Reorientational processes in solid (C 6H 6) 2Cr, (C 6H 6)Cr(CO) 3 and (C 6H 5Me)Cr(CO) 3

Abstract

The reorientational motion of the benzene rings in solid (C 6H 6) 2Cr ( 1) and (C 6H 6)Cr(CO) 3 ( 2) and of the tricarbonyl group in solid (C 6H 5Me)Cr(CO) 3 ( 3), proposed on the basis of solid-state NMR experiments (wide-line 1H and 13C MAS) is studied by means of the pairwise potential energy method. Starting from the known crystal structures of the three species, relative potential energy barriers to motion and root mean square librational amplitudes at the bottom of the potential energy wells are evaluated and compared with the available anisotropic displacement parameters of the atoms involved in the process. The results show that the crystal packing in 2 and 3 forbids rotation of the Cr(CO) 3 group as a whole, while benzene reorientation encounters low potential energy barriers in both 1 and 2, in agreement with the spectroscopic information. An alternative interpretation of the dynamic behaviour shown by 3, based on large amplitude librations of both C 6H 5Me and Cr(CO) 3 groups, is put forward.