Molecular Structure of p-Azoxyanisole, a Mesogen, Determined by Gas-Phase Electron Diffraction Augmented by ab Initio Calculations

DFT Studies of Selected Epoxies with Mesogenic Units-Impact of Molecular Structure on Electro-Optical Response

Theoretical studies of molecular structure and electric charge distribution were carried out for three epoxy compounds with different mesogenic cores. The compounds exhibit a nematic phase and form polymer networks that are potential bases for various composites. Results were compared to analogous materials with non-polar chains. A customized process involving geometry optimization of a series of conformations was employed to greatly increase likelihood of reaching global energy minimum for each molecule. All computations used Density Functional Theory (DFT) electron correlation model with the B3LYP hybrid functional. Molecular structure calculations yielded several parameters, including the magnitude and direction of the dipole moment, polarizability (α), first hyperpolarizability (β), and highest-occupied/lowest-unoccupied molecular orbital (HOMO-LUMO) energies. These parameters can help predict electronic properties of the nematic phase and the polymer network and assess their predisposition for application in electrooptical devices. In particular, the magnitude and direction of the dipole moment determine molecular alignment of liquid crystal phases in electric field, which enables controlling molecular order also in cured networks. Theoretical results were supplemented with observations of the nematics and their behavior in electric field. It was demonstrated for the studied compounds that a change in aliphatic chain polarity helps preserve and reinforce perpendicular alignment of molecules induced by electric field.

Theoretical studies on the role of π-electron delocalization in determining the conformation of N-benzylideneaniline with three types of LMO basis sets

To understand the role of pi-electron delocalization in determining the conformation of the NBA (Ph-N==CH-Ph) molecule, the following three LMO (localized molecular orbital) basis sets are constructed: a LFMO (highly localized fragment molecular orbital), an NBO (natural bond orbital), and a special NBO (NBO-II) basis sets, and their localization degrees are evaluated with our suggesting index D(L). Afterward, the vertical resonance energy DeltaE(V) is obtained from the Morokuma's energy partition over each of three LMO basis sets. DeltaE(V) = DeltaE(H) (one electron energy) + DeltaE(two) (two electron energy), and DeltaE(two) = DeltaE(Cou) (Coulomb) + DeltaE(ex) (exchange) + DeltaE(ec) (or SigmaDeltaE(n)) (electron correction). DeltaE(H) is always stabilizing, and DeltaE(Cou) is destabilizing for all time. In the case of the LFMO basis set, DeltaE(Cou) is so great that DeltaE(two) > |DeltaE(H)|. Therefore, DeltaE(V) is always destabilizing, and is least destabilizing at about the theta = 90 degrees geometry. Of the three calculation methods such as HF, DFT, and MPn (n = 2, 3, and 4), the MPn method provides DeltaE(V) with the greatest value. In the case of the NBO basis set, on the contrary, DeltaE(V) is stabilizing due to DeltaE(Cou) being less destabilizing, and it is most stabilizing at a planar geometry. The LFMO basis set has the highest localization degree, and it is most appropriate for the energy partition. In the NBA molecule, pi-electron delocalization is destabilization, and it has a tendency to distort the NBA molecular away from its planar geometry as far as possible.

Effects of molecular structure on the stability of a thermotropic liquid crystal. Gas electron diffraction study of the molecular structure of phenyl benzoate

As a model of the core of molecules forming liquid crystals, the molecular structure of phenyl benzoate (Ph-C(=O)-O-Ph) at 409 K was determined by gas electron diffraction, and the relationship between the gas-phase structures of model compounds and the nematic-to-liquid transition temperatures was studied. Structural constraints were obtained from RHF/6-31G ab initio calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coordinate analysis. Thermal vibrations were treated as small-amplitude motions, except for the phenyl torsion, which was treated as a large-amplitude motion. The potential function for torsion was assumed to be V(phi(1),phi(2)) = V(12)(1 - cos 2phi(1))/2 + V(14)(1 - cos 4phi(1))/2 + V(22)(1 - cos 2phi(2))/2, where phi(1) and phi(2) denote the torsional angles around the C-Ph and O-Ph bonds, respectively. The potential constants (V(ij)()/kcal mol(-)(1)) and the principal structure parameters (r(g)/A, angle(alpha)/deg) with the estimated limits of error (3sigma) are as follows: V(12) = -1.3 (assumed); V(14) = -0.5(9); V(22) = 3.5(15); r(C=O) = 1.208(4); r(C(=O)-O) = 1.362(6); r(C(=O)-O) - r(O-C) = -0.044 (assumed); r(C(=O)-C) = 1.478(10); <r(C-C)> = 1.396(1); angleOCO = 124.2(13); angleO=CC = 127.3(12); angleCOC = 121.4(22); ( angleOCC(cis) - angleOCC(trans))/2 = 3.0(15); ( angleC(=O)CC(cis) - angleC(=O)CC(trans))/2 = 4.8(17), where < > means an average value and C-C(cis) and C-C(trans) bonds are cis and trans to the C(=O)-O bond, respectively. The torsional angle around the O-Ph bond was determined to be 64(+26,-12) degrees. An apparent correlation was found between the contributions of the cores to the clearing point of liquid crystals and the gas-phase structures of model compounds of the cores of mesogens, i.e., phenyl benzoate, trans-azobenzene (t-AB), N-benzylideneaniline, N-benzylideneaniline N-oxide (NBANO), trans-azoxybenzene (t-AXB), and trans-stilbene. The structures of t-AB, NBANO, and t-AXB have been obtained by our research group.