Coupling constants three-bond

As resolution increases, small couplings beyond three bonds often become noticeable as line broadening of the benzylic CH2 and CH3 peaks in Figure 3.34 and Problem 7.2, respectively (see Appendix F for coupling constants). 

The coupling constants for coupling across three bonds depend on the... 

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°. 

For the determination of homonuclear or hetero-nuclear three-bond coupling constants, three fundamentally different approaches are used (1) direct measurements of splittings caused by /-couplings, (2) the so-called E.COSY-signal patterns in which the splittings can be measured as shift differences of signals and (3) special experiments which lead to modulation of signal intensities via / (Table 3). A frequent requirement for the latter is a N-enriched sample, e.g. for the /-modulated ( N, iH)-COSY experiment or the HNHA experiment. Whereas labelled compounds are routinely used for proteins and nucleic acids they are expensive for peptides and therefore rarely used. 

Eor a macromolecule such as a large protein, the steps in characterization involve, first, identification of the spin systems present, using correlated spectroscopy, and identification of neighboring amino acids. The long range noes are then assigned, and three bond coupling constants ate deterrnined. 

Table 2.8. Structural features and vicinal (three-bond) CH coupling constants Jch (Hz)...

Since fragment A was clearly assigned with the help of HH coupling constants, all of the C atoms not included in A, which, according to the CH COLOC plot, are two or three bonds apart from the equivalent protons at Sh = 6.72 (Table 43.1), belong to the benzene ring B. 

J, V, V, V nuclear spin-spin coupling constant (in Hz) through two, three, four and five bonds (geminal, vicinal, longer-range couplings)... 

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