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Electron paramagnetic resonance studies of matrix isolated inorganic radicals Tait, John Charles

Abstract

The technique of electron paramagnetic resonance (EPR) has been used to investigate the electronic structure and geometry of several small triatomic or tetraatomic radicals which have been trapped at 4.2 K in the inert matrices of neon, argon or krypton. The EPR spectra were analyzed by using an extensive computer simulation program based on a detailed general spin Hamiltonian which includes the Zeeman, nuclear Zeeman, hyperfine and quadrupole interaction terms. A general method for analyzing the EPR spectrum of a poly-crystalline sample has been discussed with particular emphasis being placed on systems for which the principal axes of the electronic g-tensor and hyperfine tensors are not coincident. The chloroperoxyl radical (C100) was previously studied in several polar environments and in an argon matrix, but the EPR parameters were somewhat in disagreement. A study of this radical, formed by the UV irradiation of chlorine dioxide (ClO₂) trapped in an inert matrix was undertaken with a view to improving the accuracy of the EPR parameters. The electronic g-tensor and hyperfine tensor were found to be non-coincident. The radical species FSO has been produced by the far UV photolysis of thionyl fluoride (F₂SO) in an argon matrix. The spin Hamiltonian parameters were obtained for this radical and found to be remarkably similar to the well known species F00. The hyperfine components were interpreted in a manner similar to F00. The chlorodisulfonyl radical (C1SS) was produced by the near UV photolysis of the dichlorodisufane molecule (S₂Cl₂) in an argon matrix. The electronic g-tensor, hyperfine coupling and quadrupole coupling constants were determined from comparison of the observed spectrum with the computer simulated spectrum. To obtain good agreement, the principal axes of the hyperfine and quadrupole tensors had to be rotated by ~10° from those of the electronic g-tensor. The radical species HSO₂ and FSO₂ were produced by the UV photolysis of HI/S0₂ or trifluoromethylhypofluorite (CF₃OF)/S0₂ mixtures in the rare gas matrices. The F₂SO₂ radical was also produced by the far UV photolysis of sulfuryl fluoride (F₂SO₂). The EPR parameters were determined assuming non-coincident axes for the hyperfine and electronic g-tensors. Another species was produced in the reaction of methyl radicals with SC>2 but the nature of the adduct is uncertain. Chlorine dioxide (ClO₂) was studied in several inert matrices and the spin Hamiltonian parameters were determined for each matrix. Partial orientation has been observed in all the matrices. The preferred orientation is with the plane of the radical parallel to the deposition surface. The spectrum has been simulated using an approximate distribution function for the molecular orientations. A second trapping site has been observed on annealing the argon matrix. This site exhibits slightly different hyperfine and Zeeman interactions and is thought to mean that the site is a substitutional one. The matrix shifts of the electronic g-tensor and hyperfine tensor are discussed in terras of the van der Waals and Pauli interaction forces.

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