The Epr Spectroscopic
            <i>D</i>
            Parameter of Localized Triplet Diradicals as Probe for Electronic Effects in Benzyl‐Type Monoradicals

Adam, Waldemar; Harrer, Heinrich M.; Kita, Fumio; Nau, Werner M.

doi:10.1002/9780470133552.ch4

Cited by 20 publications

(8 citation statements)

References 112 publications

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“…The traditional approach relies on the concept of ‘atomic spin densities’ and a simple point dipole model of spin–spin interactions. In ‘localized’ diradicals (such as cyclopentane‐1,3‐diyls), the D parameter decreases with the third power of the distance d AB between the ‘radical sites’ A and B and is proportional to the spin densities ρ at both centers (Eqn (2)):

D ~ ρ_{A} ρ_{B} / d_{AB}^{3}

…”

Section: Resultsmentioning

confidence: 99%

“…This relationship has been firmly established by elegant EPR studies on various diradicals by Adam and coworkers and it has been used to study substituent effects on the spin density distribution within these systems . It relies on the assumption that spin‐orbit contributions to the ZFS can be completely neglected (which is usually justified for hydrocarbon systems) and that the spin–spin interactions can be treated within a simple point dipole scheme.…”

Section: Resultsmentioning

confidence: 99%

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EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

Neuhaus

Winkler

Sander

2011

J of Physical Organic Chem

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The isomeric 2‐dehydro‐ and 4‐dehydro‐1,3‐benzoquinodimethanes, 18 and 19, were generated by irradiation of the corresponding triiodo compounds in cryogenic argon matrices and characterized by electron paramagnetic resonance spectroscopy. In agreement with multireference computations, both systems possess quartet ground states, whereas in the isomeric 5‐dehydro‐m‐xylylene 20 the 2B2 doublet state lies energetically below the 4B2 state, so that no characteristic quartet signals could be observed for this triradical under similar experimental conditions. The best estimates for the adiabatic doublet–quartet energy splittings (CAS(7,7)‐AQCC/cc‐pVTZ//CAS(9,9)‐RS2c/cc‐pVTZ) of 18–20 are 10.4, 7.7, and −1.3 kcal/mol, respectively. The measured zero‐field splitting parameters of 18 and 19 are discussed in terms of the contributions of carbenoid resonance structures (spin polarization of the π‐system) to the resonance hybrid of the title triradicals. Copyright © 2011 John Wiley & Sons, Ltd.

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D ~ ρ_{A} ρ_{B} / d_{AB}^{3}

…”

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

Neuhaus

Winkler

Sander

2011

J of Physical Organic Chem

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“…The homolysis of toluene to yield benzyl radical is discussed in many textbooks of organic chemistry as the prima facie example of the energetic benefits of electron and spin delocalization (Chart 1). The hyperfine couplings in electron paramagnetic resonance (EPR) spectra inform about spin distributions, 47,48 electronic structure theory shows this spin distribution to occur by spin polarization and to feature spin alternation in most cases, [49][50][51] and the EPR hyperfine coupling constants are related to the electron and spin delocalization and the degree of radical stabilization. 52,53 This view of "electron and spin delocalization" permeates the research literature [54][55][56][57] and, for example, Wu et al 57 concluded from their studies of substituent effects of neutral para-substituted toluene derivatives that "both electron-donating and electron-withdrawing groups reduce the bond dissociation energy (BDE) of the benzylic C-H bond [by 0-3 kcal/mol] because both groups cause spin delocalization from the benzylic radical center.…”

Section: Point Of Reference: Benzyl Radical and Benzoannulationmentioning

confidence: 99%

Electronic Structures and Spin Topologies of γ-Picoliniumyl Radicals. A Study of the Homolysis ofN-Methyl-γ-picolinium and of Benzo-, Dibenzo-, and Naphthoannulated Analogs

et al. 2008

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Radicals resulting from one-electron reduction of (N-methylpyridinium-4-yl) methyl esters have been reported to yield (N-methylpyridinium-4-yl) methyl radical, or N-methyl-gamma-picoliniumyl for short, by heterolytic cleavage of carboxylate. This new reaction could provide the foundation for a new structural class of bioreductively activated, hypoxia-selective antitumor agents. N-methyl-gamma-picoliniumyl radicals are likely to damage DNA by way of H-abstraction and it is of paramount significance to assess their H-abstraction capabilities. In this context, the benzylic C-H homolyses were studied of toluene (T), gamma-picoline (P, 4-methylpyridine), and N-methyl-gamma-picolinium (1c, 1,4-dimethylpyridinium). With a view to providing capacity for DNA intercalation the properties also were examined of the annulated derivatives 2c (1,4-dimethylquinolinium), 3c (9,10-dimethylacridinium), and 4c (1,4-dimethylbenzo[g]quinolinium). The benzylic C-H homolyses were studied with density functional theory (DFT), perturbation theory (up to MP4SDTQ), and configuration interaction methods (QCISD(T), CCSD(T)). Although there are many similarities between the results obtained here with DFT and CI theory, a number of significant differences occur and these are shown to be caused by methodological differences in the spin density distributions of the radicals. The quality of the wave functions is established by demonstration of internal consistencies and with reference to a number of observable quantities. The analysis of spin polarization emphasizes the need for a clear distinction between "electron delocalization" and "spin delocalization" in annulated radicals. Aside from their relevance for the rational design of new antitumor drugs, the conceptional insights presented here also will inform the understanding of ferromagnetic materials, of spin-based signaling processes, and of spin topologies in metalloenzymes.

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“…On the other hand, a wide-range tuning of diradical character for these compounds together with improving their kinetic stability are challenging issues from the viewpoint of understanding and analyzing the chemical bond nature of such transient species as well as of expanding the number of possible open-shell singlet species. In this study, therefore, as a prototypical localized diradical, we investigate 1,3-diradical compounds that are usually observed as the transition structure in the homolytic bond-cleavage and -formation reaction processes of cyclic compounds. − 1,3-Diradical compounds are expected to have a small HOMO–LUMO gap because of the balance between the weak trough-space (TS) interaction of the pπ AOs on the C 1 and C 3 sites and the weak through bond (TB) interaction of the ψ S with the pseudo-π-orbital of CH 2 at the C2 position (Figure , central part). − Because of this feature, most of 1,3-diradical compounds tend to have small singlet–triplet (ST) energy gap and to be the ground-state triplet. − …”

Section: Introductionmentioning

confidence: 99%

“…Furthermore, para donor and/or acceptor substituent effects on the lifetimes of 2,2-dimethoxy-1,3-diphenylcyclopentane-1,3-diyl derivatives were investigated theoretically and experimentally . Compared to the nonsubstituted reference compound, mono- and disubstituted compounds were found to show slightly longer lifetimes. ,,, The relationship between the lifetime and the ST energy gap of the para substituted systems was examined as well. Introduction of donor and/or acceptor groups to the conjugated systems are known to be an efficient way to modulate their optical response properties as well through the change of electronic structures.…”

Section: Introductionmentioning

confidence: 99%

Theoretical Study on Diradical Characters and Nonlinear Optical Properties of 1,3-Diradical Compounds

et al. 2014

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We investigate the relationships between the diradical character (y) and nonlinear optical (NLO) properties of open-shell 1,3-diradical compounds using the broken-symmetry density functional theory method. The 2,2-substituent effects on the structure-property relationship are clarified for several 1,3-diphenylcyclopentane-1,3-diyl derivatives, which are known as the systems with weak or intermediate π-single-bonding characters. The parent 1,3-diphenylcyclopentane-1,3-diyl (1a: X = H) is found to be almost pure diradical (y ∼ 1) owing to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The energy gap is determined by the balance of the through-space coupling with the through bond coupling effect. On the other hand, the introduction of the electron-withdrawing substituents X at the C2 position of cyclopentane-1,3-diyls (1b: X = OH, 1c: X = F) is found to decrease the y-value owing to the effects of additional through-bond interactions. As a result, 1b and 1c are found to have intermediate y. Static second hyperpolarizabilities (γ) of 1b and 1c are found to be enhanced by a factor of ∼4.5 and ∼6.4, respectively, compared with those of the pure singlet diradical 1a and those of the triplet 1a-1c. From the analysis of the third-order responses of electron density, the introduction of the 2,2-substituents is found to enhance the field-induced third-order polarizations over the whole system. We also investigate the effects of asymmetric donor/acceptor substitutions at the para positions of phenyl rings on the response properties. Although the asymmetric donor/acceptor substitutions have no significant impact on y in the present systems, they are found to provide the increase of γ from the corresponding nonsubstituted analogues. The present results have revealed strong correlation between the π-bonding character (diradical character) and third-order NLO properties in the real 1,3-diradical compounds. On the basis of the theoretically predicted correlation in the real systems, NLO measurements are speculated to be utilized as a new probe of the unique chemical bonding nature in such localized diradical compounds, which is one of the fundamental subjects in chemistry.

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The Epr Spectroscopic D Parameter of Localized Triplet Diradicals as Probe for Electronic Effects in Benzyl‐Type Monoradicals

Cited by 20 publications

References 112 publications

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

Electronic Structures and Spin Topologies of γ-Picoliniumyl Radicals. A Study of the Homolysis ofN-Methyl-γ-picolinium and of Benzo-, Dibenzo-, and Naphthoannulated Analogs

Theoretical Study on Diradical Characters and Nonlinear Optical Properties of 1,3-Diradical Compounds

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