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( 13 C) values and charge densities calculated by various semi‐empirical methods are less successful.