H-2 NMR TIME-DOMAIN ANALYSIS OF ULTRASLOW REORIENTATIONS IN SUPERCOOLED LIQUIDS

A method for evaluating H-2 NMR Stimulated echo experiments in the tim e domain is presented. It exhibits a high sensitivity to molecular reo rientation mechanisms for small angles, Reorientations with jump angle s below 25 degrees can be resolved with a precision of approximate to 1 degrees so that, e.g., rotational diffusion and finite jump angle me chanisms become distinguishable. The method, applicable to isotropic r eorientation models, is thought to extend the domain of 2D exchange sp ectroscopy where the best resolution is obtained in the large angle ra nge of anisotropic reorientational mechanisms. Application to reorient ation in the supercooled melt of ortho-terphenyl is presented. These d ata clearly show that the molecular reorientation of ortho-terphenyl m olecules cannot be described by small angular step rotational diffusio n. A better parametrization of the experimental data incorporates elem entary jump angles in the range of approximately 10 degrees. (C) 1998 Academic Press.