Comparison of low-barrier hydrogen bonds in acid salts of carboxylic acids and basic salts of betaines - FTIR study

The following isostructural complexes: [C5H5N+(CH2)(n)COO](2)HX and [C6H5(CH2)(n)COO](2)HK (n = 1-4) were synthesised and their FTIR spectra analysed. In these two series hydrogen-bonded units are similar, although they differ in that one is zwitterion. Most of the spectra are of Hadzi type (iii), characterised by an intense broad (continuum) absorption below 1600 cm(-1) typical for a short-strong hydrogen bonds, and a single nu C=O band. The O ... O distances in [C5H5N+CH2COO](2)HCl-. H2O and [C6H5CH2COO](2)HK are 2.436(6) Angstrom [Chen and Mak, J. Mol. Struct. 221 (1990) 265] and 2.443(4) Angstrom [Speakman, Structure and Bonding 12 (1972) 141], respectively, and their spectra are very similar. Addition of methylene groups breaks this similarity, as only complexes containing pyridine ring are non-linear as results of the attractive Coulombic interaction between the oppositely charged groups. Counterions interact with N+ atoms and have negligible effect on the OHO bridge, and as the result the spectra are comparable, and the O ... O distance would be also comparable. The O ... O distance in [C5H5N+CH2CH2COO](2)HBr is 2.450(6) Angstrom [Chen and Mak, Acta Cryst. Sect. C 50 (1993) 1807]. All the complexes containing phenyl ring are probably linear and the K+ ion interacts with the oxygen atoms of the carboxylate groups. On addition of methylene groups contacts of K+ ion with carboxylate groups become non-equivalent and the broad absorption moves to the higher wave numbers. This indicates that hydrogen bonds become longer and the H-bonded proton is closer to one of the carboxylate group. In the spectrum of [C6H5(CH2)(4)COO](2)HK both the nu C=O and nu(as)COO bands, respectively, at 1707 and 1641 cm(-1) were found. (C) 1999 Elsevier Science B.V. All rights reserved.