Two-bond coupling

Similar to geminul proton-proton coupling, two-bond carbon-proton coupling 2Jcli becomes more positive when the C —C—H bond angle increases [129]. 

Another approach is spin-coupled valence bond theory, which divides the electrons into two sets core electrons, which are described by doubly occupied orthogonal orbitals, and active electrons, which occupy singly occupied non-orthogonal orbitals. Both types of orbital are expressed in the usual way as a linear combination of basis functions. The overall wavefunction is completed by two spin fimctions one that describes the coupling of the spins of the core electrons and one that deals with the active electrons. The choice of spin function for these active electrons is a key component of the theory [Gerratt ef al. 1997]. One of the distinctive features of this theory is that a considerable amount of chemically significant electronic correlation is incorporated into the wavefunction, giving an accuracy comparable to CASSCF. An additional benefit is that the orbitals tend to be... 

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

Spm-spm splitting of NMR signals results from coupling of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds vicinal coupling)... 

Table 7.57 Two-Bond Carbon-Hydrogen Spin Coupling Constants 7.108...

Table 2.7. Structural features and geminal (two-bond) CH coupling constants Jqh (Hz)...

The proton-coupled spectrum is much more informative. We can see immediately which protons show a measurable coupling with the phosphorus atom, because the pattern is clearly identifiable as two triplets separated by 28.7 Hz. This, as we have already seen in Table 1, is the two-bond coupling between the phosphorus and the methine proton. The triplets (intensity 1 2 1)... 

In all cases the oxidation state of phosphorus is five, and the chemical shift range observed is only about 12 ppm. Note that the two phosphorus atoms attached to the methine carbon are non-equivalent because they are chemically different (phosphonate and phosphine oxide). We can expect the coupling between aP and bP to be large, as they are separated by two bonds, while that of aP to bP or CP will be small (coupling over five bonds). 

Figure 42 shows spectra of the simplest tetraorganotin compound, tetramethyltin. The upper spectrum was recorded with complete decoupling of all protons, the middle spectrum without. The result is a multiplet with 13 lines (n = 12), but if you work out the binomial coefficients for such a multiplet you will see that the outer two lines are too weak to be seen. The lower spectrum is the proton spectrum, which shows satellites due to two-bond tin-proton coupling to the tin-117 (inner lines) and tin-119 (outer lines) nuclei. 

The tin spectrum becomes much more interesting when the molecule under study contains two tin nuclei which can couple with each other. Figure 43 shows the spectrum of a 1,1-distannyl-l-alkene. The tin nuclei are separated by two bonds, so that a large tin coupling can be observed. The signals... 

Fig. 53 74.63 MHz CP-MAS spectrum of C(SnMe3)4, isotropic chemical shift 48.2 ppm, 2J(119Sn-119Sn) 328 Hz (the coupling visible is that between a tin-119 and a tin-117 nucleus). Both the isotropic chemical shift and the two-bond tin-tin coupling correspond to the solution values... 

Figure 3.3 provides a typical example of a proton spectrum of an n-alkyl fluoride. In this spectrum, one can clearly see the doublet signals resulting from the large two-bond F—H coupling (47 Hz), which are... 

As mentioned above, the doublet due to the large (48 Hz) two-bond F—H coupling constant can easily be seen in the proton spectrum of 2-fluoropentane (Fig. 3.6). Note also the nice doublet of doublets centered at 1.31 ppm (3/n, = 24 Hz, 3/hh = 6 Hz) deriving from the C-l methyl group, which exemplifies the significant difference in magnitude between typical three-bond F—H and three-bond H—H coupling constants. 

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