The D parameter (zero-field splitting) as a direct measure of structural end electronic effects in localized triplet 1,3-diradicals

Adam, Waldemar; Harrer, Heinrich M.; Heidenfelder, Thomas; Kammel, Thomas; Kita, Fumio; Nau, Werner M.; Şahin, Coşkun

doi:10.1039/p29960002085

Cited by 12 publications

(17 citation statements)

References 14 publications

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“…Dougherty's anticipation 6 that no special electronic effects (captodative stabilization, spin polarization, etc.) should play a significant role in triplet 1,3- diradicals was recently confirmed experimentally and theoretically for the bicyclic 1,3-cyclopentanediyl diradical derivatives 6 . 8b,c As a consequence, such 1,3-diaryl-substituted cyclopentane-1,3-diyl triplet diradicals can be described as a composite of two cumyl radical fragments 7 , which may provide the opportunity to assess electronic substituent effects on benzyl-type monoradicals through the EPR D parameter of the corresponding triplet diradicals . This was demonstrated by a direct linear dependence between the D parameters of such triplet diradicals and the a β hyperfine splitting constants of the corresponding cumyl monoradicals in terms of substituent effects. 8b,c …”

Section: Introductionmentioning

confidence: 74%

Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

et al. 1997

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The zero-field splitting D parameter was determined in a 2-MTHF glass matrix at 77 K for a large set (35 derivatives) of para- and meta-substituted 1,3-arylcyclopentane-1,3-diyl triplet diradicals 6. The D values are a sensitive function of electronic substituent effects; for convenience, the ΔD scale was defined as the difference D H − D X. Spin acceptors decrease while spin donors increase the D value relative to the unsubstituted reference system (D H). Theoretical (PM3-AUHF) α spin densities (ρα) for the corresponding cumyl monoradicals 7 display a good linear dependence (r 2 = 0.963) when plotted against the D parameters of the triplet diradicals 6. The radical stabilization energies (RSE) of the cumyl radicals 7 were semiempirically calculated as the energy difference between in-plane (full conjugation) and perpendicular (no conjugation) conformations of the aryl groups and shown to correlate linearly (r 2 = 0.947) against the experimental D parameter for the corresponding triplet diradicals 6. These linear correlations, i.e., D versus ρα and versus RSE, demonstrate that the D parameter of the localized triplet diradicals 6 reflects reliably electronic substituent effects in benzyl-type monoradicals. The spectroscopic ΔD scale correlates poorly with the reported chemical σrad scales, unless polar corrections (Hammett σpol values) are made by means of a two-parameter Hammett treatment. Then a good linear correlation (r 2 = 0.921) of the ΔD values versus the Creary σrad scale applies; as expected, the radical effects dominate (ρrad = 1.00 versus ρpol = 0.41). The advantages of the new EPR-spectroscopic ΔD scale are that polar effects are nominal and the D parameter can be measured experimentally with sufficient accuracy to probe even small and subtle electronic effects through changes in the α spin densities.

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Section: Introductionmentioning

confidence: 74%

Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

et al. 1997

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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%

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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“…The magnitude of D in 2 can be expected to resemble that of 1,3‐cylcopentane diradical in which the distance between the radical centers is 2.355 Å (at UBP86/TZVP level) compared with 2.382 Å in 2 . However, 2 exhibits a significantly lower D value of 0.041‐0.045 cm ‐1 compared to 0.084 cm ‐1 in 1,3‐cylcopentane diradical 46. The lower D value in 2 is due to the fact that unlike in cyclic‐1,3‐diradicals where the two spins are nearly parallel, in 2 the spins are almost perpendicular, which diminishes the spin–spin interaction and hence lowers D .…”

Section: Experimental and Calculated (Ubp86‐d3/tzvp) Geometries Of 1mentioning

confidence: 64%

“…The lower D value in 2 is due to the fact that unlike in cyclic‐1,3‐diradicals where the two spins are nearly parallel, in 2 the spins are almost perpendicular, which diminishes the spin–spin interaction and hence lowers D . The resulting D of ∼0.045 cm ‐1 is similar to that of 1,3‐diphenylcyclopentane ( D = 0.047 cm –1 ),46 in which the relatively low D is due to spin delocalization to the phenyl rings.…”

Section: Experimental and Calculated (Ubp86‐d3/tzvp) Geometries Of 1mentioning

confidence: 99%

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

et al. 2015

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We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopyo f atriplet diradical that is formed in athermally induced rotation around amain-group p bond, that is,the Si = Si double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350-410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest at hermal equilibrium between 1 and 2 with av ery lows inglet-triplet energy gap of only 7.3 kcal mol À1.Rotation around the p-C=Cdouble bond, leaving the s-(C= C) bond intact, is one of the most fundamental processes in chemistry and is important in many chemical and biological transformations.C onsequently,i th as intrigued chemists for decades and has been studied extensively both experimentally and theoretically. [1][2][3][4][5][6][7][8][9] One approach to investigate this rotation process is to study tetrasubstituted olefins bearing large substituents that force the C = Cd ouble bond out of its preferred planar geometry,t hus reducing the energy (and temperature) required for rotation, [10][11][12][13][14][15][16][17][18] which is about 60 kcal mol À1 for H 2 C=CH 2 .[19] Some of these highly twisted C=Cd ouble bonds exhibit ac hemical behavior suggesting that they possess some diradical character;h owever, generally diradicals were not observed spectroscopically. [15][16][17][18] In one case,t hat of bis-[dibenzo[a.i]-fluorenylidene],i tw as claimed that the singlet ground state and the diradical triplet state resulting from rotation around the C = Cd ouble bond exist in equilibrium, but spectroscopic evidence was not provided. [15][16] Disilenes,R 2 Si=SiR 2 ,t he heavier analogs of alkenes,p resent an attractive opportunity to observe triplet diradicals because the barrier to rotation around Si=Si pdouble bonds is about 15-25 kcal mol À1 , [20][21][22][23][24][25] significantly lower than around C = C p-bonds.C onsequently,d isilenes are more easily torsionally twisted than alkenes; [25][26][27][28] for example,t etrakis(di-tert-butylmethylsilyl)disilene (1)s hows av ery large torsional distortion around the Si=Si double bond, with an average SiÀSi=SiÀSi dihedral angle (q)o f 55.98 8.[29] One consequence of this large twisting around the Si = Si bond is the blue color of 1,indicating asmall HOMO-LUMO gap.[29] Thel arge twisting around the Si = Si bond in 1 with p-p*t ransition at 612 nm, raises the possibility to observe spectroscopically the 1,2-disiladiradical 2 [Eq. (1)], formed upon thermally induced rotation around the Si=Si pbond of 1.T his physical behavior is unprecedented and especially intriguing if 2 exists in thermal equilibrium with its singlet electronic ground state 1.We report here the first direct observation, using EPR spectroscopy,o fathermally accessible triplet state resulting from at hermally...

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The D parameter (zero-field splitting) as a direct measure of structural end electronic effects in localized triplet 1,3-diradicals

Cited by 12 publications

References 14 publications

Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

Electronic Effects of para- and meta-Substituents on the EPR D Parameter in 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals. A New Spectroscopic Measure of α Spin Densities and Radical Stabilization Energies in Benzyl-Type Monoradicals

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

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

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