Random orientation, electron spin resonance

Electron Spin Resonance Spectroscopy of Randomly Oriented... 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

ESR spectroscopy (a) J. E. Wertz and J. R. Bolton, in Electron Spin Resonance Elementary Theory and Practical Applications McGraw-Hill, New York, 1972. (b) J. A. Berson, in The Chemistry of the Quinonoid Compounds, Vol. II, S. Patai and Z. Rappoport, Eds., John Wiley Sons, Inc., New York, 1988. (c) W. Gordy, in Theory and Applications of Electron Spin Resonance, Vol. 15, A. Weissberger series Ed., W. West, Ed., John Wiley Sons, Inc., New York, 1980, p. 589. (d) E. Wasseiman, W. A. Yager, and L. C. Snyder, Electron spin resonance (E.S.R.) of the triplet states of randomly oriented molecules, J. Chem. Phys. 1964, 41, 1763. 

Because of the great chemical importance of those methods of trapping radicals which result in a completely randomized distribution of orientations, the task of interpreting their electron spin resonance spectra will now be considered. 

The nuclear hyperfine interaction splits the paramagnetic states of an electron when it is close to a nucleus with a magnetic moment. For a random orientation of spins and nuclei, the tensor quantities in Eq. (4.11) are replaced by scalar distributions, and the resonance magnetic field is shifted from the Zeeman field // by... 

Built-up multilayers can be obtained only for transfer achieved at a surface far beyond the plateau regime, generally around 35 mN/m. Hence the resulting films are just a disordered superposition of oken monocrystals, with a random orientation in the plane of the support, as shown in Fig. 2b. As a consequence, the normal to the support behaves as an axis of full rotation. Such a macroscopic configuration is easy to investigate. This was done by the use of two different resonance methods, namely, linear dichroism and electron spin resonance, with the following result TCNQ radical anions lie completely flat on the substrate in the dimeric form [9]. Obviously, such a structure is not compatible with an in-plane conductivity, for which TCNQ molecular planes should be perpendicular to the substrate. In addition, the complete charge transfer between pyridinium and TCNQ prevents any kind of electron conductivity in the film and, as expected, the electrical properties of the pristine film are those of an insulator. 

Abbreviations EPR - electron paramagnetic resonance HE -high-energy IPS-isopentane solution LB-Langmuir-Blodgett LE - low-energy MCD - microcrystals dispersed in a KBr disk ORTEP - Oak Ridge thermal ellipsoid plot ROC - randomly oriented crystals SC-spin-coated SCE-spin-coated film... 

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