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

The nature of hydrogen bonding within intermolecular complexes of carboxyli c acids and 1-methylimidazole (1-MeIm), 1-n-butylimidazole (1-BuIm), and 1- tert-butylimidazole (1-t-BuIm) in chloroform was characterized by Fourier t ransform infrared spectroscopy. Earlier spectroscopic studies indicated tha t carboxylic acid-1-MeIm complexes are of three types: (I) neutral complexe s with the weaker acids (pK(a) greater than or equal to 2.2) in which the a ntisymmetric carbonyl stretching frequencies are lowered relative to the fr ee acids and the ethyl esters of the acids; (II) ionic complexes of stronge r acids (pK(a) less than or equal to 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 t ype II, in which the carbonyl stretching frequencies are intermediate betwe en those for the tetrabutylammonium salts (bond order 1.5) and the free aci ds (bond order 2.0).(12) Assignment of the ionic and intermediate carbonyl stretching frequencies was verified by shifts to longer wavelengths in the O-18-labeled 2,2-dichloropropanoic acid-1-MeIm complexes. Type III complexe s have been postulated to incorporate a low barrier hydrogen bond (LBHB) be tween the N-3 of the imidazole ring and the carboxylic group. The size of t he alkyl group in 1-alkylimidazoles has no significant effect on the carbon yl stretching frequencies in any of the complexes. However, increasing bulk iness in the alkyl group increases the intensity of the type III species re lative to type II, so that the equilibrium is shifted toward low barrier hy drogen bonding in solution. The broad bands at approximately 2500 and 1900 cm(-1) in the R-COOH-1-alkylimidazole complexes are similar to those classi cally attributed tb strong hydrogen bonds. These bands are absent from the spectra of R-COOD-1-alkylimidazole complexes. Moreover, the antisymmetric c arbonyl stretching bands characteristic of the type III, or LBHB-bonded com plexes, are greatly decreased in intensity in the spectra of R-COOD-1-alkyl imidazole complexes, and are shifted to higher wavelengths nearer to those expected for the free R-COOD, The deuteron is more strongly attached to oxy gen, 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 a ssisted hydrogen. bonding.