Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR.

Dynamic nuclear polarisation (DNP) can significantly enhance the sensitivity of solid-state nuclear magnetic resonance (SSNMR) experiments by transferring the electron spin polarisation of paramagnetic species to nuclei through microwave irradiation of the sample at cryogenic temperatures. Paramagnetic species required for DNP can be provided in the form of mesoporous silica materials containing nitroxide radicals either located on the porous surface or embedded in the pore walls. The present study focuses specifically on porous materials with wall-embedded radicals that were synthesised using conventional molecular imprinting protocols. More remarkably, by changing the molecular structure of the TEMPO precursor, the theoretical distance between the oxygen atoms in a pair of wall-embedded face-to-face TEMPO radicals was increased stepwise (0.7, 0.9, 1.1, 1.3 and 1.5 nm). The thermal activation of these five series of materials led to 37 TEMPO-functionalised silica materials with different radical concentrations. Their efficiency as DNP polarising agents was subsequently investigated at 9.4 T and ∼110 K under magic-angle spinning conditions (10 kHz) after impregnating them at room temperature with an aqueous solution of isotopically enriched proline. Our results show that the highest DNP efficiency was obtained for the silica materials that exhibited the shortest theoretical oxygen-oxygen distance between the TEMPO rings, suggesting that the design rules accepted for soluble DNP polarising agents may not be transposed to these materials with wall-embedded pairs of nitroxides.

Investigating Unusual Organic Functional Groups to Engineer the Surface Chemistry of Mesoporous Silica to Tune CO2-Surface Interactions.

As the search for functionalized materials for CO2 capture continues, the role of theoretical chemistry is becoming more and more central. In this work, a strategy is proposed where ab initio calculations are compared and validated by adsorption microcalorimetry experiments for a series of, so far unexplored, functionalized SBA-15 silicas with different spacers (aryl, alkyl) and terminal functions (N3, NO2). This validation then permitted to propose the use of a nitro-indole surface functionality. After synthesis of such a material the predictions were confirmed by experiment. This confirms that it is possible to fine-tune CO2-functional interactions at energies much lower than those observed with amine species.

Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR.

Hybrid mesoporous silica materials with wall-embedded nitroxides are shown to efficiently polarize impregnated substrates in high-field dynamic nuclear polarization magic-angle spinning solid-state NMR experiments.

Numerical examination of the extended phase-space volume-preserving integrator by the Nosé-Hoover molecular dynamics equations.

This article illustrates practical applications to molecular dynamics simulations of the recently developed numerical integrators [Phys Rev E 2006, 73, 026703] for ordinary differential equations. This method consists of extending any set of ordinary differential equations in order to define a time invariant function, and then use the techniques of divergence-free solvable decomposition and symmetric composition to obtain volume-preserving integrators in the extended phase space. Here, we have developed the technique by constructing multiple extended-variable formalism in order to enhance the handling in actual simulation, and by constituting higher order integrators to obtain further accuracies. Using these integrators, we perform constant temperature molecular dynamics simulations of liquid water, liquid argon and peptide in liquid water droplet. The temperature control is obtained through an extended version of the Nosé-Hoover equations. Analyzing the effects of the simulation conditions including time step length, initial values, boundary conditions, and equation parameters, we investigate local accuracy, global accuracy, computational cost, and sensitivity along with the sampling validity. According to the results of these simulations, we show that the volume-preserving integrators developed by the current method are more effective than traditional integrators that lack the volume-preserving property.