Determining relative proton–proton proximities from the build-up of two-dimensional correlation peaks in 1H double-quantum MAS NMR: insight from multi-spin density-matrix simulations

Abstract

The build-up of intensity-as a function of the number, n(rcpl), of POST-C7 elements used for the excitation and reconversion of double-quantum (DQ) coherence (DQC)-is analysed for the fifteen distinct DQ correlation peaks that are observed experimentally for the eight separate (1)H resonances in a (1)H (500 MHz) DQ CRAMPS solid-state (12.5 kHz MAS) NMR spectrum of the dipeptide beta-AspAla (S. P. Brown, A. Lesage, B. Elena, and L. Emsley, J. Am. Chem. Soc., 2004, 126, 13230). The simulation in SPINEVOLUTION (M. Veshtort and R. G. Griffin, J. Magn. Reson., 2006, 178, 248) of t(1) ((1)H DQ evolution) FIDs for clusters of eight dipolar-coupled protons gives separate simulated (1)H DQ build-up curves for the CH(2)(a), CH(2)(b), CH(Asp), CH(Ala), NH and OH (1)H single-quantum (SQ) (1)H resonances. An analysis of both the simulated and experimental (1)H DQ build-up leads to the following general observations: (i) considering the build-up of (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the DQC corresponding to the shortest H-H distance; (ii) for the maximum intensity (1)H DQ peak at a particular SQ frequency, the recoupling time for the observed maximum intensity depends on the corresponding H-H distance, e.g., maximum intensity for the CH(2)(a)-CH(2)(b) (H-H distance = 1.55 A) and OH-CH(Asp) (H-H distance = 2.49 A) DQ peaks is observed at n(rcpl) = 2 and 3, respectively; (iii) for DQ peaks involving a CH(2) proton at a non-CH(2) SQ frequency, there is much reduced intensity and a maximum intensity at a short recoupling time; (iv) for the other lower intensity (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the same (or close to the same) recoupling time, but the relative intensity of the DQ peaks is a reliable indicator of the relative H-H distance-the ratio of the maximum intensities for the peaks at the CH(Ala) SQ frequency due to the two DQCs with the NH and OH protons are found to be approximately in the ratio of the squares of the corresponding dipolar coupling constants. While the simulated (1)H DQ build-up curves reproduce most of the features of the experimental curves, maximum intensity is often observed at a longer recoupling time in simulations. In this respect, simulations for two to eight spins show a trend towards a faster decay for an increasing number of considered spins. Finally, simulations show that increasing either the Larmor frequency (to 1 GHz) or the MAS frequency (to 125 kHz) does not lead to changes in the marked differences between the (1)H DQ build-up curves at the CH(Asp) SQ frequency for DQCs to the CH(2)(a) and OH protons that correspond to similar H-H distances (2.39 A and 2.49 A, respectively).

Probing Intermolecular Crystal Packing in γ-Indomethacin by High-Resolution 1H Solid-State NMR Spectroscopy

“…In contrast, the red solid line in Figure 3b for the multi-proton dipolar-coupled networks found in organic solids. 61 A first important observation was that, considering all DQCs associated with a specific proton, maximum intensity is observed for the DQC corresponding to the shortest H-H distance. Comparing the red solid line in Figure 3a for the OH-OH proximity of 2.38 Å to the green and pink lines in Figure 3b for the OH-CH18 and OH-CH6 proximities of 2.48 Å (CH18) and 2.89 and 3.18 Å (2 × CH6), it is evident that maximum simulated intensity is indeed for the closer OH-OH proximity.…”

Challenges in numerical simulations of solid-state NMR experiments: Spin exchange pulse sequences

“…The desire to understand processes involving complex pulse sequences and large spin systems [42,45,[60][61][62]83] has underlined the need for both flexibility to investigate any pulse sequence under any experimental conditions and possibility to address large spin systems. This need is contrasted by availability of highly popular user-friendly specialized-purpose programs [3][4][5][6][7][8][9] and more general-purpose programs [10][11][12][13] typically operating on modest spin system sizes.…”

Measurement of homonuclear magnetic dipole–dipole interactions in multiple 1/2-spin systems using constant-time DQ-DRENAR NMR

“…It is well known that advanced back-to-back (BABA) 90°pulse schemes can produce pure double quantum Hamiltonians [32][33][34].…”

“…As a result, among the numerous homonuclear dipolar recoupling methods that have been developed , no clear method of choice can be identified. A powerful approach of homonuclear re-coupling is based on the excitation of double quantum coherences using suitable excitation strategies [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. In many of these studies internuclear distances in two-spin systems and average dipolar coupling constants in many-spin systems have been determined by measuring DQ coherence intensities as a function of excitation time.…”