687-689The 4-n-heptyloxy-4´-cyanobiphenyl (7OCB) is a member of 4-n-alkoxy-4´-cyanobiphenyl homologous series. This liquid crystal series is well known for its electro-optical properties. The 7OCB compound transforms from crystal to nematic at 53.5°C and nematic to isotropic phase at 75.0°C. Using Hartree-Fock and DFT methods with 6-31G(d,p) basis set, molecular geometry, conformational analysis, electronic structure, MEP, HOMO-LUMO surfaces and molecular parameters of 7OCB molecular system have been investigated. Efforts have been made to elucidate the physico-chemical properties of the 7OCB liquid crystal

Sharma, D

Singh, N B

Tiwari, G

Tiwari, SN

English

NOPR

Journal of Scientific & Industrial Research Vol. 78, October 2019, pp. 687-689       Frontier Orbital and Conformational Analysis of a Nematic Liquid Crystal: 4-n-heptyloxy-4´-cyanobiphenyl G Tiwari1, D Sharma1, SN Tiwari1 and N B Singh2*    1Department of Physics, DDU Gorakhpur University, Gorakhpur 273009 2Department of Chemistry, SBSR Research & Technology Development Centre, Sharda University, Knowledge Park-3, Greater Noida Received 15 July 2018; revised 4 March 2019; accepted 5 June 2019  The 4-n-heptyloxy-4´-cyanobiphenyl (7OCB) is a member of 4-n-alkoxy-4´-cyanobiphenyl homologous series. This liquid crystal series is well known for its electro-optical properties. The 7OCB compound transforms from crystal to nematic at 53.5°C and nematic to isotropic phase at 75.0°C. Using Hartree-Fock and DFT methods with 6-31G(d,p) basis set, molecular geometry, conformational analysis, electronic structure, MEP, HOMO-LUMO surfaces and molecular parameters of 7OCB molecular system have been investigated. Efforts have been made to elucidate the physico-chemical properties of the 7OCB liquid crystal. Keywords: Liquid crystal, PES, MEP, HOMO, LUMO Introduction Commonly known states of matter are solid, liquid and gas. However, there are certain substances which when heated/cooled or treated with water or aqueous medium exhibit properties of both true liquids and crystals. Such substances are referred to as liquid crystals.1,2 Due to widespread use of liquid crystals in, displays, fabricating technology, industrial growth, biology, medicine and so on; scientists from almost all the disciplines have progressively focused their increased attention to the liquid crystal science.3-6  Method of calculation Electronic structure methods are being effectively used to gather information about molecular structure and examine systems where electronic effects and molecular orbital interactions are dominant.7-9  Hartree-Fock (HF) and DFT (B3LYP) methods with 6-31G(d,p) basis set have been used to optimize the molecular geometry, conformational analysis, molecular electrostatic potential (MEP) and HOMO-LUMO surfaces of the 7OCB molecule.10-12 Both HF and DFT methods were obtained from GAMESS program.13   Results and Discussion  Geometry and conformational analysis The optimized molecular geometry of 4-n-heptyloxy-4´-cyanobiphenyl (7OCB) is shown in Figure 1 along with various bond length values. In Figure 2 (a), the potential energy surface (PES) scan curve of 7OCB corresponding to two torsion angles τ1 and τ2 has been displayed. As evident from this figure, 7OCB molecule exhibits several stable conformers (dark blue regions) which are in the trans-trans conformation. Energy minimization or global minimum of trans–trans conformer of 7OCB molecule is obtained for τ1 = -90⁰ and τ2 = -180⁰ with energy value of 144.88 kcal/mol. Further, global maximum or cis-cis conformer (red region) corresponds to τ1= 0⁰ and τ2 =20⁰ with energy value of 1118.71 kcal/mol. Another maximum position is observed for τ1 = 0⁰ and τ2 = -20⁰, which indicates that these molecular conformations are energetically unfavoured.5  Molecular electrostatic potential (MEP) and HOMO-LUMO surface analysis The MEP surface on the 0.001 electron/bohr3 isosurface of electron density is depicted in Figure 2(b); which shows that electron densities are very low at the outer surface and near the hydrogen atoms (blue and light blue regions). Hence electrostatic potentials near these regions are positive. In the inner surface of the molecule (green regions), electron densities are moderately high due to the presence of carbon atoms. Further, electron densities are very high at the terminals of the molecule (yellow and red regions); which is due to the presence of electronegative atoms like oxygen and nitrogen. According to the frontier orbital theory, the chemistry of π-conjugated systems ————— *Author for Correspondence E-mail: nbsingh43@gmail.com J SCI IND RES VOL 78 OCTOBER 2019   688is largely determined by the HOMO and LUMO orbitals in the reactant molecules. Furthermore, the HOMO and LUMO of the ground-state molecule are the essential orbitals because the ground state reaction of a strong electron acceptor (or donor) causes a mixing in of an ionized or an excited electron configuration. The quantities like global softness, global hardness and other related parameters of 7OCB molecule are given in Table 1. Details of the mathematical treatments for calculating global parameters can be found elsewhere.14 A molecule with smaller HOMO-LUMO energy gap is expected to have higher reactivity and a lower stability in chemical and photo-chemical processes with electron transfer or leap. In the present investigation, HOMO-LUMO gap is found to be 0.164 a.u., elucidating that molecular reactivity of 7OCB such as ability to absorb light and to react with other species is very high.5   Conclusions It is observed that both the methods, ab-initio HF/6-31G(d,p) and DFT B3LYP/6-31G(d,p) yield almost the same geometrical parameters like bond lengths and bond angles. Conformational analysis suggests that 7OCB molecule is stable in trans-trans conformation. MEP surface shows that electrophilic ability strengthens while nucleophilic ability weakens as one maps outwards from the inner core of the 7OCB molecule. Further, lower HOMO-LUMO energy gap indicates that 7OCB is chemically highly   Fig. 1 — Optimized geometry of 4-n-heptyloxy-4´-cyanobiphenyl (7OCB) molecule.    Fig. 2 — (a) Surface plot of the relative potential energy of 7OCB molecule as a function of two constitutive torsion angles τ1 and τ2 and (b) Molecular electrostatic potential (MEP) of 7OCB molecule projected on 0.001 electron/bohr3 isodensity surface.   Table 1 — Global parameters of 7OCB molecule as calculated by HF and DFT methods. Global Parameters HF/ 6-31G(d,p) B3LYP/ 6-31G(d,p) HOMO energy -0.298 a.u. -0.219 a.u. LUMO energy 0.069 a.u. -0.055 a.u. Orbital energy gap (ΔE) 0.367 a.u. 0.164 a.u. Ionization potential (I) 0.298 a.u. 0. 219 a.u. Electron affinity (A) -0.069 a.u. 0. 055 a.u. Electronegativity (χ) 0.114 a.u. 0.137 a.u. Electronic chemical potential (µ) -0. 114 a.u. -0.137 a.u. Global hardness (η) 0.183 a.u. 0.082 a.u. Global softness (S) 5.464 a.u.-1 12.195 a.u.-1 Electrophilicity index (ω) 0.035 a.u. 0.114 a.u. Dipole Moment 7.06 Debye 7.03 Debye  SINGH et al.: FRONTIER ORBITAL AND CONFORMATIONAL ANALYSIS OF A NEMATIC LIQUID CRYSTAL   689reactive and therefore a potential liquid crystal for electro-optic applications.   Acknowledgments Authors are thankful to Dr. Mark S. Gordon, Department of Chemistry, Iowa State University for providing us the GAMESS program, August 2013.  References 1 Sonin A A. Pierre-Gilles de Gennes and physics of liquid crystals, Liq Cryst Rev, 6 (2018) 109-128. 2 Jones J C. The fiftieth anniversary of the liquid crystal display, Liq Cryst Today, 27 (2018) 44-70.  3 Mandal P, Deblal D & Majumder T P. Theoretical investigation of dielectric behavior in the region of Freedericksz transition in antiferroelectric liquid crystals,  J Mol Liq, 215 (2016) 170-174. 4 Zhou S. Recent progresses in lyotropic chromonic liquid crystal research: elasticity, viscosity, defect structures, and living liquid crystals, Liq Cryst Today, 27 (2018) 91-108. 5 Tiwari S N & Sharma D. Electronic structure, FT-IR analysis and nematic behaviour studies of para-methoxybenzylidine p-ethylaniline: Ab-initio and DFT approach, J Mol Liq, 244 ((2017)) 241-251. 6 Saha P, Majumder T P, Czerwiński M & Dabrowski R. Optical behaviour of ZnO nanocapsules and their nanocomposites mixed with ferroelectric liquid crystal  W-206E, Chin J Phys, 4 (2017) 1447-1452. 7 Bapat S, Viswanadh N, Mujahid M, Shirazi A N, Tiwari R K, Parang K, Karthikeyan M, Muthukrishnan M & Vyas R. Synthesis, biological evaluation and molecular modeling studies of Novel Chromone/ Aza-Chromone fused α-aminophosphonates as Src Kinase inhibitor, J Sci Ind Res, 78 (2019) 111-117.  8 Monga D, Soni S & Satsangi P S. Role of optical properties on photon distribution in a biological tissue using Monte Carlo method, J Sci Ind Res, 76 (2017) 154-159. 9 Ramesh K V, Solanki M, Sharma S & Deshmukh S. Molecular dynamics simulation of homology modeled 2-nitropropane dioxygenase from Mycobacterium tuberculosis, J Sci Ind Res, 74 (2015) 277-280. 10 Tiwari G, Srivastava A K, Kumar R & Kumar A. Quantum chemical and molecular docking studies on two potential hepatitis C virus inhibitors, Main Group Chem, 18 (2019) 107-121. doi: 10.1080/15421406.2019.1641987. 11 Kumar A, Srivastava A K, Tiwari S N, Misra N & Sharma D. Evolution of anisotropy, first order hyperpolarizability and electronic parameters in p-alkyl-p’-cynobiphenyl series of liquid crystals: odd-even effect revisited, Mol Cryst Liq Cryst, (2019) doi: 10.1080/15421406.2019.1641987. 12 Prasad S & Ojha D P. Optimized molecular geometries, vibrational spectra, and frontier molecular orbital of fluorinated liquid crystals-a comparative quantum chemical calculations, Mol Cryst Liq Cryst,669 (2018) 12-26. 13 Schmidt M W et al. General atomic and molecular electronic structure system. J Comput Chem, 14 (1993) 1347-1363. 14 Sharma D & Tiwari S N. Comparative computational analysis of electronic structure, MEP surface and vibrational assignments of a nematic liquid crystal: 4-n-methyl-4´-cyanobiphenyl, J Mol Liq, 214 (2016) 128-135.  

Frontier Orbital and Conformational Analysis of a Nematic Liquid Crystal: 4-n-heptyloxy-4´-cyanobiphenyl

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Frontier Orbital and Conformational Analysis of a Nematic Liquid Crystal: 4-n-heptyloxy-4´-cyanobiphenyl

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