Triple-quantum correlation NMR experiments in solids using J-couplings

Abstract

We show that triple-quantum -single-quantum (TQ-SQ) correlation spectra of crystalline and disordered solids can be obtained under MAS using pulse sequences based on through-bond J-couplings. The feasibility of the experiments in coupled spin-½ systems is demonstrated for fully 13 C-labelled L-alanine and Pb 3 P 4 O 13 crystalline compounds, considered as model three-spin and four-spin systems, respectively. In the case of phosphate glasses, we show that the obtained TQ-SQ correlation spectra provide an improved description of the glass forming network connectivities and of the chain length distribution in the disordered network. IntroductionHomonuclear double-quantum correlation spectroscopy has been shown to be a valuable tool in solid-state NMR for determining atomic connectivities and studying the structure and dynamic of ordered or disordered materials [1,2]. In this context, various experiments have been developed to obtain two-dimensional (2D) double-quantum -singlequantum (DQ-SQ) correlation spectra of coupled spin-½ systems under magic angle spinning (MAS) which is required for enhanced spectral resolution and sensitivity. Many of these methods employ pulse sequences which reintroduce the MAS-averaged through-space homonuclear dipolar interaction, to obtain DQ-SQ correlation spectra which reflect the through-space atomic proximities [3][4][5][6][7][8][9][10]. Alternatively, pulse sequences based on throughbond isotropic J-couplings, like the INADEQUATE [11] and refocused INADEQUATE [12] experiments, have also been applied efficiently to obtain through-bond DQ-SQ correlation spectra for large range of materials [12][13][14][15][16][17][18][19][20][21][22][23].In the case of inorganic glasses, for which the characterization of medium range order remains challenging, the DQ-SQ correlation experiment allows to improve the structural description obtained from NMR spectroscopy by determining the pairwise connectivities between the basic building units of the disordered network. In phosphate glasses, homonuclear through-bond [22,23] and through-space [24-25] DQ-SQ correlation methods have been applied successfully to probe P-O-P connectivities between the various network former PO 4 tetrahedral units (Q n with n bridging oxygen atoms). These experiments provide enhanced spectral resolution relative to the conventional MAS spectra allowing the distinction between Q n units bonded to different types of PO 4 to be made and giving evidence of a chain length distribution in the disordered network [22][23][24][25].Nevertheless, information about longer range order, which could be obtained from a three-spin correlation experiment, remains desirable for a better description of the disordered glassy network. Recently, various dipolar recoupling pulse sequences have been proposed to obtain 1 H and 13 C homonuclear through-space triple-quantum -single-quantum (TQ-SQ) correlation spectra in solids [26][27][28][29][30]. A modified INADEQUATE experiment was also proposed to record 13 C through-bond TQ-SQ correlatio...

Detection and use of small J couplings in solid state NMR experiments

“…It then becomes possible to build relayed transfer experiments that allow transferring repeatedly the magnetization from a starting nucleus to its first and second neighbors [15,16], as if ''walking'' across the covalent 3D network structure. It is also possible to describe experiments dedicated to counting the different NMR-active neighboring nuclei (or to editing the spectrum according to this count) with examples on pairs of nuclei such as { 31 [36,43,14,17]. These new possibilities can be applied to the study of crystalline a-cordierite magnesium aluminosilicate (Mg 2 Al 4 Si 5 O 18 ) [85].…”

“…In that sense, solid-state NMR surpasses liquid state NMR in terms of resolution and of the size of the couplings that can consequently be assessed. Many examples showing scalar couplings between spin-1/2 and quadrupolar nuclei have indeed been reported, including couplings between 13 C and [35][36][37] Cl nuclei [56], and between 31 [74,75,14] with applications to the analyses of the details of glass structures and evidence of the presence of m 3 oxygen bridges in calcium aluminate glasses [74] for example.…”

Structure and dynamics of oxide melts and glasses: A view from multinuclear and high temperature NMR

“…Because this isotropic J-coupling can be measured, it can also be used to reveal -or to spectrally edit -different polyatomic molecular units in the glass. Figure 1c and 1d respectively show the two-dimensional correlation spectra that enable the identification of through-bond connectivity between two linked PO 4 tetrahedra ( fig.1c) [6] and between three linked PO 4 tetrahedra ( fig.1d) [8]. These experiments, fully glass temperature, they begin to exchange just above glass transition with characteristic times of the order of seconds [12] and finally end up into merging in a unique line in the high temperature molten state.…”

“…That type of information can be obtained by implementing multidimensional NMR experiments that allow to evidence Dipolar [4] or J-coupling [6,7,8] interaction and further use them to build correlation experiments separating the different contributions of well defined molecular motifs. Figure 1 gives a general picture of the possibilities offered by the J-coupling mediated experiments that allow to directly evidence the P-O-P bonds bridging phosphate units through J 2 P-O-P interaction.…”

“…Summary of NMR experiments on a 60%PbO-40%P 2 O 5 glass evidencing polyatomic molecular motifs with : (a) 1D spectrum, (b) the J resolved spectrum showing doublet for Q 1 and triplet for Q 2 [7], (c) the INADEQUATE experiment evidencing pairs of phosphates (Q-Q) [6], and (d) the 3Quantum spectrum evidencing triplets of phosphates (Q-Q-Q) [8].…”