The infrared absorption spectra of acids and their compounds with amines

Abstract

The spectra of a number of mixtures of carboxylic acids and tertiary amines in solution in carbon tetrachloride and in chloroform were recorded for a series of acid;base ratios. Carbon tetrachloride was chosen as the non-interacting solvent whilst chloroform was used as the second solvent since it is known to hydrogen bond to proton acceptors. This property wasused to elucidate some of the spectra. Generally the Interaction between the acids and bases resulted in the formation of two products, having acid:base ratios of 2:1 and 1:1. Only in the case of acids weaker than iodoacetic acid with pyridine, the weakest base, does the formation of a 1:1 product occur directly. The general reaction can be written thus: For the strong acids and bases the position of each equilibrium is well to the right, but as the relative strengths of the acid and the cation of the base approach each other, the extent of Interaction decreases. The relative strengths of the acid and base not only govern the extent of the interaction, but also the products formed. For the stronger acids and bases the proton is transferred to the nitrogen atom and, in general, back hydrogen bonding occurs. The weak acids form simple hydrogen bonded complexes with the weakest of the bases studied, pyridine, but proton transferred adducts with the stronger bases. There is some evidence that the acids of intermediate strength form both a proton transferred species as well as a simple hydrogen bonded complex with pyridine, and that these two forms are in tautomeric equilibrium with each other. The actual species present are very dependent on the relative strengths of the acid and base concerned. Acid-base pairs with the same pK values, where pK = pKa (conjugate acid of base) - pKa (acid), form essentially the same type of 1:1 adducts.The spectral region in which the stretching mode of the proton Involved in hydrogen bonding occurs was of particular interest since a well separated doublet was observed. This has been discussed in terms of the theory that there is a double minimum in the potential energy surface of the proton. Temperature studies and substitution by deuterium have been carried out to obtain more information. All that can be stated however is that this is the best theory, at the moment, to explain the presence of more than one band associated with the stretching mode of the hydrogen bonded proton, but that it is definitely unwise to regard the presence of these bands as evidence for such a potential energy surface.