13C, 13C coupling constants in phenyl substituted ethylene, naphthalene, phenanthrene and cyclopentadienone. Dependence on dihedral angles. Determination of signs by the symmetrical double labelling (SDL) method. IV

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Abstract
Carbon‐13, carbon‐13 coupling constants and carbon‐13 chemical shifts have been measured in a series of phenyl substituted ethylenes and aromatics all doubly labelled with 13C at the olefinic positions (α,β‐) or at neighbouring aromatic positions, tetraphenylcyclopentadienone labelled at the 3,4‐positions, and dichlorodiphenylmethane labelled at the α‐carbon. Signs of coupling constants were determined by the symmetrical double labelling (SDL) method. Coupling constants over as many as five bonds are reported. Two‐bond couplings between carbons in the aromatic skeleton belong to different classes according to the nature of the coupling path. The magnitudes of three‐bond coupling constants between such carbons correlate linearly with π‐bond orders and a separation of the δ‐ and π‐contributions is evident. The three‐bond couplings between the 2‐position in a phenyl substituent and the olefinic β‐position or a corresponding aromatic position depend on the out‐of‐plane twist of the phenyl ring and may lead to information about the twist angle. Contrary to findings with aromatic carbonyl compounds two‐ and three‐bond couplings to the α‐carbon in the present compounds are fairly constant. The reported data suggest that the signs of coupling constants over more than two bonds alternate in aromatic systems. Carbon‐13, carbon‐13 coupling constants in naphthalene have been calculated by the INDO‐SOS method.