Orientational structure

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. The anisotropy of the orientational distribution of water has therefore been investigated in most studies of aqueous systems in inhomogeneous environments. The results can be summarized as follows. In almost all studied systems, a preference for orientations in which the water dipole moment is more or less parallel to the interface has been observed. The driving force for the avoidance of orientations that can lead to surface electrostatic 

In Ref. 49 the orientational distribution of water near the Pt(lOO) surface was investigated in great detail. In spite of the preference for adsorption of isolated water molecules through the oxygen atom, which is incorporated into the water-metal interaction potential, relatively few configurations were observed in which the dipole moment of the molecule points into the solution. The analysis will not be repeated here the interested reader is referred to Ref. 49. 

The orientational anisotropy ranges as far into the liquid phase as the density inhomogeneities do (roughly up to panel m), with increasingly less pronounced features. Slightly beyond the second maximum in the density profile the orientational distribution is isotropic, as it has to be the case for a bulk-like liquid. 

The behavior near the Pt(lOO) surface (see Ref. 150, 151) is qualitatively similar to the one on the Hg(l 11) surface. It is only noted here that with the flexible BJH model used in Ref. 150 the bimodal character of the distribution function is not observed, apparently because the flexibility of the model allows for a wider range of low energy orientations within the adlayer hydrogen bond network. 

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If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

A further increase in extension leads to irreversible changes which immediately precede the transition of the polymer into the oriented state. During this transition, the spherulites undergo considerable structural changes and are thus converted qualitatively into different structural elements i.e. macrofibrils4). After a certain critical elongation has been attained, the initial crystallites collapse and melt and a new oriented structure is formed in which the c axes of crystals are oriented in the direction of extension. 

Fairly recently, another method for obtaining polymer materials with uniaxial orientation has been developed. It is the directed polymerization i.e. the synthesis of polymers under conditions at which the material attains instanteneously the oriented structure. The formation of crystals from the macromolecules in an extended conformation occurs in those polymerizing systems simultaneously with polymerization22. ... 

T. Zavada, R. Kimmich 1998, (The anomalous adsorbate dynamics at surfaces in porous media studied by nuclear magnetic resonance methods. The orientational structure and Levy walks), J. Chem. Phys. 109, 6929. 

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). 

Fig. 9.6), the orientation smearing must first be extinguished (Sect. 9.7) before the scattering of the perfectly oriented structural entities is retrieved. 

The aim of orientation analysis is not only the quantitative description of orientation, but also the separation of orientational effects from topological ones - ultimately meaning the desmearing of imperfect orientation in order to reconstruct the scattering pattern of the perfectly oriented structural entities. 

The coefficients of the multipole expansion are computed from Eq. (9.8), and after analogous expansions of both the intensity of the perfectly oriented structural entity (i0pt, be), and of the measured intensity (/, ce), Ruland [253] obtains a set of algebraic equations among the expansion coefficients,... 

Figure 4 is an optical absorption spectrum from a multilayer assembly and shows the sharp absorption in the visible characteristic of the polydiacetylenes. Electron diffraction reveals a crystalline layered structure. However, registry between layers is less than perfect. Electron diffraction from a few layers indicates a strong possibility for growing well-oriented structures, and this is being pursued in our laboratory. 

The urethane-substituted polydiacetylenes exhibit thermo-chromic transition with low and high temperature crystal phases favoring acetylenic and butatriene backbone, respectively (4-6). Our interest in the application of epitaxial polymerization to diacetylenes has been the possibility of substrate control over orientation, structure, and the single crystal nature of thin films. 

The question now arises of what simplification is possible in the treatment of orientationally structured adsorbates and what general model can be involved to rationalize, within a single framework, a diversity of their properties. Intermolecular interactions should include Coulomb, dispersion, and repulsive contributions, and the adsorption potential should depend on the substrate constitution and the nature of adsorbed molecules. However difficult it may seem, all these factors can be taken into account if we follow the description pattern put forward in this book. Its fundamentals are briefly sketched below. 

Analyzing orientational structures of adsorbates, assume that the molecular centers of mass are rigidly fixed by an adsorption potential to form a two-dimensional lattice, molecular orientations being either unrestricted (in the limit of a weak angular dependence of the adsorption potential) or reduced to several symmetric (equivalent) directions in the absence of lateral interactions. In turn, lateral interactions should be substantially anisotropic. 

In the present book, we aim at the unified description of ground states and collective excitations in orientationally structured adsorbates based on the theory of two-dimensional dipole systems. Chapter 2 is concerned with the discussion of orientation ordering in the systems of adsorbed molecules. In Section 2.1, we present a concise review on basic experimental evidence to date which demonstrate a variety of structures occurring in two-dimensional molecular lattices on crystalline dielectric substrates and interactions governing this occurrence. 

Chapter 3 is devoted to dipole dispersion laws for collective excitations on various planar lattices. For several orientationally inequivalent molecules in the unit cell of a two-dimensional lattice, a corresponding number of colective excitation bands arise and hence Davydov-split spectral lines are observed. Constructing the theory for these phenomena, we exemplify it by simple chain-like orientational structures on planar lattices and by the system CO2/NaCl(100). The latter is characterized by Davydov-split asymmetric stretching vibrations and two bending modes. An analytical theoretical analysis of vibrational frequencies and integrated absorptions for six spectral lines observed in the spectrum of this system provides an excellent agreement between calculated and measured data. 

Substrate Molecule So Molecular lattice (Distances in A) a b a Orientational structure (Angles in degrees) 6 q> Method and Refs. 

For polar molecules, dipole-dipole interactions provide a paramount contribution to Coulomb interactions. The orientational structure is determined by the ground state of the dipole-dipole Hamiltonian ... 

A theoretical treatment of the effect caused by the competition between the sine-like angular-dependent component of the adsorption potential and dipole lateral interaction demonstrated that the values 6 are the same in the ground state and at the phase transition temperature.81 Study of the structure and dynamics for the CO monolayer adsorbed on the NaCl(lOO) surface using the molecular dynamics method has also led to the inference that angles 0j are practically equalized in a wide temperature range.82 That is why the following consideration of orientational structures and excitations in a system of adsorbed molecules will imply, for the sake of simplicity, the constant value of the inclination angle ty =0(see Fig. 2.14) which is due to the adsorption potential u pj,q>j). 

Fig. 2.14. A schematic representation of orientational structure for adsorbed molecules inclined at the same angle to the surface normal.

An inference of fundamental importance follows from Eqs. (2.3.9) and (2.3.11) When long axes of nonpolar molecules deviate from the surface-normal direction slightly enough, their azimuthal orientational behavior is accounted for by much the same Hamiltonian as that for a two-dimensional dipole system. Indeed, at sin<9 1 the main nonlocal contribution to Eq. (2.3.9) is provided by a term quadratic in which contains the interaction tensor V 2 (r) of much the same structure as dipole-dipole interaction tensor 2B3 > 0, B4 < 0, only differing in values 2B3 and B4. For dipole-dipole interactions, 2B3 = D = flic (p is the dipole moment) and B4 = -3D, whereas, e.g., purely quadrupole-quadrupole interactions are characterized by 2B3 = 3U, B4 = - SU (see Table 2.2). Evidently, it is for this reason that the dipole model applied to the system CO/NaCl(100), with rather small values 0(6 25°), provided an adequate picture for the ground-state orientational structure.81 A contradiction arose only in the estimation of the temperature Tc of the observed orientational phase transition For the experimental value Tc = 25 K to be reproduced, the dipole moment should have been set n = 1.3D, which is ten times as large as the corresponding value n in a gas phase. Section 2.4 will be devoted to a detailed consideration of orientational states and excitation spectra of a model system on a square lattice described by relations (2.3.9)-(2.3.11). 

Fig. 2.15. Planar orientational structures of nonpolar molecules on a square lattice.

To treat the orientational structure of the monolayer formed by 02 molecules on a graphite surface, allowance must be made for the fact that an oxygen molecule is characterized not only by a nonzero magnetic moment but also by a record small quadrupole moment, so that dispersion interactions prevail over quadrupole interactions at intermolecular distances shorter than 10 A.79 In addition, the adsorbate lattice parameters give rise to very small minimum intermolecular distances, a 3.3 A, the parameter b 8.1 A markedly exceeding the values a. That is why, it is sufficient to consider only the nearest-neighbor interactions in a... 

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