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This paper combines the valence bond block diabatization approach with the idea of orbital breathing. With highly compact wave functions, the breathing orbital valence bond (BOVB) method is applied to investigate several atomic and molecular properties, including the electron affinity of F, the adiabatic and diabatic potential energy curves and the dipole moment curves of the two lowest-lying 1Σ+ states, the electronic coupling curve and the crossing distance of the two diabatic states, and the spectroscopic constants of the ground states for LiF. The configuration selection scheme proposed in this work is quite general, requiring only the selection of several de-excitation and excitation orbitals in a sense like the restricted active space self-consistent field method. Practically, this is also the first time that BOVB results are extrapolated to complete basis set limit. Armed with the chemical intuition provided by valence bond theory, the classic but challenging covalent-ionic interaction in the title molecule is not only conceptually interpreted but also accurately computed. [ABSTRACT FROM AUTHOR]

A valence bond (VB) block-diagonalization approach, named VBBDA, is proposed to construct the charge-localized diabatic state explicitly within the framework of ab initio VB theory. Since the VB structure built upon the localized orbitals represents the charge localized character of the diabatic state faithfully, we are able to obtain accurate electronic coupling between diabatic states by using a very compact VB wave function. Moreover, the potential energy curves of the diabatic states and hence the crossing points of them can be accurately evaluated. The pilot applications showed that the electronic couplings computed by the VB method are consistent with the complete active space self-consistent field method and may even be close to the results of other high-level ab initio methods such as full configuration interaction and multireference configuration interaction. In addition, the computed electronic couplings show the expected exponential attenuation for the donor-acceptor systems as the distance increases. Moreover, VBBDA has the capability for handling complicated systems based on either two-state or multi-state treatment. Finally, because of the outstanding performance of the Xiamen Valence Bond software package, which is an ab initio VB program, VBBDA is capable for systems consisting more than 1000 basis functions. [ABSTRACT FROM AUTHOR]

A unique thorium‐thorium bond was observed in the crystalline tri‐thorium cluster [{Th(η8‐C8H8)(μ3‐Cl)2}3{K(THF)2}2]∞, though the claim of σ‐aromaticity for Th3 bond has been questioned. Herein, a new type of core–shell syngenetic bonding model is proposed to describe the stability of this tri‐thorium cluster. The model involves a 3c–2e bond in the Th3 core and a multicentered (ThCl2)3 charge‐shift bond with 12 electrons scattering along the outer shell. To differentiate the strengths of the 3c–2e bond and the charge‐shift bond, the block‐localized wavefunction (BLW) method which falls into the ab initio valence bond (VB) theory is employed to construct a strictly core/shell localized state and its contributing covalent resonance structure for the Th3 core bond. By comparing with the σ‐aromatic H3+ and nonaromatic Li3+, the computed resonance energies and extra cyclic resonance energies confirm that this Th3 core bond is truly delocalized and σ‐aromatic. [ABSTRACT FROM AUTHOR]

The agostic interaction is a ubiquitous phenomenon in catalytic processes and transition‐metal complexes, and hyperconjugation has been well recognized as its origin. Yet, recent studies showed that either short‐range London dispersion or structural constraints could be the driving force, although proper evaluation of the role of hyperconjugation therein is needed. Herein, a simple variant of valence bond theory was employed to study a few exemplary Ti complexes with α‐ or β‐agostic interactions and interpret the agostic effect in terms of the steric effect, hyperconjugation, and dispersion. For the complexes [MeTiCl3(dmpe)] and [MeTiCl3(dhpe)] with α‐agostic interactions, hyperconjugation plays the dominant role with comparable magnitudes in both systems, but dispersion is solely responsible for the stronger agostic interaction in the former compared with the latter. For the complexes [EtTiCl3(dmpe)] and [EtTiCl3(dhpe)] with β‐agostic interactions, however, hyperconjugation and dispersion play comparable roles, and the weaker steric repulsion leads to a stronger agostic effect in the former than in the latter. Thus, the present study clarifies the variable and sensitive roles of steric, hyperconjugative, and dispersion interactions in the agostic interaction. [ABSTRACT FROM AUTHOR]

Abstract: The concept of resonance‐enhanced hydrogen bond (RAHB) has been widely accepted and applied as it highlights the positive impact of π‐conjugation on intramolecular H‐bonds. However, electron delocalization is directional and there is a possibility that π‐resonance goes from the H‐bond acceptor to the H‐bond donor, leading to a negative impact on H‐bonds. Here we used the block‐localized wavefunction (BLW) method which is a variant of ab initio valence bond (VB) theory and able to derive strictly electron‐localized structures self‐consistently, to quantify the interplay between H‐bond and π‐resonance in the terms of geometry, energetics and spectral properties. The comparison of geometrical optimizations with and without π‐resonance shows that conjugation can indeed either enhance or weaken intramolecular H‐bonds. We further experimented with various substituents attached to either the H‐bond acceptor and/or H‐bond donor side(s) to tune the H‐bonding strength in both directions. [ABSTRACT FROM AUTHOR]