Molecular structure of 1,1,2,2-tetra-tert-butyldisilane: unusual structural motifs in sterically crowded disilanesElectronic supplementary information (ESI) available: Structural results from calculations at the HF level with the 3-21G* and 6-31G* basis sets (Table S1), geometric and amplitude restraints (Table S2), least-squares correlation matrix (Table S3), and experimental coordinates from the GED analysis (Table S4). See http://www.rsc.org/suppdata/dt/b3/b316701a/