Chemically induced dynamic nuclear polarization, III 13C-polarization observed during reduction of diazonium salts, using the pulse fourier transform technique.

Abstract

n cursors'of diamagnetic speciesL. In proton magnetic resonance spectroscopy the CIDNP-effect can readily be observed with conventional NMR-spectrometer systems. For the organic chemist, the carbon atom is of even more importance. Unfortunately the magnetic carbon isotope 13 C is present only in 1 % natural abundance. Recently,however, Lippmaa and coworkers3 have shown that the enhancement factor in I3 C polarization is high. Therefore, it should be possible to observe ‘3,_ CIDNP without I3 C enrichment in systems showing 1 H-CIDNP. For the interpretation of CIDNP effects 13 C measurements would have some advantages: (i) By using complete proton decoupling multiplet effects will disappear. Furthermore, carbon-carbon coupling is not observed. Therefore one can expect simple line spectra in 13C-CIDNP. (ii) According to the experiments published so far3, it seems that in a radical only few carbon atoms (those coupled with the free electron) are strongly polarized. In fast reactions, however, the conventional continuous wave sweep technique permits one to accumulate the 13 C spectrum only within a small frequency range. Examples of such reactions are the reduction of diazonium' and iodonium4 salts with NaBH4, NaOH, amines,or NaN02. In these cases we observed 'H-CIDNP (emission from the protons of the arenes formed). Since these reactions do not Proceed exclusively via aryl radicals, the enhancement factors for ?-I -CIDNP are