The D-HMQC MAS-NMR Technique

Abstract

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the cross-polarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to C_q values and radiofrequency field. Furthermore, the high robustness to irradiation offset makes of the D-HMQC sequence the technique of choice for the structural characterisation of materials especially at high magnetic field. We show here how the D-HMQC can be easily implemented and optimised to give access to the structural analysis of silicate-, phosphate-, carbon- and proton-containing materials. An emphasis will be on describing the most popular dipolar recoupling schemes that can be used in that sequence and providing their advantages and drawbacks.

Introduction

During the past decades, magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy has proven to be a technique of choice for the structural characterisation of solid materials. Limitations in sensitivity and resolution have been pushed back by the recent technical and methodological developments (i.e. very high magnetic field spectrometer (23 T), dynamic nuclear polarisation and implementation of sophisticated pulse sequences), leading to unprecedented level of detailed structural information. The very local order can be investigated not only through the chemical shift but also through the quadrupolar interaction in case of nuclei with spin number I > 1/2. Valuable information about the coordination state, neighbour nature or chemical environment symmetry can be derived from the study of these two interactions. The degree of disorder within the structure, resulting from bond length and angle distributions, is also reflected in the width of the NMR signal, giving information about the crystalline/amorphous character of the investigated material. Nevertheless, a complete and helpful structural characterisation also requires information about the medium-range order. Indeed, it is well admitted that the material macroscopic properties are governed by the intermediate length-scale organisation deriving from the association between the different polyhedra. If local-order analysis gives a picture of the elementary bricks, structural understanding and designing precise properties require an extended picture of the material structure. NMR is also capable of handling this kind of investigation. Interactions between the different building units can be probed using the different correlation NMR techniques implemented during the past 20 years. A classification of the numerous available correlation NMR techniques [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] is presented in Fig. 4.1, depending on (i) the nature of interacting elements (identical or different), (ii) the interaction used in the sequence (scalar or dipolar) and (iii) the qualitative or quantitative information character. When both building units under investigation share the same chemical nature (two silicate units, two protonated groups, etc.), homonuclear-based experiments are required for the analysis. Contrariwise when the two structural units are not of identical chemical nature, heteronuclear sequences must be selected. If the correlation sequences use the dipolar interaction to create correlation signal, spatial proximity will be derived from the study. Correlating signals through the scalar interaction leads to spectra carrying information about chemical connectivity. Finally, while some sequences only provide a qualitative vision of the interacting nuclei, other sequences afford quantitative data such as interatomic distances, number of neighbours or scalar coupling values. To summarise, correlation NMR is the technique of choice to analyse the medium-range structure through different points of view. The global set of data offers different and complementary visions of the interactions from which supported and precise structural models can be proposed.

Heteronuclear dipolar correlation techniques will be the scope of this chapter. More precisely, the main topics will be the edition of filtered 1D spectra or 2D correlation maps that qualitatively trace the spatial proximity between two units presenting different chemical nature. Historically, the technique of choice to achieve this kind of investigation is the cross-polarisation (CP) sequence [12]. The development of this technique in its 1D and 2D versions [16] is considered as a major breakthrough in the development of solid-state NMR. The pulse sequence, reported in Fig. 4.2A, uses a magnetisation transfer from a ‘source’ nucleus to the nucleus ‘under investigation’. Using appropriate irradiation conditions on both nuclei (fulfilling the so-called ‘Hartmann–Hahn’ (HH) conditions) allows acquiring spectra, showing evidence of spatial proximity between the two structural units, the transfer efficiency being distance-dependent.

If performing a CP experiment between two spins 1/2 can be considered as a routine procedure nowadays, optimisation on systems containing a quadrupolar nucleus implies a high level of complexity. One of the main limitations comes from the weak irradiation that is required on the quadrupolar nucleus to achieve the spin locking, leading to strong offset irradiation dependence [17]. Thus, CP experiments performed at very high magnetic field, where chemical shift differences are enhanced, strongly suffer from this limitation, rendering this kind of NMR sequence inappropriate on recent high-field spectrometers. Considering the need of through-space correlation schemes performed at high field in inorganic materials science, it appears that the limitation of CP for quadrupolar nuclei, due to offset dependence, had to be overcome using either a modification of the CP pulse sequence [18] or the implementation of a new technique.

The dipolar heteronuclear multiple-quantum coherence (D-HMQC) sequence shown in Fig. 4.2B was proposed in 2007 to replace the CP for systems containing a half-integer quadrupolar and a spin-1/2 nuclei [14], [15]. It is noteworthy that the presented pulse sequence is the indirect recoupling version since the dipolar interaction is recoupled by irradiating the non-observed nuclei (indirect channel). The basic idea was to modify the original HMQC pulse sequence (a liquid-state-derived scalar-based sequence providing information about chemical connectivity [19], [10]) by introducing additional pulse schemes on the spin-1/2 in order to reintroduce the X–Y dipolar interaction [20], [21], [22], [23]. As a consequence, the obtained spectra trace the X–Y spatial proximity through the edition of filtered 1D spectra or 2D correlation maps. The Y{X} D-HMQC sequence was designed specifically to investigate phosphate materials (X = ³¹P nucleus) and shows its efficiency in various systems including alumino-, boro- and vanado-phosphate materials (Y = ²⁷Al, ¹¹B and ⁵¹V, respectively) [24], [25].

The purpose of this chapter is to show that D-HMQC is not restricted to phosphates but can, with slight modifications of the recoupling method, be efficiently applied to other spin-1/2-containing systems like silicium- (²⁹Si), hydrogen- (¹H) and carbon (¹³C)-containing materials. The first part of the chapter will be devoted to theoretical background. The advantages of the D-HMQC method compared to the CP sequence will be clearly identified. Particular emphasis will be placed on the recoupling methods. Since different dipolar recoupling schemes can be used, specific attention will be devoted to the suitable scheme depending on the characteristics of the investigated systems.

The second part will present different examples of D-HMQC applications on materials. Special attention will be paid to the experimental conditions used for the acquisitions and to the structural information derived from the analysis. We have chosen to dissociate this part in subparts focusing on specific spin-1/2-containing materials. The examples reported here have been extracted from our already published contributions, from unpublished works and from publications of others NMR groups around the world.