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

✍ Scribed by Stefan Berger; Sigrid Hauff; Paul Niederer; Anton Rieker

• Book ID: 104236804
• Publisher: Elsevier Science
• Year: 1972
• Tongue: French
• Weight: 244 KB
• Volume: 13
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(01)84879-6

✦ Synopsis

There is a steady increase in the number of reactions in which the phenomenon of CIDNPsis observed, providing evidence for the existence of radical pairs as pren 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