Preface

This Special Issue (SI) is a tribute to the tremendous contributions to Chemistry made by Gilbert Lewis, following both the 70th anniversary of his passing in 1946 and the 100th anniversary of his landmark paper “The atom and the molecule”, and to Linus Pauling, who by establishing the connection between the then-recent quantum theory and Lewis’ model laid the foundation of Valence Bond theory. The focus of this SI, as well as the contributions therein, have been carefully selected so as to balance the weighting of methods development and applications, making the contents of the issue of interest to both theoreticians and practicing chemists alike.</p

The Breathing Orbital Valence Bond Method in Diffusion Monte Carlo: C-H Bond Dissociation ofAcetylene

This study explores the use of breathing orbital valence bond (BOVB) trial wave functions for diffusion Monte Carlo (DMC). The approach is applied to the computation of the carbon-hydrogen (C-H) bond dissociation energy (BDE) of acetylene. DMC with BOVB trial wave functions yields a C-H BDE of 132.4 {+-} 0.9 kcal/mol, which is in excellent accord with the recommended experimental value of 132.8 {+-} 0.7 kcal/mol. These values are to be compared with DMC results obtained with single determinant trial wave functions, using Hartree-Fock orbitals (137.5 {+-} 0.5 kcal/mol) and local spin density (LDA) Kohn-Sham orbitals (135.6 {+-} 0.5 kcal/mol)

The V state of ethylene: valence bond theory takes up the challenge

Natural Science Foundation of China [21120102035, 21273176, 21290193]; Israel Science Foundation (ISF) [1183/13]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 a (0) (2) , is in the range of accepted values obtained by large-scale state-of-the-art molecular orbital-based methods. The sigma response to the fluctuations of the pi 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 sigma 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 a (0) (2) 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 pi 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

Structure-Property Relationships for Exciton and Charge Reorganization Energies of Dipolar Organic Semiconductors: A Combined Valence Bond Self-Consistent Field and Time-Dependent Hartree-Fock and DFT Study of Merocyanine Dyes

We present an analysis of the optoelectronic properties of merocyanine dyes by means of valence bond self-consistent field (VBSCF), time-dependent Hartree-Fock (TD-HF), density functional theory (DFT), and high-level ab initio calculations. The electronic structure of merocyanines can be described as a superposition of two resonance structures, a neutral one and a zwitterionic one. Calculated valence bond (VB) weights for these resonance structures demonstrate the importance of strong accepting groups when increasing the weight of the zwitterionic structures of different merocyanines. The dependence of exciton and charge reorganization energies on VB weights composition is analyzed, demonstrating that the special case of equal contributions of both structures, the so-called cyanine limit, goes along with minimal exciton and charge reorganization energies. For the latter, it is shown that the external (outer-sphere) reorganization energy plays a crucial role. Furthermore, a careful investigation of the excited-state behavior of merocyanines indicates that a possible excited-state torsion might be another important parameter for merocyanine-based optoelectronic devices, whereas internal (inner-sphere) charge reorganization energies of a variety of merocyanines are in a typical range for molecular semiconductors

Charge-Shift Bonding Emerges as a Distinct Electron-Pair Bonding Family from Both Valence Bond and Molecular Orbital Theories

Israel Science Foundations (ISF) [1183/13]; Natural Science Foundation of China [21120102035, 21273176, 21290193]The charge-shift bonding (CSB) concept was originally discovered through valence bond (VB) calculations. Later, CSB was found to have signatures in atoms-in-molecules and electron-localization-function and in experimental electron density measurements. However, the CSB concept has never been derived from a molecular orbital (MO)-based theory. We now provide a proof of principle that an MO-based approach enables one to derive the CSB family alongside the distinctly different classical family of covalent bonds. In this bridging energy decomposition analysis, the covalent ionic resonance energy, RECS, of a bond is extracted by cloning an MO-based purely covalent reference state, which is a constrained two-configuration wave function. The energy gap between this reference state and the variational TCSCF ground state yields numerical values for RECS, which correlate with the values obtained at the VBSCF level. This simple MO-based method, which only takes care of static electron correlation, is already sufficient for distinguishing the classical covalent or polar-covalent bonds from charge-shift bonds. The equivalence of the VB and MO-based methods is further demonstrated when both methods are augmented by dynamic correlation. Thus, it is shown from both MO and VB perspectives that the bonding in the CSB family does not arise from electron correlation. Considering that the existence of the CSB family is associated also with quite a few experimental observations that we already reviewed (Shaik, S.; Danovich, D.; Wu, W.; Hiberty, P. C. Nat. Chem., 2009; I, 443-449), the new bonding concept has passed by now two stringent tests. This derivation, on the one hand, supports the new concept and on the other, it creates bridges between the two main theories of electronic structure

On the Nature of Blueshifting Hydrogen Bonds

US National Science Foundation [CHE-1055310, CNS-1126438]; China Scholarship Council (CSC); National Science Foundation of China [20873106]; Ministry of Science and Technology of China [2011CB808504]The block-localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F3CH with NH3 and OH2 shows that blueshifting is a long-range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long-range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short-range n(Y) >sigma*(X H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F3CH center dot center dot center dot NH3 and blueshifting in F3CH center dot center dot center dot OH2. This mechanism works well in the series FnCl3-nCH center dot center dot center dot Y (n=0-3, Y=NH3, OH2, SH2) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O-H bond of water even with the electron transfer between benzene and water completely quenched. A distance-dependent analysis for this system reveals that the long-range electrostatic interaction is again responsible for the initial lengthening and redshifting

A review of structural properties and synthesis methods of solid electrolyte materials in the Li2S - P2S5 binary system

International audienceAll-solid-state-batteries (ASSBs) are one of the most promising post-lithium-ion technologies that can increase the specific energy density and safety of secondary lithium batteries. Solid sulfide electrolytes are considered as promising candidates to be used in ASSBs owing to high ionic conductivities. In particular, solid electrolytes in the Li2S - P2S5 binary system have attracted considerable attention as they are composed of low-cost elements and they provide ionic conductivity values comparable to those of liquid electrolytes (> 10(-4) Scm(-1)). In this review, the structural properties and synthesis methods of materials in the binary system are summarized. Distinctions in local structures and Li-ion conduction properties between glassy, glass-ceramic, and crystalline materials are highlighted. Possible mechanisms are proposed for the fast ionic conduction observed in glass ceramics. Important parameters of each synthesis method are suggested and the relationships between structure, synthesis and material properties are discussed. The goals of this review are to provide greater understanding of the state-of-the-art in the field, and to point out the overlooked aspects for application

Multicenter Bonding in Ditetracyanoethylene Dianion: A Simple Aromatic Picture in Terms of Three-Electron Bonds

The nature of the multicenter, long bond in ditetracyanoethylene dianion complex [TCNE]₂^2– is elucidated using high level <i>ab initio</i> Valence Bond (VB) theory coupled with Quantum Monte Carlo (QMC) methods. This dimer is the prototype of the general family of pancake-bonded dimers with large interplanar separations. Quantitative results obtained with a compact wave function in terms of only six VB structures match the reference CCSD­(T) bonding energies. Analysis of the VB wave function shows that the weights of the VB structures are not compatible with a covalent bond between the π* orbitals of the fragments. On the other hand, these weights are consistent with a simple picture in terms of two resonating bonding schemes, one displaying a pair of interfragment three-electron σ bonds and the other displaying intrafragment three-electron π bonds. This simple picture explains at once (1) the long interfragment bond length, which is independent of the countercations but typical of three-electron (3-e) CC σ bonds, (2) the interfragment orbital overlaps which are very close to the theoretical optimal overlap of 1/6 for a 3-e σ bond, and (3) the unusual importance of dynamic correlation, which is precisely the main bonding component of 3-e bonds. Moreover, it is shown that the [TCNE]₂^2– system is topologically equivalent to the square C₄H₄^2– dianion, a well-established aromatic system. To better understand the role of the cyano substituents, the unsubstituted diethylenic Na⁺₂[C₂H₄]₂^2– complex is studied and shown to be only metastable and topologically equivalent to a rectangular C₄H₄^2– dianion, devoid of aromaticity