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Electron spin resonance study of some triplet state molecules Gardner, Christopher Leonard

Abstract

Hydrazoic acid has been photolysed in a krypton matrix at 4°K and the products studied by electron spin resonance spectrometry. This study showed that NH₂ radicals have been produced as a secondary product in the reaction. In addition a broad, intense resonance at g = 2 and a weak, half field resonance has been tentatively assigned to the imine (NH) radical. This suggestion is shown to be consistent with theoretical considerations. Diazomethane has also been photolysed in krypton and carbon monoxide matrices at 4°K, and the products studied by e.s.r., in an attempt to detect the methylene (CH₂) radical. The results of this study were complicated and a complete analysis was not possible. It is suggested that some of the features may be explained in terms of an overlap of the spectre from CH₃ and CH radicals. There are difficulties in such an explanation however. A study has been made on the line shapes of poly-crystalline samples of aromatic triplet state molecules. It is shown how the experimentally observed spectra of the photoexcited triplet states of axially symmetric molecules such as triphenylene and non-axially symmetric molecules such as naphthalene and phenanthrene can be explained in terms of a line shape calculated from a first order perturbation treatment. This model gives a good explanation of the observed line shapes, however better agreement with the observed field positions is obtained if a second order correction is included. Line shape calculations have also been made for molecules, such as the substituted imines, where spin-spin interaction is large. It is shown that the calculation is in agreement with the observed spectra of phenylimine and benzenesulfonylimine. In addition, it is shown how the experimentally determined value of the spin-spin interaction constant, D, can be related to the spin density on the nitrogen of the substituted imines. The spin densities calculated in this way are in good agreement with spin densities calculated on the basis of the Hückel theory.

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