Through-space contributions

Originally the constant for the field/inductive effect was called the inductive constant ((jj). The field (through-space) contribution is, however, clearly larger than the inductive effect proper, i. e., the through-bond contribution. 

The unpaired electron(s) on the metal, Fe(III) in the case of the nitrophorins, act as beacons that illuminate the protons in the vicinity of the metal, by causing shifts (isotropic shifts) of the resonances from those observed in a diamagnetic protein, for example, the NO-bound forms of the nitrophorins. These shifts allow much to be learned about the intimate details of the electron configuration at the iron center. The two contributions to the isotropic shifts are the contact (through bonds) and dipolar or pseudocontact (through space) contributions, and these are discussed in considerable detail elsewhere 92-94). For the purposes of this chapter, a detailed physical treatment of these two contributions is not necessary or even desirable, but rather, the differences in the proton NMR spectra of the heme substituents, and what they tell us about the electronic environment at the iron(III) center, the effect of... 

A study of platinum-fluorine coupling constants in compounds of the type rrarcs-Pt(PEt3)2(X)CF3 CF F2 (where F1 is trans to the metal) shows that while /(Pt-F3) correlates linearly with other measures of the trans-influence of X, /(Pt-F1) and 7(Pt-F2) do not, and reasons for this, including a through-space contribution which depends on the orientation of the C-F bond with respect to the co-ordination plane of the platinum, are discussed. (148)... 

The external magnetic field Bq induces currents in bonds, lone pairs and inner shell orbitals of our molecular systems, and these currents are the source of the chemical shifts. Adopting the picture of localized bond orbitals, through-space contributions are caused by charge distributions of bonds that are not directly connected to the nucleus under consideration. Provided that the charge distributions are sufficiently far away so that their electron density in the vicinity of the nucleus can be neglected, this type of interaction can be approximated by the McConnell equation ... 

From these results it was concluded that hydrogen bonds are dominated by polarization, but in general the proton chemical shift is influenced by through-space contributions (see previous chapter) as well. Since through-space contributions affect the anisotropy even more than the chemical shift mean value, no good correlations between distances and proton CS anisotropies are observed. 

F. Fayon, D. Massiot, M.H. Levitt, J.J. Titman, D.H. Gregory, L. Duma, L. Emsley, S.P. Brown, Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids, J. Chem. Phys. 122 (2005) 194313. 

Abraham has argued that for highly fluorinated propane fragments, /fp are given approximately by the values shown in Scheme 3, and suggested that the large values for/(eclipsed) arise from a through-space contribution. ... 

---