¹⁴N overtone transition in double rotation solid-state NMR

Solid-state NMR transitions involving outer energy levels of the spin-1 14N nucleus are immune, to first order in perturbation theory, to the broadening caused by the nuclear quadrupole interaction. The corresponding overtone spectra, when acquired in conjunction with magic-angle sample spinning, result in lines, which are just a few kHz wide, permitting the direct detection of nitrogen compounds without the need for labeling. Despite the success of this technique, “overtone” resonances are still broadened due to indirect, second order effects arising from the large quadrupolar interaction. Here we demonstrate that another order of magnitude in spectral resolution may be gained by using double rotation. This brings the width of the 14N solid-state NMR lines much closer to the region commonly associated with high-resolution solid-state NMR spectroscopy of 15N and demonstrates the improvements in resolution that may be possible through the development of pulsed methodologies to suppress these second order effects

14N overtone NMR under MAS: signal enhancement using symmetry-based sequences and novel simulation strategies

Overtone 14N NMR spectroscopy is a promising route for the direct detection of 14N signals with good spectral resolution. Its application is currently limited, however, by the absence of efficient polarization techniques for overtone signal enhancement and the lack of efficient numerical simulation techniques to aid in both the development of new methods and the analysis and interpretation of experimental data. In this paper we report a novel method for the transfer of polarization from 1H to the 14N overtone using symmetry-based R-sequences that overcome many of the limitations of adiabatic approaches that have worked successfully on static samples. Refinement of these sequences and the analysis of the resulting spectra have been facilitated through the development of an efficient simulation strategy for 14N overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker–Planck equation

14N overtone NMR under MAS: signal enhancement using cross-polarization methods

Polarization transfer methods are widely adopted for the purpose of correlating different nuclear species as well as to achieve signal enhancement. Polarization transfer from 1H to the 14N overtone transition (Δm=2) can be achieved using cross polarization methods under magic-angle spinning conditions, where spin locks of the order of several milliseconds can be obtained on common bio-solids (α-glycine and N-acetylvaline). Signal enhancement factors up to 4.4 per scan, can be achieved under favorable conditions, despite MHz-sized quadrupolar interaction. Moreover, we present a detailed theoretical treatment and accurate numerical simulations which are in excellent agreement the unusual experimental matching conditions observed for cross-polarization to 14N overtone

Expanding beyond the micropore: active-site engineering in hierarchical architectures for Beckmann rearrangement

The ability to engineer discrete solid-acid centers within hierarchically porous architectures that contain micropores with interconnected mesopores offers the potential to overcome the spatial restraints and diffusional limitations imposed by conventional microporous zeotype catalysts, which often lead to decreased catalyst lifetimes and restricted substrate scope. By employing a one-step soft-templating approach, coupled with detailed physicochemical and spectroscopic characterization, isolated, solid-acid sites can be suitably tailored and discretely modulated within the micropores and mesopores. The design strategy facilitates a synergistic enhancement in catalytic activity, selectivity, substrate versatility, and longevity, compared to analogous microporous zeotypes that have been extensively employed in the chemical industry, as solid-acid catalysts, for the production of ?-caprolactam (precursor for Nylon-6)

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Solid-state NMR transitions involving outer energy levels of the spin-1 14N nucleus are immune, to first order in perturbation theory, to the broadening caused by the nuclear quadrupole interaction. The corresponding overtone spectra, when acquired in conjunction with magic-angle sample spinning, result in lines, which are just a few kHz wide, permitting the direct detection of nitrogen compounds without the need for labeling. Despite the success of this technique, “overtone” resonances are still broadened due to indirect, second order effects arising from the large quadrupolar interaction. Here we demonstrate that another order of magnitude in spectral resolution may be gained by using double rotation. This brings the width of the 14N solid-state NMR lines much closer to the region commonly associated with high-resolution solid-state NMR spectroscopy of 15N and demonstrates the improvements in resolution that may be possible through the development of pulsed methodologies to suppress these second order effects