Additive Potential Model Research Articles

Spontaneous striped pattern formation in thin chiral nematic liquid crystal layers

Periodic modulations in chiral nematic liquid crystals (CLCs) are used in several optical and photonic devices. A remarkable way of fabricating and controlling such patterns is in a thin wetting layer that grows as temperature is decreased. Here, we study this stripe formation process experimentally and by means of Monte Carlo simulations. We report on samples of CLCs that present, after the wetting transition, a stripe/fringe formation occurring either through a nucleation process or homogeneously across the cell. These patterns depend on the ratio of the pitch to thickness of the layer, elastic anisotropy, and anchoring strength. Although such process is well documented for CLCs under applied field, we show here that similar textural transitions occur in thin layers mediated by the weak anchoring at the CLC-isotropic interface; it happens for values of pitch to thickness ratio much smaller than the ones reported in field driven situation. The Monte Carlo method takes action in simulating the experimentally observed textures by using a pairwise additive potential model for CLCs. A qualitative comparison with the experimental phenomena is possible to be made in which both the stripes formation and their behavior agree with the experiments.

A Simple Additive Potential Model for Simulating Hydrogen Peroxide in Chemical and Biological Systems.

Hydrogen peroxide (H2O2) has numerous industrial, environmental, medical, cosmetic, and biological applications. Given its importance, we provide a simple model as an alternative to experiment for studying the properties of pure liquid H2O2 and its concentrated aqueous solutions, which are hazardous, and for understanding the biological roles of H2O2 at the molecular level. A four-site additive model is calibrated for H2O2 based on the ab initio and experimental properties of the gaseous monomer and the density and heat of vaporization of liquid H2O2 at 0 °C. Our model together with the TIP3P water model reproduce the ab initio binding energies of (H2O2) m, H2O2· nH2O, and nH2O2·H2O clusters ( m = 2, 3 and n = 1, 2) calculated at the MP2 level using the 6-311++G(d,p) or the 6-311++G(3df,3pd) basis set. It yields structure, the self-diffusion coefficient, heat capacity, and densities at temperatures up to 200 °C of the pure liquid in good agreement with experiment. The model correctly predicts the hydration free energy of H2O2 and reproduces the experimental density of aqueous H2O2 solutions at 0-96 °C. Investigation of the solvation of H2O2 and H2O in aqueous H2O2 solutions reveals that, as in the gas phase, H2O2 is a better H-bond donor but poorer acceptor than H2O and the bonding stability follows the order Op-Hp···Ow > Ow-Hw···Ow ≥ Op-Hp···Op > Ow-Hw···Op. Stronger H-bonding in H2O/H2O2 mixtures than in the pure liquids is consistent with exothermic heats of mixing and explains why the observed density and vapor pressure of the aqueous solutions are higher and lower, respectively, than expected from ideal mixing. Results also show that H2O2 adopts a skewed equilibrium geometry in gas and liquid phases but more polar cis and nonpolar trans conformations also are accessible and will stabilize H2O2 in environments of different polarity. In sum, our simple model presents a reliable tool for simulating H2O2 in chemistry and biology.

The evolution of intermediate-range order in molten network-forming materials

The atomistic origin of the intermediate-range order (IRO) is investigated for an archetypal network-forming liquid. A pairwise additive potential model is chosen which is augmented with a description of the (many-body) anion polarization. The anion polarizability and system temperature are both systematically varied in order to control the network topology. The change in the IRO is monitored via the construction of Bhatia-Thornton structure factors which highlight the effect of chemical composition and network topology. The atomistic origin of the first-sharp diffraction peak in the concentration-concentration function, S(CC)(k), is discussed in terms of the connectivity of the polyhedral network. The atomistic origin of the IRO is discussed by reference to previous analyses.

Molecular Conformations in a Phospholipid Bilayer Extracted from Dipolar Couplings: A Computer Simulation Study

This paper describes an analysis of NMR dipolar couplings in a bilayer formed by dimyristoylphosphatidylcholine (DMPC). The couplings are calculated from a trajectory generated in a molecular dynamics (MD) simulation based on a realistic atom-atom interaction potential. The analysis is carried out employing a recently developed approach that focuses on the construction of the conformational distribution function. This approach is a combination of two models, the additive potential (AP) model and the maximum entropy (ME) method, and is therefore called APME. In contrast to the AP model, the APME procedure does not require an intuition-based choice of the functional form of the torsional potential and is, unlike the ME method, applicable to weakly ordered systems. The conformational distribution function for the glycerol moiety of the DMPC molecule derived from the APME analysis of the dipolar couplings is in reasonable agreement with the "true" distributions calculated from the trajectory. Analyses of dipolar couplings derived from MD trajectories can, in general, serve as guidelines for experimental investigations of bilayers and other complex biological systems.

Tetraethyl stannane: structure, conformations, and orientational order when dissolved in a nematic liquid crystalline solvent

Quantum chemical calculations are used to investigate the structure and the distribution of conformers of tetraethyl stannane, and these are compared with those of tetraethyl methane. The calculations predict that the most abundant conformers in tetraethyl stannane are those with symmetry C1, in contrast to tetraethyl methane, in which the D2d and S4 conformers are dominant. The structural and conformational information on tetraethyl stannane are then used, together with the additive potential model, to explain the finite quadrupolar splittings observed in the deuterium NMR spectrum of tetraethyl stannane dissolved in a nematic liquid crystalline solvent. This analysis emphasizes that to understand the orientational order of a flexible molecule in a liquid crystalline phase it is essential to consider the symmetry of individual conformers rather than that of an average structure.

The temperature and density dependence of fluid xenon self-diffusion coefficients: a comparison between experimental, theoretical and molecular dynamics results

The molecular dynamics (MD) simulation technique has been employed to investigate the self-diffusion coefficients of neat liquid and supercritical (SC) Xenon over a wide range of thermodynamic conditions. The fluid has been studied at densities in the range 0.0700–3.2141 g/cm 3 and temperatures from 203 to 343 K. The simulations were based on a pair-wise additive potential model with ɛ/ k B = 212 K and σ = 0.3905 nm, which is presented for the first time here. The self-diffusion coefficients obtained are shown to be in good agreement with experimental values reported by Prins and coworkers [P.W.E. Peereboom, H. Luigjes, K.O. Prins, An NMR Spin-echo study of self-diffusion in Xenon, Physica A 156 (1989) 260], covering PVT data up to 1500 bar. Theoretical results for self-diffusion coefficients from various theoretical and empirical models are tested against experimental and MD results. In addition, the structure of the fluid is presented in terms of the pair distribution functions and possible correlation of its local behaviour with diffusivity is briefly discussed.

Conformational distribution functions extracted from residual dipolar couplings: A hybrid model based on maximum entropy and molecular field theory

This paper describes a new approach for analysis of residual dipolar couplings (RDCs). The method, which focuses on construction of the conformational distribution function, is applied to 4-n-pentyl-4′-cyanobiphenyl in the nematic phase. The RDCs are calculated from a trajectory generated in a molecular dynamics (MD) simulation, based on a realistic atom–atom interaction potential. Computer simulation is an attractive method for investigating theoretical models for partially ordered systems since the answer is provided: we know the true orientational order and molecular structure. Our new approach is based on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: the additive potential (AP) model and the maximum entropy (ME) method. These models suffer, however, from serious limitations: the AP model requires a priori knowledge of the functional form of the torsional potential, whereas the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented here (which we call APME) does not require knowledge of the functional form of the intramolecular potential and is applicable to weakly ordered systems. This makes the APME model a potentially useful tool for investigations of conformations in biomacromolecules dissolved in dilute aqueous liquid crystals. In the investigation reported in the present study, the results from the APME analysis are in excellent agreement with the true molecular structure in the MD simulation. The estimation of the validity range indicates that the APME approach is applicable to weakly ordered systems as well as to conventional nematic mesophases.

A Pairwise Additive Potential for the Elastic Interaction Energy of a Chiral Nematic

We have developed a pairwise additive potential model to describe the macroscopic elastic interactions of a chiral nematic liquid crystal. The potential is obtained from the expression for the elastic free energy density discretized onto a cubic lattice and mapped onto a suitable expansion in scalar invariants. The value of the pair potential is explored by means of a Monte Carlo lattice computer simulation. It allows, in a simple and efficient way, the simulation of liquid crystal devices and samples in confined geometries, taking into account the effect of the temperature of the sample and of the thermal fluctuations in the director distribution, as well as the elastic interactions.

Conformational distribution function of a disaccharide in a liquid crystalline phase determined using NMR spectroscopy.

A new approach is proposed for analysis of NMR parameters in general, and residual dipolar couplings in particular. The method, which enables construction of the conformational distribution function, is applied to a disaccharide dissolved in a dilute liquid crystal. Our approach rests on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: (i) the additive potential model (AP) and (ii) the maximum entropy method (ME). These models suffer, however, from serious limitations: the AP model requires an a priori knowledge of the functional form of the torsional potential, while the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented herein (APME) is valid in the low-order limit. The intraresidue dipolar couplings were used to determine the orientational order, while the conformational distribution function is constructed from the interresidue dipolar- and J couplings, together with NOEs. Very good agreement between experimental and calculated NMR parameters was observed. An attempt to analyze the experimental data using a single molecular conformation resulted in significantly larger errors. This study demonstrates that the APME method can be used as a tool in structure determination of flexible molecules in dilute liquid crystals.

Monte Carlo and reverse Monte Carlo simulations on molten zinc chloride

Monte Carlo and reverse Monte Carlo simulations on molten ZnCl2 were performed on the basis of the total atom pair correlation functions, recently determined with X-ray diffraction experiments at T=623 and 873 K. A new pair-wise additive potential model is presented that is based on an intermediate-range oscillating function for the Zn–Zn interaction in combination with a modified Coulomb potential. This potential mimics the effect of induced anion polarization, which has been found as to be the reason for the highly directional interactions between the cations and anions in the melt. With this simple potential model, the experimentally determined total atom pair correlation functions and the total structure factor, the prepeak included, are well reproduced. The analysis of the configurations, generated with the Monte Carlo method, confirms the existence of a network structure with rings and chains of small and medium size consistent with corner connected [ZnCl4] units of high tetrahedricity. In contrast, the reverse Monte Carlo method generates more disordered structures and is obviously not able to give a correct description of the ion arrangement in molten ZnCl2.

A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of He79Br2

Pump-probe spectra of HeBr2 in vibrational states v′=10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0g+)←B(3Π0u+) spectra of Br2. The experimental results are compared to theoretical calculations on the B←X spectra using atom-atom model potentials and performing a thermal average over transitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of v′=44, where the Δv=−1 channel closes, and in the region of v′=48 where the Δv=−2 channel closes. For v′=43, and v′=44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold effects associated with the Δv=−1 channel closing. Similar sensitivity to the potential due to the Δv=−2 channel closing impairs the agreement between experiment and theory for v′=48. Below v′=43, the rotational distributions for Δv=−1 and Δv=−2 are quite similar. Above v′=43 the peaks of the rotational distributions for Δv=−2 move to higher values of j. These results are compatible with the theoretical conclusion that dissociation shifts from a direct mechanism to one involving intramolecular vibrational distribution in the region of the closing of the Δv=−1 channel. Although the simple additive potential model used in this work succeeds in reproducing most of the experimental data for this system, further improvements in the potential energy surface will be required to achieve precise agreement between experiment and theory for large Br-Br separations.

Proton two-dimensional multiple pulse NMR experiments on the nematic liquid crystal MBBA

Two types of 1H two-dimensional (2D) multiple pulse experiments were performed on the nematic liquid crystal MBBA (4'-methoxybenzylidene-4-n-butylaniline). In one of them, a multiple pulse sequence is applied in the t 2 dimension to separate the 1H–1H dipolar spectra by the chemical shifts. The other type is a 2D exchange NMR experiment with multiple pulse sequences in both t 1 and t 2 periods by which spin diffusion between protons with different chemical shift values is observed through cross-peaks. The data were analysed by an additive potential model. The dipolar spectra obtained by the first experiment were found to be sensitive to the twist angles of the two phenyl rings from the -CH=N- plane. The twist angles were determined to be 33° for the N-side phenyl ring and 20° for the C-side phenyl ring.

Proton two-dimensional multiple pulse NMR experiments on the nematic liquid crystal MBBA

Two types of 1H two-dimensional (2D) multiple pulse experiments were performed on the nematic liquid crystal MBBA (4'-methoxybenzylidene-4-n-butylaniline). In one of them, a multiple pulse sequence is applied in the t 2 dimension to separate the 1H–1H dipolar spectra by the chemical shifts. The other type is a 2D exchange NMR experiment with multiple pulse sequences in both t 1 and t 2 periods by which spin diffusion between protons with different chemical shift values is observed through cross-peaks. The data were analysed by an additive potential model. The dipolar spectra obtained by the first experiment were found to be sensitive to the twist angles of the two phenyl rings from the -CH=N- plane. The twist angles were determined to be 33° for the N-side phenyl ring and 20° for the C-side phenyl ring.

An NMR study of the conformational flexibility of phenyl acetate dissolved in a nematic liquid crystalline solvent

Abstract The 1H, 2H and 13C NMR spectra of phenyl acetate, phenyl acetate-[13CO] and phenyl acetate-[C2H3] dissolved in a nematic liquid crystalline solvent have been analysed to yield dipolar coupling, D ij. These have been interpreted using the additive potential model to provide information on the molecular conformation, resulting in three possible shapes for V(φ), the potential energy for rotation about the ring-oxygen bond. A comparison with the results of molecular orbital calculations leads to the conclusion that a potential with a minimum at 54.4° ± 0.1° is the most probable.

Relativistic impulse approximation in an additive potential model.

Relativistic impulse approximation in an additive potential model.

The potential model of colored quarks: Success for single-hadron states, failure for hadron-hadron interactions

The success of the additive potential model of colored quarks for the masses, decay rates, and other properties of single mesons and baryons does not imply that this model can yield the observed meson-nucleon and nucleon-nucleon interactions. We give a comprehensive discussion of this issue. In agreement with previous authors, we conclude that, on the contrary, this model predicts inverse-power color-analog van der Waals potentials between separated hadrons which are in substantial contradiction with experimental data. We also discuss pathologies of non-abelian confining potentials, and show that the hamiltonian is unbounded below for an arbitrary number of quarks and antiquarks in a definite color state for all color states, except the singlet, triplet, and antitriplet.