Determination of absolute configuration in molecules with chiral axes by vibrational circular dichroism: A C₂-symmetric annelated heptathiophene and a D₂-symmetric dimer of 1,1′-binapththyl

The absolute configurations of two large molecules that possess chiral axes, but no chiral centers have been determined by vibrational circular dichroism (VCD): an annealed heptathiophene (a helical molecule with C₂-symmetry) and a π-conjugated chiral derivative of o-tetraphenylene (a D₂-symmetric dimer of 1,1-binapthyl). In both cases, the size of these molecules exceeds the current limit of published structures for which VCD has been used to determine the absolute configuration. In the case of the annelated heptathiophene, 3 different elements, and 11 total (7 S atoms, 2 Si atoms, and 2 Cl or Br atoms), beyond the second row in the periodic table are included in the calculated structure. In the case of the tetraphenylene molecule, a total of 40 C atoms constitutes the hydrocarbon structure, which is a new upper limit for the number of atoms beyond H for which VCD has been calculated for the determination of absolute configuration. The excellent agreement between observed IR and VCD spectra and spectra calculated at the density functional theory (B3LYP/6-31G*) level for these molecules provides definitive determination of their absolute configurations and establishes a new regime of molecular size and elemental variety for which accurate comparisons of VCD calculations to experiment can be conducted.