Spectroscopic conformational study of PMS 952, a specific imidazoline ligand. Experimental results

X-ray analysis of PMS 952 displays two kinds of very strong hydrogen bonds, an intramolecular one stabilizing the conformer A and an intermolecular on e that associates conformers A to form two infinite chains. The H-1 NMR stu dy of vicinal coupling constants based on dihedral angle values, evidences that PMS 952 takes up an eclipsed conformation, in CCl4 and CDCl3 solutions , closely related to the conformation in solid state. Deuterium exchange in methanol-d(4) points out the presence of a tautomeric form. However, as th ere is spectral evidence neither from NMR nor from FTIR studies for such a tautomer, this specie cannot be taken into account to explain the complexit y of the FTIR spectra. NMR data also reveal that the imidazoline ring freel y rotates while FTIR analysis allows characterizing two conformers (A and B ), with lifetime shorter than NMR time scale. These two conformers are stab ilized by two different intramolecular H-bonds. Taking into account that co nformer A could exist in two states (monomer A and (A)(n)-chains), an equil ibrium between A, (A)(n), and B entities allowed the assignment of all main bands of the IR spectra, assigned to nu NH, nu OH, nu CH, and nu CN modes. The strength of hydrogen bonds is solvent dependent. Polar solvents, eithe r proton donor or acceptor stabilized the conformer A, while CCl4 favors th e conformer B.