Minimizing Lineshape Distortions in Static Ultra-wideline Nuclear Magnetic Resonance of Half-Integer Spin Quadrupolar Nuclei

Abstract

Chirped excitation using frequency-swept wideline uniform rate smooth truncation (WURST) pulses in combination with Carr-Purcell-Meiboom-Gill acquisition (WCPMG) is currently the state-of-the-art method for the direct observation of the central transition (CT) in static ultra-wideline nuclear magnetic resonance (NMR) of half-integer spin quadrupolar nuclei. However, CT lineshape distortions and an inefficient, large number of transmitter steps in frequency-stepped acquisition are two major drawbacks. Here, we identify three main sources for lineshape distortions occurring in WCPMG NMR spectra of the CT: (I) distortions due to inaccurate setting of the radio frequency field strength, (II) chirped-excitation artifacts, and (III) distortions due to non-selective irradiation. A new and efficient approach for the acquisition minimizing these distortions is presented using low sweep rates (R ≤ 5 kHz/μs) and sweep widths (Δ ≤ 600 kHz). We further demonstrate that such an acquisition strategy also minimizes the number of transmitter steps in ultra-wideline NMR. This is achieved from numerical simulations and theoretical analysis of the orientational dependence for the quadrupolar-perturbed Zeeman states and their transition frequencies. The theoretically derived strategies are validated experimentally, allowing us to set up guidelines for the optimum recording of wideline and ultra-wideline WCPMG NMR spectra.

Field‐stepped ultra‐wideline NMR at up to 36 T: On the inequivalence between field and frequency stepping

“…A numerical simulation in SIMPSON [46] was conducted using WURST/CPMG pulse sequence parameters that are identical to those used for the experimental acquisition (Figure 4c), in order to model the overlap between the 35 Cl CT and ST patterns for the β EFG ≈ 54.74 isochromats. The full quantum mechanical simulation shows a depletion of signal intensity that matches well with what is observed experimentally and agrees with previous observations for CT powder patterns acquired with WURST/CPMG [50,51]. The 79 Br powder pattern in Figure 3b was also simulated by including a 81 Br ST pattern (Figure 4e), revealing a similarly good match with the experimental feature at approximately +1.6 MHz.…”

“…This originates from a subset of isochromats associated with both the CT and ST patterns of the same nucleus with similar resonance frequencies that are within the WURST sweep width. For these isochromats, the rf irradiation is no longer selective for the CT, causing not only coherence transfer and leakage from the CT to other transitions but also changes in the effective rf nutation frequency of the WURST pulses, both of which contribute to signal depletion [50]. This artifact is more pronounced when the EFG tensor is close to axial symmetry with its largest component at or near the magic angle (i.e., the angle between the largest component of the EFG tensor V 33 and B 0 is β EFG ≈ 54.74 ), similar to the "magic-angle hole" effect in crosspolarization spectra.…”

Broadband adiabatic inversion cross‐polarization to integer‐spin nuclei with application to deuterium NMR

“…When the two spectra acquired with LtoH and HtoL frequency sweeps are overlayed (Figure 3c), which is typically referred to as a "skyline" projection, [44] there is a relatively good match with the ideal pattern shape (Figure 3d). However, the relative intensity of the highfrequency half of the powder pattern is greater than the low-frequency half.…”

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

“…Notably, when sweeping from low (high) to high (low) frequencies, Δ is positive (negative). 33 The linear frequency sweep is typically symmetric about the rf-carrier frequency ν 0 (likewise in Hz) and implemented by a quadratic phase profile (given in radians)…”

“…where I denotes the spin-quantum number and c is constant offset parameter (in Hz), which is determined by the exact experimental conditions. 33 Rotating Powder and Modulations by MAS. In solid materials, the magnetic shielding of a nucleus is typically orientational dependent due to the anisotropic shielding caused by the surrounding electrons.…”