Identification and Characterization of Molecular Bonding Structures by ab initio Quasi-Atomic Orbital Analyses

Abstract

The quasi-atomic analysis of ab initio electronic wave functions in full valence spaces, which was developed in preceding papers, yields oriented quasi-atomic orbitals in terms of which the ab initio molecular wave function and energy can be expressed. These oriented quasi-atomic orbitals are the rigorous ab initio counterparts to the conceptual bond forming atomic hybrid orbitals of qualitative chemical reasoning. In the present work, the quasi-atomic orbitals are identified as bonding orbitals, lone pair orbitals, radical orbitals, vacant orbitals and orbitals with intermediate character. A program determines the bonding characteristics of all quasi-atomic orbitals in a molecule on the basis of their occupations, bond orders, kinetic bond orders, hybridizations and local symmetries. These data are collected in a record and provide the information for a comprehensive understanding of the synergism that generates the bonding structure that holds the molecule together. Applications to a series of molecules exhibit the complete bonding structures that are embedded in their ab initio wave functions. For the strong bonds in a molecule, the quasi-atomic orbitals provide quantitative ab initio amplifications of the Lewis dot symbols. Beyond characterizing strong bonds, the quasi-atomic analysis also yields an understanding of the weak interactions, such as vicinal, hyperconjugative and radical stabilizations, which can make substantial contributions to the molecular bonding structure.

Clarifying the quantum mechanical origin of the covalent chemical bond

“…The interaction of different electron configurations need not be associated directly with either electron delocalization in space or its anticipated effects on kinetic energy, based on a particle-in-a-box picture. The perspective that wavefunction interference is the origin of the covalent bond has been found by not only this work, but also Ruedenberg, Nascimento, Bacskay, and many others 6 , 9 – 15 , 20 , 21 , 23 , 24 , 35 – 44 . The fact that this picture is recovered by many models speaks to its primacy as the fundamental origin of bonding, while systematic kinetic energy lowering appears in only certain models and in certain systems.…”

“…By contrast, recent work by Ruedenberg and co-workers [11][12][13][14][15][16] indicates that the KE-lowering is critical in all bonds. This qualitative difference is not because of any error on their part or ours.…”

“…For the last nearly 60 years, this KElowering paradigm has been used to explain the quantum origin of covalent bonds via extrapolation from H þ 2 and H 2 [5][6][7][8][9][10][11][12] . Recently, work has been done to define a schema to allow this theory to be tested with other molecules [11][12][13][14][15][16] . However, the very strong assumption that the results for hydrogen are universal to other covalent bonds deserves scrutiny by alternative approaches.…”

The Basics of Covalent Bonding in Terms of Energy and Dynamics

“…More recently, Ruedenberg and coworkers [97] have developed a quasi-atomic orbital (QUAO) analysis, where the QUAOs are rigorous counterparts to the bond forming hybrid orbitals and allow straightforward analyses of bonding in a molecule, determining bond orders, kinetic bond orders, hybridizations, and local symmetries. Of particular interest are the kinetic bond orders that provide computationally efficient energy-based quantitative estimates of covalent bonding.…”

“…Indeed the earliest quantum mechanical computations, ab initio and semi-empirical, focused on questions of bonding, but gradually the balance shifted and nowadays the majority of quantum chemical calculations, performed typically on supercomputers, focus on modeling chemical processes, structure, and properties. Yet, as chemists we want to, indeed need to, understand bonding and considerable effort is directed to the extraction of simple-to-understand bonding information from complex wave functions that are often characterized by millions of numbers [90][91][92][93][94][95][96][97][98][99][100][101][102][103][104]. In contrast, this paper is concerned with the fundamental aspects of bonding, a problem of long standing, rather than the immediate interpretation of results from large scale quantum chemical calculations.…”