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

Overtone N-14 NMR spectroscopy is a promising route for the direct detection of N-14 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 H-1 to the N-14 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 N-14 overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker-Planck equations.