TOPOLOGICAL RESONANCE ENERGIES OF THIENOPYRIMIDINES

Topological resonance energies of isomeric thienopyrimidines are repor ted. They show all thienopyrimidines to be aromatic compounds, but thi eno[3,4-d]pyrimidine to be by 20% less aromatic than thieno[2,3-d]pyri midine and thieno[3,2-d]pyrimidine. This result is also in agreement w ith a simple resonance-theoretic argument according to which thieno[2, 3-d]pyrimidine and thieno[3,2-d]pyrimidine should be more aromatic tha n thieno[3,4-d]pyrimidine because they possess two resonance structure s while the latter isomer only one. The HOMO-LUMO energy separation in dicates that thieno[3,4-d]pyrimidine should be more reactive than eith er of the two remaining isomeric thienopyrimidines. It is conjectured that, despite a relatively high aromaticity, thieno[3,4-d]pyrimidine w as never isolated because it is a rather reactive compound.