Systematic analysis of substituent effects on the 13C chemical shifts of the ring carbons in trimethyl-cyanurates and -thiocyanurates

Abstract

The carbon-13 n.m.r. spectra of 23 of the 24 possible isomers of O- and S-substituted trimethyl-s-triazine are reported. The 69 chemical shifts of the ring carbons distributed over a 42.4 p.p.m. range have been assigned on the basis of a classification into 16 groups obtained by combining a number of structural characteristics which identify the different carbon sites. This assignment allows a prediction of the resonance peaks of the only isomer not synthesized so far. Shielding and deshielding effects of substituents modifying the charge density at each carbon are shown to combine and determine the actual distribution of chemical shifts into 16 classes each made of groups of three or four peaks and spanning a frequency range 3–8 p.p.m. wide. These groups were found to correlate with the π-electron delocalisation energy, to the π-atomic orbital electron densities, and to the trend observed in thermal isomerisations and methanolysis.