The study of weakly adsorbed molecules by 13C NMR: Reaction products of propanal on silica

The application of 13C NMR to the study of the reactions of molecules weakly adsorbed on solid surfaces is illustrated by the system propanal on silica. The reaction is shown to produce a polyether type oligomer, probably the cyclic trimer. This conclusion seems to contradiet that from previous infrared work, namely that an aldol, an unsaturated aldehyde and/or an ester would be formed. It is now shown that the formation of a cyclic trimer is not incompatible with the IR spectra; it is merely that the amounts of aldehyde and/or ester have probably been overestimated in the past. The present study shows that 13C NMR, although not a very sensitive technique, can be used to obtain a more quantitatively correct analysis of a mixture that is physically adsorbed on a solid surface.