Influence of electrostatic interactions on complexes with short O center dot center dot center dot O hydrogen bonds in basic salts of pyridine betaines and acid salts of omega-phenyloalkanocarboxylic acids

The isostructural complexes [C5H5N+(CH2)(n)COO](2)HX and [C6H5(CH2)(n)COO]( 2)HK (n = 1-4), which differ in their counterions and charge on the ring, w ere synthesized, and their powder FT-LR spectra analyzed. All complexes con taining a charged pyridine ring are of Hadzi type iii, characterized by an intense broad (continuum) absorption below 1600 cm(-1) typical of a short-s trong hydrogen bond (SSHB) with a delocalized proton and a single vC=O band . The positively charged nitrogen atoms interact electrostatically with the X- ion and, additionally, with one of the oxygen atoms of the carboxylic g roup, producing a more or less symmetric environment of the H-bonded proton , and stabilizing the SSHB. The broad absorption of [C6H5CH2COO](2)HK is ve ry similar to that of other pyridine complexes. Upon addition of methylene groups the broad absorption moves to higher wavenumbers, the O ... O distan ce is elongated, and the H-bonded proton becomes more localized. In the spe ctrum of [C6H5(CH2)(4)COO](2)HK the vC=O and v(as)COO bands were found at 1 704 and 1641 cm(-1), respectively, which shows that the H-bonded proton is asymmetrically located. The observed variation of absorption with the numbe r CH2 groups reflects changes of contacts between the K+ ion and COO- group s.