Dipolar and J- Derived Solid State NMR Techniques and First Principles Calculations Applied to the Structure of Silicoph
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Dipolar and J- Derived Solid State NMR Techniques and First Principles Calculations Applied to the Structure of Silicophosphates and to the Characterization of Phosphonate Grafting on Silica Nanoparticles Cristina Coelho1, Nadia Benharbya1, Cristel Gervais1, Thierry Azaïs1, Laure Bonhomme-Coury1, Guillaume Laurent1, Francesco Mauri2, and Christian Bonhomme1 1 Laboratoire de Chimie de la Matière Condensée de Paris, Universite P et M Curie, Paris, 75252, France 2 Laboratoire de Mineralogie Cristallographie de Paris, Universite P et M Curie, Paris, 75252, France
ABSTRACT J-derived (HMQC, INEPT) and D-derived (double and triple resonance) experiments were applied to the detailed characterization of crystalline and amorphous silicophosphate derivatives. 31P/29Si and 1H/31P/29Si CP MAS experiments were suitable for the description of complex silicophosphate gels, which can act as precursors for biocompatible materials. First principles calculations involving the GIPAW approach (first developed by Mauri and Pickard) were applied for the determination of CSA (29Si, 31P, 17O) and quadrupolar (17O) parameters. Excellent agreement between experimental and calculated data was obtained.
INTRODUCTION Silicophosphate gels and derivatives can act as useful biocompatible materials in various medical fields such as bone repair [1]. Such compounds present also very promising physical properties such as proton conductivity [2]. Recent developments in solid state NMR, including J-derived techniques [3,4], but also triple resonance experiments, open new opportunities for the detailed structural characterization of such compounds [5,6]. The spectroscopic goal to be reached is the characterization of Si-O-P bonds in terms of J and dipolar interactions. The solid state NMR methodology applied in this work is the following: J and CP experiments involving the {31P/29Si} spin pair have been successfully implemented by using the Si5O(PO4)6 crystalline phase as a standard compound [7-9]. Then, the extension to 2D experiments (HMQC, INEPT, HETCOR) has been applied to the characterization of complex silicophosphate crystalline phases including Si5O(PO4)6 and SiP2O7 polymorphs. The silicophosphate gels obtained at low temperature (T < 150°C) contain both crystalline and amorphous components. The behavior of the crystalline derivatives in terms of J-mediated coherences and 1H/(29Si,31P) CP transfer is comparable to the one observed for Si5O(PO4)6 and SiP2O7. On the contrary, it is now well established that the CP transfer is strongly reduced at room temperature for the amorphous components. We suppose that proton mobility averages the 1H/(29Si,31P) heteronuclear dipolar couplings. Such mobility could be related to proton conductivity [2]. At low temperature (T = -35°C), the CP efficiency increases, allowing the editing of the corresponding CP MAS spectra. The solid state NMR methodology described above has been applied to the study of
chemical grafting of phosphorus containing molecules (H3PO4, phosphonates) on silica nanoparticles
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