¹³C, ¹³C coupling constants and ¹³C chemical shifts of aromatic carbonyl compounds. Effects of ortho‐ and peri‐interactions involving the carbonyl substituent

Carbon‐13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives 13 C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non‐hindered molecules and molecules in which the carbonyl substituent is subject to ortho ‐ or peri ‐interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range 13 C, 13 C coupling constants involving the carbonyl carbon, in particular 2 J and 3 J , on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. 2 J (s‐ t )> 2 J (s‐ c ) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s‐ t and s‐ c refer to the orientation of the CO group relative to the aromatic bond in question with respect to the connecting single bond). For ketones 3 J ( t ,s‐ c )< 3 J ( t ,s‐ t ), and both of these 3 J ( t ) values decrease with increasing angle of twist, θ, about the single bond, whereas 3 J ( c ,s‐ c ) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three‐bond coupling path). Generally it is found that 3 J ( t )> 3 J ( c ) and 3 J ( t )> 2 J , confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.