Abstract
The singlet-triplet energy spacing has attracted considerable attention in diradical chemistry. We have previously found that 2,2-dialkoxy substituents locate the singlet state below the triplet state in cyclopentane-1,3-diyl localized diradicals with the energy gap of 7.5 kcal/mol. In the present study, we have found the notable singlet-triplet energy gap, ΔEST = ES - ET = ca. -15 - -20 kcal/mol, in 2,2,2',2'-tetraalkoxy-cyclobutane-1,3-diyl localized diradicals by using DFT calculations (UB3LYP/6-31+G(d,p)). The quite large energy gap can be reasonably explained by the two sets of pseudo π interaction between the symmetric non-bonded molecular orbital and the C-O σ* orbital.
