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

Magnitudes and signs of 13 C 13 C coupling constants in compounds of the type Ph 13 CR 1 R 2 13 CR 1 R 2 Ph 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 2 J (C‐2, C‐α) are always found positive, 2 J (C‐1, C‐β) in the present compounds are found to be negative or about zero. 3 J (C‐3, C‐α) has the same sign as 2 J (C‐2, C‐α). A 4 J and a 5 J were observed in trans ‐stilbene.