Raman and IR studies of isomerization and self-association of β,β′-dichloropivalic acid

Processes of conformational changes, hydrogen bonding and phase transitions of beta,beta'-dichloropivalic acid have been studied by means of Raman and IR spectroscopy, and quantum chemistry (ab initio, semi-empirical AM1 and PM3) calculations. Both the AM1 and PM3 as well as ab initio methods give the conformer with eclipsed C-C and C=O bonds (cis conformer) as the lowest energy conformer. They also predict the ti-ans conformer to be rather stable (Delta E = 0.2-0.9 kJ/mol). The enthalpy differences calculated from experimental IR spectra of beta,beta'-dichloropivalic acid solution in C2Cl4 are 52 +/- 4 kJ/mol and 22 +/- 5 kJ/mol for the monomer tt cyclic dimer and open dimer ct cyclic dimer equilibria, respectively. The high temperature solid phase of beta,beta'-dichloropivalic acid is found to be a plastically crystalline phase, and this phase is formed on cooling of the melt only. A description of the molecular motion of beta,beta'-dichloropivalic acid in the melt as well as in the high temperature solid phase in terms of rapid isotropic reorientations is appropriate. (C) 1998 Elsevier Science B.V. All rights reserved.