ALKYL CHAIN DYNAMICS IN PEROVSKITE BILAYERS AND THEIR VARIATION WITH STRUCTURAL PHASE AS STUDIED BY POWDER AND SINGLE-CRYSTAL NMR

A single crystal magnetic resonance study of C4Cd, [(C4H9NH3)(2)Cd]Cl- 4, specifically deuterated at a variety of positions is reported. This class of compound forms bilayered structures and C4Cd itself undergoe s at least three phase transitions over the temperature range studied, 120 to 350 K. The experimental results yield a detailed picture of th e variation of molecular ordering in the butyl chain with temperature and structural phase and bear substantial resemblance to H-2 NMR resul ts reported for Lipid membranes. In the high and intermediate phases, complete order tensors are determined in the bilayer frame which are c ompared (unsuccessfully) with the disordered high temperature x-ray st ructure of the isomorphous manganese analog, C4Mn. On the time scale o f 10(-5) s, the butyl chain is nearly stationary at 120 K where it is found to occupy the expected all-trans configuration which runs parall el to the bilayer normal. At temperatures between the intermediate pha se and 120 K, a mosaic structure is observed in which the C-D bonds oc cupy for long times (>10(-3) s) a distribution of orientations. A qual itative model for the high and intermediate phases based on gauche-tra ns isomerization is presented. To explain the measured order tensors, we assume that (a) gauche-trans isomerization is coupled to molecular reorientation and (b) the configurations are distorted from ideal diam ond lattice configurations so as to make molecular packing more compac t. It is suggested that in the mosaic structure, configurations contai ning gauche sequences remain populated but the isomerization rate is d ramatically slowed. (C) 1996 American Institute of Physics.