Preferred orientation in filtercakes of kaolinite

The orientational order parameter of dense colloidal dispersions of plate-like particles as a function of volume fraction is measured using neutron diffraction. This non-invasive experimental approach directly provides the full particle orientation distribution from which the order parameters can be calculated. The orientation parameters are shown to be linked to the solids fractions of the cakes and the macroscopic permeability of the samples. However, this study suggests that, although orientation can be relevant for a given system, other factors can have a stronger influence, for example, the degree of dispersion or colloidal stability of the clay and may be the principle factor that controls permeability. In addition, we report enhanced ordering of these materials under the influence of an external cross-flow field.     

Dipole moments of symmetrical molecular systems

It is shown that quantization of nuclear motion causes the intrinsic dipole moment of a molecular system to depart from the classical representation, e.g., it is different from zero for symmetrical molecules. A formula is derived for the mean dipole moment ¯p as a function of temperature with allowance for the internal motion of the nuclei, which is functionally related to the dipole moment. Calculations are performed for ammonia with allowance for the inversion splitting, which is due to tunneling between two equivalent equilibrium configurations having their dipole moments in opposite directions. The temperature coefficient of ¯p may be positive or negative, in accordance with the relation between the tunneling frequency and the temperature; the formula usually employed is valid only in the limiting case of low frequencies and high temperatures. A deduction is given for the criterion for instability of the maximally symmetrical configuration with respect to odd nuclear displacements (dipole distortions); this is based on a simple model system having an inversion center, a totally symmetric ground state, and a triply degenerate odd excited state of the T_1u type. The experimental consequences of the results are discussed, as well as the concept of symmetry for a molecular system in which the maximally symmetrical configuration is unstable.     

Crystallization in oriented polymers: A framework for analyzing orientation distributions

A framework is presented for analyzing the changes that occur in the orientation distribution of polymer segments when crystallization occurs in an initially amorphous precursor. Existing theories which relate crystalline orientation distribution to that of the precursor can be incorporated in this framework in a straightforward manner. The orientation distribution of segments that remain uncrystallized is inferred from a kinetic analysis of the incorporation of precursor segments into growing crystals. The relevance of such a framework in understanding polymer fabrication processes is discussed briefly with examples in fiber formation from the melt.     

Elimination of the effect of orientation distributions in fiber diagrams

Summary A general mathematical treatment is given for the problem of eliminating the effect of orientation distributions in fiber diagrams. The method involves the inversion of an integral transform the kernel of which is determined by an orientation function already derived in an earlier paper (Ruland andTompa (3)). The inversion can be obtained by a development of the intensity distributions into a series of Legendre polynomials.

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Zusammenfassung Es wird eine allgemeine mathematische Behandlung des Problems der Eliminierung des Einflusses von Orientierungsverteilungen in Faserdiagrammen durchgeführt. Die Methode erfordert die Inversion einer Integraltransformierten, deren Kern durch eine schon in einer früheren Arbeit (Ruland undTompa (3)) entwickelte Orientierungsfunktion gegeben ist. Die Inversion kann mit Hilfe einer Reihenentwicklung der Intensitätsverteilungen nach Kugelfunktionen durchgeführt werden.

     

Excited mixed-valence states of symmetrical donor-acceptor-donor pi systems

We investigated the spectroscopic properties of a series of four bistriarylamine donor-pi-bridge-donor D-pi-D compounds (dimers), composed of two asymmetric triarylamine chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were recorded and compared with those of the corresponding D-pi monomers. Bilinear Lippert-Mataga plots indicate a major molecular reorganization of the excited state in polar media for all compounds. The excited states of the dimers are described as mixed-valence states that show, depending on the chemical nature of the pi bridge, a varying amount of interactions (couplings). We found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling. Whether the first excited-state potential energy surface of the dimers is described by single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a symmetry broken CT state with partial positive charge at the triarylamine donor and negative charge at the pi bridge. The [2.2]paracyclophane bridged dimer is an example of a weakly coupled system because the spectroscopic behavior is very similar to the corresponding p-xylene monomer. In contrast, anthracene as well as p-xylene bridges mediate a stronger coupling and reveal a significant cooperative influence on the optical properties.     

Barriers to conformational transformations in symmetrical linear conjugated systems

A quantum-chemical study has been carried out on conformational transformations in the ground and first excited states of symmetrical polymethine dyes and related α, ω-disubstituted polyenes. The magnitude of the trans-cis isomerization barriers depends on the length of the conjugation chain, position of the rotated bond, electron-donor capacity of the terminal groups, and occupancy of the electron shell. The rotation of molecular fragments in the excited state may lead to a change in the nature of the first electronic transition. Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanskaya ul., Kiev 02094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 3, pp. 162–168, May–June, 2000.     

Transformations of bicyclic systems containing an isothiazole ring

Isothiazolium salts, which are readily hydrolyzed to heterocyclic o-formyl disulfides, were obtained by alkylating bicyclic systems containing an isothiazole ring. The initial isothiazole-containing systems are stable in water and react with nucleophiles such as the hydroxyl ion with opening of the isothiazole ring and formation of o-cyanodisulfides.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 223–226, February, 1985.     

Excluded volume theory of rigid plates in solution using continuous orientation distributions

A new analytical theory has been developed for the thermodynamics of dilute‐to‐moderately concentrated solutions of rigid plates of arbitrary dimensions. The approach presented differs from most current theories by allowing for a continuous distribution of plate orientations rather than a small set of discrete orientations. With a few simple approximations, the theory is made tractable enough to allow for the straightforward determination of the thermodynamic properties. The results of the theory, which contains no adjustable parameters, are in good agreement with the results of Monte Carlo simulations of spatially sparse distributions of plates created by random sequential addition. An example set of the free energy of mixing curves and a phase diagram are also presented, and the prospects for future development of the theory are briefly discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2366–2377, 2000     

Optical transitions of symmetrical mixed-valence systems in the Class II-III transition regime

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the donor and acceptor sites is so great that two separate minima are no longer discernible and the energy surface features a single minimum. The electron is delocalized and the system has its own unique properties. The Robin and Day classification has enjoyed considerable success and most of the redox systems studied to date are readily assigned to Class II. However the situation becomes much more complicated when the system shows borderline Class II/III behavior. Such "almost delocalized" mixed-valence systems are difficult to characterize. In this article spectral band shapes and intensities are calculated utilizing increasingly complex models including two to four states. Free-energy surfaces are constructed for harmonic diabetic surfaces and characterized as a function of increasing electronic coupling to simulate the Class II to III transition. The properties of the charge-transfer absorption bands predicted for borderline mixed-valence systems are compared with experimental data. The treatment is restricted to symmetrical (delta G0 = 0) systems.     

Transformations of thermodynamic systems with the retention of projective invariants

Equations that determined the transformations of thermodynamic properties with the retention of projective invariants and had a solution for thermodynamic systems whose fundamental equation satisfied a certain phenomenological condition were obtained. (The geometric meaning of this condition is the possibility of projective bending of the surface described by the fundamental equation.) It was shown that, for a mixture of ideal gases, there were three sets of solutions, and, for a mixture of real gases in the region of convergence of the virial expansion (with an accuracy to at least the forth virial coefficient inclusive), there was one solution. A second solution appeared if the virial coefficients of intermolecular interactions were related by an additional equation.     

Synthesis with silicon derivatives: non symmetrical derivatization of symmetrical diamines

The conversion of symmetrical diamines to monoamides via the use of cyclic silicon-nitrogen compounds as intermediates is described.     

Investigation of molecular orientation distributions by polarized raman scattering and polarized fluorescence

It is assumed that the intensity of the vibrational Raman scattering from each of the structural units (molecular segments or crystallites) in an oriented polymer solid is determined by a symmetric second-rank tensor. The distribution of orientations of the principle axes of the tensors is expanded in a series of generalized spherical harmonics, Z_lmn(θ)e^?imψe^?in?, and it is shown that relations among the coefficients in this expansion can be obtained from suitable intensity measurements using polarized radiation. If the orientation of the tensor axes within the structural unit is known (in the general case, for several Raman lines), then the coefficients M_lmn of a similar expansion for axes fixed in the units can be obtained for l, m, and n even and l ≤ 4. The limitation to even m follows from the assumption that the solid has at least orthotropic statistical symmetry but the limitations on n and l arise from the nature of the Raman effect. Some simple special cases are considered and some orientation-independent intensity sums are derived. It is shown that, with the simplifying assumption usually made, the theory of the polarized fluorescence method for determining molecular orientations is a special case of the theory for the Raman method.     

Orientation-dependent interactions in polymer systems: Segmental orientation,miscibility, and self-organization

Three co-operative phenomena that are supposed to originate in the orientation-dependent interactions among polymer segments are discussed and attempted to be interpreted in terms of a common set of polymer models.     

Multifunctional polycondensation distributions: A kinetic approach to mixed monomer systems

The size distributions of a number of multifunctional polycondensations which contained mixtures of monomers of different functionalities were derived from kinetic reaction schemes. The distributions obtained were in agreement with those derived by other authors on similar systems. The distribution functions differed, however, from those obtained by a kinetic method by using mean functionalities of the monomer mixtures; the reasons for the differences were explored. Size distribution parameters also obtained were used to consider the differences between actually observed gel points and gel points calculated for multifunctional polycondensing systems.     

Understanding ligand distributions in modified particle and particlelike systems

Chemical modification of nanoparticles or particlelike systems is ubiquitously being used to facilitate specific pharmaceutical functionalities or physicochemical attributes of nanocrystals, proteins, enzymes, or other particlelike systems. Often the modification process is incomplete and the functional activity of the product depends upon the distribution of functional ligands among the different particles in the system. Here, the distribution function describing the spread of ligands in particlelike systems undergoing partial modification reactions is derived and validated against a conjugated enzyme model system by use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF). The distribution function is shown to be applicable to describe the distribution of ligands in a wide range of particlelike systems (such as enzymes, dendrimers, or inorganic nanocrystals) and is used to establish guidelines for the synthesis of uniformly modified particle systems even at low reaction efficiencies.     

Preferred conformers in n-propanol: A PCILO study

Preferred conformers around the central C-C and C-O bonds in n-propanol have been determined using the quantum-mechanical PCILO method and compared with those observed experimentally by microwave spectroscopy. The agreement between theory and experiment is excellent, thus minimizing time consuming searches for microwave transitions corresponding to the preferred conformers.