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We have investigated self-assembled stearic acid films adsorbed on an Al 2 O 3 film by means of electron paramagnetic resonance (EPR) as well as near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stearic acid molecules are orientated in the film as shown by the NEXAFS data. Doping of the film with spin-labels in order to make the films accessible to EPR spectroscopy distorts the structure only slightly as long as the concentration of the spin-labels is low. The temperature-dependent EPR spectra reveal a distinct dependence of the onset of the rotational motion of the spin-label with variation of the location of the spin-label along the chain. The analysis of the EPR line shape allows to gain insight into the internal motion of the molecular chain.

The system di-tert-butyl nitroxide (DTBN) adsorbed on a thin film of γ-Al 2 O 3 (111) grown on a NiAl(110) single crystal has been studied with various surface science methods, including TPD, XPS, NEXAFS, and ESR line-shapes analysis. In the monolayer regime, two strong chemisorbed species with adsorption energies of 120 and 150 kJ/mol, respectively, are found after adsorption at 40 K. One species is ESR active and reveals distinct dynamic behavior above 200 K through cw ESR line-shape analysis. The second species is oriented with an angle of about 70°between the surface normal and the N-O axis of the molecule, as the NEXAFS data show. Missing of the π* resonance and the ESR inactivity suggest a direct participation of the half-filled π* orbital located at the nitrogen atom in the bonding mechanism. Above 200 K, both adsorbed species exchange with each other. This could be followed by monitoring the temperature-dependent ESR intensity. Adsorption at room temperature leads to the formation of a third unstable species which is ESR active but shows a different ESR spectrum in comparison with the other ESR-active species. The accessibility of the nitrogen atom for bonding is discussed on the basis of high-quality ab initio calculations.

Are radicals involved in the activation of a heterogeneous Ziegler–Natta catalyst? The activation of titanium centers in an active Ziegler–Natta model system initiated by alkyl aluminum compounds has been shown by ESR spectroscopy under ultrahigh‐vacuum conditions to proceed, in the case of trimethylaluminum, by a radical mechanism (see picture). In contrast, no indication of radical production is found when the ethyl derivative is used for activation.