Going local: Exploring intricate bonding patterns with localized quasi-atomic orbitals

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Duchimaza Heredia, Juan
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Mark S. Gordon
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The Department of Chemistry seeks to provide students with a foundation in the fundamentals and application of chemical theories and processes of the lab. Thus prepared they me pursue careers as teachers, industry supervisors, or research chemists in a variety of domains (governmental, academic, etc).

The Department of Chemistry was founded in 1880.

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As chemists move to correct the approximations necessary to solve a quantum chemical problem, it remains an important task to extract all the information available from the theoretical treatment of a chemical system to keep methods grounded in chemical and physical principles. To this extent, it is important to build a bridge between theoretical methods and experimental observations. Such a bridge is developed by transformations of molecular orbitals into quasi-atomic orbitals (QUAOs) that align more with the concept of localized chemical bonds.

The QUAOs are the rigorous ab initio counterparts to the conceptual bond forming atomic hybrid orbitals of qualitative chemical reasoning. An automated analysis is developed to identify a QUAO as a bonding orbital, lone pair orbital, radical orbital, vacant orbital or orbital with intermediate character. The program determines the bonding characteristics of all QUAOs 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.

The origin of bonding in the rare gas containing molecules HXeCCH, HXeCCXeH, and HXeOXeH is explored using the QUAO analysis. The analysis suggests significant covalent bonding for Xe-Y (Y = C, O) as well as Xe-H, both bonds using the same pσ-type orbital on Xe. These covalent interactions are established by substantial charge shifts from Xe to Y as well as to H. Accordingly, a covalent three-center four-electron bond links the atoms H-Xe-Y. Based on the analysis, electrostatic interactions do not play a significant role in the Xe-Y or Xe-H bonding.

A comprehensive analysis of the bonding structure of the disilyl zirconocene amide cation {Cp2Zr[N(SiHMe2)2]}+ is conducted by application of the QUAO analysis. An emphasis is placed on describing a previously characterized three-center two-electron interaction between zirconium, hydrogen, and silicon that presents structural and spectroscopic features similar to that of agostic bonds. By expressions of the first order density matrix in terms of the QUAOs, bonds orders, kinetic bond orders, and the extent of transfer of charge become available to determine the electronic nature of the Zr-H-Si bond. The QUAOs demonstrate the importance of vicinal interactions in the stabilization of the molecule. In addition, the evolution of the QUAOs during reactions with Lewis bases reveals the role of the Zr-H-Si interaction in facilitating the reaction.

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Sat Dec 01 00:00:00 UTC 2018