Hydrogen bonding in complexes of carboxylic acids with 1-alkylimidazoles: steric and isotopic effects on low barrier hydrogen bonding

Abstract

The nature of hydrogen bonding within intermolecular complexes of carboxylic acids and 1-methylimidazole (1-MeIm), 1-n-butylimidazole (1-BuIm), and 1-tert-butylimidazole (1-t-BuIm) in chloroform was characterized by Fourier transform infrared spectroscopy. Earlier spectroscopic studies indicated that carboxylic acid–1-MeIm complexes are of three types: (I) neutral complexes with the weaker acids (pK_a  2.2) in which the antisymmetric carbonyl stretching frequencies are lowered relative to the free acids and the ethyl esters of the acids; (II) ionic complexes of stronger acids (pK_a ⩽ 2.0) in which the carbonyl stretching frequencies are slightly lower than those for the tetrabutylammonium salts of the acids; (III) depolarized partially ionic complexes coexisting with type II, in which the carbonyl stretching frequencies are intermediate between those for the tetrabutylammonium salts (bond order 1.5) and the free acids (bond order 2.0).¹² Assignment of the ionic and intermediate carbonyl stretching frequencies was verified by shifts to longer wavelengths in the ¹⁸O-labeled 2,2-dichloropropanoic acid–1-MeIm complexes. Type III complexes have been postulated to incorporate a low barrier hydrogen bond (LBHB) between the N³ of the imidazole ring and the carboxylic group. The size of the alkyl group in 1-alkylimidazoles has no significant effect on the carbonyl stretching frequencies in any of the complexes. However, increasing bulkiness in the alkyl group increases the intensity of the type III species relative to type II, so that the equilibrium is shifted toward low barrier hydrogen bonding in solution. The broad bands at approximately 2500 and 1900 cm^–1 in the R-COOH–1-alkylimidazole complexes are similar to those classically attributed to strong hydrogen bonds. These bands are absent from the spectra of R-COOD–1-alkylimidazole complexes. Moreover, the antisymmetric carbonyl stretching bands characteristic of the type III, or LBHB-bonded complexes, are greatly decreased in intensity in the spectra of R-COOD–1-alkylimidazole complexes, and are shifted to higher wavelengths nearer to those expected for the free R-COOD. The deuteron is more strongly attached to oxygen, whereas the corresponding proton is more free to engage in low-barrier hydrogen bonding. Spectroscopic data indicate that R-COOH–1-alkylimidazole complexes are unexpectedly strong in CHCl₃, perhaps because of resonance assisted hydrogen bonding.