Low-load rotor-synchronised Hahn-echo pulse train (RS-HEPT)1H decoupling in solid-state NMR: factors affecting MAS spin-echo dephasing times

Abstract

Transverse dephasing times T(2)' in spin-echo MAS NMR using rotor-synchronised Hahn-echo pulse-train (RS-HEPT) low-load (1)H decoupling are evaluated. Experiments were performed at 300 and 600 MHz for (13)CH-labelled L-alanine and (15)NH(delta)-labelled L-histidine.HCl.H(2)O, together with SPINEVOLUTION simulations for a ten-spin system representing the crystal structure environment of the (13)CH carbon in L-alanine. For 30 kHz MAS and nu(1)((1)H) = 100 kHz at 300 MHz, a RS-HEPT T(2)' value of 17 +/- 1 ms was obtained for (13)CH-labelled L-alanine which is approximately 50% of the XiX T(2)' value of 33 +/- 2 ms. Optimum RS-HEPT decoupling performance is observed for a relative phase of alternate RS-HEPT pi-pulses, Deltaphi = phi'- phi, between 40 and 60 degrees . For experiments at 600 MHz and 30 kHz MAS with (13)CH-labelled L-alanine, the best RS-HEPT (nu(1)((1)H) = 100 kHz) T(2)' value was 3 times longer than that observed for low-power continuously applied sequences with nu(1)((1)H) < or =40 kHz, i.e. corresponding to the same average power dissipated in the probe. A marked improvement in RS-HEPT (1)H decoupling is observed for increasing MAS frequency: at 55.6 kHz MAS, a best RS-HEPT T(2)' value of 34 +/- 5 ms was recorded for (13)CH-labelled L-alanine. Much improved RS-HEPT broadband performance was also observed at 55.6 kHz MAS as compared to 30 kHz MAS.

Unexpected effects of third-order cross-terms in heteronuclear spin systems under simultaneous radio-frequency irradiation and magic-angle spinning NMR

“…2 the spin-echo period. The fitted time constants, which are consistent with previous experimental observations, 29 are about three times larger than those observed with homonuclear decoupling using the same 1 H RF power. One significant distinction between the two spin-echo experiments shown in Fig.…”

Fast acquisition of multi-dimensional spectra in solid-state NMR enabled by ultra-fast MAS

“…Kotecha et al recently compared the efficiency of different decoupling sequences at high spinning speeds depending on the multiplicity of the carbon under consideration in ubiquitin samples [17]. Griffin et al have also recently discussed the use of low-load decoupling during spin-echo experiments [18]. Low-field decoupling is thus an important alternative, in particular for the study of biological substrates with high dielectric constants, where prolonged high-power irradiations can lead to undesired, often disastrous, heating of the sample.…”

Recent Advances in Solid‐State MAS NMR Methodology for Probing Structure and Dynamics in Polymeric and Supramolecular Systems

“…The latest generation of MAS probes support rotation frequencies in excess of 60 kHz, offering new possibilities for crosspolarisation [237] and 1 H decoupling. [238][239][240] Moreover, recently developed microcoil designs allow the investigation of very small sample volumes together with very high B 1 magnetic fields. [241] While the focus of this article has been supramolecular and polymeric systems, a major focus of solid-state NMR research is directed to large biomolecules.…”