New methods for nitrogen-14 solid state NMR

Abstract

Nitrogen is one of the most abundant elements. It plays a significant role in different scientific disciplines, like materials science, chemistry, biochemistry and pharmaceutical chemistry. Most NMR studies of nitrogen have been performed on samples enriched with 15N. To avoid enrichment and overcome the problem of using NMR where isotopic enrichment is not feasible, NMR of 14N that has 99.6% natural abundance is a viable alternative. However, the 14N spin quantum number 1 and high quadrupolar interactions lead to a few challenges for NMR investigation. Recently, two approaches appeared which overcome the complications associated to the first order quadrupolar line broadening, namely indirect detection using vicinal nuclei and overtone transition. Paper 1 describes a novel version of HMQC experiments, which requires the application of a moderate RF field to the 14N nucleus to correlate the 14N with a 13C ‘spy’ nucleus, which provides good sensitivity on natural abundance and labelled materials. In addition, overtone 14N NMR spectroscopy is a promising route for the direct detection of 14N signals with good spectral resolution. Overtone NMR is the main topic of my research. To overcome the poor efficiency of the overtone transition, polarization transfer techniques have been implemented. Papers 2 and 4 present respectively polarization transfer methods from 1H to the 14N overtone using symmetry-based R-sequences and cross polarization methods. Signal enhancements of 6.7 and 2 have been obtained when cross polarization methods and symmetry-based R-sequences are used respectively. Paper 3 shows results obtained with the DOR (double rotation) technique to reduce 14N overtone linewidth, with 14N overtone spectra with linewidth reduced by nearly one order of magnitude. Analysis of the results has been facilitated through the use of a new simulation strategy for 14N overtone NMR spectroscopy of spinning samples, using Spinach library.

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Identifiers

ORCID for Marina Carravetta: orcid.org/0000-0002-6296-2104

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