The Effect of Acids on the Infrared Spectra of Schiff Bases—i. Non‐conjugated Imines

The Fourier‐transform infrared spectra of CHCl 3 solutions containing a non‐conjugated imine, R 1 HC=NR 2 and an acid of the series HCl, HBr, HI, trichloro‐, dichloro‐, monochloro‐, monobromo acetic acids and propionic acid have been recorded. Both the N + H and double bond stretching regions are examined. The results show that HCl, HBr and HI protonate the imine completely, trichloroacetic acid to about 90‐95%, the other haloacetic acids to 55‐80% and propionic acid to <10%. Considered as model studies for the problem of protonation in rhodopsins our results lead to the conclusion that with the relatively weak acids available in the pigments full protonation requires a stabilization mechanism which should be further investigated.