Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy

Abstract

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including (119)Sn, (199)Hg and (195)Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr-Purcell-Meiboom-Gill experiment (for instance with WURST-CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.

Ultrastabilization of Zeolite Y Transforms Brønsted–Brønsted Acid Pairs into Brønsted–Lewis Acid Pairs

“…[11] This steaming process deliberately removes ap ortion of Al from the framework, creating extra-framework aluminum (EFAL) species,w hich are typically LAS.R emoval of Al from the zeolite framework stabilizes the remaining framework structure,h ence the resulting zeolite is called ultrastabilized Y( USY). [13a] Herein we provide evidence for the transformation of BAS-BAS pairs into BAS-LAS pairs,e mploying 1 Hd oublequantum (DQ) MAS NMR spectroscopy [14] and novel 27 Al wideline NMR excitation methods,that is, 27 Al{ 1 H} WURST-CPMG [15] (WCPMG) and 27 Al{ 1 H} BRAIN-CP/WCPMG NMR [16] (BRAIN-CP). [12b,c, 13] Based on early catalytic tests, am odel was proposed in which enhanced activity occurs if one of the Al atoms of aB AS pair is removed from the framework (forming aL AS), thereby withdrawing electron density by interacting with the remaining BAS.…”

Natural abundance 14N and 15N solid-state NMR of pharmaceuticals and their polymorphs

“…The BRAIN-CP (broadband adiabatic 6 inversion CP) pulse sequence is utilized to enable polarization transfer over a wide frequency range (an order of magnitude larger than conventional CP experiments). 109 A major advantage of this sequence is the ability to perform efficient CP at low radio frequency (rf) power levels, 109 In all cases, high-power proton decoupling was applied, with the decoupling RF fields between 30 and 82 kHz. Due to the limited excitation bandwidths associated with the WURST pulses, all spectra were acquired using the previously mentioned VOCS method, 98,99 with frequency increments equal to an integer multiple of the spikelet spacings arising from the CPMG portion of the pulse sequence.…”

Broadband MAS NMR spectroscopy in the low-power limit

“…Many pulse shapes have been introduced to solve this problem, such as the hyperbolic secant pulse [9], the Wide, UniformRate Smooth-Truncation (WURST) pulse [10], and the tanh/tan pulse [11]. These broadband pulses have many uses in a variety of magnetic resonance applications and in the case of solid-state studies, WURST pulses have been used to great effect in ultra-wideline spectroscopy [12] of static powders [13,14,15], and have also been used in broadband adiabatic inversion cross polarization (BRAIN-CP) experiments of static powders [16,17].…”