the effective fragment potential (EFP) method, which is a polarized force field approach developed by Gordon et al. (J. Chem. Phys. 1996, 105, 1968). This hybrid method enables one to take the first-solvation shell and heterogeneous solvation effects into account explicitly with VB wave function. Therefore, the nature of chemical bonding and the mechanism of chemical reactions with explicit solvent environments can be explored at the ab inito VB level. In this paper, the hydrated metal-ligand complexes [M(2+)L](H(2)O)(n) (M(2+): Mg(2+), Zn(2+); L: NH(3), CH(2)O) are studied by the VBEFP method. Resonance energy and bond order are computed, and the influence of the solvent coordination and hydrogen bonding to the metal-ligand bonding are explored in the paper.

An analysis of the dynamic σ polarization in the V state of ethene

AbstractThe importance of the dynamic σ polarization (absent in methods where the σ skeleton is treated at a mean‐field level) for the correct description of the V state of the ethene molecule has been recognized by many authors in the past. In this article, this physical effect is analyzed and it is seen as arising from the sum of two contributions: the polarization of the σ CC bond and of the σ CH bonds. In both cases it is described in a valence bond scheme and the types of excitations needed in a molecular orbital frame to introduce such effects are identified. The effect of the dynamic σ polarization on the spatial extent of the V state (〈x2〉) is described. The analysis here reported has been used in a recent article (Angeli, J Comp Chem 2009, 30, 1319) for the accurate calculation of the V state and of its vertical excitation energy. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2436–2447, 2010

The Nature of Resonance in Allyl Ions and Radical

A recent valence bond scheme based on Lewis structures, the valence bond BOND (VBB) method (BOND: Breathing Orbitals Naturally Delocalized) method (Linares, M.; Braida, B.; Humbel, S. J. Phys. Chem. A 2006, 110, 2505-2509), is applied to explore the nature of resonance in allyl systems. Whereas allyl radical is correctly described by the resonance between the two traditional Lewis structures, a third "long-bonded" structure, which apparently creates a pi bond between the two distant carbon atoms, appears to plays an important role in allyl ions description. The similar vertical resonance energy (VRE) for both allyl ions is rather moderate (approximately 37 kcal/mol) in the two-structure description but is significantly enhanced when the long-bonded structure is included into the VBB wave function (by up to 20 kcal/mol). The allyl radical is much less resonant and is correctly described by the traditional two-structure picture. The development of VBB Lewis structures into "pure" valence bond determinants enlightens the role of the third structure in the description of allyl ions. The existence of a long bond between the two distant carbon atoms is clearly ruled out. Charge equilibration effect is shown to be a minor factor. The third structure is finally attributed to one- and three-electron bonding character revealed in the pi systems of the cation and anion, respectively. This makes these systems two surprising examples of odd electron bonding within a singlet state. Last, the two-structure description of allyl radical is improved by addition of missing ionic structures.

Covalent Excited States of Polyenes C2nH2n+2 (n = 2-8) and Polyenyl Radicals C2n-1H2n+1 (n = 2-8): An Ab Initio Valence Bond Study.

The ab initio valence bond (VB) methods, VBSCF and VBCI, are applied to the ground states and the covalent excited states of polyenes C2nH2n+2 (n = 2-8) and polyenyl radicals C2n-1H2n+1 (n = 2-8). The excitation energy gap was computed at the ab initio VB level, which is in good agreement with the semiempirical VB method, VBDFT(s), and the experimental values as well as with the molecular orbital theory based methods, CASPT3 and MRCI. The ab initio VB wave functions of systems are also in very good agreement with those of the VBDFT(s) method, even though the former is based on the ab initio VB scheme while the latter is a semiempirical Hückel type method, in which no orbital optimization procedure is performed. The computational results show that the ab initio VB method is capable now of providing numerical accuracy not only for bond forming and breaking processes, as shown in the past, but also for excitation energies, as shown here. In addition, the computational results validate the efficiency of the VBDFT(s) method, which is a simple VB model with less computational effort but which provides intuitive insights into the excited states of conjugated molecules.

Quantifying resonance through a Lewis Valence Bond approach: application to haloallyl and carbonylcations

A recent Valence Bond scheme based on Lewis structures, the VBB method (M. Linares, B. Braida and S. Humbel, J. Phys. Chem. A, 2006, 110, 2505 2509) is applied to resonance effect quantification. An accurate evaluation of this effect is provided by targeting pi interactions only, while all other factors remain constant. Valence Bond theory allows us to circumvent two main shortcomings of other approaches, i.e. the lack of a quantitative aspect, and the difficulty to properly separate resonance from other effects. The pi effect of fluorine and chlorine atoms is found to be comparable and quite significant (approximately 20 kcal mol(-1)), in both haloallyl and protonated carbonyl cations. The validity of the resonance model for carbonyl compounds is confirmed. Resonance in protonated formamide is indeed found to be significantly larger than in formic acids, itself being more resonant than the formyl fluoride cation. Comparisons with other methods of resonance effect evaluation are also made.

Lewis-Based Valence Bond Scheme: Application to the Allyl Cation

A method for expressing the wave function in terms of Lewis structures is proposed and tested on the allyl cation. This computational scheme is called valence bond BOND (VBB). The compact VBB wave function gives consistent results with the breathing orbital valence bond method (BOVB) for the resonance energy of the allyl cation (54 and 55 kcal/mol for VBB and BOVB, respectively). The optimization of the sigma orbitals, in such a way they adapt to each resonance structure, makes use of the breathing orbital effect. It is shown that this "breathing" of the sigma frame is more efficient in the resonant hybrid than in the localized state, so that a resonance energy of 63 kcal/mol is obtained at this level of computation.

Optimum valence bond scheme: theoretical study of small clusters of elements in the second and third row of periodic table

Using optimum valence bond scheme which reduces the computation effort, we study systematically the properties of the critical structures (critical points) of small clusters (up to four atoms) of elements in the second and third row of the periodic table. We also show the evolution process of the clusters from two atoms to four atoms. By examining the electronic structures of all clusters, we can understand why the four-atom clusters for specific elements can have three-dimensional structures with Td symmetry. More interestingly, comparing the variation of the binding energy of such small clusters with the melting points and boiling points of corresponding pure element materials, we can understand the effect of the critical structures in the melting and boiling processes.

To understand matter from a point of view of clusters

We present an optimum valence bond scheme to study important stable geometric structures of clusters, by combining the characteristics of frontier molecular orbitals and the first-principle molecular dynamics simulation. It is interesting to note that even for small size clusters it has already provided clues in some macroscopic properties of the second- and the third-row elements in the period table (e.g. melting and boiling point).

First-principle molecular dynamics study of clusters:optimum valence bond scheme

We present an optimum valence bond scheme to study stable geometric structures of clusters, by combining the characteristics of valence bonds and the first-prin ciple molecular dynamics simulation. With limited computational effort, we can o btain the properties of cluster Xm with m as large as possible, and th e properties of all clusters Xi (i<m), e.g., their stable geometric struct ures, electronic properties, etc. Consequently we can elucidate the evolution la w of clusters. We illustrate the optimum valence bond scheme with the second_row elements. The properties of three-atom clusters interestingly reflect the evolv ement law with atomic number of macroscopic properties of corresponding elements (melting and boiling points).

The valence bond calculations for conjugated hydrocarbons having 24-28 ?-electrons

A novel algorithm is introduced for coding all Slater determinants in the covalent space with conserved SZ, the z component of total spin S for a classical valence bond (VB) model. It effectively minimizes the search time and the storing space in the central memory of the computer. In cooperation with symmetry reductions based on molecular point group and spin inversion, the VB calculations have been extended to benzenoid hydrocarbons of up to 28 π-electrons that have 4×107 configurations. The low-lying states of benzenoids with 24, 26, and 28 π-electrons have been obtained for 62 species. To rationalize the aromaticity of benzenoids in a VB scheme, the resonance energy per hexagon (REPH) is defined. A linear correlation between the REPH and the energy gap of the ground (singlet) state and the first excited (triplet) state for 89 benzenoids is established. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 856–869, 2000

VBDFT(s): a Hückel-type semi-empirical valence bond method scaled to density functional energies. Application to linear polyenes

A semi-empirical valence bond (VB) method, called VBDFT(s), is described and applied to C n H n+2 conjugated hydrocarbons. The method is a Hückel-type VB scheme with energies scaled to density functional theory energies based on a single parameter; the matrix element λ due to spin transposition between bonded atoms. Total energies, excitation energies, resonance and π-bond energies are presented for the various species. The insight that can be derived from the corresponding wavefunctions is discussed.

Valence bond analysis of the Kondo chain Hamiltonian

Accurate extrapolations for the ground state energy per site of the one - dimensional Kondo chain system is obtained from exact finite system calculations carried out employing a valence bond scheme. An analysis of the ground state wave function indicates that the localized spin is quenched for all nonzero values of the Kondo exchange constant in one dimension.

Ab-initiovalence bond calculations on the He-He potential curve using small bases

In studying methods for the ab-initio calculation of Van der Waals interactions which can be extended to larger molecules, we have tested the Valence Bond scheme on the He2 system. This method looks promising, as it appears to yield in one consistent formalism both the attractive dipole—dipole terms and the repulsive exchange terms with reasonable accuracy, using only a simple orbital basis and a very small number of VB structures.

Quantum-Mechanical Potential-Energy Curve for the Lowest 1Σu+ State of He2

The potential-energy curve of the lowest 1Σu+ state of He2 is computed in a valence bond scheme using a 17-term wavefunction of Slater orbitals. A maximum in the potential curve occurs at 5.22 a0 with a computed height of 0.153 eV. A rigorous upper bound of 0.364 eV is found for the potential maximum. The rationalized dissociation energy for the best wavefunction is 1.719 eV at Re=2.11 a0, and a rigorous lower bound on the dissociation energy is 1.508 eV.

Spin-Density Distributions in Conjugated Ligands of Paramagnetic Chelates from NMR Contact Interaction Shifts

Electron-spin-density distributions in a variety of π systems have been obtained from proton-contact interaction shifts in paramagnetic nickel II chelates of N,N′-disubstituted aminotroponeimines. The results have furnished a great deal of new information regarding the detailed manner in which π electrons are distributed via conjugative effects. Spin-density distributions in these systems calculated by the valence-bond scheme are shown to be in quite good general agreement with experimental determinations. Effects attributable to hyperconjugation have been observed and the conjugating abilities of linking groups C=C, N=N, NH, O, S, and SO2 have been assessed. It is shown also that metal-ligand π-bond formation results in transfer of 0.1 of an unpaired electron from nickel to the π system of each of the ligands of these bischelates.

The carbon‐chlorine bond in the chloronaphthalenes

AbstractThe dipole moments of 1 and 2‐chloronaphthalene are discussed by the method of molecular orbitals. If an “effective” electron charge is assumed in agreement with the dipole moment of 1‐chloronaphthalene, the moment of 2‐chloronaphthalene can be calculated in complete agreement with the experimental value.The calculated energy levels are in nearly quantitative agreement with the long‐wave limits of U.V. absorption of the two compounds. From the calculation follow the values 14.2% and 13% for the double‐bond character of the C‐Cl bond in 1 and 2‐chloronaphthalene, respectively. Starting from this result the acid and basic strengths of the naphtholes and the naphthylamines are discussed by consideration of a very simple valence bond scheme.