Carbon-13 NMR spectra of coumarin and methoxycoumarins—a reinvestigation of charge density/Chemical Shift Relations

The carbon-13 NMR spectra of coumarin, 6-, 7-, 8-methoxycoumarin, and 5,7-, 7,8-, 5,8- and 6,7-dimethoxycoumarin have been measured and assigned. It is shown that substituent induced chemical shifts S(δ) in the mono- and disubstituted systems correlate well with the HMO atom-atom polarisibilities π_ij of the parent compound: S_i(δ_i) = 80.13 π_ij with a standard deviation of 1.42 ppm and a correlation factor of 0.994. Correlations between δ(¹³C) values and charge densities calculated by various semi-empirical methods are less successful.