On the Ordering of Orbital Energies in High-Spin ROHF<sup>†</sup>

Glaesemann, Kurt R.; Schmidt, Michael W.

doi:10.1021/jp101758y

Cited by 45 publications

(34 citation statements)

References 52 publications

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“…The adopted approaches were found to accurately describe the orbital configuration in several earlier studies of different SOMO-HOMO converted radicals [6][7][8] . We find that orbital arrangements obtained using single-reference methods are strongly dependent on the method, basis set, type of wave function and even software code 41 .…”

Section: Methodsmentioning

confidence: 97%

Switching radical stability by pH-induced orbital conversion

et al. 2013

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In most radicals the singly occupied molecular orbital (SOMO) is the highest-energy occupied molecular orbital (HOMO); however, in a small number of reported compounds this is not the case. In the present work we expand significantly the scope of this phenomenon, known as SOMO-HOMO energy-level conversion, by showing that it occurs in virtually any distonic radical anion that contains a sufficiently stabilized radical (aminoxyl, peroxyl, aminyl) non-π-conjugated with a negative charge (carboxylate, phosphate, sulfate). Moreover, regular orbital order is restored on protonation of the anionic fragment, and hence the orbital configuration can be switched by pH. Most importantly, our theoretical and experimental results reveal a dramatically higher radical stability and proton acidity of such distonic radical anions. Changing radical stability by 3-4 orders of magnitude using pH-induced orbital conversion opens a variety of attractive industrial applications, including pH-switchable nitroxide-mediated polymerization, and it might be exploited in nature. AbstractIn most radicals the singly-occupied molecular orbital (SOMO) is the highest-energy occupied one

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Section: Methodsmentioning

confidence: 97%

Switching radical stability by pH-induced orbital conversion

et al. 2013

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“…Subsequent orbital relaxation can further rearrange orbital energy levels. Since, as noted above, the orbitals themselves are neither observable, nor unique (in fact, erroneous non‐aufbau occupations can be observed with restricted open‐shell methods depending on the canonicalization parameters of the α and β electrons in Fock matrix, see Ref ), a rigorous support for the non‐aufbau occupation can be obtained a posteriori from the ionization potentials and electron affinities (or redox potentials in solution), measured using, e.g., cyclic voltammetry (CV), as well as from the determination of the spin state and magnetic properties of the corresponding open‐shell oxidation/reduction products. Conservation of the initial unpaired electron and preferential formation of polyradical species upon electron(s) removal from or addition to such non‐aufbau molecules is an intriguing feature on its own, and, moreover, leads to the fascinating diradical chemistry.…”

Section: Quasi Closed‐shell Moleculesmentioning

confidence: 99%

Theory and practice of uncommon molecular electronic configurations

Gryn’ova

Coote

Corminbœuf

2015

WIREs Comput Mol Sci

102

112

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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. How to cite this article:WIREs Comput Mol Sci 2015Sci , 5:440-459. doi: 10.1002Sci /wcms.1233 INTRODUCTIONT he electronic configuration and state of a molecule characterize the distribution of electrons across the corresponding one-electron wavefunctions, i.e., orbitals. 1 This model description, albeit approximate, is nonetheless indispensable in explaining and predicting molecular geometry and various chemical and physical properties. As orbitals comprise not only the spatial component, but also the spin, they also give rise to such key features as the nature of configuration shell-open or closed-and the multiplicity of the electronic state. While the majority of stable organic molecules have a closed-shell singlet ground state, species with unpaired electrons display unique chemical reactivity and are capable of carrying magnetism and conductivity functionalities. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Although historically the latter have been the domain of metals and, in particular, transition metal complexes, in the recent years the focus has ...

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“…However, it should be noted that KROHF is not always equivalent to KRHF in closed‐shell cases. The necessary and sufficient condition in KROHF to reproduce the KRHF spinor energies is

true(A_{CC} + B_{CC} = 1 true) \land true(A_{VV} + B_{VV} = 1 true),

as discussed by Glaesemann and Schmidt for nonrelativistic ROHF. This condition normalizes the Fock matrix when assembled in the KROHF procedure, and consequently gives the normalized spinor energies.…”

Section: Numerical Assessmentsmentioning

confidence: 98%

Development of spin‐dependent relativistic open‐shell Hartree–Fock theory with time‐reversal symmetry (II): The restricted open‐shell approach

Nakano

Nakamura

Seino

et al. 2017

Int J of Quantum Chemistry

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An open‐shell Hartree–Fock (HF) theory for spin‐dependent two‐component relativistic calculations, termed the Kramers‐restricted open‐shell HF (KROHF) method, is developed. The present KROHF method is defined as a relativistic analogue of ROHF using time‐reversal symmetry and quaternion algebra, based on the Kramers‐unrestricted HF (KUHF) theory reported in our previous study (Int. J. Quantum Chem., doi:). As seen in the nonrelativistic ROHF theory, the ambiguity of the KROHF Fock operator gives physically meaningless spinor energies. To avoid this problem, the canonical parametrization of KROHF to satisfy Koopmans' theorem is also discussed based on the procedure proposed by Plakhutin et al. (J. Chem. Phys. 2006, 125, 204110). Numerical assessments confirmed that KROHF using Plakhutin's canonicalization procedure correctly gives physical spinor energies within the frozen‐orbital approximation under spin–orbit interactions.

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On the Ordering of Orbital Energies in High-Spin ROHF^†

Cited by 45 publications

References 52 publications

Switching radical stability by pH-induced orbital conversion

Switching radical stability by pH-induced orbital conversion

Theory and practice of uncommon molecular electronic configurations

Development of spin‐dependent relativistic open‐shell Hartree–Fock theory with time‐reversal symmetry (II): The restricted open‐shell approach

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On the Ordering of Orbital Energies in High-Spin ROHF†

Cited by 45 publications

References 52 publications

Switching radical stability by pH-induced orbital conversion

Switching radical stability by pH-induced orbital conversion

Theory and practice of uncommon molecular electronic configurations

Development of spin‐dependent relativistic open‐shell Hartree–Fock theory with time‐reversal symmetry (II): The restricted open‐shell approach

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On the Ordering of Orbital Energies in High-Spin ROHF^†