Multiple-resonance irradiations in NMR for high-resolution structural characterization of fluorinated solids – IRMMAF

The route we are exploring is based on the use of multiple-channel NMR probes coupled to receivers working in parallel. Increasing the number of channels gives access to new measurements (eg. 13C-27Al); increasing the number of receivers allows simultaneous acquisitions of NMR spectra. Coupling multiple-channel probes with parallel receivers therefore open a new field of development for the strategy of NMR measurements.

Associated to non-uniform sampling methods, these developments will allow considerable reduction of NMR measurement time. Higher dimensionality NMR experiments will be accessible in reasonable amount of time. This ensemble of NMR measurements, coupled to diffraction and structure modeling, will open access to a better comprehension of the structure of fluorinated hybrid solids.

The coupling of multiple-channel probes and parallel receivers also opens new perspective and thinking about strategies of NMR measurements. This approach can be extended to numerous NMR probes and chemical systems, most solid-state NMR labs being equipped with triple-resonance probes.

Communication (poster): 54th ENC conference, Asilomar (USA) Avril 2013

Porous MOFs have exceptional properties in gas storage, catalysis, drug delivery... Fluorinated MOFs seem even more promising for specific applications. The central bottleneck for the improvement of their properties is the understanding of their formation. Though this bottleneck is being investigated since decades, the rational phase design is still limited by characterization methods that would uncover with a high enough resolution the structure of these solids, including the periodic and non-periodic parts which are directly related to their conditions of formation. Diffraction methods have provided a large number of significant contributions. However they only allow resolution of the “average crystal structures”, and these methods have considerable difficulties to describe, for nanoporous materials, the agents responsible of the geometry of their pores, the “templating agents”. Therefore, in order to open this bottleneck, very high-resolution structural investigation techniques are needed. Because it is more local in character, solid-state nuclear magnetic resonance (NMR) has recently emerged as one of the most powerful technique, in combination with diffraction, to address the specificity of porous solids like zeolites or metal organic frameworks (MOFs). However, the key to the success of the use of solid-state NMR is the resolution that can be obtained on the NMR spectral signatures of the various structural features of a compound. If going to high magnetic field and ultra-fast magic angle spinning (MAS) is one of the possible solution to get high-resolution, it is however highly expensive thus limited to a few large facility NMR centres in France and in Europe, and hardly accessible for the many measurements required to unlocked the understanding of structure formation. We have recently shown that an alternative solution to retrieve the resolution in fluorinated samples was to use specific NMR multiple-irradiation measurement probes, which are efficient starting at moderate magnetic field and MAS frequency. To extend these techniques for the description of the more complex materials that are Fluorinated MOFs, several new requirements are needed: 1) Additional irradiation channels and larger flexibility in the accessible tuning frequencies. This will broaden the type of simultaneous nuclei that can be measured, thus widen the range of possible NMR experiments, without loss of resolution; 2) more robust NMR probes, since the requirement in terms of radio-frequency (RF) power are getting higher with the multiplication of RF channels. This is all the more important when quadrupolar nuclei and multiple-decouplings are involved. This will also provide access to higher dimension (> 2D) NMR experiments; 3) Transposition to NMR probes operating at high magnetic field. The probe electronic technology developed successfully for several years by Bruker Biospin has probably reached its limits, and to further improve robustness and efficiency, incremental improvements from existing state of the art are not possible. These limits have an important impact on the whole NMR community since they represent a barrier for the race to higher magnetic fields (that only makes sense if it is accompanied by efficient probes) and to robust multi-purpose probes. The electronic limits have been identified by Bruker Biospin. New developments in digital electronics already exist, and the company is willing to make all the necessary efforts to achieve the step-change of probe technology. It is therefore the purpose of this IRMMAF ANR project to come along with Bruker Biospin by showing feasibility studies for multiple-irradiation in terms of NMR resolution and structural investigation, developing NMR methodology and spectroscopy theory to understand the phenomena that will go along with the multiple-irradiations. Finally, the applications of these developments should provide new key structural information in layered or porous F-MOFs.