Carbon‐13 chemical shifts of α,β‐unsaturated acids

Abstract

Abstract Carbon‐13 spectra of 59 α,β‐unsaturated carboxylic acids have been measured. Large differential shieldings of ethylenic carbons in the cis and trans isomers of acrylic acid derivatives were found (altogether 15 isomeric Z ‐ and E ‐pairs of acids were investigated). The origin of differential shieldings is complex and both molecular ground states as well as changes in excited states appear to be involved. All measured 13 C chemical shifts can be described by additive parameters that provide a straightforward new technique by which structural assignments can be made for a wide variety of isomeric mono‐, di‐ and tri‐ substituted α,β‐unsaturated acids.