13C?13C coupling constants in derivatives of trans-stilbene and tetraphenylethylene. Determination of relative signs II

Magnitudes and signs of ¹³C? ¹³C coupling constants in compounds of the type Ph¹³CR¹R²? ¹³CR¹R²Ph have been determined and the results are discussed in a broader context. Two types of coupling constants, J(C-i, C-α) and J(C-i, C-β), between aromatic carbon atoms and the benzylic carbons, probably with different coupling mechanisms, are considered. Whereas ²J(C-2, C-α) are always found positive, ²J(C-1, C-β) in the present compounds are found to be negative or about zero. ³J(C-3, C-α) has the same sign as ²J(C-2, C-α). A ⁴J and a ⁵J were observed in trans-stilbene.