Scalar interactions with protons

IX. SCALAR INTERACTIONS WITH PROTONS In the earlier Sections we indicated that most measurements of the proton electron Overhauser effect that have been reported may be interpreted in terms of dipolar interactions between the proton and the free-radical electrons the proton polarization is inverted. On the other hand, the results with nuclei show that there exists a dominant scalar interaction that often results in positive enhancement. 

Fig. 6.6 Schematics of hydrogen bonds between purine and pyrimidine bases with indicated trans-hydrogen bond scalar interactions and related coupling constants, which can be measured using NMR. In addition to correlations between exchangeable protons and nitrogens, also a relayed transfer to nonexchangeable aromatic protons, shown by a dashed arrow, can be employed. The...

D spectra are in principle possible for heteronuclei coupled by either dipolar or scalar interactions. However, the magnetic moments of heteronuclei are sizably smaller than that of the proton, and since cross relaxation depends on the square of the magnetic moment it appears that this is a serious limitation for the observation of NOESY or ROESY cross peaks. However, as already discussed, in scalar-coupled systems the relevant coherences build up with sin(nJ/jt). Since Jjj in directly bound 13C- H and l5N- H moieties is of the order of 102 Hz, as opposed to about 10 Hz between proton pairs, it is conceivable that scalar correlation experiments are successful. Heterocorrelated spectra have the advantage of allowing one to detect signals of protons attached to carbons or nitrogens when they are within a crowded envelope. 

In this section, we will only discuss a specific system the enhancement of proton spin relaxation in aqueous solutions of paramagnetic ions, which to our knowledge is the only case of paramagnetic relaxation, which has been studied with MD simulations. Just as in the description of dipole-dipole relaxation, we will only treat the through space dipole-dipole interaction, and not the scalar interaction. This is experimentally well motivated in the case of Ni ions [23]. 

In contrast to NOEs, scalar coupling relies on localized direct interaction. They have therefore potentiality to define precise local structure information. They can also be used to obtain stereospecific assignments (e.g., for C J1 methylene protons) in combination with intra-residual and sequential NOEs.72... 

Systems containing both proton and fluorine nuclei provide a good test of theory. If the proton data are interpreted in terms of pure dipolar coupling, then a diffusion coefficient can be calculated for the protons. This may be then compared with that obtained from the fluorine data, when scalar interaction is present. As both nuclei are on the same molecule the results should be identical. Unfortunately, diffusion coefficients have only been calculated on a few occasions, but this is a point to be borne in mind for future testing of theories. ... 

There are a couple of special methods of separating the contribution of dipolar relaxation in solution. One is by the NOE factor which is the fractional difference in the signal intensity of one spin with and without irradiation applied to another spin system. For a sample containing protons and carbon-13 in the motionally narrowed limit, this factor should be 2 if the relaxation takes place through the dipolar and the scalar interactions. Thus, the departure from 2 of the NOE factor is an indication of other relaxation mechanisms. Clearly, any other pairs of spin systems with NOE s can be treated this way, with appropriate limiting NOE factors. See, for example, Noggle and Shirmer listed in Appendix A for more details. 

Quinting and Cai [62] carried out high-resolution C-NMR and proton NMR measurements to determine the tacticity of poly(n-butyl methacrylate) (PBMA) with particular focus on the peak assignments for the n-butyl side chain. Free-radical and anionic PBMA were examined, with the former being predominantly syndiotactic and the latter isotactic. Proton NMR resonances for the n-alkyl chain of these polyacrylics show a combination of effects from configurational sensitivity and homonuclear scalar interactions. A combination of J-resolved proton NMR and proton- C-heteronuclear correlated 2D-NMR spectra was used to characterise the long-range chemical shift effects due to tacticity. 

NMR can, in principle, provide complementary information on motional processes in liquid crystals. The dipole-dipole interaction between a C-H pair and the quadrupolar interaction when the proton is replaced with a deuteron share the same principal interaction axis. In the case where the carbon is not directly bonded to a proton, there is still dipole-dipole relaxation by nearby protons, but it is also necessary to include an additional relaxation mechanism, the modulation of the chemical shift anisotropy. Proton spin decoupling is necessary to give well-resolved chemically shifted lines in the mesophase of liquid crystals. Furthermore, it is not practical to determine individual spectral density parameters from measured relaxation rates. Proton-proton dipolar interactions may not be ignored even when observation is exclusively confined to the resonant spin [5.31]. This is because proton relaxation causes population flow among the proton spin levels through dipolar (or scalar) coupling. As a consequence, cross-... 

Widmalm in a short review based mostly on the results of his laboratory has presented a perspective on structures of carbohydrates. He has underlined the central position of the solution state NMR spectroscopy in these studies starting with analysis of the primary structure of glycans (components and sequence), followed by conformational and dynamics analysis to the study of interaction with proteins. He has also anticipated that among other NMR parameters DFT calculated vicinal proton-proton, proton-carbon and carbon-carbon scalar couplings will aid these investigations. 

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