Steric and Electronic Situation in the 4-X-4′-[(4′′-Y-Phenyl)ethynyl]biphenyl Homologous Series: A Joint Theoretical and Spectroscopic Study

Abstract

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.

Aluminium Functionalized Germanes: Intramolecular Activation of Ge–H Bonds, Formation of a Dihydrogen Bond and Facile Hydrogermylation of Unsaturated Substrates

“…Δ δ values are a measure for the polarity of C≡C triple bonds which influences their reactivity in secondary reactions. [8j], All compounds show two absorptions in the IR spectra at about 2180 and 2145 cm –1 for the stretching vibrations of the triple bonds. The presence of Ge bound H atoms in 3 is evident from low‐field 1 H NMR resonances at δ = 4.35 (alkyl groups) and δ = 6.01 (aryl groups) as well as intense absorptions for ν GeH at 2029 and 2073 cm –1 in the IR spectra.…”

Concerning the Electronic Control of Torsion Angles in Biphenyls

“…Some interesting examples are the studies of the influence of θ on the binding properties of polyoxa‐bridged compounds 1 ,16 on the variation of intramolecular triplet energy transfer in tethered biphenyl‐based mixed‐metal Ru–Os bis(terpyridine) complexes 2 ,17,18 on the changes in the conductivity in substituted biphenyls with terminal amino groups19 or in constrained 2,2′‐bridged biphenyls 3 ,20 and their related push–pull biphenyl derivatives 4 (Figure 1). 21 Considering these precedents, it is remarkable that, as far as we are aware, the only study of the variation of the torsion angle with the electronic character of the substituents in the phenyl rings have recently been reported for 4‐Y‐4′‐[(4″‐X‐phenyl)ethynyl]biphenyls 5 22. Although this study was not systematic, the authors concluded that the calculated DFT rotational barriers and the equilibrium angles ( θ eq ) of the biphenyl moieties are barely sensitive to substituent effects 22…”

Silicon–Halogen Bond Activation in Mixed Si/Al Compounds and an Approach to Intramolecular Stabilized Silylium Ions

“…In the 1 H NMR spectra the signals of the SiH protons are observed at about δ = 4. The polarization of the C≡C triple bonds as indicated by the difference of the chemical shifts of C α and C β in the 13 C NMR spectra[23d], decreases from about Δ δ = 42 in the alkyl‐substituted alkynes 4i and 4j to Δ δ = 19.4 and 10.7 in the aryl‐substituted alkynes 4h and 4k . The characteristic signals for the stretching vibrations ν(C≡C) and ν(Si‐H) are found in the IR spectra at about 2160 and 2110 cm –1 , respectively.…”

“…The signals of the chlorosilanes 3 are without exception shifted to a lower field compared to the hydrosilanes 4 due to the replacement of the H atom with a more electronegative Cl atom. The Δ δ (C α C β ) values are indicative for the polarization of the alkynyl groups which influences the selectivity of the following hydroalumination reactions. [23d] They are large for most compounds with values close to 40 except of 3h and 3k (aryl groups attached to the C≡C bonds) which show smaller values of 20.8 and 12.2, respectively.…”