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The Electronic Structure and Bonding in Some Small Molecules.

George B Bacskay¹

¹School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia.

Molecules (Basel, Switzerland)

|March 13, 2025

Summary

This study explores molecular orbital (MO) and valence bond theories for various molecules. Advanced computational methods reveal limitations in current MO theories, especially for dicarbon (C2) molecules, suggesting a need for improved theoretical models.

Keywords:

covalent bonding density functional theory electronic structure molecular orbital theory quantum chemistry valence bond theory

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Area of Science:

Computational Chemistry
Quantum Chemistry
Theoretical Chemistry

Background:

Understanding molecular electronic structures is fundamental in chemistry.
Existing molecular orbital (MO) and valence bond (VB) theories provide frameworks for electronic structure.
The accurate description of certain molecules, like dicarbon (C2), remains a challenge for standard theories.

Purpose of the Study:

To discuss the electronic structures of first- and second-row homonuclear diatomics, XeF2, and dimers of NO and NO2.
To evaluate the performance of various quantum chemical methods in describing these electronic structures.
To identify limitations in current molecular orbital theories and propose extensions.

Main Methods:

Application of molecular orbital (MO) and valence bond (VB) theories.
Utilized advanced computational techniques: restricted and unrestricted Hartree-Fock (RHF/UHF) self-consistent field (SCF), complete active space SCF (CASSCF), multi-reference configuration interaction (MRCI), coupled cluster CCSD(T), and unrestricted Kohn-Sham (UKS) density functional theory.
Employed a polarized triple-zeta basis set for calculations.

Main Results:

Electronic structures of specified molecules were analyzed using multiple theoretical approaches.
Restricted Hartree-Fock (RHF) theory demonstrated significant deficiencies in describing the dicarbon (C2) molecule.
The study highlights the necessity of multi-configuration treatments for accurate electronic structure descriptions of molecules like C2.

Conclusions:

Current molecular orbital (MO) theories, as typically taught, are insufficient for accurately describing certain molecular systems.
Advanced computational methods confirm the inadequacy of simpler models for molecules with complex electronic correlations.
Undergraduate MO theories should be extended to incorporate multi-configuration treatments for a more comprehensive understanding of electronic structures.