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

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(δ)‐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 ν 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 ∼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 π–pulses, Δϕ = ϕ′− ϕ, between 40 and 60° . For experiments at 600 MHz and 30 kHz MAS with 13 CH‐labelled L ‐alanine, the best RS‐HEPT (ν 1 ( 1 H) = 100 kHz) T 2 ′ value was 3 times longer than that observed for low‐power continuously applied sequences with ν 1 ( 1 H) ⩽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. Copyright © 2007 John Wiley & Sons, Ltd.