Following Molecular Mobility During Chemical Reactions: No Evidence for Active Propulsion

The measured changes in self-diffusion of small molecules during reactions have been attributed “boosted mobility”. We demonstrate the critical role of changing concentrations of paramagnetic ions on nuclear magnetic resonance (NMR) signal intensities, which lead to erroneous measurements of diffusion coefficients. We present simple methods to overcome this problem. The use of shuffled gradient amplitudes allows accurate diffusion NMR measurements, even with time-dependent relaxation rates caused by changing concentrations of paramagnetic ions. The addition of a paramagnetic relaxation agent allows accurate determination of both diffusion coefficients and reaction kinetics during a single experiment. We analyze a copper-catalyzed azide-alkyne cycloaddition ‘click’ reaction, for which boosted mobility has been claimed. With our methods, we accurately measure the diffusive behavior of solvent, starting materials and product, and find no global increase in diffusion coefficients during the reaction. We overcome NMR signal overlap using an alternative reducing agent to improve the accuracy of the diffusion measurements. The alkyne reactant diffuses slower as the reaction proceeds, due to binding to the copper catalyst during the catalytic cycle. The formation of this intermediate was confirmed by complementary NMR techniques and density functional theory calculations. Our work calls into question recent claims that molecules actively propel or swim during reactions, and establishes that time-resolved diffusion NMR measurements can provide valuable insight into reaction mechanisms.

Magnetic control over the fractal dimension of supramolecular rod networks

<p>Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd³⁺-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure formation of other supramolecular or coordination polymers that contain paramagnetic ions.</p>

Structural Analysis of the BaWO4 Crystal Doped with Ce and Codoped with Na Ions Based on g-shift Parameters

The relationship between the g-shift and the local structure of the Ce3+ paramagnetic center with axial symmetry were investigated for four BaWO4 single crystals doped with Ce and codoped with Na. Based on g-shift the displacements of Ce3+ ions are determined. The g-shift method yields displacements of impurity ions in good agreement with the superposition model (SPM) and the perturbation methods (PM) predictions. The structural analysis of the paramagnetic ions and its surrounding in the BaWO4 unit cell was also conducted.

Magnetic Control over the Topology of Supramolecular Rod Networks

Understanding and controlling supramolecular polymerization are of fundamental importance to create advanced materials and devices. Many stimuli have been explored in the past decades, but magnetic fields and field gradients have received little attention. This is because magnets do not provide enough magnetic energy to overcome thermal noise at the single molecule level. Here we show that significant changes in network topology of Gd³⁺-decorated supramolecular polymer rods can nevertheless be observed using magnetic fields of order 1 T at room temperature. The structure of the rod networks is influenced during a slow diffusive process over a timescale of hours by the anisotropy of the demagnetizing field. Our approach opens opportunities to control and tune structure formation of many supramolecular and coordination polymers using a variety of rare earth or other paramagnetic ions.

Magnetic Control over the Topology of Supramolecular Rod Networks

Understanding and controlling supramolecular polymerization are of fundamental importance to create advanced materials and devices. Many stimuli have been explored in the past decades, but magnetic fields and field gradients have received little attention. This is because magnets do not provide enough magnetic energy to overcome thermal noise at the single molecule level. Here we show that significant changes in network topology of Gd³⁺-decorated supramolecular polymer rods can nevertheless be observed using magnetic fields of order 1 T at room temperature. The structure of the rod networks is influenced during a slow diffusive process over a timescale of hours by the anisotropy of the demagnetizing field. Our approach opens opportunities to control and tune structure formation of many supramolecular and coordination polymers using a variety of rare earth or other paramagnetic ions.