Theoretical Study of Tetramethyl- and Tetra-tert-butyl-Substituted Cyclobutadiene and Tetrahedrane

The tetramethyl and tetra- tert -butyl derivatives of cyclobutadiene and tetrahedrane have been studied with ab initio and density functional methods. The ring in tetra- tert -butylcyclobutadiene displays very unequal bond lengths (1.354, 1.608 Å) and confirms the earlier suspicion that the low-temperature X-ray structure was distorted. The C−C single bonds have the longest separations found to date between sp 2 -hybridized carbons. Tetra- tert -butyltetrahedrane, which prefers T over T d symmetry, is calculated to be 1.5 kcal/mol more stable than tetra- tert -butylcyclobutadiene (B3LYP/6-311+G(d)//B3LYP/6-31G(d)). The dications, C 4 R 4 2+, dianions, C 4 R 4 2-, and dilithiated species, Li 2 C 4 R 4 (R = H, Me, t -Bu), also were studied to determine the effect of substitution on structure and stability. Finally, NICS (nucleus-independent chemical shifts) values were calculated and showed, as expected, the dications and dianions (4 n + 2 π-electrons) to be aromatic (negative NICS(0)) and the cyclobutadienes to be antiaromatic (positive NICS(0)).