High-resolution 14N NMR in polycrystalline solids

14N overtone nuclear magnetic resonance of rotating solids

By irradiating and observing at twice the 14N Larmor frequency, overtone (OT) nuclear magnetic resonance (NMR) is capable of obtaining 14NOT spectra without first-order quadrupolar broadening. Direct excitation and detection of the usually "forbidden" double-quantum transition is mediated by the perturbation from the large quadrupole interaction to the spin states quantized by the Zeeman interaction. A recent study [L. A. O'Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)] has shown that 14NOT NMR under magic-angle spinning (MAS) can yield high-resolution spectra with typical second-order quadrupolar line shapes allowing the measurement of 14N chemical shift and quadrupolar coupling parameters. This article has also shown that under MAS the main 14NOT peak is shifted by twice the sample spinning frequency with respect to its static position. We present the theory of 14NOT NMR of static or rotating samples and the physical picture of the intriguing spinning-induced shift in the second case. We use perturbation theory for the case of static samples and Floquet theory for rotating samples. In both cases, the results can be described by a so-called OT parameter that scales down the 14NOT radio-frequency (rf) excitation and signal detection. This OT parameter shows that the components of the rf field, which are transverse and longitudinal with respect to the magnetic field, are both effective for 14NOTrf excitation and signal detection. In the case of MAS at angular frequency ωr , the superposition of the excitation and detection components in the OT parameter makes either the +2ωr or -2ωr term the dominant 14NOT signal, depending on the sense of sample spinning with respect to the magnetic field. This leads to an apparent 14NOT signal shifted at twice the spinning frequency. The features of 14NOT NMR spectra for both static and rotating samples are illustrated with simulations. The spinning induced shift and its dependence on the spinning direction are confirmed experimentally by reversing the spinning direction and the field of the 36 T series-connected hybrid magnet at the US National High Magnetic Field Laboratory.

Proton-detected 3D (14)N/(14)N/(1)H isotropic shift correlation experiment mediated through (1)H-(1)H RFDR mixing on a natural abundant sample under ultrafast MAS

In this contribution, we have demonstrated a proton detection-based approach on a natural abundant powdered l-Histidine HCl-H2O sample at ultrafast magic angle spinning (MAS) to accomplish (14)N/(14)N correlation from a 3D (14)N/(14)N/(1)H isotropic shift correlation experiment mediated through (1)H finite-pulse radio frequency-driven recoupling (fp-RFDR). Herein the heteronuclear magnetization transfer between (14)N and (1)H has been achieved by HMQC experiment, whereas (14)N/(14)N correlation is attained through enhanced (1)H-(1)H spin diffusion process due to (1)H-(1)H dipolar recoupling during the RFDR mixing. While the use of ultrafast MAS (90kHz) provides sensitivity enhancement through increased (1)H transverse relaxation time (T2), the use of micro-coil probe which can withstand strong (14)N radio frequency (RF) fields further improves the sensitivity per unit sample volume.

Indirect Detection of Nitrogen-14 in Solid-State NMR Spectroscopy

NMR spectra of (14)N (spin I=1) are obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring "spy" nucleus with S=1/2, such as (13)C or (1)H, to single- or double-quantum transitions of (14)N nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter are also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolar coupling constant C(Q) (typically a few megahertz), on the asymmetry parameter eta(Q) of the (14)N nucleus, and on the orientation of the internuclear vector r(IS) between the I ((14)N) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional averaging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in (13)C or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.

A solid-state 14N magic-angle spinning NMR study of some amino acids

Experimental strategies for the acquisition of high-quality 14N magic-angle spinning (MAS) NMR spectra of the simple amino acids, which exhibit 14N quadrupole coupling constants (C(Q)) on the order of 1.2 MHz, are devised. These are the first useful 14N MAS spectra reported for nitrogen compounds having a C(Q)(14N) value in excess of 1 MHz. The complete manifolds of spinning sidebands (ssbs), i.e., about 300 ssbs for a spinning frequency of 6.0 kHz, have been observed in the 14N MAS NMR spectra of a series of amino acids. In their crystal structure these amino acids all exhibit the zwitterionic form and thus the 14N MAS NMR spectra represent those of a rotating -NH(3)(+) group and not of an amino (-NH(2)) group. Computer simulations combined with fitting of simulated to the experimental ssb intensities result in the determination of precise values for the 14N quadrupole coupling (C(Q)) and its associated asymmetry parameter (eta(Q)) for the nitrogen sites in these molecules. For some of the amino acids the 14N MAS NMR spectra exhibit overlap between the manifolds of ssbs from two different nitrogen sites in accordance with their crystal structures. Computer analysis of these spectra results in two different sets of (C(Q), eta(Q)) values which mainly differ in the magnitudes for eta(Q).

A simple method for the preparation of pseudopure states in nuclear magnetic resonance quantum information processing

The use of nuclear magnetic resonance (NMR) to carry out quantum information processing (QIP) often requires the preparation, transformation, and detection of pseudopure states. In our previous work, it was shown that the use of pairs of pseudopure states (POPS) as a basis for QIP is very convenient because of the simplicity in experimental execution. It is now further demonstrated that the product of the NMR spectra corresponding to two sets of POPS that share a common pseudopure state has the same peak frequencies as those of the common (single) pseudopure state. Examples of applying two different quantum logic gates to a 5-qubit system are given.

14N chemical shifts and quadrupole coupling constants of inorganic nitrates

The isotropic chemical shift and the nuclear quadrupole coupling constant for (14)N were obtained for 14 inorganic nitrates by solid-state MAS NMR measurements at two different field strengths, 9.4 and 11.7 T. The compounds studied were polycrystalline powders of AgNO(3), Al(NO(3))(3), Ba(NO(3))(2), Ca(NO(3))(2), CsNO(3), KNO(3), LiNO(3), Mg(NO(3))(2), NaNO(3), Pb(NO(3))(2), RbNO(3), Sr(NO(3))(2), Th(NO(3))(4)center dot4H(2)O, and UO(2)(NO(3))(2)center dot3H(2)O. Even though the spectra show broadening due to (14)N quadrupole interactions, linewidths of a few hundred hertz and a good signal-to-noise ratio were achieved. From the position of the central peaks at the two fields, the chemical shifts and the nuclear quadrupole coupling constants were calculated. The chemical shifts for all compounds studied range from 282 to 342 ppm with respect to NH(4)Cl. The nuclear quadrupole coupling constants range from 429 kHz for AgNO(3) to 993 kHz for LiNO(3). These data are compared with those available in the literature.

Filtering of spinning sidebands in 1D MAS NMR spectra

Spinning sidebands (SSBs) in the MAS NMR spectrum of a polycrystalline solid are related to the principal values of the chemical shift or quadrupole coupling tensors. At present, 2D methods are widely used to sort out the SSBs for each isotropic peak. Here a simple and efficient method for separating the SSBs in 1D MAS NMR spectra is described. It is based on finding the optimal spinning rate with a mathematical algorithm and subsequently treating the spectra with filtering functions.

Spin-1/2 and beyond: a perspective in solid state NMR spectroscopy

Novel applications of solid state nuclear magnetic resonance (NMR) to the study of small molecules, synthetic polymers, biological systems, and inorganic materials continue at an accelerated rate. Instrumental to this uninterrupted expansion has been an improved understanding of the chemical physics underlying NMR. Such deeper understanding has led to novel forms of controlling the various components that make up the spin interactions, which have in turn redefined the analytical capabilities of solid state NMR measurements. This review presents a perspective on the basic phenomena and manipulations that have made this progress possible and describes the new opportunities and challenges that are being opened in the realms of spin-1/2 and quadrupole nuclei spectroscopies.