Correlation of Coupling Constants with Other Physical Properties

4 CORRELATION OF COUPLING CONSTANTS WITH OTHER PHYSICAL PROPERTIES 

Theory suggests and experiment confirms that coupling constants can be related to a number of physical parameters. Among the most important are (1) hybridization, (2) dihedral bond angles, and (3) electronegativity of substituents. 

One of the most fruitful theoretical contributions to the interpretation of coupling constants has been the valence bond treatment by Karplus71 of 3JHH in ethanelike fragments, Ha—C —Cb—Ht. The most interesting conclusion is that this coupling depends drastically on the dihedral angle between the H —Ca and the Cj,—Hj bonds. The calculated results were found to fit approximately the relation 

The solid line in Fig. 5.2 is a plot of Eq. 5.3 with the parameters A = 7, B = — 1, and C = 5 Hz. It is apparent that large values of J are predicted for cis (9 = 0°) and tram (6 = 180°) conformations and small values for gauche (60° and 120°) conformations. These predictions have been amply verified, and the Karplus relation is of great practical utility in structure determinations. 

FIGURE 5.2 Calculated variation of vicinal and allylic proton-proton coupling constants with dihedral angle between the C—H bonds shown. 

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Correlation of Coupling Constants with Other Physical Properties... 

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