Dresden 2003 – wissenschaftliches Programm

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CPP: Chemische Physik und Polymerphysik

CPP 20: POSTER A

CPP 20.64: Poster

Montag, 24. März 2003, 19:00–21:00, ZEU/250

MAGNETIC FIELD DEPENDENCE OF CIDNP IN ACYL-ALKYL-BIRADICALS OF DIFFERENT LENGTH IN PLASTIC-CRYSTAL-MATRICES — •Karsten Miesel¹, Alexandra Yurkovskaya², and Hans-Martin Vieth¹ — ¹Institute of Experimental Physics, Free University of Berlin, D-14195 Berlin, Germany — ²International Tomography Center, Institutskaya 3a, 630090 Novosibirsk, Russia

The efficiency of Chemically Induced Dynamic Nuclear Polarization (CIDNP) depends on the external magnetic field and is strongest in regions of level crossing between singlet and triplet spin states. Analysis of the resonance shape of field dependence of CIDNP in singlet-triplet transitions gives information on the magnetic resonance parameters, in particular about the exchange interaction J in intermediate radical pairs and its modulation by molecular mobility of the radicals. Here we present a study of the field dependence of ¹H-CIDNP in a series of acyl-alkyl biradicals of different lengths in liquid solution and in plastic crystals of cyclohexane-d₁₂. Short lived biradicals are formed under UV laser irradiation of a series of cyclic aliphatic ketones. Fast field cycling allows to measure the CIDNP field dependence between 0.05mT and 7T. When going from liquid to the plastic phase a drastic increase of the width of magnetic field dependence and a shift of the maximum positions is detected. The results will be discussed with respect to the polarization mechanism and the molecular mobility in the matrix.