Li cation–aromatic organic radical complex in a zeolite studied by electron spin echo envelope modulation spectroscopy

Doetschman, David C.; Gilbert, D.C.; Dwyer, David W.

doi:10.1016/s0301-0104(00)00094-x

Cited by 8 publications

(8 citation statements)

References 17 publications

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“…In Cu-ZSM-5 the nuclear hfc was found to be 161 MHz, corresponding to a spin population transfer to the Cu cation of 2.7% . In a recent ESR study of the Li-phenalenyl radical in Y-zeolite the measured isotropic hfc of 59 MHz (sign undetermined) corresponds to a 16% spin transfer to the Li ion . In the calculations of Webster and Macrae, the effective charge on the Na, due to its binding with the C 6 H 6 Mu radical, is 0.88, which suggests a comparable spin transfer of 12%, neglecting lattice contributions.…”

Section: Experimental Results In Naymentioning

confidence: 93%

“…While most studies of this nature in zeolites have been of radical cations, − there are also several reports of ESR studies of neutral free radicals in zeolites, − ,− some involving stable free radicals. ,, Radicals are often formed in geminate pairs by photolysis of selected organic precursors, and recombination reactions can be fast, though mitigated by the restricted zeolite geometry at lower temperatures. ,, Stable nitroxide radicals have also been observed on the external surfaces of two MFI zeolites 12 (silicalite and ZSM-5). An important class of neutral organic free radicals are those formed directly by H-atom addition to unsaturated bonds, which might generally be expected from Bronsted active OH centers in zeolites via specific hydrogen exchange reactions. , Of particular interest here is H-atom transfer to benzene, leading to the classic cyclohexadienyl radical, C 6 H 7 , as a basis for more complex aromatic systems in faujasites. ,,,,, The only report 13 of the direct observation of C 6 H 7 in a zeolite, formed by H-atom addition, is from the radiolysis of HZSM-5. However no studies of its dynamics were reported and, to our knowledge, there have been no studies reported at all of this radical in faujasites, where radical recombination is more facile .…”

Section: Introductionmentioning

confidence: 99%

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Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

et al. 2002

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The adsorption and dynamical behavior of the muonated cyclohexadienyl radical (C 6 H 6 Mu) in NaY zeolite, formed by muonium (Mu) addition on adsorbed benzene, was investigated by the muon spin resonance (µSR) technique, primarily at loadings of 2-3 C 6 H 6 molecules per supercage of NaY. The dynamics of this radical are expected to be the same as its isotopic analogue, C 6 H 7 , for which there are no similar data available. Both TF-µSR and ALC-µSR spectra were recorded, with the most detailed information provided by the positions and line widths of the avoided level crossing resonances. In concert with 2 H NMR, neutron diffraction and molecular dynamics studies of the parent benzene molecule, as well as current theoretical calculations, the dominant adsorption site for the C 6 H 6 Mu radical is believed to be the S II Na cation, within a supercage, which gives rise to three observed ALC lines, corresponding to two different orientations for the muon (proton) of the CHMu methylene group: pointing toward (endo) and away (exo) from the Na cation. The cation interaction gives rise to unprecedentedly large (≈20%) shifts in hyperfine coupling constants, indicative of a strong bond formed with the π electrons of the C 6 H 6 Mu radical. An additional but weaker resonance line is also seen, which is interpreted as being due to adsorption at the window sites between supercages. The ALC lines associated with the C 6 H 6 Mu radical bound to both the Na cation and window sites are all broad, ≈1 kG, change little with temperature and exhibit mainly static line shapes over the whole temperature range studied, from 3 to 322 K. This indicates a much stronger host-guest interaction for C 6 H 6 Mu, particularly with the Na cation, than is known for benzene, to the extent that this site acts as an effective trap for the free radical, over the critical µSR time scale of 50 ns.

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Section: Experimental Results In Naymentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

et al. 2002

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“…The resulting spin density on the cation nucleus results in an, as yet, unresolved nuclear hyperfine structure. The hyperfine interaction has been observed recently, however, for Li + , Na + , and Cs + in X- and Y-zeolites loaded with phenalenyl with electron spin echo envelope modulation spectroscopy UV, and ODMR spectroscopic studies of these zeolites have shown indirect evidence that Lewis acid−base complex formation between neutral, diamagnetic guests with aromatic π-electron systems and exchanged alkali metal cations occurs at various temperatures.…”

Section: Introductionmentioning

confidence: 96%

A Magnetic Resonance Study of Complex Formation between the Neutral Phenalenyl Radical and Alkali Metal Ions in Dilute Alcohol Solutions

et al. 2000

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In previous work from our laboratories we have characterized the molecular motion of phenalenyl in the supercage of cation-exchanged X- and Y-zeolites using continuous wave−electron paramagnetic resonance (CW-EPR) and pulsed-EPR techniques. The trends in activation energies with cation size and Lewis acidity and in EPR line widths provided circumstantial evidence for a weak covalent interaction between the neutral phenalenyl π-system and the cations. This work on phenalenyl and alkali metal cations in solution provides 7Li nuclear magnetic resonance (NMR) and CW-EPR spectroscopic evidence for this type of interaction. Moreover, to our knowledge it is the first example of a neutral radical−alkali metal ion complex in solution. In rigorously dried alcohol solutions this interaction results in the formation of a complex between neutral phenalenyl and Li+ or Na+, which is in equilibrium with solvated phenalenyl and the respective cation. The forward and reverse rates for this equilibrium for Li+ and phenalenyl in methanol allow for an analysis of 7Li NMR data acquired at 320 K in the fast exchange limit, which yields an equilibrium constant of 29 ± 4 M-1. Analysis of the CW-EPR data for Na+ and phenalenyl in methanol at 295 K in the slow exchange limit produces an isotropic Na hyperfine frequency for the phenalenyl−Na+ complex of 4.2 ± 0.1 MHz. This hyperfine frequency reflects a Fermi contact with the Na nucleus of ca. 0.5% of the nonbonding π-electron spin density from phenalenyl.

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“…It was not only possible to prove the direct coordination of the adsorbate nitroxide radicals to the Li + cations but also to determine the NO−Li bond length and the geometrical structure of the DTBN:Li + adsorption complex. Only a few applications of the HYSCORE experiment for nuclei with nuclear spin I = 3/2 such as the 7 Li isotope or I = 5/2 like 25 Al are known, − but to our knowledge this method has not been utilized for the investigation of I = 7/2 nuclei such as 133 Cs. Therefore, before discussing the experimental 133 Cs HYSCORE data, the characteristics of such spectra as well as the influence of the hf interaction on the cross-peak ridges are discussed.…”

Section: Introductionmentioning

confidence: 99%

¹³³Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs⁺ Adsorption Complex in CsNaY Zeolite

Gutjahr¹,

Böttcher²,

Pöppl³

2003

J. Phys. Chem. B

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133Cs hyperfine sublevel correlation (HYSCORE) spectroscopy has been employed to characterize the structure of adsorption complexes of di-tert-butyl nitroxide (DTBN) with cesium cations in zeolite CsNaY. The experimental 133Cs HYSCORE data proved the direct coordination of the adsorbed DTBN molecules to the Cs+ ions and revealed nonambiguously the existence of two different types of adsorption complexes. Evaluation of the orientation selective 133Cs HYSCORE spectra provided the 133Cs hyperfine coupling tensors and thus information about the geometrical structure of those complexes. For one type of adsorption complexes, a complex geometry was obtained where the Cs+ ion is located within the molecular mirror plane of the DTBN radical with an oxygen−cesium cation bond length of 0.25 nm. For this, the isotropic Cs hyperfine coupling was negative. The second Cs+−DTBN complex is characterized by a bent structure. With an isotropic 133Cs hyperfine coupling of 9 MHz, the unpaired electron spin density in this Cs+−DTBN complex localized at the Cs ion was 0.36%.

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Li cation–aromatic organic radical complex in a zeolite studied by electron spin echo envelope modulation spectroscopy

Cited by 8 publications

References 17 publications

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

A Magnetic Resonance Study of Complex Formation between the Neutral Phenalenyl Radical and Alkali Metal Ions in Dilute Alcohol Solutions

¹³³Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs⁺ Adsorption Complex in CsNaY Zeolite

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Li cation–aromatic organic radical complex in a zeolite studied by electron spin echo envelope modulation spectroscopy

Cited by 8 publications

References 17 publications

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

A Magnetic Resonance Study of Complex Formation between the Neutral Phenalenyl Radical and Alkali Metal Ions in Dilute Alcohol Solutions

133Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs+ Adsorption Complex in CsNaY Zeolite

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¹³³Cesium HYSCORE Investigation of the Di-tert-butyl Nitroxide−Cs⁺ Adsorption Complex in CsNaY Zeolite