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

The following isostructural complexes: [C5H5N+(CH2)(n)COO](2)HX and [C6H5(C H2)(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 diffe r in that one is zwitterion. Most of the spectra are of Hadzi type (iii), c haracterised by an intense broad (continuum) absorption below 1600 cm(-1) t ypical 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 2.43 6(6) Angstrom [Chen and Mak, J. Mol. Struct. 221 (1990) 265] and 2.443(4) A ngstrom [Speakman, Structure and Bonding 12 (1972) 141], respectively, and their spectra are very similar. Addition of methylene groups breaks this si milarity, as only complexes containing pyridine ring are non-linear as resu lts of the attractive Coulombic interaction between the oppositely charged groups. Counterions interact with N+ atoms and have negligible effect on th e 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+CH2CH2CO O](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 addit ion of methylene groups contacts of K+ ion with carboxylate groups become n on-equivalent and the broad absorption moves to the higher wave numbers. Th is indicates that hydrogen bonds become longer and the H-bonded proton is c loser 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 c m(-1) were found. (C) 1999 Elsevier Science B.V. All rights reserved.