Sensitivity-enhanced multiple-quantum MAS NMR for half-integer spin quadrupolar nuclei using WURST-amplitude shaped pulses

Solid‐State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane‐Phosphane Frustrated Lewis Pairs and Secondary Adducts

“…Moreover, the time resolution of the WURST phase profiles (linear frequency sweep corresponding to a quadratic phase profile) can be chosen according to the intended bandwidth covered by the frequency sweep ( nyquist limit if necessary). In addition, the rounded edges of WURST pulses typically allow the application of higher rf‐field strengths [70] than the PM‐saturation pulses (rectangular‐shaped rf‐amplitude), which generally favor saturation performances, in particular for strong quadrupolar couplings. [ 67 , 68 , 69 ] Herein, we have evaluated the saturation performance for WURST‐80 pulses with different sweep widths applied to 27 Al (I=5/2) using spin‐density matrix analysis (SIMPSON, [72] see Supporting Information for further details), not considering spin‐lattice relaxation effects.…”

Frequency-Swept Ultra-Wideline Magic-Angle Spinning NMR Spectroscopy

“…The extent of the rf-amplitude truncation and the roundness of the amplitude profile are controlled by shape parameter N . Typical amplitude profiles and corresponding shape parameters, indicated as WURST-N, can be found elsewhere. ,, The velocity of the frequency sweep, referred to as the sweep rate, is defined as R = Δ/τ rf in s –2 and is known to determine the optimum WURST amplitude for WCPMG acquisition under static conditions according to where I denotes the spin-quantum number and c is constant offset parameter (in Hz), which is determined by the exact experimental conditions …”

“…eq ), as it is the case here, this is a short interval about the midpoint of irradiation, , where the maximum rf-amplitude ν rf,max is reached regardless of the choice for the shape parameter N ≥ 2 (again see Figure S2 in the Supporting Information). The possibility of incorporating small WURST shape parameters proves extremely beneficial when applying optimum rf-amplitudes ν rf,max opt,MAS for WURST pulses with large sweep rates, according to eq .…”

Pushing the Detection Limit of Static Wideline NMR Spectroscopy Using Ultrafast Frequency-Swept Pulses

“…We have recently discussed the properties of these different amplitude profiles when employed in constantfrequency rf-pulses. 36 During irradiation, the rf-carrier Δν rf (t) of a WURST pulse is swept linearly over the frequency interval Δ, referred to as the sweep width. For a symmetric rf-carrier profile, i.e., Δν rf (t) = Δν rf (τ rf − t), the corresponding rf-phase can be written as…”

“…Clearly, this in turn compromises the available bandwidth for uniform excitation; for N = 10, this is reduced to about half of its nominal value Δ. On the other hand, the more pronounced amplitude truncation provides additional advantages for the experimental application of UF WURST pulses, since the corresponding, smoother amplitude profiles allow the safer application of high peak rf-amplitudes 36 required for optimum excitation conditions. Furthermore, the ringdown voltages are significantly reduced, avoiding pulse artifacts in the acquisition window, which might be particularly advantageous when investigating low-γ nuclei or performing experiments at low temperatures.…”