The Electronic Properties of Diradicals

Salem, Lionel; Rowland, Colin

doi:10.1002/anie.197200921

Cited by 1,007 publications

(916 citation statements)

References 75 publications

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“…1,13 Some chemists prefer the name "biradical", 14 but others call molecules with two positive charges "dications" and molecules with two negative charges "dianions". Therefore, calling molecules that, at least potentially, have two unpaired electrons "diradicals" has the virtue of a consistency.…”

Section: Wave Functions For Diradicals and The Qualitative Predictionmentioning

confidence: 99%

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The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

Lineberger

Borden

2011

Phys. Chem. Chem. Phys.

106

107

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This Perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet-triplet energy differences ( ST ) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring  ST are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of  ST in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this Perspective are HN, CH 3 N, PhN, CH 2 , trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D 8h cyclooctateraene (COT). All of these diradicals have been studied in one and, in most cases, in both of the authors' laboratories. The studies of OXA and D 8h COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability of NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the cabonyl stretching frequency in the singlet state of OXA and to establish that D 8h COT violates the strictest version of Hund's rule. However, in some cases, it is not qualitative theories but the results of high-level calculations that motivate experimentalists to test how quantitatively accurate the calculations are. In other cases calculations are performed to see how well they reproduce experimental results that have already been obtained. When the results of calculations and experiments are found to be at odds with each other, additional experiments and/or calculations almost inevitably follow.Investigations of the energy differences between the lowest singlet and triplet state (E ST ) of organic diradicals 1 have provided many illustrations of the above types of synergies. Qualitative theories have resulted in computational and/or experimental tests, and calculations and experiments have not only stimulated each other but, in some cases, also led to the development of qualitative theories.For example, in response to the results of experiments and calculations on cyclobutadiene and trimethylenemethane, one of the coauthors of this review (WTB) helped to develop a qualitative theory, which allows the qualitative prediction of the approximate size and sign of

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Section: Wave Functions For Diradicals and The Qualitative Predictionmentioning

confidence: 99%

“…10, has been transformed into an "open-shell" singlet wave function,  S0 ', in eq. (13). Clearly, if one wishes to use the terms "open-shell" and "closed-shell" to describe the two, lowest-energy.…”

Section: Relativementioning

confidence: 99%

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

Lineberger

Borden

2011

Phys. Chem. Chem. Phys.

106

107

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“…The "color spectrum" of the N DA distribution map over the molecule atoms or of the relevant map of the chemical activity of its atoms is determined by a difference between N DA values for atoms of different groups and makes the ground for *The term "excess electrons" widely used to describe the electron structure of diradicals [12,13] is introduced to denote the fact that the number of valence electrons of each atom from fullerene molecules is by a unit larger than the number of attached atoms. **In accord with [14], N D and N DA are determined as the atom-local selectivity of the molecule chemical activity.…”

Section: "Chemical Portraits" Of C 60 and C 70 Moleculesmentioning

confidence: 99%

“Chemical portrait” of fullerene molecules

Sheka

2006

J Struct Chem

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541.572.7+546.26 Chemical activity of fullerene molecules is associated with a partially radical character of these molecules caused by the presence of effectively unpaired electrons (EUE). Values of the total number of N D and partial distribution N D of EUE over the atoms in a singlet state of molecules 60 and 70 have been calculated. "Chemical portraits" of molecules are presented, and N D values are proposed to be used as indicators for chemical activity of atoms and to predict thereby the position of favorable atom-atom contacts in addition reactions with participation of these molecules. Bases of "computational synthesis" procedures for fullerene 60 derivatives are exemplified by initial phases of its fluorination. Keywords: quantum chemistry, chemical activity, fullerene 60 and 70 , computational synthesis.Present ideas of chemical activity of fullerenes, mostly of 60 molecule, come to two basic statements. The first is that fullerene behaves like an electron-deficient polyalkene [1, 2], and not like an electron-enriched aromatic molecule as was originally supposed [3]. Rigid localization of double bonds in the molecule, confidently shown experimentally, is explained [1,2] by the presence of pentagon molecules in the structure precluding -electrons from conjugation. However, the electronic mechanism by which the pentagon structures localize double bonds remains entirely obscure.The second statement concerns selectivity of the chemical activity of molecules. The opinions on this point have divided. Some researchers incline to that the molecule has 30 equivalent reaction centers (by the number of double bonds) due to which most chemical reactions are not selective [2]. It is, however, difficult to reconcile this statement with the presence of primary derivatives of 60 , hydrides and halides, mostly of a specific stoichiometric composition, for example, 60 H 36 , C 60 Hal 24 , C 60 Hal 36 , C 60 Hal 48 [4,5]. Moreover, in obtaining the metallofullerenes, it has been established from the very beginning [1,6] that transition metals are forming an octahedron surrounding for the internal 60 nucleus. Thus the idea of atom-local selectivity (regioselectivity) of the molecule has appeared. The extension of this approach to organic derivatives of fullerene has led to a model suggested by Wudle [2,7] the core of which is a structural factor -pyracyclene fragment responsible for each elementary act of addition. In the author's opinion [2], localization of molecule the chemical activity within the fragment is caused by its pentagons. The presence of six such fragments in the structure of the 60 molecule made it possible to explain the formation of organic pseudo-octahedron covalent derivatives, fullerene stars with added products of different complexity [8].Attempts to give the quantum chemical (QC) description of 60 derivatives have been made repeatedly (see, for example, detailed works [9-11] and references therein). These computations, however, were aimed not so much at understanding the nature of the molecule c...

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“…The understanding of magnetic interactions is essential to analyze and interpret, among others, neutron diffraction experiments and magnetic susceptibility measurements. Most important is the fact that magnetic interactions are at the hearth of molecular based magnets [1][2][3][4][5][6] or of High-T c superconductivity [7] and largely dominate the chemistry of radicals [1,6,8]. Also, the description of magnetic states of damaged DNA, RNA or protein oxidation adducts by radicals has an increasing interest because of their relevance to understand the biological activity of important carcinogenic and antitumor compounds.…”

Section: Introductionmentioning

confidence: 99%

“…Still, one can use the non-relativistic Hamiltonian and handle magnetic interactions through a proper introduction of spin coordinates and spin symmetry. In this formalism, magnetic interactions in systems with localized magnetic moments result from important electronelectron correlation effects [1,8,15] which, to provide accurate theoretical estimates of J i j values, must be appropriately described. This requires going beyond the usual mean field theories and therefore, the electronic structure of magnetic systems provide severe test cases for theoretical methods.…”

Section: Introductionmentioning

confidence: 99%

Spin Symmetry Requirements in Density Functional Theory: The Proper Way to Predict Magnetic Coupling Constants in Molecules and Solids

et al. 2006

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Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Abstract In this paper it is argued that the use of density functional theory (DFT) to solve the exact, non-relativistic, many-electron problem, for magnetic systems requires imposing space and spin symmetry constraints exactly in the same way as it is currently done in ab initio wave function theory. This strong statement is supported on pertinent calculations for selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids. These calculations include several wave function methods of increasing accuracy and different forms of the exchangecorrelation functional. The comparisons of numerical results carried out always within the same standard Gaussian Type Orbital atomic basis set show that imposing or not the spin and space constraints (restricted or unrestricted formalisms) leads to contradictory results. Therefore, it appears that, in the case of the Heisenberg magnetic constant, the present exchange-correlation functionals may provide reasonable numerical results although for the wrong physical reasons thus evidencing the failure of the current DFT methods to properly describe magnetic systems.

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The Electronic Properties of Diradicals

Cited by 1,007 publications

References 75 publications

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“Chemical portrait” of fullerene molecules

Spin Symmetry Requirements in Density Functional Theory: The Proper Way to Predict Magnetic Coupling Constants in Molecules and Solids

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