Comparison between Theoretical and Experimental Deformation Density of 1,3,5,7-Tetra-t-Butyl-S-Indacene

Abstract

1,3,5,7-Tetra-t-butyl-s-indacene is a twelve-membered fused-ring compound; the stabilization of the compound by bulky t-butyl groups is discussed. The distribution of bonding electron density is depicted in terms of deformation density using the experimental X-X method, a multipole expansion model and a calculation according to the extended Hückel molecular orbital method (EHMO). The molecules crystallize according to the space group P21/n with cell parameters a = 9.700(3), b = 11.746(3), c = 10.858(2)Å, β = 107.67(2)°, Z=2 at 100K. The molecule has a center of inversion( ) and belongs to the symmetry point group Ci. The packing of two unique t-butyl groups in the asymmetric unit appears significantly different. However, the map of deformation density in the plane of the ring shows pseudo D2h symmetry. The accumulation of density at the midpoint of the bonds is observed as expected. The theoretical deformation densities agree with the experimental ones. Analyses of the molecular-orbital wavefunctions provide a good illustration of the aromatic π-electron system. A theoretical study of a series of substituents (-H, - CH3, -t-butyl ) on s-indacene illustrates the stabilization effect of the t-butyl group on the 12-π-electron ring system.