Rotational diffusion measurements of suspended colloidal particles using two‐dimensional exchange nuclear magnetic resonance

We present here an experimental and theoretical study of the application of two‐dimensional exchange nuclear magnetic resonance spectroscopy (NMR) 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 (CSA). 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 poly(methyl methacrylate) (PMMA) particles which were synthesized with a 20% enrichment in ¹³C 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 ¹³C‐(carbonyl)‐methacrylate monomer and the small quantities of 20% enriched ¹³C‐(carbonyl)‐poly(methyl methacrylate) microspheres used for this study are presented.