The influence of the position of a chiral substituent on undecathiophene chain. A DFT study

Undecathiophenes substituted by chiral and achiral groups were studied using DFT method. The substituent shift from one end to the other was monitored for: relative energy, the SRCM chirality measure, the chiral Wiener index, the HOMO-LUMO gap, HOMA and NICS aromaticity indices, and theoretical VCD spectrum. For both, all-trans and one-cis conformations, structures substituted at the end were ca. 10 kJ/mol more stable than the other isomers. The SRCMs fluctuations with the conformation made this parameter not useful, but, the topological chiral Wiener index displayed regular yet non-linear changes with the substituent position enabling differentiation of the chiral systems. The HOMO-LUMO gap increased with the substituent shift towards the central ring. It varies regularly with the Wiener index for all-trans but irregularly for one-cis conformers, indicating strong response of the gap on slight conformational changes. The aromaticity indices adopted to linear systems indicated the all-trans to be a bit less delocalized than the one-cis isomers. Finally, the calculated VCD spectra were indicated possibility to monitor the position of the chiral substituent along the all-trans and one-cis structures.