Coupling through space

In some particular instances, a large coupling between fluorine and a hydrogen, a carbon, a nitrogen, or another fluorine that may be separated by many bonds (four, five, six, or more) is observed. Such 

Although initially mainly of theoretical interest, through-space couplings are now considered an essential element of structure, in particular stereochemical elucidation, and there was an early study in which the fluorine-fluorine TS couplings in 19F-labeled amino acids were used to elucidate the folding of a protein.21 

Sederholm and his co-workers suggested some time ago that a through-space mechanism operated for F—F coupling. This suggestion was at first rejected, but has since been restated in a modified form. Recent evidence has shown that when two fluorine atoms are in close proximity in a molecule, a large coupling results, even though the atoms are separated by several bonds. 

Kobrina, L. V. Adasova, and V. I. Mamatyuk, Izvest. Sibirsk. Otdel. Akad. Nauk 

---

Dipolar coupling The direct through-space coupling interaction between two nuclei. It is responsible for nOe and relaxation, and represents the... 

Homonuclear 19F-19F experiments are the most commonly carried out, and they are also the most easily implemented on conventional NMR spectrometers. Among such experiments, 19F COSY correlation spectroscopy is probably the 2D 19F NMR technique most frequently encountered, mainly because of through space couplings that can make it otherwise difficult to infer definitive structural information from the presence or magnitude of observed correlations. It has been found to be particularly useful in the analysis of fluoropolymers. 

Examples of fluorine chemical shift and coupling constant data are given in Scheme 5.56. Note the significant four- and five-bond F—F and F—H coupling in these compounds, which no doubt is due in part to through-space coupling. 

Hz, Jfcu.h-i = 0 Hz (cis and trans relative to C-2). Also, a through-space coupling-constant was considered to be operative between Ftrans and H-5 in these compounds254,264 and a number of corresponding xylofuranoses.254... 

The dipolar-coupling Hamiltonian (TTy) describes the through-space coupling between two nuclear spins l and Ij. The dipolar coupling has an rk- dependence, and is key to the determination of internuclear distances in both solid-state and solution-state NMR. The high-field truncated form of the dipolar Hamiltonian is given by... 

This variability may be related to the sensitivity of fluorine to through-space coupling as mentioned in Section III.A.l. 

The separations between the transition metal atoms in complexes are such that direct through-space coupling of their spins is not significant except, of course, for direct metal—metal bonding. Rather, the coupling of the spins occurs by polarization of intervening diamagnetic atoms. 

Scheme 3.10 Through-space coupling between two 1,3-diaxial fluorine substituents...

---