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 dipóle 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 (Θ_eq) and the torsional barriers ΔE(0°) and AE(90°) are not very sensitive to the substituent effects. We have found evidence for the similarity in the π-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 μ(D) and the difference between the ¹³C NMR chemical shifts of the acetylenic carbon atoms (Aáci-cn [ppml). The obtained results contribute to a better understanding of the structure-activity relationship for potential, liquid crystalline systems.