Steric and electronic situation in the 4-X-4'-[(4''-Y-phenyl)ethynyl]biphenyl homologous series: a joint theoretical and spectroscopic study.

In this work we have studied the rotational barriers, the polarization of the acetylenic triple bond, and the molecular dipole moments in the 4-X-4'-[(4''-Y-phenyl)ethynyl]biphenyl homologous series using the density functional theory (DFT) and 1D/2D NMR spectroscopy. This series of compounds constitutes an effective base for the acquisition of liquid crystals. The equilibrium angle (theta(eq)) and the torsional barriers DeltaE(0 degrees) and DeltaE(90 degrees) are not very sensitive to the substituent effects. We have found evidence for the similarity in the pi-conjugation of the Y-substituted and X,Y-disubstituted compounds, the latter with mesomorphic properties, by means of the graphic analysis of the relationship between the molecular dipolar moment mu(D) and the difference between the (13)C NMR chemical shifts of the acetylenic carbon atoms (Deltadelta(CI[triple bond]CII) [ppm]). The obtained results contribute to a better understanding of the structure-activity relationship for potential liquid crystalline systems.