An examination of the nature of localized molecular orbitals and their value in understanding various phenomena that occur in organic chemistry

James J P Stewart

doi:10.1007/s00894-018-3880-8

Abstract

While canonical molecular orbitals have been used in computational chemistry for almost a century, the use of localized molecular orbitals is relatively new, and generating them has been difficult until recently. This has impeded their routine use in modeling chemical systems and reactions so that, even though localized molecular orbitals can now be generated easily, their usefulness in interpreting chemical phenomena has not been properly appreciated. Localized molecular orbitals can provide useful insights into chemical phenomena such as two-electron bonds, π delocalization, and lone pairs. A potentially important application would be interpreting the phenomena that occur in chemical reactions, in particular those reactions which can be described using the Lewis curly-arrow electron pushing convention. This paper considers how canonical and localized molecular orbitals are generated, their usefulness and limitations, and some issues that could be considered controversial regarding their nature, and it presents examples of the usefulness of LMOs in describing six chemical systems and one reaction.

Highlights

Quantum theoretical calculations have produced many kinds of results that have been extremely useful for predicting the properties of chemical systems
Little attention has been paid to the properties and uses of localized molecular orbitals; attention has focused instead on canonical molecular orbitals
& Each LMO in a chemical system can be related to a specific element in a Lewis structure diagram

Summary

Introduction

Quantum theoretical calculations have produced many kinds of results that have been extremely useful for predicting the properties of chemical systems. This paper belongs to the Topical Collection Tim Clark 70th Birthday Festschrift acteristic of an observable calculable is that it is the result of an operator acting on a system’s state wavefunction, Ψ, in contrast to those quantities that involve only the wavefunction of an individual electron or a pair of electrons, ψi. These latter quantities are calculable and have real expectation values, they have no experimental equivalent, so comparison with any observable property of a real system is impossible. In describing and interpreting chemical phenomena, especially those involved in reactivity and reactions, the use of calculated non-observables such as individual molecular orbitals has undoubtedly been of considerable value

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