Coupling constants two-bond

It is evident from Fig. 57 that the sensitivity of 2/to bond angle (i.e., the greatest slope) is near 90°. This plot can be used effectively to estimate 

Bond and Dihedral Angles (deg.) and Corresponding 2/( HSi15N) and 3/(1HSiN13C) in Penta- and Hexacoordinate Hydrido-Complexes 

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H and 13C NMR Data. The H chemical shifts of CF2H protons of difluoromethyl ethers lie between 6.00 and 6.3 ppm, with a significantly enhanced F—H two-bond coupling constant of around... 

Three-bond coupling constants present even more problems concerning the question of solvent dependence than do the two-bond coupling constants. 

The values of 1Jch couplings for C-2 and C-3 correspond to those observed for pyran-4-one. Two-bond coupling constants for these atoms are typically 6-7 Hz, whilst 3/m couplings of 9 Hz have been observed for C-8a and of 4-5 Hz for C-4a. The couplings within the aromatic ring are similar to those observed for related compounds such as coumarins <79JST(56)87>. 

Table 3.9. Two Bond Coupling Constants 2JCH of Selected Alkyl, Alkenyl, and Alkynyl Compounds. ...

Figure 2.12 shows the carbon NMR spectrum for 1,2-dibromofluoroethane. As usual, the protons are decoupled, so that the only couplings that one can see are those between fluorine and carbon. Two signals are observed at 33.8 and 89.1 ppm, with one-bond and two-bond coupling constants of 257 and 23.5 Hz, respectively. (The multiplet at 77 ppm derives from the solvent, CDC13.)... 

Ipso (one-bond) coupling of H to C is consistently between 165 and 172 Hz, whereas its two-bond coupling constants (0-5 Hz) are usually much smaller than three-bond H-C couplings (4-10 Hz). 

The doublet for the four equatorial fluorines appears at +83.8 ppm, while the pentet for the single axial fluorine appears at 77.6 ppm. The two-bond coupling constant is 161 Hz. 

Fig. 2 Influence of the choice of the exchange-correlation potentials and of relativistic effects on the 31p-31p two-bond coupling constants in cis- and fra s-M(CO)4(PH3)2, M is Cr, Mo or W. (Graphics courtesy of Kaupp [97])...

Fig. 57. Two-bond coupling constants as a function of the deviation of H-Si-N bond angle from 90° in hydrido complexes.87...

EXAMPLE 9.2 Using the spin-pairing model above, predict the sign of the two-bond coupling constant (27hh) n H-13C-H. 

Although this model is somewhat naive, it does help us rationalize why one- and three-bond H-H, C-H, and C-C coupling constants are usually positive, while two-bond coupling constants are usually negative. 

TABLE 9.3 Some Representative Two-Bond Coupling Constants 2... 

Table 1 summarizes the Si chemical shifts and Si Si one and two bond coupling constants of all trisilanes synthesized so far. The coupling constants were measured using INADEQUATE pulse techniques. 

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