Muonium adduct of benzaldehyde: a novel probe of cation–molecule interactions in zeolite catalysts and of solvation and electronic substituent effects

Abstract

The adduct radicals (ArCHOMu˙) have been formed by muonium addition to the carbonyl group of benzaldehyde and its derivatives. The muon coupling is found to be highly sensitive to the nature of substituents in the benzene ring, being increased by electron releasing groups. This bears relevance to their properties in Group 1 and Group 2 exchange zeolite-X, in which the coupling is decreased by interactions between the aromatic π-electrons and the cation: the effect increasing in the order of decreasing ionic radius, as the electron withdrawing power of the ring is increased by polarization of the π-electrons towards the cation. For the radical derived from benzaldehyde itself, a strong dependence is also found on the solvent in which it is dissolved. It is suggested that this may be used in the study of solvent effects, particularly H-bonding, pertinent to organic and bio-organic media. Electronic substituent effects are also studied in substituted PhCHOMu˙ radicals, and in cyclohexadienyl radicals formed by muonium addition to the aromatic ring, the latter showing evidence of captodative stabilization, when an electron donor substituent is present with the electron accepting CHO group.