Hydrogen Bonding and Tautomerism in Anils of Salicylaldehydes and Related Compounds. A Study of Deuterium Isotope Effects on13C Chemical Shifts

Deuterium-induced chemical shift differences on the 13C (DIS) are reported for a series of Schiff bases of salicylaldehydes, 1-(phenyliminomethyl)naphthalen-2-ol and 4-chloro-1,7-phenanthrolin-10-ol. Provided that corrections for the hydrogen donor atom and diamagnetic effects of the surrounding groups are made, plots of logarithms of two bond DIS vs. the chemical shift of the exchangeable proton are linear for compounds in which tautomeric equilibria are absent or strongly displaced in favor of one form. Molecules with comparably populated tautomers in fast exchange fall off this correlation. By this method, anils of salicylaldehydes were found to exist as hydroxy forms with localized hydrogen bonds while 1-(phenyliminomethyl)naphthalen-2-ol and 4-chloro-1,7-phenanthrolin-10-ol were found to be tautomeric mixtures dominated by the hydroxy forms. DIS also revealed that 4-chlor-1,7-phenanthrolin-10-ol exists in solution as an aggregate of two molecules.