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- Liu, W, et al.
(författare)
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Structure and dynamics of radicals in zeolite matrices : ESR and theoretical studies
- 2003
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Ingår i: Applied Magnetic Resonance. - 0937-9347 .- 1613-7507. ; 24:3-4, s. 285-302
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Tidskriftsartikel (refereegranskat)abstract
- Amine radical cations of the type R3N.+ and [R3NCH2](.+), R = CH3, C2H5, C3H7, and nitric oxide, NO, have been used to probe the bonding to the surface and the dynamics of the radicals trapped in the confined space of cages or channels in the zeolite. Regular continuous-wave electron spin resonance (ESR) was employed to study the internal motion of the cation radicals formed by gamma-irradiation of amines and related ammonium ions, introduced during the synthesis of the zeolites Al-offretite, SAPO-37, SAPO-42 and AlPO4-5. The ESR spectra of [(CH3)(3)NCH2](.+) radical cation in several studied systems changed reversibly with temperature, indicating dynamical effects. Free rotation about the >N-CH2 bond of the [(CH3)(3)NCH2](.+) species was found to occur in the temperature range of 110 to 300 K, while the rotation about the >N-CH3 bonds was hindered. The observations confirm the theoretical prediction on the basis of density functional theory calculations, which indicate that the corresponding barriers are of the order of 0.3 and 7 kJ/mol, respectively. The radical cations of the type R3N.+ with R = C2H5, C3H7 were found to undergo a different type of dynamics, involving a two-jump process of the methylene hydrogens next to the nitrogen. A cage or channel size effect on the stability and molecular dynamics was inferred in some cases. Pulsed ESR was employed to study the (NO)(2) triplet-state dimers in Na-A type zeolite, with the purpose to resolve the interaction with surface groups, and to elucidate the role of the zeolite on stabilizing the triplet rather than the usual singlet state. Measurements performed at 5 K gave rise to Fourier transform spectra that were assigned to the dimer species interacting with one or more Na-23 nuclei, with approximative parameters A(Na-23) = (4.6, 4.6, 8.2) MHz and Q(Na-23) = (-0.3, -0.3, 0.6) MHz for the hyperfine and nuclear quadrupole coupling tensors, respectively. The values are of similar magnitude as those determined for the NO---Na+ complex. The stability of the triplet-state structure was attributed to an unusual geometric structure imposed by the zeolite matrix, with the N-O bonds along a line as in [O-N---Na+---N-O], which according to UHF ab initio calculations has a triplet ground state.
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