The Effect Of Acids On The Infrared-Spectra Of Schiff-Bases .1. Nonconjugated Imines

The Fourier-transform infrared spectra of CHCl3 solutions containing a non-conjugated imine, R1HC=NR2 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.