Reducing the spin–spin interaction of stable carbon radicals

Green, Uri; Aizenshtat, Zeev; Ruthstein, Sharon; Cohen, Haim

doi:10.1039/c3cp50533b

Cited by 33 publications

(30 citation statements)

References 16 publications

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“…The original lignocellulosic materials and compounds gave ESR spectra with radicals with higher g-values than their derived chars, indicating higher levels of oxygen-containing radicals (Tables 5 and 6). The chars prepared under slow heating could be fitted by two spectra with g-values (2.0026e2.0046), indicating two radical types which were assigned to oxygen-centered radicals (g ¼ 2.0038e2.0047) [12] and carbon-centered radicals (g ¼ 2.0025e2.003) based on their g-values [12,61,62].…”

Section: G-factors and Radical Typesmentioning

confidence: 99%

Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

Trubetskaya

Jensen

et al. 2016

Biomass and Bioenergy

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a b s t r a c tThe concentration and type of free radicals from the decay (termination stage) of pyrolysis at slow and fast heating rates and at high temperatures (above 1000 C) in biomass char have been studied. A roomtemperature electron spin resonance spectroscopy study was conducted on original wood, herbaceous biomass, holocelluloses, lignin and their chars, prepared at high temperatures in a wire mesh reactor, an entrained flow reactor, and a tubular reactor. The radical concentrations in the chars from the decay stage range up between 7$10 16 and 1.5$10 18 spins g À1. The results indicated that the biomass major constituents (cellulose, hemicellulose, lignin) had a minor effect on remaining radical concentrations compared to potassium and silica contents. The higher radical concentrations in the wheat straw chars from the decay stage of pyrolysis in the entrained flow reactor compared to the wood chars were related to the decreased mobility of potassium in the char matrix, leading to the less efficient catalytic effects of potassium on the bond-breaking and radical re-attachments. The high Si levels in the rice husk caused an increase in the char radical concentration compared to the wheat straw because the free radicals were trapped in a char consisting of a molten amorphous silica at heating rates of 10 3 e10 4 K s À1. The experimental electron spin resonance spectroscopy spectra were analyzed by fitting to simulated data in order to identify radical types, based on g-values and line widths. The results show that at high temperatures, mostly aliphatic radicals (g ¼ 2.0026e2.0028) and PAH radicals (g ¼ 2.0027e2.0031) were formed.

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Section: G-factors and Radical Typesmentioning

confidence: 99%

Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

Trubetskaya

Jensen

et al. 2016

Biomass and Bioenergy

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“…Moreover, these dark‐green solution gave almost identical EPR spectra, implying the generation of similar paramagnetic species (Figure S30 in SI). The highly symmetric EPR resonance with a g ‐value ( g= 2.0023) close to that of the free electron in combination with the dark‐green color of the reaction mixture further suggested the formation of carbon‐centered radicals within delocalized aromatic systems …”

Section: Resultsmentioning

confidence: 96%

“…The highlys ymmetric EPR resonance with a g-value (g = 2.0023) close to that of the free electron in combination with the dark-green color of the reaction mixture further suggested the formation of carbon-centered radicals within delocalized aromatics ystems. [8] Based on the successful synthesis of complexes 1-2,w er easoned that the unprecedented reaction pathway of L 3-5 H with Ca[N(SiMe 3 ) 2 ] 2 (THF) 2 wasp robablyt riggered by the introduction of two isopropyl groups to the quinolyl rings, which would have led to considerable repulsion between the tetradentate ligand and the silylamido group in the targeted calcium complexes. Therefore, we sought to introduce as maller amino group,N (SiHMe 2 ) 2 ,w hich we envisioned would allow for the synthesis of stable heavy alkaline earth metal complexesw ith the help of internal metal···HÀSi agostici nteractions.…”

Section: Introductionmentioning

confidence: 99%

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C−N Bond Cleavage Reactions Induced by C−H Bond Activations

Yang

Wang

2016

Chemistry An Asian Journal

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Five bis(quinolylmethyl)-(1H-indolylmethyl)amine (BQIA) compounds, that is, {(quinol-8-yl-CH ) NCH (3-Br-1H-indol-2-yl)} (L H) and {[(8-R -quinol-2-yl)CH ] NCH(R )[3-R -1H-indol-2-yl]} (L H) (L H: R =Br, R =H, R =H; L H: R =Br, R =H, R =iPr; L H: R =H, R =CH , R =iPr; L H: R =H, R =nBu, R =iPr) were synthesized and used to prepare calcium complexes. The reactions of L H with silylamido calcium precursors (Ca[N(SiMe R) ] (THF) , R=Me or H) at room temperature gave heteroleptic products (L )CaN(SiMe ) (1, 2), (L )CaN(SiHMe ) (3 a, 4 a) and homoleptic complexes (L ) Ca (D3, D5). NMR and X-ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C-Si, Ca⋅⋅⋅H-Si or Ca⋅⋅⋅H-C agostic interactions. Unexpectedly, calcium complexes ((L )CaN(SiMe ) ) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C-N bond cleavage processes as a consequence of intramolecular C-H bond activation, leading to the exclusive formation of (E)-1,2-bis(8-isopropylquinol-2-yl)ethane.

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“…The ESR spectra of original lignocellulosic material showed higher g‐values than the biomass chars, indicating higher levels of oxygen‐centered radicals, as shown in the Supporting Information (Table S‐2). The chars prepared under slow heating could be fitted by two spectra, each with a center g‐value in the interval (2.0026–2.0046), indicating two radical types, which were assigned to oxygen‐centered radicals ( g =2.0038–2.0047) and carbon‐centered radicals ( g =2.0025–2.003) based on their g‐values …”

Section: Resultsmentioning

confidence: 99%

“…The chars preparedu nder slow heatingc ould be fitted by two spectra,e ach with ac enterg -value in the interval( 2.0026-2.0046), indicatingt wo radicalt ypes, which were assigned to oxygen-centered radicals (g = 2.0038-2.0047) [14] and carboncenteredr adicals (g = 2.0025-2.003) based on their gvalues. [14,23,24] DFT structure and energy calculations…”

Section: Resultsmentioning

confidence: 99%

The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements

2018

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Pyrolysis is a promising way to convert biomass into fuels and chemicals. This reaction is complex and inevitably involves a cascade of radical reactions that lead to char formation, in which some radicals become trapped and stabilized. Their nature is difficult to characterize, and in this respect computational chemistry can be a strong supplementary tool to electron spin resonance spectroscopy and other experimental methods. Here biomass char radicals and oxidation reactivity are studied experimentally, and density functional theory is used to predict the thermodynamic stability and g‐values of carbon‐ and oxygen‐centered radicals of polyaromatic char models including defect structures. Hydroxylated and especially certain dihydroxylated structures provide exceptional stabilization of oxygen‐centered radicals. Hydrogen bonding plays a crucial role, and it is proposed that hydrogen atom transfer couples radical localizations. This is a new proposal on the structural requirements for stabilization of char radicals, which impacts our understanding of pyrolysis mechanisms and char reactivity.

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Reducing the spin–spin interaction of stable carbon radicals

Cited by 33 publications

References 16 publications

Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C−N Bond Cleavage Reactions Induced by C−H Bond Activations

The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements

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