Low barrier hydrogen bonds in sterically modified Schiff bases

Five derivatives of an ortho-hydroxy Schiff base (2-(N-alkyl-alpha-iminoethyl)phenols) with very short intramolecular hydrogen bonds (d(O(H)...N)less than or equal to2.500 Angstrom) were synthesised. The crystal structures were determined. The steric repulsion of the substituted methyl group results in an unusual strengthening of the hydrogen bonds, decreasing the barrier for the proton movement within the hydrogen bridge, which leads to a delocalization of the proton position. The very strong influence of the character of the substituent in the phenol ring as well as the character of the N-alkyl chain on the proton distribution is demonstrated for these hydrogen bonds from the so-called inversion range.