13C NMR-spectroscopic studies on the electronic structure of simple polymethines

The 13 C-NMR chemical shifts δ of the simplest polymethines of the general formula equation image are reported and correlated with the corresponding π-electron densities (PPP approximation). The 13 C-H coupling constants J(CH) of the polymethine fragments CH are discussed in relation to the charge-induced alternation of the CCC bond angles within the polymethine chain. The changes in the 13 C chemical shifts induced in merocyanines by the addition of the shift reagent Eu(DPM) 3 are attributed to a superposition of pseudocontact and contact interactions between the 13 C nuclei of the polymethine chain and the associated paramagnetic europium complex.