Influence of the order parameter on the anchoring energy of liquid crystals

No theory of the polar and azimuthal anchoring energies of liquid crystals (LCs) has been developed on a molecular level, despite the scientific and practical topicality of the problem. The interaction energies of mesogenic molecules with graphite and polyethylene surfaces calculated previously by the method of atom-atom potentials are in good agreement with the experimental data, but, at the same time, the calculated polar and azimuthal anchoring energies are larger than their experimental values by one and two orders of magnitude, respectively. To explain these values, the anchoring energy has been assumed to depend not only on the interaction with the surface but also on the interaction between the LC molecules arranged in the model in the form of quasi-layers. The mesogenic molecules have been modeled by rods with virtual C’ atoms (carbon atoms with hydrogen atoms attached to them) “threaded” on them. The molecule orientation has been specified by the polar and azimuthal angles θ _i , φ _i and θ _j , φ _j relative to the directors of the ith and jth layers. The derived polar and azimuthal anchoring energies as well as their dependences on the order parameter have turned out to be close to the experimental data.     

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE _b in the binding energies of core-level electrons in the initial (ΔE _i) and final (ΔE _f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE _i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE _f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E _i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE _f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.     

Can 2‐acylpyrroles form an intramolecular hydrogen bond?

The formation of intramolecular hydrogen bonding by certain N‐substituted 2‐acylpyrroles has been demonstrated by B3LYP/aug‐cc‐pVDZ calculations, the quantum theory of atoms in molecules, and the natural bond orbital method. Total electron energy densities H_BCP at the bond critical point of the H?O bond were applied to analyze the strength of these interactions. The relations between quantum theory of atoms in molecules, carbonyl stretching vibrational modes ν_{C = O}, and natural bond orbital parameters associated with the formation of the C–H?O interaction have been established. The short contacts were found experimentally in the crystal structure of a new 2‐acylpyrrole derivative 5‐chloro‐2‐oxopentyl‐1‐(5‐chloro‐2‐oxopentyl)pyrrolo‐2‐carboxylate. The influence of 2‐ and N‐substitution of 2‐acylpyrroles on C‐H?O interaction energy is discussed. It was found that the methylene group may act as a proton donor leading to a red‐shift or blue‐shift phenomenon of the ν_C–H stretching mode. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical analysis of trends in hydrogen bonding involving halogen acceptors (F−At) covalently bonded to a group 14 atom (C−Pb)

In this work, extensive quantum-chemical calculations have been carried out to identify and elucidate trends in the hydrogen-bonding (HB) interaction involving halogen acceptors covalently bonded to a group 14 atom. A series of 25 heterodimers composed of MH₃X (where M = C?Pb and X = F?At) and HNC molecules have been selected as model complexes stabilised by the HB interaction occurring between the X atom of MH₃X and the H atom of HNC. The interaction energy (E_int) between MH₃X and HNC in the MH₃X···HNC complexes falls in the range from ?2.7 to ?10.8 kcal/mol, indicating weak or medium strength of HB in these complexes. The strength of HB in the complexes remains consistent with the well-known HB weakening as X gets heavier. Regarding the effect of M on E_int, the gradual strengthening of HB is observed while descending group 14, but only from M = Si to M = Pb. The trends in E_int are compared with various HB-related parameters obtained from vibrational analysis, the natural bond orbital (NBO) method, the symmetry-adapted perturbation theory (SAPT) and the quantum theory of atoms in molecules (QTAIM). The parameters that present clear (possibly linear) relationships with E_int have been selected to characterise the effect of M and X on the HB interaction.     

Comparisons between statistics of low energy collision cascades generated by full molecular dynamics and by its binary collision approximation

Abstract

Collision cascades in Cu, Au and Cu₃Au are generated by full molecular dynamics (MD) and by its binary collision approximation (BCA) with the Marlowe program. Cu and Au primaries have 1 keV initial energy.

The same Molière repulsive potential is used in both models for close encounters. In the MD model, this potential is carefully splined to the pair component of the N-body potential developed by Ackland and Vitek. In the BCA, this N-body interaction is roughly modeled by a constant isotropic 4 eV binding energy of the target atoms to their rest positions.

Time distributions of the number of atoms moving with a total energy higher than a threshold value E _d are compared and discussed. Recoil range distributions during the cascade development are discussed as well. The agreement between MD and BCA is fairly good in all cases for E _d larger than about 3 eV. In the case of smaller E _d-values, the BCA may result in an overestimate of the number of moving atoms in the late development of the cascades. This discrepancy is suggested to originate in the lack of attractive forces between the moving particles and the surrounding atoms in the BCA.     

Properties of ground state and anomalous quantum fluctuations in one-dimensional polaron-soliton systems—the effects of electron-two-phonon interaction and non-adiabatic quantum correlations

Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expansion scheme, the properties of the ground state and the anomalous quantum fluctuations are investigated in a strongly coupled electron-phonon system with special consideration of the electron-two-phonon interaction. The effective renormalization (αi) of the displacement of the squeezed phonons with the effect of the squeezed-coherent states of phonon and both the electron-displaced phonon and the polaron-squeezed phonon correlations have been combined to obtain the anomalous quantum fluctuations for the corrections of the coherent state. Due to these non-adiabatic correlations, the effective displacement parameter αi is larger than the ordinary parameter α (0) i . In comparison with the electron-one-phonon interaction (g) corrected as αig, we have found the electron-two-phonon interaction (g1) corrected as αi2 g1 is enhanced significantly. For this reason, the ground state energy (E(2) 0 ) contributed by the electron-two-phonon interaction is more negative than the single-phonon case (E(1) 0 ) and the soliton solution is more stable. At the same time, the effects of the electron-two-phonon interaction greatly increase the polaron energy and the quantum fluctuations. Furthermore, in a deeper level, we have considered the effect of the polaron-squeezed phonon correlation (f-correlation). Since this correlation parameter f > 1, this effect will strengthen the electron-one and two-phonon interactions by fαig and f2αi2 g1, respectively. The final results show that the ground state energy and the polaron energy will appear more negative further and the quantum fluctuations will gain further improvement.     

The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.     

Molecular dynamic simulation of core–shell structure: study of the interaction between modified surface of nano-SiO2 and PAMAA in vacuum and aqueous solution

The interaction between acrylamide acrylicacid copolymer (PAMAA) and the modified surface of nano-SiO₂ is investigated using the molecular dynamic (MD) simulation. The binding energies (E_binding) of interface, the concentration profiles of PAMAA and functional groups (carboxyl and acylamino) of corresponding model, the mean square displacements (MSD) and diffusion coefficients (D) of PAMAA in four systems with different modifiers are all calculated at 325 K in vacuum. Vinyl trimethoxy silane (VTEOS) shows best modification effect in the systems mentioned above. Furthermore, the effects of temperature on the interaction between VTEOS modified surface of nano-SiO₂ and PAMAA are studied at 300, 325, 350, 375 and 400 K in aqueous solution. Interesting results show that, water molecular layer reduces with the increase of temperature, and then improves the interaction between PAMAA and VTEOS modified surface of nano-SiO₂. The corresponding E_binding of interface, the radial distribution functions (RDF) of carbon atoms on the surface and oxygen atoms of water molecules, the concentration profiles of PAMAA on the surface of nano-SiO₂, the MSD and D of PAMAA are all studied seriously to find the reason of this counterintuitive phenomenon.     

Bulk chemical properties of new elements from one-atom-at-a-time data

For the volatile (oxo)halides of known elements the energies of desorption from the surface of fused silica measured in radiochemical gas-solid chromatographic experiments proved close to the sublimation energies. This could hardly be expected for interaction of isolated molecules with bare surface of ionic SiO₂. Because such regularity shows great promise for evaluation of bulk properties of transactinoid compounds we need to understand the origin of apparent inconsistency. The clue seems to be the real structure of the surface. It is initially rough (from μm down to nm scale), inherently heterogeneous at the molecular level, and contains atoms and groups with excessive energy. Exposing to ambient air produces numerous surface ≡SiOH groups. The halogenating agents employed in transactinoid studies chemically modify such surface: it gets tightly covered by halogen atoms and fragments of the agent molecule attached to the Si and O atoms. Now the molecules of new compounds cannot touch SiO₂ lattice; moreover, in some initially geometric wells (due to roughness), they get surrounded by halogen atoms bonded to the surface — the situation resembling that in their own bulk condensed phase. Hence, there must be sites with desorption energies up to the sublimation energy.     

Influence of the hydrogen bonding on the basicity of selected macrocyclic amines

The optimized minimum‐energy geometries of different macrocyclic amines and their protonated structures were determined by using ab initio and density functional theory (DFT) calculations. All the gas phase optimizations and energy calculations were performed at the DFT/B3LYP/6‐311++G(d,p) level of theory. The HF/6‐31 + G(d,p) level was used for all single point calculations in the solution phase. Geometry optimizations indicate that the most stable structures are stabilized by intramolecular hydrogen bonds. The proton affinity (PA) of macrocyclic amines is controlled by the strength of intramolecular hydrogen bonds of macrocyclic amines. These hydrogen bonds strongly influence the basicity of heteroatoms in macrocycles. The highest PA value among the studied macrocyclic amines was found to be 264.9 kcal mol^?1 for structure 7. This is comparable with PA of proton sponges such as 1,8‐bis(dimethylamino)naphthalene. The solution phase calculations were carried out in the dimethyl sulfoxide solution as a commonly used solvent in organic reactions. Natural bond orbital analysis was performed to calculate the charge transfers and the second‐order interaction energies (E⁽²⁾) between the donor and acceptor. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of hydrogen bonds. QTAIM studies showed that the intramolecular hydrogen bonds in these structures are electrostatic (closed‐shell) interactions as well as partially covalent and partially electrostatic in nature. Copyright © 2012 John Wiley & Sons, Ltd.     

Optical and EPR investigations of a new hydrogen center in potassium chloride

In KCl interstitially diffusing neutral hydrogen atoms (H _i ⁰ ) can be trapped near substitutional Li⁺ ions. The new center possesses a characteristic absorption band at 250nm, at the long wavelength side of the knownH _i ⁰ absorption band (235nm). EPR measurements reveal, that the hydrogen atoms are closely connected to the Li⁺ ions at a cation lattice site forming (LiH)⁺ molecules. Below 70K hydrogen and lithium occupy a fixed relative position to each other in a (111) crystal direction: the hydrogen shows a superhyperfine interaction to three equivalent Cl^– neighboring ions. Above 70K the hydrogen exhibits a superhyperfine interaction to six equivalent chlorine neighbors, interpreted by a rapid reorientational motion of the (LiH)⁺ molecule within the cation vacancy. The high temperature interaction parameters can be explained as the time average of the low temperature values. A qualitative explanation of the optical absorption of the (LiH)⁺ center is given.     

Ab Initio Investigation of the Microspecies and Energy in Hydrated Strontium Ion Clusters

Quantum chemistry calculations were used to study the structure and energy of strontium (Sr) ion hydrated clusters [Sr(H₂O)_1?25]²⁺. The saturated hydration number of the first hydration layer of Sr²⁺ was 8, and the hydration distance was 2.58 Å. The second hydration layer had 1–9 hydration numbers, and the hydration distance was in the range of 4.4–4.6 Å. This work also developed the relationship between the thermodynamic data (average water binding energy E_n and successive water binding energy ΔE_n,n?1, etc.) of the aforementioned low-energy structure and the hydration structures. The first hydration layer was formed by the strong electrostatic interaction between Sr²⁺ and water molecules, and the decrease in ΔE_n,n?1 was relatively large. Hydrogen bonds were formed between water molecules of the second hydration layer and water molecules of the inner layer, and the decrease in ΔE_n,n?1 was relatively small. When one water molecule was added beyond the second hydration layer, ΔE_n,n?1 was close to the hydrogen bond energy 8.88 kcal/mol (37.1 kJ/mol) of dimer water molecule, indicating that there was very weak interaction between Sr²⁺ and the water molecules beyond the second hydration layer.     

低能原子沉积在Pt(111)表面的分子动力学模拟

采用嵌入原子方法的原子间相互作用势,利用分子动力学方法模拟了六种贵金属原子(Ni,Pd,Pt,Cu,Ag,Au)分别在Pt(111)表面低能沉积的动力学过程.结果表明:随着入射能量从0.1eV升高到200eV,基体表面原子是按层迁移的,沉积过程对基体表面的影响和沉积原子在基体表层的作用均存在两个转变能量(ET1≈5eV,ET2≈70eV).当入射能量低于5eV时,基体表面几乎没有吸附原子和空位形成,沉积原子在基体表层几乎没有注入产生;当入射能量在5—70eV范围内时,沉积原子在基体表层有注入产生,其注入深度小于两个原子层,即为亚注入,此时吸附原子主要由基体表层原子形成,基体表面第三层以下没有空位形成;当入射能量高于70eV时,沉积原子的注入深度大于两个原子层,将会导致表面以下第三层形成空位,并且空位产额随入射能量的升高而急剧增加.基于分子动力学模拟的结果,对低能沉积作用下的薄膜生长以及最优沉积参数的选择进行了讨论.     

Study on the relations of sensitivity with energy properties for HMX and HMX-based PBXs by molecular dynamics simulation

Molecular dynamics simulation was applied to study the structure and energy properties of β-HMX (β-cyclotetramethylene tetranitramine) crystal and its composite PBXs (polymer-bonded explosives) with F₂₃₁₁ as a polymer binder under different temperatures and F₂₃₁₁ concentrations. The interface interaction energy of HMX and F₂₃₁₁, the interaction energy E_N–N between N atoms in N–NO₂ trigger bond in HMX molecules, and the cohesive energy density (CED) are presented and analyzed. A meaningful finding is that there exists correlation between E_N–N and the sensitivity of β-HMX and its composites, i.e. the less the E_N–N is, the larger the sensitivity is. Additionally, molecular interactions are inherently disclosed by using pair correlation function (PCF) to analyze the interfacial structure between (1 0 0)HMX crystal surface and F₂₃₁₁ molecular chain.     

Conformational studies of ethylene glycol and its two methyl ether derivatives

As a theoretical analysis of the conformational equilibria of ethylene glycol, methoxyethanol and dimethoxyethane, the energy of each stable conformational isomer (rotamer) of these molecules was calculated for various temperatures and solvent dielectric constants. Classical semi-empirical potential functions were used. Besides intrinsic potentials for rotation about single bonds, intramolecular dispersion and repulsive interaction, dipole-dipole interaction and hydrogen bonding energies were included. Interaction with the solvent was considered only in terms of a continuous dielectric medium interacting with the local dipoles and quadrupoles of the molecule. For each rotamer, the dihedral angles giving the lowest energy were determined. From the energies of each rotamer, Boltzmann distributions of populations were obtained, and total concentrations were calculated in various physically distinguishable states, e.g. those with and without internal hydrogen bonds, or those in which the central C-C bond takes a trans or a gauche conformation. It is shown that the equilibrium constants, K _HB and K _TG, for these two cases are not identical. While changes in the dielectric constant may alter strongly the geometries and energies of individual rotamers, their effect on the average geometries and on the two equilibrium constants is small. The same is true of temperature changes, and is due to the presence of several rotamers in each of the physical states considered. Thus the small temperature dependence of some observed physical properties is shown to be consistent with the distribution of molecules over several conformational states. In solution, the fraction of ethylene glycol molecules with two free OH groups (i.e. without an intramolecular hydrogen bond) is predicted to be at least 20 per cent. This shows that the presence of three-dimensional hydrogen-bonded structures in the liquid, which we propose, is possible in principle.     

The DFT study on non‐conjugated polymer host materials based on styrene derivatives for phosphorescent polymer light‐emitting diodes

A series of poly(4,4‐vinyltriphenylamine) based non‐conjugated polymer as host molecules are designed and studied by density functional theory. The results show that the substituent has a great influence on the properties of polymer. The parent molecule directly linked para‐carbazole, β‐pyrrole and triphenylamine are favorable to hole injection, and para‐carbazole could significantly increase E_T of the host molecules. The large changes of structural parameters between the lowest triplet state and ground state can cause the decrease of E_T. Moreover, parent molecule directly linked carbazole and triphenylamine units possess strong intramolecular charge transfer and low singlet and triplet energy difference (?E_ST). The calculated results also show that all designed host molecules are suitable for green emitter by comparing with the E_T. S₁ → S₁ and T₁ → T₁ energy transfer mechanism between host and guest is thermodynamically feasible. In addition, host–guest model is built to study the charge transfer nature, and the results indicate that a good intermolecular charge transfer can be achieved between host and guest materials. In the designed host molecules, the N atom of parent molecule linked para‐carbazole substituent shows a great potential for the green phosphorescent polymer light‐emitting diodes. Copyright © 2015 John Wiley & Sons, Ltd.     

Deuterium-deuterium fusion by an activated precursor

Summary A general scheme is proposed for the interpretation of the phenomena involving low-energy hydrogen-isotope fusion. This scheme is especially developed for the interpretation of the fusion rate observed after the impact of heavy-water clusters (D₂O) _n , 25≲n≲1350, onto targets of titanium deuteride TiD. It is shown that 1) the impinging energy of large clusters or molecules is equiparted among a lot of target atoms which are brought in collective motion; 2) data can conveniently be represented in an Arrhenius plot; 3) this plot suggests that fusion is a thermally activated process from a metastable precursor; 4) the activation energy for the precursor formation isE ^*≃2E ₀ (E ₀ being the electron binding energy in the hydrogen atom), and 5) the activated precursor can reasonably be identified with the metastable binuclear heliumlike (D⁺D⁺)2e⁻ atom.     

Effect of molecular structural elements of chloro-substituted derivatives of naphthalene on intramolecular interactions, which determine T1 ? S0 nonradiative transition

By calculating the matrix elements of operators of adiabatic and nonadiabatic interactions, which determine the process T ₁ ^S ↝ S ₀ in molecules of naphthalene and its 1,4- and 2,3-dichloro-substituted derivatives, we study how various factors of intramolecular interactions affect this process. The effects of α and β chlorine substituents on spin-orbit (SO) interactions in the carbon backbone of the molecule and in its chlorine atoms, where the compensation effect of SO interactions was revealed, are separated. It is found that high-frequency out-of-plane promoting modes (corresponding to vibrations of CCH groups) differently affect SO interactions in the carbon backbone of molecules and in chlorine atoms.     

Nature of gallium deep centres in lead telluride based semiconductors

Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal, interaction of various gallium-containing molecules with defect-free crystal as well as with native defects in PbTe was considered. Formation energies for different point defects created in PbTe as a result of interaction the Ga₂Te molecules, Ga₂ dimers and single Ga atoms with a host crystal were calculated using density functional theory. Particularly Ga_Pb and Ga_i together with formation of accompanied self interstitials Pb_i in various charge states were examined. In addition we propose the new type of defects - the impurity complex (2Ga)_Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest the double donor behaviour and DX-like properties of this defect together with extremely low formation energy values. Namely, (2Ga)_Pb centres are preferably formed under Ga₂Te doping while (Ga₂)_Pb+Pb_i ones are formed under Ga₂ or Ga doping. In all cases, formation energies are negative and resulting defect concentration is determined by reaction kinetics only. Mechanisms of the lead vacancy compensation with the vapour phase doping are considered as well.