Three-bond couplings

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 50.2. Partial structures from the CH COLOC plot. Each partial structure B-I is deduced from the two- or three-bond couplings Jew for the H atoms of B-I (with italic values)...

Following a detailed NMR study of the 3-substituted thietane dioxides 188 it was concluded that the three-bond coupling constants can be safely used for stereochemical assignments in this series in particular the 7rih4<4Hz (Table 7, R =H, X = C) is consistent with an equatorial-equatorial interaction. This indicates an axial preference for the 3-substituent R (i.e. 188b) in both liquid and solid phases, and also suggests that the ring is puckered . 

A typical 19F NMR spectrum of a compound with a secondary CF2 group, that of 2,2-difluoropentane, is given in Fig. 4.3. Its single signal appears as a sextet at -90.8 ppm with identical three-bond coupling of 18.6 Hz to both its vicinal CH2 and its CH3 protons. 

The 13C NMR spectrum in Fig. 5.10 exhibits a quartet at 123.8ppm with a one-bond F—C coupling of 278 Hz, another quartet at 46.9 ppm, with a two-bond F—C coupling of 28 Hz, and a small quartet at 198.2 ppm (for the carbonyl) with a still observable three-bond coupling of 2.5 Hz. 

Substantial long-range (four-bond) couplings with the CF3 groups can be observed, whereas little three-bond coupling is evident. 

Figure 19 Schematic effect of the STAR operator on 2JCH and 3,/CH couplings. The vicinal component of magnetization in the long-range response that is two-bond coupled to a protonated carbon experiences modulation, which serves as a pseudo-evolution for this coupling. In contrast, the vicinal component of magnetization in the long-range response that is three-bond coupled to a protonated carbon does not exhibit a F, skew. Homonuclear modulation during the evolution period f, is still present, as the full experiment is not a constant-time experiment.

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

The observed ya-SCS(X) values in 7-exo-substituted norcaranes 97 (225) and those of M(CH3)3 (M = Si, Ge, Sn, or Pb) in cyclohexyl and bicy-clo[2.2. l]heptane derivatives (133) were later interpreted on the same basis. The back-lobe-overlap treatment was further supported by interpretations of H and 13C contact shifts of aliphatic amine signals in the presence of nickel acetyl-acetonate and by INDO calculations (226,227). Additional support came from extensive investigations of the structure dependence of three-bond coupling constants 3JCX (X = H, C, or F) (228,229), although the interpretation of these data has been subjected to criticism (230). 

---