Ultra-Wideline Solid-State NMR Spectroscopy

Abstract

Although solid-state NMR (SSNMR) provides rich information about molecular structure and dynamics, the small spin population differences between pairs of spin states that give rise to NMR transitions make it an inherently insensitive spectroscopic technique in terms of signal acquisition. Scientists have continuously addressed this issue via improvements in NMR hardware and probes, increases in the strength of the magnetic field, and the development of innovative pulse sequences and acquisition methodologies. As a result, researchers can now study NMR-active nuclides previously thought to be unobservable or too unreceptive for routine examination via SSNMR. Several factors can make it extremely challenging to detect signal or acquire spectra using SSNMR: (i) low gyromagnetic ratios (i.e., low Larmor frequencies), (ii) low natural abundances or dilution of the nuclide of interest (e.g., metal nuclides in proteins or in organometallic catalysts supported on silica), (iii) inconvenient relaxation characteristics (e.g., very long longitudinal or very short transverse relaxation times), and/or (iv) extremely broad powder patterns arising from large anisotropic NMR interactions. Our research group has been particularly interested in efficient acquisition of broad NMR powder patterns for a variety of spin-1/2 and quadrupolar (spin > 1/2) nuclides. Traditionally, researchers have used the term "wideline" NMR to refer to experiments yielding broad (1)H and (2)H SSNMR spectra ranging from tens of kHz to ∼250 kHz in breadth. With modern FT NMR hardware, uniform excitation in these spectral ranges is relatively easy, allowing for the acquisition of high quality spectra. However, spectra that range in breadth from ca. 250 kHz to tens of MHz cannot be uniformly excited with conventional, high-power rectangular pulses. Rather, researchers must apply special methodologies to acquire such spectra, which have inherently low S/N because the signal intensity is spread across such large spectral breadths. We have suggested the term ultra-wideline NMR (UWNMR) spectroscopy to describe this set of methodologies. This Account describes recent developments in pulse sequences and strategies for the efficient acquisition of UWNMR spectra. After an introduction to anisotropically broadened NMR patterns, we give a brief history of methods used to acquire UWNMR spectra. We then discuss new acquisition methodologies, including the acquisition of CPMG echo trains and the application of pulses capable of broadband excitation and refocusing. Finally, we present several applications of UWNMR methods that use these broadband pulses.

DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal–Organic Frameworks

“…11 In addition, uniform excitation of such broad spectra cannot be achieved at any single transmitter offset. 12,13 It is necessary to perform a piecewise acquisition of ultrawideline spectra using the variable offset cumulative spectrum (VOCS) acquisition method. 12 Of course, the broad NMR line widths exacerbate the sensitivity problems, particularly in the case of dilute surfacebound species.…”

Biosilica and bioinspired silica studied by solid-state NMR

“…Along these lines there goes the development of frequency-stepped methods, which allow for the coverage of large quadrupolar [83] or paramagnetic patterns [84]. Several strategies [85] have been proposed to improve experimental sensitivity in NMR.…”

Spin–Spin Coupling between Quadrupolar Nuclei in Solids: 11B–75As Spin Pairs in Lewis Acid–Base Adducts

“…14 Table 1 provides a list of 75 As and 11 B NMR properties relevant to this study. Arsenic-75 has a moderate nuclear electric quadrupole moment and a fairly large Sternheimer antishielding factor (γ ∞ = −7.09) 15 which leads to generally large nuclear quadrupolar coupling constants (e.g., values on the order of 100 MHz or larger are common).…”

“…24,27−43 In solids, C Q ( 75 As) values are mostly known from nuclear quadrupole resonance (NQR) 20−24 and microwave spectroscopy experiments. 16−19 Note that information obtained from 75 As NQR studies of nonsymmetric molecules in powdered samples is limited, as C Q ( 75 As) and the quadrupolar asymmetry parameter, η( 75 As), are difficult to determine independently from experiment due to 75 As possessing a nuclear spin number of I = 3/2, giving the nucleus only one NQR transition (m I = ± 1/2 ↔ m I = ± 3/2) dependent on both parameters in zero-field conditions. 44 There are, however, techniques available to get around this limitation.…”