Use of molecular orbital (MO) theory allows for an understanding of the observed properties (shape, reactivity) of molecules. The tag should be applied to all questions related to MO theory, from questions about the qualitative use of the theory to questions about the underlying quantum mechanics and applications in computational chemistry.
Molecular orbital theory (MO) is a theoretical model for bonding in molecules that may be used to deduce/rationalize molecular structure and reactivity.
Unlike traditional theories of bonding, in MO theory, electrons are considered to be delocalized into ‘molecular orbitals’, formed through the linear combination of atomic orbitals (mathematically through a combination of the wavefunctions corresponding to the atomic orbitals).
No IUPAC definition exists for molecular orbital theory, however, the concept of a molecular orbital is defined:
A one-electron wavefunction describing an electron moving in the effective field provided by the nuclei and all other electrons of a molecular entity of more than one atom. Such molecular orbitals can be transformed in prescribed ways into component functions to give 'localized molecular orbitals'. Molecular orbitals can also be described, in terms of the number of nuclei (or 'centers') encompassed, as two-centre, multi-centre, etc. molecular orbitals, and are often expressed as a linear combination of atomic orbitals. An orbital is usually depicted by sketching contours on which the wavefunction has a constant value (contour map) or by indicating schematically the envelope of the region of space in which there is an arbitrarily fixed high (say 96%) probability of finding the electron occupying the orbital, giving also the algebraic sign (+ or −) of the wavefunction in each part of that region.
Molecular orbital (MO) theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule. The spatial and energetic properties of electrons within atoms are fixed by quantum mechanics to form orbitals that contain these electrons. While atomic orbitals contain electrons ascribed to a single atom, molecular orbitals, which surround a number of atoms in a molecule, contain valence electrons between atoms.
Applicability of the molecular-orbital-theory tag:
- molecular-orbital-theory should be applied to any question dealing with the underlying orbitals behind structure or reactivity, whether dealing with the qualitative use of a molecular orbital scheme, or with the mathematical derivation of the orbitals themselves.
- In most cases, the molecular-orbital-theory tag is not standalone and should be applied in addition to other relevant tags. For example:
- A question dealing with the computational generation of molecular orbitals should be tagged with molecular-orbital-theory and computational-chemistry.
- A question dealing with the molecular orbitals of a transition metal complex should be tagged with molecular-orbital-theory and an additional tag such as coordination-compounds.
- Even a question dealing only with the theory itself would need tagging with molecular-orbital-theory and the quantum-mechanics tag, as this is the underlying basis for MO theory.
The following tags are related to molecular-orbital-theory, with multiple questions on chemistry.se already tagged with one of more of the following:
Many books have been published on the topic of molecular orbital theory, some of the more readable are listed below:
- Atkins', P; Quanta, Matter and Change; W. H. Freeman: New York, 2009
- Albright, T. A.; Orbital Interactions in Chemistry; Wiley: New Jersey, 2013
- Fleming, I; Molecular Orbitals and Organic Chemical Reactions (Reference edition); Wiley: Sussex, 2010