In switched-angle spinning spectroscopy ͑SAS͒ a sample is spun about different angles, ␤, relative to the magnetic field, during various periods of the experiment. In the present work, SAS is combined with two-dimensional exchange spectroscopy in order to correlate carbon-13 chemical shift tensors of the carbonyl ͑1͒ and hydroxyl ͑2͒ carbons of tropolone. Experiments were performed on a sample enriched to 25 at. % in each of these sites ͑at different molecules͒. At this level of enrichment the dominant exchange mechanism between the two sites involves spin diffusion. The experiment consists of a preparation period during which the sample spins at the magic angle and the magnetization of one of the sites is quenched by means of a selective pulse sequence. During the rest of the experiment the sample spins with its axis away from the magic angle except for a short period just before the detection where the axis is switched to the magic angle in order to select the magnetization to be detected. Experiments were performed for all four possible combinations of the initial and final magnetizations, thus providing chemical shift correlations between carbons 1,1Ј, 2, and 2Ј in the two magnetically inequivalent ͑but symmetry related͒ molecules in the unit cell. Combining these results with the known crystal structure of tropolone ͑neglecting a small tilt between the perpendicular to the molecular plane and the crystallographic c-axis͒ provides information on the orientation and magnitude of the chemical shift tensors of the two types of carbons. The principal values ͑in ppm͒ are xx 1 ϭ65, yy 1 ϭ33, zz 1 ϭϪ98, xx 2 ϭ77, yy 2 ϭ17, and zz 2 ϭϪ94. Assuming zz to be perpendicular to the molecular plane, the orientations of the yy 's are 12°off the C 1 ϭ0 bond ͑toward the hydroxyl carbon͒ for carbon 1 and 10°off the C 3 ϭC 2 bond ͑away from the carbonyl carbon͒ for carbon 2.

Deuterium twodimensional exchange nuclear magnetic resonance by rotorsynchronized magic angle spinning J. Chem. Phys. 98, 7699 (1993); 10.1063/1.464578The determination of the reorientational angle distribution in twodimensional exchange nuclear magnetic resonance spectroscopy on powder samplesWe present here an experimental and theoretical study of the application of two-dimensional exchange nuclear magnetic resonance spectroscopy ͑NMR͒ to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time-correlation function where the NMR interaction is represented by the chemical shift anisotropy ͑CSA͒. Time-correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time-correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two-dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two-dimensional exchange NMR experiments on a colloidal suspension of spherical poly͑methyl methacrylate͒ ͑PMMA͒ particles which were synthesized with a 20% enrichment in 13 C at the carbonyl site. Rotational diffusion time-correlation functions determined from the experimental exchange spectra are consistent with the composition of the colloidal suspension. Detailed explanations of the syntheses of the enriched methyl 13 C-͑carbonyl͒-methacrylate monomer and the small quantities of 20% enriched 13 C-͑carbonyl͒-poly͑methyl methacrylate͒ microspheres used for this study are presented.