PHOTO-CIDNP INVESTIGATION OF THE DEPROTONATION OF AMINIUM CATIONS

The photoreactions of triallylamine and triethylamine with a variety of triplet sensitizers containing carbonyl functions were studied in acetonitrile-d3 by using pseudo-steady-state measurements of chemically induced dynamic nuclear polarization (CIDNP). These reactions are hydrogen abstractions formally, but they are known to proceed in two steps: electron transfer from the amine to the excited sensitizer, followed by deprotonation of the resulting aminium cation to give an alpha-aminoalkyl radical. From the CIDNP intensity patterns in the reaction products, we determined the intermediates (radical ion pairs or pairs of neutral radicals) that give rise to the observed polarizations. The products of free radicals were found to be identical regardless of their polarization patterns; hence, both types of intermediates lead to the same escape products. This is explained by two pathways for the deprotonation of an aminium cation. Within the cage, the proton is transferred to the sensitizer anion; outside the cage, it can be abstracted by the amine itself. Both processes yield free alpha-aminoalkyl radicals, which react further to the products, but in the former case the escaping radicals carry polarizations from the neutral pair, whereas in the latter their spin polarizations originate from the radical ion pair. The rate of the deprotonation within the cage relative to the cage lifetime (approximately nanoseconds) therefore determines the observed CIDNP patterns. The rate constant for deprotonation of aminium cations by the amine must be faster than the rate of degenerate electron exchange of these species (approximately 1 x 10(8) L mol-1 s-1, as found for similar compounds) but slower than diffusion control by about 1 order of magnitude. The free enthalpies for both reactions were estimated from experimental data (redox potentials and pK(a) values) and from the heats of formation calculated by AM1. A correlation between the observed polarization patterns and the differences in the free enthalpies of the radical ion pair and the pair of neutral radicals was found, indicating that DELTAG-degrees of the in-cage proton transfer, from the aminium cation to the sensitizer radical anion, is a controlling factor of the investigated photoreactions. The deprotonation outside the cage, by the amine as a base, is fairly exergonic for the compounds studied. With these calculations, the different behavior of other amines (e.g., N,N-dimethylaniline) can be rationalized as well. With the sensitizer anthraquinone, these compounds undergo only reversible electron-transfer reactions, but their radical cations are not deprotonated in these experiments.