Scalar Coupling

From NMR Wiki

1 Introduction
2 Uses of scalar coupling
3 Related articles
4 Dependence of scalar coupling on molecular conformation
5 References

Introduction

Scalar coupling is the isotropic part (independent on the molecular orientation) of the J coupling. The J coupling [1] arises due to indirect interaction between the two nuclear spins, where the interaction is mediated by the electrons participating in the bond(s) connecting the nuclei. Magnitude of J coupling is typically larger when nuclei are separated by smaller number of bonds. Fast molecular tumbling in fluids averages some of the J coupling, but some observable coupling remains. From the physical point of view, J coupling is proportional to the product of gyromagnetic ratios of the isotopes involved.

Terms "scalar coupling" and J coupling are frequently used as synonyms, however in principle J coupling is anisotropic while scalar coupling is the isotropic component of the former. In practice, anisotropy of J coupling is rarely observed since it is only possible in aligned media and it can be hard to distinguish from the much stronger dipolar coupling component.

When rare isotopes (such as ¹³C at natural abundance) couple with an abundant nucleus (e.g. proton), each dominant peak in the spectrum of the abundant nucleus will be symmetrically surrounded by two small peaks arising due to the coupling with the rare isotope. Those peaks are called satellite signals.

J-coupling is usually quite easy to suppress, by decoupling, on the heteronucleus, or even by applying decoupling on the same kind of nucleus, homonuclear decoupling, this could help to make spectra simpler. For instance ¹³C spectra are most frequently recorded with ¹H decoupling. The RF power necessary for suppression of J coupling is lower in comparison to that used in the case of dipolar coupling.

In liquid state NMR most magnetization (or coherence) transfer is done through the J-coupling. Experiments like COSY, TOCSY, HSQC, HMQC, etc. rely on this coupling.

Sometimes scalar coupling cannot not be observed when one of the nuclei has very fast relaxation. For example couplings of ¹H atoms to ¹⁴N in amide groups is not observable because of the fast ¹⁴N relaxation.

Uses of scalar coupling

Since scalar coupling is mediated by chemical bonds, its observation greatly aids assignment of molecular fragments, further dependence of scalar coupling on molecular conformation helps of conformation and relative stereochemistry.

J coupling is heavily used in the liquid state NMR, particularly for transfer of magnetization in the multidimensional experiments such as COSY, TOCSY, HSQC, etc. For that purpose a delay for evolution of the J-coupling is introduced in the pulse sequence.

Dependence of scalar coupling on molecular conformation

The fact that magnitude of scalar coupling does not depend on the molecular orientation does not contradict another fact that scalar coupling does depend on molecular geometry and in particular - molecular conformation.

Aliphatic ³J_HH vicinal coupling parametrization is described in the paper by Haasnoot et al [2]. Parametrization takes into account electronegativities of groups attached to carbon atoms, H-C-C-H torsion angle and stereospecific orientations of substituents relative to protons. This is the best parametrization of vicinal ¹H-¹H scalar coupling so far (as of year 2008).