Vicinal couplings, dihedral angle dependence

Extension of this type of analysis to scalar coupling Interactions Involving other nuclei requires establishment of the dihedral angle dependence of the vicinal couplings. Three approaches have been used (1) Measurement of coupling constants In conformatlonally constrained molecules (2) correlation of observed couplings with rotamer distributions In flexible molecules deduced previously on the basis of values (3)... 

Vicinal coupling constants strongly depend on the dihedral angle, <]) (Karplus equation) ... 

Hz (Table 3). The dihedral-angle dependence of vicinal Vpoch coupling and Vpccc has been... 

In cyclic systems the effect of the electronegative substituent X on the vicinal coupling constant is dependent on the steric disposition of X with respect to the coupled vicinal protons. The maximum effect of the substituent in reducing the vicinal coupling constant is observed when the substituent and a proton on the vicinal carbon are trans-coplanar. In Figure 2.52 the substituent X is in an equatorial disposition and is not trans-coplanar with respect to any of the coupled protons. However, in Figure 2.53 the substituent X and the axial proton attached to C-2 are trans-coplanar. Thus, while the dihedral angles between the protons (connected with thick lines) are about 60° in both cases, J,e is 5.5 Hz ( 1 Hz) when the substituent X is equatorial... 

Representative values for Vpx coupling constants through intervening C, N, O, or other heteroatoms are presented in Table V. As might be expected from the dihedral-angle dependence of vicinal Vhcch coupling constants,... 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

The vicinal coupling (protons on adjacent carbon atoms) h-c-c-h have values 0 - 16 Hz depending mainly on the dihedral angle (f). 

The magnitude of carbon-fluorine coupling attenuates with coupling distance in alkyl and cycloalkyl fluorides, as shown in Tables 4.20(b) and 4.21. Analogously to vicinal proton-proton, carbon-proton and carbon-carbon coupling (Sections 3.2.2.4, 3.2.4.2), 37cf depends on the dihedral angle enclosed by the C—C—C —F connection, and Karplus cosine relations such as eq. (4.7) have been derived empirically [266] and theoretically [267],... 

Coupling between protons on vicinal carbon atoms depends primarily on the dihedral angle between the H—C—C and the C—C —H planes. This angle can be visualized by an end-on view (Newman projection) of the bond between the vicinal carbon atoms... 

The important fact is that the magnitude of Vab (e.g., between Ha and Hb in Figure 9.5) is (in addition to its dependence on y values and s-character effects) a function of the dihedral angle (6) between the nuclei. The Karplus equation (named for the discoverer of the relationship) for vicinal H-H coupling has the form... 

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