Dipolar interactions measurement

Thus, identification of all pairwise, interproton relaxation-contribution terms, py (in s ), for a molecule by factorization from the experimentally measured / , values can provide a unique method for calculating interproton distances, which are readily related to molecular structure and conformation. When the concept of pairwise additivity of the relaxation contributions seems to break down, as with a complex molecule having many interconnecting, relaxation pathways, there are reliable separation techniques, such as deuterium substitution in key positions, and a combination of nonselective and selective relaxation-rates, that may be used to distinguish between pairwise, dipolar interactions. Moreover, with the development of the Fourier-transform technique, and the availability of highly sophisticated, n.m.r. spectrometers, it has become possible to measure, routinely, nonselective and selective relaxation-rates of any resonance that can be clearly resolved in a n.m.r. spectrum. 

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

Tessari, M., Vis, H., Boelens, R., Kaptein, R., Vuister, G. W. Quantitative measurement of relaxation interference effects between Hn CSA and H- N dipolar interaction correlation with secondary structure. J. Am. Chem. Soc. 1997, 119, 8985-8990. 

To measure distances in the wider temperature range, this procedure was modified. Relaxation of the carotenoid occurs through several different mechanisms including the dipolar-dipolar interaction. Assuming that kAA is the rate constant of the dipolar-dipolar interaction and K=(k,l + k2 + k3 +. ..) is the sum of the rate constants of all other relaxation pathways, we can extract kAA from the following equation ... 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

The measured spin relaxation parameters (longitudinal and transverse relaxation rates, Ri and P2> and heteronuclear steady-state NOE) are directly related to power spectral densities (SD). These spectral densities, J(w), are related via Fourier transformation with the corresponding correlation functions of reorientional motion. In the case of the backbone amide 15N nucleus, where the major sources of relaxation are dipolar interaction with directly bonded H and 15N CSA, the standard equations read [21] ... 

In addition, cross-correlation effects between 15N CSA and 1H-15N dipolar interaction could be measured, see e.g. Refs. [24-26] ... 

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