Steric and Electronic Situation in the 4-X-4′-[(4′′-Y-Phenyl)ethynyl]biphenyl Homologous Series: A Joint Theoretical and Spectroscopic Study Pedro D. Ortiz, ,†,| Reynier Suardı ´az, ‡,§ Laura de Vega, † Gunther Hennrich,* ,† and Pedro J. Ortiz* ,‡ Departamento de Quı ´mica Orga ´nica y Departamento de Quı ´mica-Fı ´sica Aplicada, UniVersidad Auto ´noma de Madrid, Madrid, 28049, Spain, and Departamento de Quı ´mica-Fı ´sica, UniVersidad de La Habana, La Habana 10400, Cuba ReceiVed: NoVember 24, 2009; ReVised Manuscript ReceiVed: January 5, 2010 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 (θ eq ) and the torsional barriers ΔE(0°) and ΔE(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 13 C NMR chemical shifts of the acetylenic carbon atoms (Δδ CItCII [ppm]). The obtained results contribute to a better understanding of the structure-activity relationship for potential liquid crystalline systems. 1. Introduction Liquid crystals are substances that show anisotropic properties as solids, combined with a degree of fluidity characteristic of liquids. 1,2 For this singular behavior, liquid crystals are of great importance for high quality displays and other modern device manufacture. 3-7 Thermal and dynamic properties of mesogenic compounds are strongly influenced by both electronic and molecular structure of individual building units, as well as the intermo- lecular interactions operative in the bulk. 8-11 The mesomorphic behavior of an organic compound can be varied by modifying its molecular structure. 12 For example, the molecular order in the liquid crystal phase depends on the mesogenic core structure and its geometry, polarizability, molecular conformation, and permanent dipole moment. In this work we have studied molecules of the 4-X-4′-[(4′′- Y-phenyl)ethynyl]biphenyl homologous series, with the biphenyl fragment being a common structural motif for thermotropic calamitic liquid crystals 13 (Scheme 1). It is important to obtain information about the effect of the terminal substituents (X, Y) on the biphenyl fragment and the carbon-carbon triple bond. For biphenyl systems, Johansson and Olsen reported values very close to the experimentally obtained equilibrium angle and torsion barriers using a com- bination of coupled cluster, density functional theory, and thermal corrections. 14-16 In order to obtain geometrical and spectroscopic data, keeping the computational effort within reasonable limits, we explore the possibility whether useful results can be achieved using the density functional theory (DFT) approach as a theoretical method. DFT calculations have proved to be an accurate method for the evaluation of different properties for a plethora of molecular systems including biphenyl systems. 14,17,18 2. Methods 2.1. Theoretical Calculations. The geometry optimizations and the calculation of frequencies were performed at the DFT 19,20 level using the B3LYP functional and 6-311G(d,p) basis set. 21,22 The 13 C NMR nuclear chemical shielding tensors were calculated by the gauge-including atomic orbital (GIAO) method. 23,24 In our case, it can be assumed that the length of side chains does not play a significant role in the electronic properties of the target molecules. Therefore, all alkyl side chains have been replaced by CH 3 groups in the corresponding theoretical models, in order to reduce calculation complexity. For comparison with experimental NMR data, the δ(ppm) relative chemical shifts were calculated by referring the absolute shielding tensor obtained by DFT to the absolute shielding tensor of tetramethylsilane (TMS), 184.5 ppm, which was calculated at the same level of theory. The value of σ iso for TMS obtained by us is very close to that obtained after correction by vibrational averaging, for bulk susceptibility, for temperature, and in relation to a secondary standard, 185.4 ppm. 25 These results confirm the appropriateness of the basis set and functional used in our chemical shifts calculation. All quantum chemical calculations are performed using the Gaussian 03 program package. 26 * Authors to whom correspondence should be addressed. (P.J.O.) E-mail: pedro@fq.uh.cu; (G.H.) E-mail: gunther.hennrich@uam.es. † Departamento de Quı ´mica Orga ´nica, Universidad Auto ´noma de Madrid. ‡ Universidad de La Habana. § Departamento de Quı ´mica-Fı ´sica Aplicada, Universidad Auto ´noma de Madrid. | Present address: Departamento de Quı ´mica Inorga ´nica, Universidad de La Habana, La Habana 10400, Cuba. SCHEME 1: Molecular Structure of 4-X-4′-[(4′-Y-Phenyl)etheny]biphenyl J. Phys. Chem. A 2010, 114, 2939–2944 2939 10.1021/jp911163t  2010 American Chemical Society Published on Web 02/01/2010