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WIREs Comput Mol Sci
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Theory and practice of uncommon molecular electronic configurations

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The electronic configuration of the molecule is the foundation of its structure and reactivity. The spin state is one of the key characteristics arising from the ordering of electrons within the molecule's set of orbitals. Organic molecules that have open‐shell ground states and interesting physicochemical properties, particularly those influencing their spin alignment, are of immense interest within the up‐and‐coming field of molecular electronics. In this advanced review, we scrutinize various qualitative rules of orbital occupation and spin alignment, viz., the aufbau principle, Hund's multiplicity rule, and dynamic spin polarization concept, through the prism of quantum mechanics. While such rules hold in selected simple cases, in general the spin state of a system depends on a combination of electronic factors that include Coulomb and Pauli repulsion, nuclear attraction, kinetic energy, orbital relaxation, and static correlation. A number of fascinating chemical systems with spin states that fluctuate between triplet and open‐shell singlet, and are responsive to irradiation, pH, and other external stimuli, are highlighted. In addition, we outline a range of organic molecules with intriguing non‐aufbau orbital configurations. In such quasi‐closed‐shell systems, the singly occupied molecular orbital (SOMO) is energetically lower than one or more doubly occupied orbitals. As a result, the SOMO is not affected by electron attachment to or removal from the molecule, and the products of such redox processes are polyradicals. These peculiar species possess attractive conductive and magnetic properties, and a number of them that have already been developed into molecular electronics applications are highlighted in this review. WIREs Comput Mol Sci 2015, 5:440–459. doi: 10.1002/wcms.1233 This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Ab Initio Electronic Structure Methods
Various arrangements (determinants) of two electrons in two molecular orbitals and the corresponding values of the secondary spin quantum number.
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Non‐aufbau occupations formed by electron addition. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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Non‐aufbau distonic radical anions.
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Spin‐polarized donors with radicals other than NN. Red dots denote spin‐up (α) electrons.
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Systems combining conductivity and magnetism, realized in practice.
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Conductivity and magnetism in non‐aufbau high‐spin polyradicals.
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Oxidation‐induced ferromagnetic coupling in high‐spin polyradical product. Red dots denote spin‐up (α) electrons.
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Rationalization of orbital energy level conversion on the basis of the disjoint and nondisjoint character of the frontier orbitals.
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Spin polarization of the donor's HOMO.
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Various (nitroxyl) nitroxide spin‐polarized donors.
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Non‐aufbau occupations in tempo dithiolate complexes. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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VO(oep).
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aufbau principle of orbital occupation.
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Spin‐state reversal by ionic resonance contributors. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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Protonation‐induced magnetization. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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Systems with borderline triplet/closed‐shell singlet preference. Red dots denote spin‐up (α) electrons.
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Spin‐state crossover by external stimuli. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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Schematic orbital diagram of singlet and triplet tropylidene.
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Topology‐driven ground state multiplicity. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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High‐ and low‐spin polyradicals. Red dots denote spin‐up (α) electrons and blue dots represent spin‐down (β) electrons.
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Dynamic spin polarization.
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Energies of ground and excited states of different multiplicities as a function of (a) H–H distance in H2 homolytic dissociation, (b) twist angle around C–C bond in ethylene, and (c) metal–ligand separation in a transition metal complex.
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Structure and Mechanism > Computational Materials Science
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Structure and Mechanism > Molecular Structures

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