Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry.

This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores.

An EPR Study on Highly Stable Nitroxyl-Nitroxyl Biradicals for Dynamic Nuclear Polarization Applications at High Magnetic Fields.

Nitroxide biradicals are efficient polarizing agents in dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance. Many recently reported radicals possess substantial DNP efficiency in organic solvents but have poor solubility in water media which is unfavorable for biological applications. In this paper, we report DNP efficiency at a high magnetic field for two water-soluble biradicals resistant to reducing media. Water solubility was achieved by obtaining the radicals in the form of quaternary ammonium salts. Parameters of hyperfine interaction and exchange interaction were quantified by EPR spectroscopy, and their influence on the DNP effect was determined. The resistance of the biradicals to strongly reducing media was characterized. High stability was achieved using tetraethyl substituents and pyrrolidine moieties.

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization.

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin-spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1'-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

Solvent-free dynamic nuclear polarization enhancements in organically modified mesoporous silica.

High-field dynamic nuclear polarization is a powerful tool for the structural characterization of species on the surface of porous materials or nanoparticles. For these studies the main source of polarization are radical-containing solutions which are added by post-synthesis impregnation of the sample. Although this strategy is very efficient for a wide variety of materials, the presence of the solvent may influence the chemistry of functional species of interest. Here we address the development of a comprehensive strategy for solvent-free DNP enhanced NMR characterization of functional (target) species on the surface of mesoporous silica (SBA-15). The strategy includes the partial functionalization of the silica surface with Carboxy-Proxyl nitroxide radicals and target Fmoc-Glycine functional groups. As a proof of principle, we have observed for the first time DNP signal enhancements, using the solvent-free approach, for 13C{1H} CPMAS signals corresponding to organic functionalities on the silica surface. DNP enhancements of up to 3.4 were observed for 13C{1H} CPMAS, corresponding to an experimental time save of about 12 times. This observation opens the possibility for the DNP-NMR study of surface functional groups without the need of a solvent, allowing, for example, the characterization of catalytic reactions occurring on the surface of mesoporous systems of interest. For 29Si with direct polarization NMR, up to 8-fold DNP enhancements were obtained. This 29Si signal enhancement is considerably higher than the obtained with similar approaches reported in literature. Finally, from DNP enhancement profiles we conclude that cross-effect is probably the dominant polarization transfer mechanism.

A Simplified Method of Synthesis to Obtain Zwitterionic Cellulose under Mild Conditions with Active Ionic Moieties.

A simplified procedure to synthesize zwitterionic cellulose by means of N-protected aspartic anhydride under mild conditions was developed. The preparation of modified cellulose samples was carried out under heterogeneous, aqueous conditions by reacting NH4OH-activated cellulose with aspartic anhydrides N-protected with trifluoroacetyl (TFAc) and carbobenzyloxy (Cbz). Modified cellulose samples Cel-Asp-N-TFAc and Cel-Asp-N-Cbz were characterized by Fourier Transform Infrared (FTIR) and 13C solid state Nuclear Magnetic Resonance (NMR) spectroscopy. The functionalization degree of each cellulose sample was determined by the 13C NMR signal integration values corresponding to the cellulose C1 vs. the Cα of the aspartate residue and corroborated by elemental analysis. In agreement, both analytical methods averaged a grafting degree of 20% for Cel-Asp-N-TFAc and 16% for Cel-Asp-N-Cbz. Conveniently, Cel-Asp-N-TFAc was concomitantly partially N-deprotected (65%) as determined by the ninhydrin method. The zwitterion character of this sample was confirmed by a potentiometric titration curve and the availability of these amino acid residues on the cellulose was inspected by adsorption kinetics method with a 100 mg L-1 cotton blue dye solution. In addition, the synthesis reported in the present work involves environmentally related advantages over previous methodologies developed in our group concerning to zwitterionic cellulose preparation.