Topological Resonance Energies of Thienopyrimidines

Topological resonance energies of isomeric thienopyrimidines are reported. They show all thienopyrimidines to be aromatic compounds, but thieno[3,4-d]pyrimidine to be by 20% less aromatic than thieno[2,3-d]pyrimidine and thieno[3,2-d]pyrimidine. This result is also in agreement with a simple resonance-theoretic argument according to which thieno[2,3-d]pyrimidine and thieno[3,2- d]pyrimidine should be more aromatic than thieno[3,4-d]pyrimid- ine because they possess two resonance structures while the latter isomer only one. The HOMO-LUMO energy separation indicates that thieno[3,4-d]pyrimidine should be more reactive than either of the two remaining isomeric thienopyrimidines. It is conjectured that, despite a relatively high aromaticity, thieno[3,4-d]pyrimidine was never isolated because it is a rather reactive compound.