Coupling over three bonds

For three-bond couplings we are back on better-trodden ground. In saturated X-C-C-Y units the coupling constant Jxy depends primarily on the dihedral angle t(X-C-C-Y) and can be described by 

H-C-C-H systems, (c) Karplus curve for 7( Sn H) in l-(trimethylstannyl)propane based on calculated and exper- 

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Inter-residue C- H coupling in disaccharides has been used to determine the orientation of the glycosidic linkage. Coupling over three bonds between... 

The 3C-13C coupling constants for coupling over three bonds depend on the dihedral angle in the same way as the vicinal H- H (see Chapter 5.1) and JC- -H coupling constants. Maximum values of ca. 4-6 Hz are observed for dihedral angles of 0° and 180° and minimal values around 0 Hz at 90°. 

In the 31P 1H NMR spectrum in a chloroform- solution two main signals appear. Each signal is flanked by two pairs of 195Pt satellites. One pair is due to the coupling of 31P with 195Pt over one bond, the other pair to the coupling with 195Pt over three bonds. The relevant parameters are... 

Coupling between protons over three bonds provided the most important early stereochemical application of NMR spectroscopy. In 1961, Karplus derived a mathematical relationship between V(HCCH) and the H-C-C-H dihedral angle (f). The simple formula... 

Compound (14) can easily be identified by Si/ H HETCOR experiments indicating the couplings of Si nuclei over two or three bonds to the BCH and BCHj protons, respectively ((5 Si = 3.0), and over three bonds to the C-2 Mes protons (c) Si = —7.0). The Si NMR spectrum also allows the observation of C satellite signals. The relatively small coupling constant /( Si- C(2)) = 18.2 Hz is typical for the accumulation of electropositive substituents at carbon <79ZN(B)1270,91SA(A)849>. 

The long-range coupling constants over three bonds (V) are larger than those over two bonds (z7) for thiophenes, selenophenes, and tellurophenes substituted furans seem to be anomalous in this.23 Linear correlations are obtained when the relative shifts of the furans, selenophenes, and tellurophenes are plotted against the relative shifts of the substituted thiophenes. A comparison of the slopes of these straight lines indicates that the ease of transmission of the substituent effects to the 5-carbon increases in the series furan < thiophene < selenophene < tellurophene. 

Correlated spectroscopy (COSY) was among the first two-dimensional (2D) NMR experiment realized [447, 448] and it is still among the most useful NMR experiments. COSY generates cross peaks in the 2D spectrum at the intersection of resonances of coupled spins (Fig. 14.48). In proteins cross peaks are observed for gem-inal, i.e. over two bonds, and vicinal, i.e. over three bonds, protons and in small peptides also couplings over four bonds may be detected. Thus the COSY spectrum allows the identification of spin systems for the assignment. However, apart from peptides, the overlap and degeneracy in chemical shifts is likely to prevent one from obtaining entire spin systems exclusively from the COSY spectrum additional experiments are required. 

D. C/H Spin-Spin Coupling Constants Over Three Bonds. 382... 

We have recently reported the first observation and measurement of spin-spin interactions in neutral hexacoordinate silicon chelates [1] which extend across the N— Si coordinative bond and over two, three, and even four bonds. These coupling constants were highly sensitive to small geometrical modifications in the complex. The major geometrical requirement found for spin-spin coupling over two bonds (N—>Si-F or N—>Si-H) in hexacoordinate complexes la-lc was that the corresponding bond angle be 90° or very close to it. 

The majority of reported spin coupling data refers to directly bonded nuclei, there are also a number of two bond couplings, but no data for couplings over three or more bonds. The absolute signs of the couplings are not known. 

Note that (1) in saturated systems proton-proton couplings are seldom observed beyond three bonds, but (2) in aromatic and heterocyclic systems, four- and even five-bond coupling is commonplace. This is because spin coupling is transferred by electrons. Where you have extended conjugation, you can expect to observe coupling over a greater number of bonds. 

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