Determining accurate molecular sizes in solution through NMR diffusion spectroscopy

Abstract

This tutorial review deals with the methodological procedures that can be used to obtain accurate molecular sizes in solution from diffusion NMR spectroscopy. The critical aspects associated with the estimation of the size of molecules from the measured translational self-diffusion coefficient, using the Stokes-Einstein equation, are highlighted and discussed. From a theoretical point of view, it is shown how to take into account the size of the solute with respect to that of the solvent and its non-spherical shape using the appropriate correction factors in the frictional coefficient. From a practical point of view, the advantages of introducing an internal standard in the sample are presented. Initially, non-aggregating systems are considered in an attempt to clarify what hydrodynamic dimensions mean. Successively, aggregating systems are addressed showing how it is possible to understand the aggregation level and derive the thermodynamic parameters for some illustrative aggregation processes.

Lithium, sodium and potassium picolyl complexes: syntheses, structures and bonding

“…The resulting crystalline product was revealed to be a 4-picH solvated polymeric structure [4-picK (4-picH)] (3) which does not contain Me6TREN as displayed in figure 5. As can be discerned from figure 5 there are 35 clearly three distinct types of picoline entities present in this structure (labeled A-C). Two of the rings (B and C) bridge between two potassium atoms while the third (A) is a terminal ligand, giving potassium an overall coordination number of five and a distorted square pyramidal coordination sphere, with N2' in the polymeric chain serves as a sixth coordination site to give a distorted octahedral geometry (figure 5, bottom) as well as confirming that this picolyl unit is the deprotonated one (vide infra).…”

“…We have consequently 30 attempted to prepare Me6TREN stabilized monomers of 2-and 4-picolyl-lithium, -sodium and -potassium and present our findings herein. 35 We commenced by studying the 4-picolyl M series (where M = Li, Na, or K) in the presence of our potentially tetradentate Lewis donor. Specifically the lithium salt was prepared in situ (from 4-picoline and nBuLi) while the heavier congeners were generated via a Lochmann-Schlosser superbase (nBuLi/MOtBu) approach, 14 40 filtered, washed with hexane to remove LiOtBu and dried (equation 1).…”

Towards the Prediction of Global Solution State Properties for Hydrogen Bonded, Self‐Associating Amphiphiles

“…However, the hydrodynamic diameter ( d H ) of the (thio)urea‐anion species obtained via the Stokes–Einstein equation24 from representative 1 H NMR DOSY studies, with 1 ( d H =1.15 nm), 4 (1.11≤ d H ≥1.12 nm), 24 (1.26≤ d H ≥1.29 nm), 27 (1.42≤ d H ≥1.43 nm) and 31 (1.61≤ d H ≥1.66 nm)17 at 55.56 m m did not show any evidence of these large aggregates. The size of aggregate obtained from both 1 H NMR DOSY and DLS should be treated as an estimate, as these methods assume that the aggregate structures are spherical, large in comparison to the coordinated solvent sphere,25 and in the case of 1 H NMR DOSY experiments maybe further complicated through the presence of fast exchange processes 26. However, given the discrepancy in size (>100 nm) of the structures observed by these two complimentary methods, it is hypothesized that either the larger species observed by DLS exist in concentrations that are too low to be observed by standard solution state 1 H NMR techniques or, alternatively, the size and consequent slow tumbling of these large aggregates results in signals too broad to be resolved by 1 H NMR.…”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…[42] The experimental conditions and data processing are particularly important to obtain accurate values of Dt and useful instructions can be found in the reference [39], which illustrates, among other things, the importance of the internal standard and what to do in the presence of non-spherical species.…”

“…[65] Firstly, it is shown how converting the values of Dt in volumes, using the corrected version of the Stokes-Einstein equation, [39] can make the technique more useful and intuitive. The main advantage is that we can easily predict the volume of the adducts (Eq.…”