14N overtone NMR spectra under magic angle spinning: experiments and numerically exact simulation

It was recently shown that high resolution 14N overtone NMR spectra can be obtained directly under magic angle spinning (MAS) conditions [L. A. O\u2019Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)]10.1016/j.cplett.2011.08.030. Preliminary experimental results showed narrowed powder pattern widths, a frequency shift that is dependent on the MAS rate, and an apparent absence of spinning sidebands, observations which appeared to be inconsistent with previous theoretical treatments. Herein, we reproduce these effects using numerically exact simulations that take into account the full nuclear spin Hamiltonian. Under sample spinning, the 14N overtone signal is split into five (0, \ub11, \ub12) overtone sidebands separated by the spinning frequency. For a powder sample spinning at the magic angle, the +2\u3c9r sideband is dominant while the others show significantly lower signal intensities. The resultant MAS powder patterns show characteristic quadrupolar lineshapes from which the 14N quadrupolar parameters and isotropic chemical shift can be determined. Spinning the sample at other angles is shown to alter both the shapes and relative intensities of the five overtone sidebands, with MAS providing the benefit of averaging dipolar couplings and shielding anisotropy. To demonstrate the advantages of this experimental approach, we present the 14N overtone MAS spectrum obtained from L-histidine, in which powder patterns from all three nitrogen sites are clearly resolved.Peer reviewed: YesNRC publication: Ye

A multinuclear NMR and quantum chemical study of solid trimethylammonium chloride

The solid salt, trimethylammonium chloride (TMAC), is investigated by a combination of NMR spectroscopic techniques and quantum chemical calculations. Chemical shift and nuclear quadrupolar interaction parameters have been measured for 35Cl, 1H/2H, and 15N/14N. These parameters have also been calculated as a function of the hydrogen position in the N\u2022\u2022\u2022H\u2022\u2022\u2022Cl fragment. Overall, the measured parameters are consistent with a structure in which the hydrogen is completely transferred to the nitrogen (i.e., N\u2013H\u2022\u2022\u2022Cl). The high hydrogen chemical shift (10.9 ppm by 2H CP/MAS) and relatively small deuterium quadrupolar coupling constant (127 kHz) indicate a moderately strong N\u2013H\u2022\u2022\u2022Cl hydrogen bond. A pronounced deuterium isotope effect on the 35Cl quadrupolar coupling constant is observed.Peer reviewed: YesNRC publication: Ye

Crystal structure based design of signal enhancement schemes for solid-state NMR of insensitive half-integer quadrupolar nuclei

A combination of density functional and optimal control theory has been used to generate amplitude- and phase-modulated excitation pulses tailored specifically for the 33S nuclei in taurine, based on one of several reported crystal structures. The pulses resulted in significant signal enhancement (stemming from population transfer from the satellite transitions) without the need for any experimental optimization. This allowed an accurate determination of the 33S NMR interaction parameters at natural abundance and at a moderate magnetic field strength (11.7 T). The 33SNMRparameters, along with those measured from 14Nusing frequency-swept pulses, were then used to assess the accuracy of various proposed crystal structures.Peer reviewed: YesNRC publication: Ye