Abstract: The principles for the rational design of systems suitable for the study of steric effects are defined. A suitable molecular framework [6-(2-X-phenyl)-SY-1, l-dimethylindan, 11 was synthesized and a study of internal rotation by dynamic NMR spectroscopy (DNMR) of 33 derivatives, differing principally in the nature of the molecular fragment (X), showed the following.(i) For 1 (Y= Me; X= halogen) the rotational barriers (AG') increase smoothly and ...