Heteronuclear dipolar coupling constant

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. 

The technique mainly applied on isotopic labeled compounds has been used in many different cases. By fitting the experimental dephasing curve of a sample of C- N labeled (10%) glycine a C- N dipolar coupling constant of 195 Hz was obtained that corresponds to a C- N internuclear distance of 2.47 A. The limit value of the heteronuclear dipolar coupling constant is around 25 Hz, then useful structure information can be obtained at distances that cannot exceed 5 A in the case of C- N and 7.5 A in the case of C- P distances. 

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

Key Words Heteronuclear dipolar couplings, REDOR spectroscopy, Structure, Second moments. Constant time, SIMPSON... 

As shown in the preceding two sections, the constant time version of REDOR, CT-REDOR, may be applied as an expedient alternative to the existing REDOR versions in the presence of strong heteronuclear dipolar couplings. In these cases, only few data points are available for the data analysis, which especially in the case of multiple-spin systems renders an evaluation of the second moments impossible. The efficiency of the dipolar recoupling may be intentionally reduced either via a dislocation of the dephasing Ti-pulses from the centre of the rotor period or via an application of non-Ti-dephasing pulses. A variation of the pulse position fpp... 

By monitoring the H MAS spectra, it has been suggested that the loss of water from bone tissue is an irreversible process under air drying conditions.38 It has been shown that CP transfer within an isolated heteronuclear spin pair will exhibit a characteristic oscillation, from which the dipolar coupling constant can be accurately determined.197 The technique of LG-CPMAS can probe this oscillation pattern due to a dominating 31P-1H interaction in a dipolar coupled network, if any. However, such oscillation is seldom observed in 31P 1H CPMAS measurements of biominerals because of the damping effect of the homonu-clear dipole-dipole interaction. Thus, it is rather remarkable that the buildup curve of 31P 1H LG-CPMAS obtained for a sample of powdered femoral cortical bone, which is partially hydrated, shows a distinctive... 

D is ealled the dipolar coupling constant and resonance frequency of the observed nuclei. The heteronuclear dipolar Hamiltonian can be expressed as... 

T.N. Pham, T. Liptaj, K. Bromek, D. Uhrin, Measmement of small one-bond proton-carbon residual dipolar coupling constants in partially oriented C natmal abundance oligosaccharide samples. Analysis of heteronuclear JcH-modulated spectra with the BIRD inversion pulse, J. Magn. Reson. 157 (2002) 200—209. 

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