Additivity of ring geometry distortion effects in unsaturated five-membered heterocyclic rings

Empirical rules for the prediction of structures are valuable, providing aids to understanding and prediction that complement sophisticated computational methods. In this work, the structures of unsaturated five-membered rings have been predicted using sets of parameters describing distortions caused by substitutions in benzene and other rings. Effects on the structures of unsaturated five-membered rings due to both ring-member and substituent changes are found to be additive. Moreover, distortions caused by chlorine substitution observed for aromatic six-membered rings can be directly transferred to unsaturated five-membered rings with satisfying accuracy. The analysis has made use of data obtained by study of the structures of 2,5-dichlorothiophene and 3,4-dichloro-1,2,5-thiadiazole by gas-phase electron diffraction and computational chemistry methods.