Structure and polarised IR and Raman spectra of the solid complex betaine–trichloroacetic acid

Abstract The structural chemistry and phase transitions of the molecular complex of betaine with trichloroacetic acid (C 7 H 12 Cl 3 NO 4 , BET–TClA) have been investigated by means of single crystal and powder X-ray diffraction, thermal analysis (DSC) and FTIR and FT-Raman measurements. BET–TClA crystallises in the orthorhombic space group Pnma with Z =4 in a layer structure of hydrogen bonded betainium cations (BETH + ) and trichloroacetate anions (TClA − ). The structural irregularities, which have been found for the TClA − sublattice, are treated on the basis of a combined model of orientational disorder and thermal libration. DSC as well as temperature dependent FTIR measurements resolve two reversible low temperature phase transitions at 177 and 187 K, respectively, of which only one was detectable with the help of powder X-ray diffraction. In addition, the FTIR measurements point to the existence of a further, hitherto unknown, low temperature phase transition below 100 K.