Violation of Hund's rule by spin polarization in molecules

Kollmar, Herbert; Staemmler, Volker

doi:10.1007/bf00549021

Cited by 133 publications

(141 citation statements)

References 26 publications

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“…The residue was chromatographed (1% ether/hexanes) to provide 1. 15,143.35,141.16,133.10,129.96,128.71,126.26,122.16,120.73,38.04,33.12. MS M*+ 288,290 (100:100),273,275 (33:33),194(30).…”

Section: Conformation and High-spin Organic Moleculesmentioning

confidence: 98%

Conformational effects on high-spin organic molecules

Silverman¹,

Dougherty

1993

J. Phys. Chem.

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“…The residue was chromatographed (1% ether/hexanes) to provide 1. 15,143.35,141.16,133.10,129.96,128.71,126.26,122.16,120.73,38.04,33.12. MS M*+ 288,290 (100:100),273,275 (33:33),194(30).…”

Section: Conformation and High-spin Organic Moleculesmentioning

confidence: 98%

Conformational effects on high-spin organic molecules

Silverman¹,

Dougherty

1993

J. Phys. Chem.

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“…Nevertheless, a physical model in which, in order to minimize the Coulombic repulsion between electrons of opposite spin, each of the electrons in the lowest  MO tends to become more localized at the pair of carbons at which an electron of the same spin is localized in the GVB MOs, provides a reasonable description of the singlet wave function for square CBD. 16,17 In contrast to the case in singlet CBD, triplet CBD has a uniform distribution of spin in the nonbonding  MOs. Consequently, in the triplet, dynamic spin polarization cannot reduce the Coulombic repulsion between the electrons in the nonbonding MOs and the electron of opposite spin in the bonding  MO.…”

Section: Gvb Mos Of Square Cyclobutadienementioning

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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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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“…Thus, to a first approximation, the lowest singlet and the lowest triplet states have the same energy. Since dynamic spin polarization [24] between the two electrons with an opposite spin is important in the singlet state in this case, the singlet state becomes a ground state. On the other hand, when NBMOs have atoms in common, a situation that is described as ''non-disjoint'' arises, and a triplet ground state is expected since in this state the Pauli principle prevents the two electrons in the NBMOs from simultaneously appearing in the AOs that the two NBMOs have in common.…”

Section: Squid Measurements Of Photoproducts From Dendrimers (3a 6amentioning

confidence: 99%

Synthesis of Phenylacetylene Dendrimers Having Sterically Congested Diazo Units and Characterization of Their Photoproducts

et al. 2004

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The compound (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,4,6‐tribromophenyl)diazomethane (1a), a precursor for persistent triplet carbene, was found to be stable enough to survive Sonogashira coupling reaction conditions to give (4‐tert‐butyl‐2,6‐dimethylphenyl)(2,6‐dibromo‐4‐ethynylphenyl)diazomethane (2a). The diazomethane 2a was used as a starting compound for synthesis of phenylacetylene dendritic molecules having peripheral diazo groups by the convergent method. Thus, tridendrons with peripheral groups bearing 3 (3a), 6 (6a), and 12 (8a) diazo units were prepared, in which diazo groups were introduced so as to generate polycarbenes in a ferromagnetic fashion. In order to characterize the spin states of carbenes generated by the photolysis of these dendrimers, the magnetic susceptibility of the photoproducts was measured. Although the data for the photoproduct of the tris(diazo) compound were fitted with S = 2.5, those for the photoproducts of hexakis and dodecakis(diazo) compounds were fitted with S = 1.62, a value that is much smaller than the theoretically predicted value and even smaller than that of triscarbene. The results are interpreted in terms of the disjoint‐nondisjoint concept. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004

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Violation of Hund's rule by spin polarization in molecules

Cited by 133 publications

References 26 publications

Conformational effects on high-spin organic molecules

Conformational effects on high-spin organic molecules

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

Synthesis of Phenylacetylene Dendrimers Having Sterically Congested Diazo Units and Characterization of Their Photoproducts

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