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Grafting on nanoparticles: is solid-state NMR a convenient tool of investigation?


The physicochemical study of organic/inorganic hybrid materials benefits from the latest developments of solid-state NMR. The NMR interactions can be considered as structural "spies," leading to the implementation of multidimensional experiments. The goal is definitely the characterization of the interfaces of hybrid materials, which play a key role in the properties of materials. These interactions are: (i) the chemical shift, which is directly related to the chemical environment of the nuclei. (ii) the homo- and heteronuclear dipolar interactions, which are spatial in nature and related to the internuclear distances. (iii) the scalar through bond J interaction, which is characteristic for chemical bonding between nuclei. (iv) the quadrupolar interaction (for I > 1/2) which corresponds to the interaction between the quadrupolar moment of a given nucleus and the local electric field gradient. (1) These interactions bear a huge amount of structural information such as: the local symmetry, the connectivities between nuclei, the size of domains at the nanometric scale .... (2-4) Figure 1 shows basic NMR techniques which can be combined in isotropic/anisotropic correlation experiments.

The NMR interactions are anisotropic in nature and static experiments on powders lead to featureless spectra (with no spectral resolution). The Magic Angle Spinning (MAS) of the samples leads to the complete averaging of the chemical shift anisotropy (CSA) and dipolar interactions: it follows that highly resolved spectra can be recorded even for powdered samples. The MAS technique works well for I = 1/2 spins, but fails for quadrupolar nuclei characterized by strong quadrupolar interaction and also for strongly coupled highly abundant nuclei (such as protons). As a matter of fact, MAS is the technique of choice for D and J correlation experiments.

The CP experiment is based on a transfer of magnetization between an abundant spin system ([.sup.1.H], [.sup.19.F], [.sup.31.P] ...) and a low-abundant spin system ([.sup.13.C], [.sup.29.Si] ...). (5) This transfer can be interpreted as a dipolar recoupling under MAS. This dipolar recoupling occurs at the so-called Hartmann-Hahn condition, (6) which is related to the RF fields on both channels. The CP experiment is fundamental for chemist: (i) it allows a significant increase in signal to noise ratio. (ii) "editing" sequences dealing with dipolar filtering can be implemented. (7) (iii) measurement of internuclear distances by solid-state NMR can be achieved. (8,9) Moreover, the CP MAS experiment can be extended to 2D versions (HETCOR) by adding a [t.sub.1] evolution on a given channel. Most articles published so far in the literature deal with [.sup.1.H]/X CP experiments, but we have shown recently that this experiment could be safely extended to more "exotic" spin pairs such as: [.sup.31.P]/[.sup.29.SI], [.sup.29.Si]/[.sup.13.C], [.sup.51.V]/[.sup.29.Si] .... (10) Several examples dealing with the [.sup.31.P]/[.sup.29.Si] spin pair will be presented. (3,4,11,12)



The MAS-J spectroscopy is one of the latest developments in solid-state NMR. (13,14) The experimental schemes are mainly related to pulse sequences already proposed in the frame of solution state NMR (HMQC, J-Res, INEPT ...). In the homonuclear version (see for instance the INADEQUATE sequence in Fig. 1), X-O-X fragments are evidenced. In the heteronuclear version, the presence of X-O-Y fragments is demonstrated. It must be emphasized that D and J experiments are complementary in the sense that chemical and spatial connectivities can be studied simultaneously. In that sense, solid-state NMR is a perfect tool of investigation for interfaces and hybrid materials.

[.sup.1.H] solid-state NMR is also an interesting tool of investigation for the study of hybrid materials, as protons are present not only in the organic components of a given material but also in the inorganic components (Si-OH, P-OH, adsorbed [H.sub.2]O molecules ...). The homonuclear [.sup.1.H]/[.sup.1.H] interaction is homogeneous in nature (2): it follows that the MAS technique does not permit the complete averaging of the dipolar interaction in the case of strongly coupled proton networks. However, at very fast MAS frequency (>35 kHz) and at very high magnetic field [B.sub.0], 1D [.sup.1.H] spectra show a reasonable resolution (at least for hybrid materials). Figure 2 shows the [.sup.1.H] study of self-associated hybrid systems involving strong H bond networks and ureidopyrimidinone molecules. (15,16) The gain in resolution at 750 MHz is spectacular. The de-shielded resonances can be assigned to the protons involved in the H bond network. The ureidopyrimidinone molecules can be silylated showing then Si(OEt)[.sub.3] groups. Hydrid materials can be obtained by hydrolysis and condensation of the OEt groups. In the final materials, correlations of pairs of protons can be established by double quantum [.sup.1.H]/[.sup.1.H] experiments, involving homonuclear dipolar recoupling. (17,18) The obtained experiment is a 2D correlation with one single-quantum dimension (corresponding to a filtered MAS spectrum) and one double-quantum dimension corresponding to coupled nuclei (via the dipolar interaction). The proximity of two [.sup.1.H] nuclei is characterized by an on-diagonal correlation peak for identical chemical shifts and off-diagonal correlation peaks for different chemical shifts. Figure 3 presents such correlation experiment for ureidopyrimidinone derivatives and the presence of correlation peaks. From such 2D spectra, safe structural models can be proposed.




Silicophosphates can be considered as potential biomaterials. Their structural characterization involved the set up of J and D techniques, such as [.sup.31.P]/[.sup.29.Si] CP MAS, J-MAS-HMQC, J-MAS-INEPT experiments. (11,12) Triple resonance experiments [.sup.1.H]/[.sup.31.P]/[.sup.29.Si] were also implemented in order to characterize double transfers between the involved nuclei. Figure 4 shows a 2D [.sup.31.P]/[.sup.29.Si] HMQC experiment dealing with a complex mixture of silicophosphates.

Obviously, the J experiments can be extended to the study of grafting of molecules at the surface of nanoparticles, such as silica. The grafting is generally postulated in the literature, but rarely proved by spectroscopic techniques. Figure 5 shows a schematic view of the solid-state NMR approach. Si-O-Si and Si-O-P bonds can be indeed characterized by a large panel of homonuclear and heteronuclear J experiments. Intensive work is on the way in the laboratory.


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11. Lejeune, C, Coelho, C, Azais, T, Bonhomme-Coury, L, Maquet, J, Bonhomme, C, "Studies of Silicophosphate Derivatives by [.sup.31.P]/[.sup.29.Si] CP MAS NMR." Solid State NMR, 27 242-246 (2005)

12. Coelho, C, Azais, T, Bonhomme-Coury, L, Maquet, J, Massiot, D, Bonhomme, C, "Application of the MAS-J-HMQC Experiment to a New Pair of Nuclei {[.sup.29.Si], [.sup.31.P]}: [Si.sub.5]O(P[O.sub.4])[.sub.6] and Si[P.sub.2][O.sub.7] Polymorphs." J. Magn. Reson., 179 106-111 (2006)

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15. (a) Moreau, JJE, Vellutini, L, Wong Chi Man, M, Bied, C, Bantignies, JL, Dieudonne, P, Sauvajol, JL, "Self Organized Hybrid Silica with Lamellar Structure." J. Am. Chem. Soc., 123, 7957-7958 (2001). (b) Moreau, JJE, Vellutini, L, Wong Chi Man, M, Bied, C, "Shape Controlled Bridged Silsesquioxanes: Hollow Tubes and Spheres." Chem. Eur. J., 9, 1594-1599 (2003). (c) Moreau, JJE, Pichon, BP, Wong Chi Man, M, Bied, C, Pritzkow, H, Dieudonne, P. Bantignies, JL, Sauvajol, JL, "A Better Understanding of the Self Structuration of Bridged Silsesquioxanes." Angew. Chem., 43, 203-206 (2004)

16. Arrachart, G, Auto-assemblage d'organosilices par reconnaissance moleculaire, PhD thesis, Universite de Montpellier 2, December 2005

17. (a) Schnell, I, Langer, B, Sontjens, SHM, van Genderen, MHP, Sijbesma, RP, Spiess, HW, "Inverse Detection and Heteronuclear Editing in [.sup.1.H]-[.sup.15.N] Correlation and [.sup.1.H]-[.sup.1.H] Double Quantum NMR Spectroscopy in the Solid State under Fast MAS." J. Magn. Reson., 150, 57-70 (2001). (b) Armstrong, G, Alonso, B, Massiot, D, Buggy, M, Solid State NMR Study of Ureidopyrimidinone Model Compounds, Magn. Reson. Chem., 43 405-410 2005. (c) Schnell, I, Langer, B, Sontjens, SHM, Sijbesma, RP, van Genderen, MHP, Spiess, HW, "Quadruple Hydrogen Bonds of Ureidopyrimidinone Moities Investigated in the Solid State by [.sup.1.H] Double Quantum MAS NMR Spectroscopy." Phys. Chem. Chem. Phys., 4, 3750-3758 (2002)

18. Graf, R, Demco, DE, Gottwald, J, Hafner, S, Spiess, HW, "Dipolar Couplings and Internuclear Distances by Double-Quantum Nuclear-Magnetic-Resonance Spectroscopy of Solids." J. Chem. Phys., 106 885-895 (1996)

[c] FSCT and OCCA 2007

C. Bonhomme ([mailing address]), C. Gervais, S. De Monredon, C. Coelho

Laboratoire de Chimie de la Matiere Condensee de Paris--UMR 7574, Universite P. et M. Curie--T54 E5, 4, place Jussieu, Paris Cedex 05 75252, France e-mail:
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Author:Bonhomme, C.; Gervais, C.; De Monredon, S.; Coelho, C.
Publication:JCT Research
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Date:Mar 1, 2008
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