One-Bond Couplings V

A one-bond coupling occurs when a single bond links two spin-active nuclei. The bonding electrons in a single bond are assumed to avoid each other such that when one electron is near nucleus A, the other is near nucleus B. According to the Pauli Principle, pairs of electrons in the same orbital have opposed spins therefore, the Dirac model predicts that the most stable condition in a bond is when both nuclei have opposed spins. Following is an illustration of a bond the nucleus of the 

It is not unusual for coupling constants to depend on the hybridization of the atoms involved. values for C—coupling constants vary with the amount of s character in the carbon hybrid, according to the following relationship  

Notice the specific values given for the couplings of ethane, ethene, and ethyne in Table 5.1. 

Using the Dirac nuclear-electronic spin model, we can also develop an explanation for the origin of the spin-spin splitting multiplets that are the results of coupling. As a simple example, consider a bond. Recall that a atom that has one hydrogen attached appears as a doublet (two 

236 Nuclear Magnetic Resonance Spectroscopy Part Three Spin-Spin Coupling 

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D-INADEQUATE spectroscopy can conveniently disclose the connectivity of each carbon atom embedded in a molecule and has found widespread use in structural elucidation of organic compounds. This technique is extremely powerful when the size of one-bond coupling (V) is much larger than that of long-range couplings... 

The two spectra in Figures 6.23 and 6.24 demonstrate coupling between and P. In the first compound, shown in Figure 6.23, the carbon atom of the methyl group at about 12 ppm is split by one adjacent phosphorus atom into a doublet with a coupling constant equal to 56.1 Hz (919.3 -863.2 = 56.1 Hz). Notice that the n -h1 Rule predicts how the pattern will appear (doublet). The nuclear spin number for phosphorus is the same as for a proton and for a fluorine atom (. This interaction is an example of one-bond coupling (V). 

There are two reports [23 24] of vanadium phosphorus one-bond couplings. ( V, I = 7/2, 99.8% abundant) and both of these resulted from the direct measurement of the metal NMR spectrum. Data are given... 

The one-bond couplings, which are determined almost exclusively by the Fermi contact term, have been treated in some detail, and they have been used to good effect to reveal details of the structural situation particularly at the 15N centre, as discussed in Section V. 

Fig. 3.17. Homonuclear carbon one-bond coupling constants observed for the six possible combinations of carbon nuclei in organic compounds the ranges of Jcc are represented as a function of the product of. characters,. v, s, of the coupling nuclei. The straight line follows the equation Jc.ci = 730 sx s2 - 17 (Hz) [133].

J or V nuclear spin-spin coupling constant (in Hz) through one bond (one-bond coupling)... 

Only few reports on Se/ N couplings exist. One-bond coupling constants, V( Se, N), are probably positive 60.1 Hz for CF3Se-NH2,... 

V NMR (94.73 MHz) spectrum of aTHF solution containing the anion fran5 -[V( N2)2(dmpe)2] (inset). The spectrum shows the quintet due to one-bond coupling (four equivalent P) ... 

This fotir-bond coupling (V) is called allylic coupling. The r electrons of the double bond apparently help to transmit the spin information from one nucleus to the other, as shown in Figine 5.20. 

V. NMR DETERMINATIONS OF STRUCTURE IN SOLUTION A. Substituent Effects on One-bond Couplings... 

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