Comprehensive examinations of the motional properties (rotational correlation time τ(R)) and the spin exchange ω(SS) of the spin probe TEMPOL have been carried out using ESR spectroscopy in two different solvents. For the first time, the dynamic parameters τ(R) and ω(SS) have been determined simultaneously by simulation of spectra measured at three different ESR frequencies (L-, X-, and Q-band) between 293 and 500 K using a dynamic model based on a stochastic fitting program and, for comparison, two alternative models involving the shift of the hyperfine lines and considering the line broadening due to spin exchange in a wide range of conditions. Possibilities and limits of the used models are shown upon comparing the obtained results of the spin exchange. Moreover, the analysis of the ESR spectra gave evidence for the existence of cage effects that produce re-encounters of the spin probes. This has been done for the activation energies, which have been calculated from the temperature dependence of the rate constants of the spin exchange. From the ratio of the activation energies and the influence of the viscosities on the dynamics of the examined systems in n-octanol and an ionic liquid, conclusions can be drawn for the re-encounter effects in solvent cages. However, in contrast to n-octanol, the dynamics of the spin probe in the ionic liquid depends on specific and anisotropic interactions. The temperature dependence of the Q-band measurements required the development of a novel Q-band cavity.
For the first time, heterogeneous catalytic reactions have been monitored by in situ EPR spectroscopy in the Q-band using a homemade heatable probe head equipped with a flow reactor. The reactions of Al(2)O(3)-supported TEMPO with NO and H(2) as well as of SiO(2)/Al(2)O(3)-supported H(4)PVMo(11)O(40) with methanol and formaldehyde were studied up to 400 degrees C. TEMPO radicals are immobilized on the support in positions which impose a different reactivity to NO and H(2). This may be due to different accessibility, which changes during thermal treatment. By combined evaluation of anisotropic X- and Q-band spectra with a complex hyperfine structure (e.g., from VO(2+)), spin Hamiltonian parameters can be derived with higher precision, since limits of the specific resolution in both frequency bands are compensated for. In addition to VO(2+), Mo(5+) is formed above 180 degrees C depending on the O(2) content of the feed, which is easily discriminated in the Q-band but not in the X-band.
Oxidation of benzoyl-and dibenzoyl-hydrazine with silver oxide or thermal decomposition of azodibenzoyl in solution gives inconclusive indicactions of the production of free benzoyl radicals. Much better evidence of the formation of free aroyl radicals has been obtained from studies of the photolyses of both azodibenzoyl and p-nitroazodibenzoyl in solution which show that the radicals react by dimerisation and by addition to undecomposed substrate in preference to attacking the surrounding solvent. Reaction schemes are proposed to account for the formation of the main reaction products.Oxidising actions and Diels-Alder condensations of azodibenzoyl are also reported.THIS work was planned as a study of the reactivity of the free benzoyl radicals by using processes which, unlike the homolytic chain reaction of benzaldehyde, do not involve competitive reactions of other radicals. Reactions of free alkyl radicals of a single type
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