Edgardo Garcia

Spontaneous ferroelectric order in a bent-core smectic liquid crystal of fluid orthorhombic layers.

The ferroelectric properties of bent-core liquid crystalline molecules emerge from ordering within the smectic layers. Macroscopic polarization density, characteristic of ferroelectric phases, is stabilized by dipolar intermolecular interactions. These are weakened as materials become more fluid and of higher symmetry, limiting ferroelectricity to crystals and to smectic liquid crystal stackings of fluid layers. We report the SmAPF, the smectic of fluid polar orthorhombic layers that order into a three-dimensional ferroelectric state, the highest-symmetry layered ferroelectric possible and the highest-symmetry ferroelectric material found to date. Its bent-core molecular design employs a single flexible tail that stabilizes layers with untilted molecules and in-plane polar ordering, evident in monolayer-thick freely suspended films. Electro-optic response reveals the three-dimensional orthorhombic ferroelectric structure, stabilized by silane molecular terminations that promote parallel alignment of the molecular dipoles in adjacent layers.

Theoretical study of CH4-CH4, CHF3-CH4, CH4-H2O, and CHF3-H2O dimers.

We have studied systems with typical hydrogen bonding and others with interaction involving hydrogen. CH(4)-CH(4), CH(4)-H(2)O, CHF(3)-CH(4), and CHF(3)-H(2)O dimers were studied using MPWB1K, PBE1PBE, MP2, and QCISD levels of theory with a large number of basis functions. The Pople 6-31+G(2d), 6-311++G(2d,2p), and 6-311++G(3df,3pd) as well as Dunning augmented aug-cc-pVDZ and aug-cc-pVTZ basis sets were used. The dimer geometries were fully optimized. An optimal basis set was determined for these systems to achieve a suitable compromise between accuracy and computational feasibility. A proper strategy was found for the electronic property calculations of dimers studied: the use of aug-cc-pVDZ as the optimal basis set at MP2 level. Dipole moments, polarizabilities, BSSE effects, and DeltaZPE were also analyzed for these dimers.

HFF : A FORCE FIELD FOR LIQUID CRYSTAL MOLECULES

Abstract Soft material simulations require an accurate representation of the intermolecular and intramolecular potential energy surface in order to achieve realistic predictions for their properties. The conformational potential of the molecules has a profound effect on many properties of molecular aggregates. This is usually a serious weakness of most commercial empirical force fields. In this paper we describe a force field (HFF), specifically designed for liquid crystal molecules. Intramolecular parameters are obtained from ab initio quantum mechanics calculations on model molecules which are substructures of liquid crystal molecules. HFF reproduces ab initio conformational energy profiles for model molecules with errors much smaller than commercial force fields. Our focus is not on creating general purpose force fields but on strategies to develop reliable force fields on-demand for specific needs.

QUANTUM CHEMISTRY BASED FORCE FIELDS FOR SOFT MATTER

Abstract We describe the use of ab initio electronic structure calculations in the development of high-quality classical interaction potentials for liquid crystal modeling. Our focus is on methods for the rapid, on-demand creation of force fields for use in mean field theory based calculations of materials properties, employed for routine pre-synthesis evaluation of novel liquid crystalline materials. The role of quantum chemistry in the development of intermolecular interaction potentials for large-scale simulations of soft matter is also discussed, and directions for future work are outlined. The utility of quantum chemistry derived force fields for liquid crystal modeling is illustrated by two example applications: mean field theory based prediction of the spontaneous polarization density P of ferroelectric liquid crystals, and large-scale simulation studies of the nanosegregation of polymer precursors in smectic liquid crystal hosts.

Rovibrational energy and spectroscopic constant calculations of CH 4 ⋯ CH 4, CH 4 ⋯ H 2 O, CH 4 ⋯ CHF 3, and H 2 O ⋯ CHF 3 dimers

In this work, we performed a thorough investigation of potential energy curves, rovibrational spectra, and spectroscopic constants for dimers whose interactions are mediated by hydrogen bonds and other hydrogen interactions. Particularly, we deal with CH 4 ⋯ CH4, CH 4 ⋯ H2 O, CH 4 ⋯ CHF3, and H 2O ⋯ CHF3 dimers by employing accurate electronic energy calculations with two different basis sets at the MP2 level of theory. Following this, the discrete variable representation method was applied to solve the nuclear Schrödinger equation, thus obtaining spectroscopic constants and rovibrational spectra. The harmonic constant, ωe, presents a direct relation to the strength of dimer interactions. As a general rule, it was found that a decrease of interatomic distances is followed by the increase of De for all dimers. This behavior suggests that the interaction of CH 4 ⋯ CH4 is the weakest among all dimers, followed by CH 4 ⋯ CHF3, CH 4 ⋯ H2 O and the strongest interaction given by the H 2O ⋯ CHF3 dimer.

Prolog representation of molecular structures and pattern recognition

Abstract Research regarding representation of chemical structures in computers has been performed since the 1970s, with two central goals: information manipulation in chemical data bases and computer assisted molecular synthesis. This work is involved with the latter subject. We show the need for a general flexible and efficient form of representation based on weighted graphs. Some algorithms for structural pattern recognition, such as functional groups, reaction sites, rings, isomorphism and topological symmetry have been developed. These algorithms were implemented in Prolog, a declarative language, used because of its facilities in symbolic computations and its natural back-tracking capability. With the use of this representation we have created a working environment that is simple and effective for the automatic manipulation of chemical structures. This framework also allows easy transformations between different representations, such as connectivity matrices, line notation or topological codes based on path counts, among others, allowing exploration of the particular advantages of each.

New SmAPF Mesogens Designed for Analog Electrooptics Applications

We have previously reported the first realization of an orthogonal ferroelectric bent-core SmAPF phase by directed design in mesogens with a single tricarbosilane-terminated alkoxy tail. Given the potentially useful electrooptic properties of this phase, including analog phase-only electrooptic index modulation with optical latching, we have been exploring its “structure space”, searching for novel SmAPF mesogens. Here, we report two classes of these—the first designed to optimize the dynamic range of the index modulation in parallel-aligned cells by lowering the bend angle of the rigid core, and the second expanding the structure space of the phase by replacing the tricarbosilane-terminated alkyl tail with a polyfluorinated polyethylene glycol oligomer.