Dipolar heteronuclear

W. Sommer, J. Gottwald, D.E. Demco, H.W. Spiess, Dipolar heteronuclear multiple-quantum NMR spectroscopy in rotating sohds, J. Magn. Reson. A 113 (1995) 131-134. 

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the crosspolarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to Cq values and... 

A novel crystalline form of the boron-containing antibacterial drug (5 )-3-(aminomethyl)-7-(3-hydroxypropoxy)benzo[c][l,2]oxaborol-l(3H)-ol hydrochloride has been studied by solid-state NMR and single-crystal X-ray diffraction techniques. After determination of the crystal structure by X-ray diffraction, solid-state NMR spectroscopy of this form is performed to obtain structural information using experimental approaches based on dipolar correlation, chemical shift analysis, and quadrupolar interaction analysis. solid-state NMR experiments at 16.4 T using MAS and homonuclear dipolar decoupling, 2D solid-state NMR experiments based on and dipolar heteronuclear correlation, and DFT... 

While the combination of the heteronuclear dipolar decoupling and MAS provides a mean to obtain high-resolution isotropic spectra in solids, the serious problem still remains in addition to the relatively small magnetic moment and low natural... 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Heteronuclear and homonuclear dipolar recoupling under magic-angle spinning... 

For the dilute aH spins surrounded by a large number of the 2H spins, the heteronuclear aH-H dipolar interaction is dominant, lifting the spectral overlap between the 1H packets. H spin diffusion is driven by the flip-flop term of the 1H-1H dipolar interaction, which becomes secular in the presence of spectral overlap. Thus, spin diffusion would be accelerated if it had not been for the 1H-2H dipolar interaction. In order to confirm this prediction, they used another RF channel of the OPENCORE... 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The REDOR experiment has formed the basis for a large number of ideal pulse type recoupling experiments, and later finite pulse variants, for heteronuclear dipolar recoupling. These include experiments such as frequency selective REDOR (FS-REDOR) [80], TEDOR (Transferred Echo DOuble Resonance) [25], and 3D variants of TEDOR [81, 82], which have found important applications, e.g., for measurement of intemuclear 13C-15N distances in biological solids. We should also mention that rotor-encoded variants of TEDOR, such as REPT, HDOR [83], and REREDOR [84], have been proposed for 1H13C dipolar recoupling under high-speed MAS conditions. 

Dipolar recoupling may also be accomplished using continuous rf irradiation as demonstrated in the heteronuclear and homonuclear case by the CP at MAS conditions (or for low-y heteronuclear spins called double-cross-polarization, DCP [103]) and HORROR (homonuclear rotary resonance) [26] experiments, respectively. These experiments may easily be described by transforming the description into the interaction frame of the rf irradiation using (14a) exploiting... 

Addressing first the heteronuclear case (i.e., the DCP experiment introduced by Schaefer and coworkers in 1984), the dipolar coupling may be expressed as in (10) that upon heteronuclear truncation of the transverse terms may be recast in irreducible tensor operator form (7Z = T[ 0, Sz = Tf 0) as... 

The PAR experiment may be described by considering two heteronuclear dipolar coupling interactions with one proton (7) in common, i.e., a Hamiltonian of the type... 

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