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Identification and Characterization of Molecular Bonding Structures by ab initio Quasi-Atomic Orbital Analyses
The quasi-atomic analysis of <i>ab initio</i> electronic wave functions in full valence spaces,
which was developed in preceding papers, yields oriented quasi-atomic
orbitals in terms of which the <i>ab initio</i> molecular
wave function and energy can be expressed. These oriented quasi-atomic
orbitals are the rigorous <i>ab initio</i> 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 <i>ab initio</i> wave functions. For the strong bonds in a molecule,
the quasi-atomic orbitals provide quantitative <i>ab initio</i> 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