On the efficiency of different EHF wavefunctions

A new computational scheme for the spin-extended Hartree-Fock (EHF) equations is shown to be suitable for large Π-electron systems. It is demostrated that the triplet EHF wavefunction with equal numbers of orbitals of different spins gives a much larger energy decrease than that with the maximum total-spin projection value.     

Thermal characterization of the bromopride recrystallized from different solvents and at different temperature conditions

Polymorphism is the ability of a substance to crystallize into different crystalline states. Different polymorphs vary according to their physical and chemical properties, and therefore, the presence of different crystalline forms of a drug may change some physicochemical properties of certain medicines. The goal of this work was to detect polymorphic forms of a compound used as a drug, bromopride. For the purpose of this study, recrystallizations were carried out from different solvents and at distinct temperatures, with the aim of obtaining different crystalline forms, which were characterized by dynamic thermal analysis (DTA) and X-ray powder diffraction. The samples obtained through recrystallization in different temperature conditions and solvents showed peculiar DTA curves and diffractograms profiles, indicating the presence of crystalline forms distinct from each other.     

Preliminary study of the retention behavior for different compounds using cryogenic chromatography at different initial temperatures

The Kováts indices (I) of 24 volatile organic compounds were measured at the different initial temperatures by a laboratory-made cryogenic chromatography. The results show that the correlations of the Kováts indices and the temperature are nonlinear at low initial temperature. Seven QSRR models were built using the heuristic method at different initial temperatures by calculating the quantitative chemical parameters of 24 organic compounds to study the relationship between the retention behavior and the molecular parameters. By studying the relationship between carbon and retention behavior, the capacity factor, and resolution of selected compounds, the contribution of gas–liquid interfacial adsorption process, which influences the retention behavior of solutes, is becoming more and more important with the decrease of the initial temperature.     

On the test of different atomic exchange functionals

The yet unknown exchange-correlation energy functional and the lack of a systematic way to find such a functional has led to propose different expressions for exchange, on the one hand, and correlation on the other, by several authors. A common factor in the design of exchange functionals is to reproduce different kinds of energies rather than the important atomic shell structure. We present a strategy for testing the behavior of exchange functionals based on the reproduction of atomic shell structure. A variety of well-known exchange functionals are tested on He (closed shell), Li (open shell), and Be (closed subshell). Important differences are found among them. Finally, we propose an exchange energy functional that includes a Becke-88-type expression and, in addition, a nongradient correction term that leads to improved shell structure behavior. © 1995 John Wiley & Sons, Inc.     

On the vibronic structure of all-trans-retinal in different environments

The great sensitivity of the electronic structure of a polyene in different environments is visualized through the all-trans-retinal molecule in solvents with different polarity. A vibronic structure in the ¹B_u⁺ ← ¹A_g⁻ transition of all-trans-retinal is detected in the second-derivative spectrum of the ordinary band-shape function. It consists mainly of a 1480 cm⁻¹ progression. Attempts to simulate this are presented. A correlation between the resolution in the vibronic structure and the polarity of the environment of retinal is found. A second excited state of all-trans-retinal may be responsible for the changes in the vibronic structure in the different solvents.     

On the measurement of the moisture content in different matrix materials

Twelve different samples were analyzed for their water content by the classical oven-drying method and a commercial moisture analyzer. Nominal values have been set by Karl Fischer titration and gravimetry. The decomposition during the drying process of samples containing organic compounds was checked by thermogravimetry coupled with Fourier transform infrared spectrometric and mass spectrometric detection. Summarizing all measurement results, the commercial moisture analyzer demonstrated an average bias for the determination of water contents, which is much smaller than the stated uncertainty and thus is negligible. Compared to the classical oven-drying method, the commercial moisture analyzer shows a smaller bias, more ease in handling and less time consumption per analysis. When validated by a reference procedure like Karl Fischer titration the commercial moisture analyzer is the method of choice for routine analysis.     

A field theoretical description of states with different orbitals for different spins

A field theoretical formulation is given for the method of different orbitals for different spins (DODS ). For an infinite system DODS describes an antiferromagnetic state and to account for this spin ordering a Gorkov-type of factorization is introduced. A corresponding gap equation is derived, where a non-zero solution indicates the presence of long-range order in the system. Actually the formulation given is general enough as to render DODS as a special case. As shown, we may also obtain a ferrimagnetic as well as a density wave state solution depending on the special characteristics of the system at hand. A related type of antiferromagnetism is described by the Overhauser spin density wave (SDW ) state and also this theory is formulated in a field theoretical language. The similarities and differences between DODS and SDW are discussed. The energy expressions for the two states are given within the Hartree-Fock approximation. It is proposed that the SDW state could be used to partly account for the correlation problem in molecules, as well as the method of DODS which has previously been employed for that purpose.     

Unified study of the different physical properties of amorphous polymers

Uptil now it has not been possible to explain the different physical properties of amorphous polymers using a model based on a single conceptual scheme. In this paper, a phenomenological model is proposed which tries to explain the mechanical, optical and thermal properties (both thermal conductivity and expansivity) of amorphous polymers. The model has similarities with the composite model, proposed by the present authors, which has proved to be successful in interpreting the different physical properties of semicrystalline polymers. The present model considers the bulk form of the polymer as an aggregate of microscopic units possessing intrinsic physical properties. On drawing, the development of anisotropy in different physical properties is supposed to be due to the development of preferred orientation of these units. The development of the preferred orientation has been estimated directly from birefringence data. The agreement between the calculated and experimental values of the elastic modulus, thermal conductivity and thermal expansivity of PVC, PMMA and PS is found to be reasonable good.     

Biomimetic synthesis of calcium carbonate with different morphologies under the direction of different amino acids

Using three different amino acids (AAs) as organic matrices, including the highly nonpolar hydrophobic l-valine, the positively charged l-arginine and the less polar uncharged l-serine, calcium carbonate (CaCO₃) with different morphologies and polymorphs were synthesized by a facile gas diffusion reaction based on biomimetic strategy. Compared with the control cubic calcite obtained in the absence of AAs, the product from l-valine was cubic calcite aggregates assembled by nano-platelets. The product from l-arginine was spherical vaterite aggregates assembled by spherical nanoparticles. The product from l-serine was the mixture of cubic calcite and spherical vaterite. The structures and properties of the side chains of the AAs exerted the significant effects on the nucleation and growth of the CaCO₃. The formation mechanisms of the CaCO₃ in the presence of AAs are preliminarily discussed. The results suggest that the polymorphs and morphologies of the inorganic nanomaterials might be easily adjusted through the careful selection of the organic matrices.     

The rheological similarity of the physical networks of different origin

It has been established that the Newtonian viscosity of solutions of styrene-acrylic acid (4 mol%) copolymers inionomeric form (0–50 mol%) in DMPh is proportional to exp n (n being the degree of neutralization). The principle of temperature-time-concentration superposition is applicable for the dependence of shear modulus on temperature (20–100°), frequency (10^?3–1.5 Hz), decree of neutralization (0–50 mol%) and concentration (15–45 g/dl). These facts can be explained in terms of the rheological similarity of physical networks connected with macromolecular chain entanglements and with salt group interactions. Some details of the model for concentrated polymer solutions are discussed.     

Theoretical investigations on the thiol–thioester exchange steps of different thioesters

As the rate-determining step in native chemical ligation reactions, the thiol–thioester exchange step is important in determining the efficiency of the ligations of peptides. In the present study, systematic theoretical calculations were carried out on the relationships between the structure of different thioesters and the free energy barriers of the thiol–thioester exchange step. According to the calculation results, the thiol–thioester exchange step is disfavored by the steric hindrance around the carbonyl center, while the electronic effect(i.e. conjugation and hyper-conjugation effects) becomes important when the steric hindrance is insignificant.     

Estimating the maximum coordination number for spheres of different sizes

Eduljee, G.H. and McDermott, C., 1984. Estimating the maximum coordination number for spheres of different sizes. Fluid Phase Equilibria, 18: 103–108.The maximum coordination number is determined from a physical model in which the coordinatng spheres are packed on the surface of the central sphere to form a series of triangles. The efficiency of packing is given by a parameter α, which is 60° for the closest possible packing and greater than 60° for looser packing. The model is applied to computer-simulated coordination number data using the principles of spherical trigonometry to obtain a value of α that provides the best match. The results compare favourably with those of empirical fitting equations.     

Effect of aminoacids on the critical micellization concentration of different surfactants

The effect of aminoacids (DL-glycine, DL-alanine, DL-serine, L-leucine, L-lysine, DL-phenylalanine, DL-tyrosine, and L-aspartic acid) on the critical micellization concentration (CMC) of nonionic, anionic, and cationic surfactants is investigated. It is established that, as the hydrophobicity of aminoacids rises, the CMC values of ionic and nonionic surfactants increase and decrease, respectively. An exception is aspartic acid, which reduces CMC values irrespective of the nature of surfactants.     

Investigation of different influences on the fatigue behaviour of industrial rubbers

Load conditions used typically for fatigue life investigations can differ strongly from the conditions for real rubber products. For example, the frequency of the laboratory measurements is increased and the product load curve is simplified to a sine. In this paper, industrial rubber blends (SBR/BR/NR blends) under tension–compression load are used. First, the influence of a higher frequency (5 Hz) compared to the product relevant frequency (1 Hz) is investigated. A higher frequency does not influence the fatigue life but certainly the sample temperature and material behaviour. This is further investigated by varying the ambient temperature for 1 Hz measurements and the strain rate. Second, a non-sinusoidal wave form depicting the product loading case is selected. The load oscillates between tension and compression with dwell periods in every cycle. The results are comparable to those of a sine wave with the same frequency.     

A comparative study of the bonding in different halides of iridium

X-ray photoelectron spectroscopy and multiple scattering Xα calculations have been applied to a series of iridium halide complexes in order to corroborate the nature of the bondings inherent in this class of compounds. Our results seem to substantiate contentiously that higher oxidation states of iridium favor the formation of covalent bonds. This conclusion is based on the observation that (1) successive bombardment of the iridium species by Ar ions almost definitely leads to a configuration in which iridium is bound to at most one halide ion, and (2) the theoretical charge per ligand ion approaches systematically a value of {1?} in the limit as the formal oxidation state of iridium approaches {1+}. The theoretical results are further arthenticated by the fact that the experimental ionization energy of the Ir(4f) level in the different iridium halide complexes studied is seen to decrease as a result of exposure to Ar ions.     

Analysis of alkaloids from different chemical groups by different liquid chromatography methods

Alkaloids are biologically active compounds widely used as pharmaceuticals and synthesised as secondary methabolites in plants. Many of these compounds are strongly toxic. Therefore, they are often subject of scientific interests and analysis. Since alkaloids — basic compounds appear in aqueous solutions as ionized and unionized forms, they are difficult for chromatographic separation for peak tailing, poor systems efficiency, poor separation and poor column-to-column reproducibility. For this reason it is necessity searching of more suitable chromatographic systems for analysis of the compounds.     

The absorption spectrum of the pyrene excimer in different solvents

The absorption of the pyrene excimer obtained using modulation excitation spectrophotometry in different solvents is shown to be solvent dependent.     

Microbial desulfurization of different coals

Coal is the most important nonrenewable energy source of fossil origin. It is also the most common fuel in thermal power plants. However, during coal incineration in power plants, high sulfur content of coal poses serious environmental problems owing to sulfur dioxide emission. We studied the application of microbial methods for removal of sulfur from three types of high sulfur coals—two samples collected from Assam and Rajasthan in India and one from Libiaz, Poland. These coal samples were desulfurized using indigenous Acidithiobacillus sp. After investigation of the effect of various parameters, the conditions optimized for the maximum removal of total sulfur (91.87% for lignite, 63.13% for Polish coal, and only 9.44% for Assam coal) were as follows: initial pH of 1.5 (2.5 in the case of Assam coal), particle size of 45 μ, pulp density of 2% (w/v), incubation period of 30 d at −35°C in presence of 44.2 g/L of ferrous sulfate in the media with shaking at 140 rpm. Poor removal of sulfur in the case of Assam coal was owing to extensive precipitation of jarosites. In addition, the sulfur in Assam coal is mostly found in organic form, which is difficult to remove with Acidithiobacillus sp. The removal of sulfur from the three coal samples was demonstrated with photomicrographic studies.