Jean-Paul Amoureux

Triple, quintuple and higher order multiple quantum MAS NMR of quadrupolar nuclei.

The optimization of the coherence transfers involved in five, seven and nine-quantum versions of the recently discovered MQMAS technique, is analysed numerically. Data reported in this paper may serve as starting parameters for the experiment set up. An analysis of the intensity and resolution given by each type of experiment is performed, which confirms the need to use very high rf fields for MQ transfers. It follows that five-quantum is achievable rather easily but the use of seven and nine-quantum MAS experiments becomes increasingly difficult due to the demand for high rf power and decreasing sensitivity. The advantages of using the z-filter MQMAS method with respect to a two-pulse sequence are analysed. The method for qualitatively and quantitatively interpret the MQMAS spectra is described. The nature of the spinning side bands along the multiple quantum dimension is explained. It is shown that the rotor synchronization can be conveniently used to eliminate these side bands, but only for 3QMAS experiments. The use of the multiple-quantum method in combination with static samples and VAS, DAS and DOR techniques is finally discussed.

2D multiquantum MAS-NMR spectroscopy of 27Al in aluminophosphate molecular sieves

Abstract Two-dimensional multiquantum MAS-NMR spectroscopy (2D-MQ) has been applied to the discrimination of the different 27 Al sites in several aluminophosphate molecular sieves. It is shown that a new quintuple-quantum MAS-NMR method (2D-5Q) gives an extraordinary improved resolution compared to the DOR or DAS technique. The pure-absorption 2D spectra allow the direct determination of the number of different species, the correct determination of their actual chemical shifts and quadrupolar parameters. In addition, the method gives incomparable information about the distributions of both the chemical shifts and quadrupolar coupling constants. These findings are demonstrated by the experimental spectra of the ALPO-11 and VPI-5 aluminophosphate molecular sieves.

Understanding the High Photocatalytic Activity of (B, Ag)-Codoped TiO2 under Solar-Light Irradiation with XPS, Solid-State NMR, and DFT Calculations

The origin of the exceptionally high activity of (B, Ag)-codoped TiO(2) catalysts under solar-light irradiation has been investigated by XPS and (11)B solid-state NMR spectroscopy in conjunction with density functional theory (DFT) calculations. XPS experimental results demonstrated that a portion of the dopant Ag (Ag(3+)) ions were implanted into the crystalline lattice of (B, Ag)-codoped TiO(2) and were in close proximity to the interstitial B (B(int.)) sites, forming [B(int.)-O-Ag] structural units. In situ XPS experiments were employed to follow the evolution of the chemical states of the B and Ag dopants during UV-vis irradiation. It was found that the [B(int.)-O-Ag] units could trap the photoinduced electron to form a unique intermediate structure in the (B, Ag)-codoped TiO(2) during the irradiation, which is responsible for the photoinduced shifts of the B 1s and Ag 3d peaks observed in the in situ XPS spectra. Solid-state NMR experiments including (11)B triple-quantum and double-quantum magic angle spinning (MAS) NMR revealed that up to six different boron species were present in the catalysts and only the tricoordinated interstitial boron (T*) species was in close proximity to the substitutional Ag species, leading to formation of [T*-O-Ag] structural units. Furthermore, as demonstrated by DFT calculations, the [T*-O-Ag] structural units were responsible for trapping the photoinduced electrons, which prolongs the life of the photoinduced charge carriers and eventually leads to a remarkable enhancement in the photocatalytic activity. All these unprecedented findings are expected to be crucial for understanding the roles of B and Ag dopants and their synergistic effect in numerous titania-mediated photocatalytic reactions.

Optimized multiple-quantum magic-angle spinning NMR experiments on half-integer quadrupoles

It has been recently shown that second-order anisotropies can be removed from solid phase NMR spectra of quadrupolar nuclei via the combined use of magic-angle spinning (MAS) and bidimensional multiple-quantum (MQ) spectroscopy. The present study investigates the conditions under which the acquisition of such high-resolution MQMAS NMR spectra are optimized. The excitation and conversion pulse lengths that maximize 0 → ±3(t1) → − 1 (t)2) coherence transfer pathway signals for arbitrary spin numbers were calculated, as were the pulse lengths that provide NMR spectra free from dispersive line shape distortions. For the case of spin-5/2 nuclei, the conditions which optimize experiments involving quintuplequantum coherences were also determined.

Synergistic effect of zeolite in an intumescence process: study of the carbonaceous structures using solid-state NMR

This work deals with the chemical effect of zeolite 4A added to the intumescent ammonium polyphosphate (APP)–pentaerythritol (PER) system [fire retardant (FR) polyethylene-based formulation]. The study of the thermal behaviour of the system using thermogravimetric (TG) analysis shows that the presence of the zeolite leads to an increase in the stability of the material at high temperature (T > 500 °C). Chemical analysis and cross-polarisation dipolar-decoupled magic-angle spinning (CP–DD–MAS)13C NMR reveals that the materials resulting from the thermal treatment of the APP–PER and APP–PER/4A systems are formed by carbonaceous and phosphocarbonaceous species and that the zeolite enhances the stability of the phosphocarbonaceous species. DD–MAS 31P NMR indicates that this stability may be due to an absence of pyrophosphate species in the heat-treated APP–PER/4A system. Micro-Raman spectroscopy and 1H NMR of the solid state provides information on the structure of the carbonaceous material. Thermal treatment led to the formation of a ‘turbostatic carbon’ with a local structure of this ‘carbon species’, with the zeolite permitting retention of the comparatively unorganised carbon species in the high-temperature range. Moreover, a low-resolution solid-state 1H NMR study of the spin–lattice and spin–spin relaxations of the samples allows an evaluation of the size of the slow relaxation domains using the Goldman–Shen procedure. It is shown that the zeolite hinders the formation of small molecules in the polyaromatic network. Finally, the relationship between the formation of a coherent macromolecular network and the improved FR performance of the APP–PER/4A system is proposed.

Chemical bonding differences evidenced from J-coupling in solid state NMR experiments involving quadrupolar nuclei

Small scalar J-coupling between quadrupolar nuclei and spin 1/2 can be measured in inorganic solids using J-Resolved experiments and further used to acquire 2D J-HQMC heteronuclear correlation, giving detailed insight into the chemical bonding scheme.

Beyond the silica surface by direct silicon-29 dynamic nuclear polarization

Buried truth: High-field magic angle spinning dynamic nuclear polarization (MAS DNP) enhances the sensitivity of solid-state NMR spectroscopy, but only for protonated surfaces. Direct 29 Si DNP using the biradical TOTAPOL (see picture) circumvents this limitation by producing a 30-fold enhancement of subsurface 29 Si NMR signals in mesoporous silica, a material with applications in photonics, nanotechnology and catalysis.

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC.

We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R3) irradiation, or on two continuous rotor-synchronized modulations (SFAM1 and SFAM2), or on four symmetry-based sequences (R2(1)1,SR4(1)2,R12(3)5,R20(5)9), or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR4(1)2 and SFAM (SFAM1 or SFAM2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM1 and SFAM2 will be the most efficient methods.

Unified representation of MQMAS and STMAS NMR of half-integer quadrupolar nuclei

Abstract High resolution solid state NMR has been achieved for nuclei that exhibit quadrupolar interaction to the second order by MQMAS and more recently by STMAS. Both methods extend to several variants, 3QMAS, 5QMAS, … for MQMAS as well as ST 1 MAS, ST 2 MAS,… for STMAS. These experiments contain the same information content i.e. anisotropic second-order powder pattern and isotropic terms. A unified representation allows us to display the 2D spectra independently of the method used. Direct comparison of any such spectra is therefore straightforward and does not need any further renormalization.

Through-space R3-HETCOR experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR

We present several new methods that allow to obtain through-space 2D HETCOR spectra between spin-1/2 and half-integer quadrupolar nuclei in the solid state. These methods use the rotary-resonance concept to create hetero-nuclear coherences through the dipolar interaction instead of scalar coupling into the HMQC and refocused INEPT experiments for spin n/2 (n>1). In opposite to those based on the cross-polarization transfer to quadrupolar nuclei, the methods are very robust and easy to set-up.