Abstract
13C NMR spectra of the cyclic ureas 1–9 and thioureas 10–18 have been recorded in CDCl 3 as solvent. N-Methylation has been found to cause shifts of the δ- 13C values of the carbonyl or thiocarbonyl group which are proportional to the number of methyl groups. No general linear correlation exists between δ- 13CO and δ- 13CS values. Several linear regressions are examined in order to investigate the relation between calculated charge densities q, bond orders Q, and the chemical shifts δ of the carbonyl or thiocarbonyl carbon atom. The best fit with the experimental values is obtained for the relation δ- 13C A = a+ bq A+ cΣ Q AB+ dq π A.
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