Accurate Molecular Size Determination by Diffusion Ordered NMR Spectroscopy Based on an Improved Diffusion Model

Using the molecular volume, a semiempirical model for diffusion was developed, which allows an accurate interpretation of diffusion coefficients obtained by diffusion-ordered spectroscopy (DOSY) NMR. While the measurement of diffusion coefficients with DOSY NMR is well-established, correlating the obtained diffusion coefficients with molecular descriptors is difficult. Various models for small molecule diffusion have been developed in the past for organic, organometallic or coordination compounds, but none are applicable for all different substance classes. In this work we first analyze the different models using a dataset of 95 molecules and show that models relying on the common assumption of a spherical molecule, and with it the loss of information through the introduction of a radius-parameter, produce large errors. Our new model therefore uses the molecular volume as molecular descriptor. This new model describes the diffusion of organic, organometallic and coordination compounds independent of molecular weight or density within a 2σ error range of 12%. The utility of the new model is demonstrated in the analysis of a range of uranyl complexes, where errors of -2% to 8% are obtained.