A new look at the reduced-gradient-following path

 The mathematical structure of the reduced-gradient-following (RGF) path introduced by Quapp et al. (1988 J. Comput. Chem. 19:1087) is reviewed and analyzed. We report two new algorithms to evaluate the RGF path. The RGF path is also compared mathematically and computationally with the gradient extremals path. An example of the evaluation of the RGF path is also reported. Received: 21 May 2001 / Accepted: 27 September 2001 / Published online: 9 January 2002     

The mechanism for the conversion of uric acid into allantoin and dehydro-allantoin : A new look at an old problem

The reaction of uric acids 1 with iodine in alkaline solution yields, on acidification, new dehydro-allantoins 11, or normal oxidation products, allantoins 13, depending on whether an excess or a stoichiometric amount of oxidant was used. The structure and regiochemistry of dehydro-allantoins 11 was established by chemical, spectroscopic, and ¹⁴C-labelling methods. These experimental results, in combination with other data, generate a new mechanistic scheme for the uricolytic pathway to allantoin, ruling out the intervention of a symmetrical transition state prior to the decarboxylation step.     

A new look at the nido-undecaborate system

The structure of the nido-undecaborate anion, [B₁₁H₁₄]⁻, has been re-examined because of what appear to be discrepancies that were observed between our determination of the structure of the anion in [(Cp₂Zr)₂B₅H₈][B₁₁H₁₄] (1) and previously published structures. The structure of 1 indicated the presence of two bridging H atoms and another pseudo-bridging one whereas those of a series of published structures indicate the presence of a plane of symmetry with two bridging H atoms and one endo-H atom. Thus, we undertook a series of structural determinations and also a computational study at the B3LYP/6-31++G(d,p) level. In addition to 1, the species studied included [NBnEt₃][B₁₁H₁₄] (2), [NBnEt₃][7-Br-nido-B₁₁H₁₃] (3) and [NBnEt₃][7-(η¹-dppm)-nido-B₁₁H₁₂] (4). Our structure of 2 indicated the presence of two bridging H atoms and an endo-hydrogen atom with some bridging character but that of 3 contained three bridging atoms. As expected the structure of 4 contains two bridging H atoms. Calculations of bond parameters fit well with the experimental data as do the ¹¹B NMR chemical shifts. The latter were calculated for the average of the two open face configurations, one with two bridging and one endo-hydrogen and the other with three bridging hydrogen atoms. The difference in energies for these two open face configurations is calculated to be 0.36 kJ/mol, which effectively suggests that the two structures are equally favored.     

A new look at bonding in trialuminides: reinvestigation of TaAl3

Single crystals of TaAl3 were grown at high temperatures from an Al-rich, binary solution. TaAl3 adopts the D0(22) structure type, space group I4/mmm with a=3.8412(5) A, c=8.5402(17) A, and Z=2. The structure type, which is the preferred structure for all group 5 trialuminides and TiAl3 as well as the high-temperature form of HfAl3, is a binary coloring of the face-centered-cubic (fcc) arrangement. The distribution of Ta atoms creates a three-dimensional network of vertex and edge-sharing square pyramids of Al atoms. Temperature-dependent electrical resistivity and magnetic susceptibility measurements are consistent with TaAl3 being a metallic compound with a relatively low density of states at the Fermi surface. Furthermore, tight-binding electronic structure calculations are utilized to describe the bonding in these compounds and to compare their stability with respect to the alternative fcc-related, e.g., the D0(23) (ZrAl3-type) and the L1(2) (AuCu3-type), structures. A modified Wade's rule argument provides insights into the structural preferences.     

A new look at XRD patterns of archaeological ceramic bodies

In this paper, the general rules of phase transformations in calcareous clay bodies during firing were used to estimate the equivalent firing temperature (EFT) of seventeenth century polychrome Persian haft rang tiles based on their X-ray diffraction (XRD) patterns. The novelty of this work is, however, that it handles the XRD patterns of archaeological clay bodies with principal component analysis (PCA) in order to have a new look at their firing and thermal behaviour. Statistically handling the XRD patterns, different clusters were discriminated in the clay bodies whose mineralogical composition showed various proportions of amorphous and quartz contents. The results showed interesting trends in the different clusters in terms of the EFT, quartz content and the density of the bodies. The present work uses PCA to have a new look at XRD patterns of archaeological clay bodies and, moreover, to interpret the PCA results in order to estimate the EFT of a large number of archaeological clay bodies.     

A fast method for the determination of plutonium in soil and sediment samples

A so-called hour method for the determination of plutonium based on extraction chromatography is validated by its application to several certified soil and sediment samples from the International Atomic Energy Agency's Seibersdorf Laboratory. The results are in good agreement with the reference values. Chemical yields range between 47%–66%, with a mean value of (56±6)%. The possible use of this method for²⁴¹Pu determination is also demonstrated.     

Studies with dithizone: Part 35. A new look at the structure of dithizone in solution

The traditional view that solutions of dithizone in organic solvents comprise equilibrium mixtures of thione- and thiol-forms which are individually responsible for the characteristic strong absorption bands around 620 and 440 nm is examined critically. It is shown that experimental values of pH_{$\text{1}\text{2}$}, pK and R (the peak ratio) can legitimately be used in calculations although they are compounded of parameters (partition coefficients, acid dissociation constants, etc.) relating to both of the alleged tautomeric forms and the equilibrium constant, K_T governing the interconversion. Published attempts to calculate K_T from spectral data alone are shown to be unreliable.     

A new look at fluorescence depolarization and the dynamics of anisotropic rotational diffusion

Existing derivations of the time-dependent fluorescence anisotropy of an asymmetric molecule constitute a straightforward application of Favro's work on the rotational diffusion equation (RDE), and make no contribution to the elucidation of rotational dynamics as such. A new approach is developed, and the problem formulated in the parlance of conventional reaction kinetics by demonstrating, with the aid of the method of moments, that the RDE is completely equivalent to a set of ordinary linear differential equations, formally identical with those used to describe a first-order series-parallel reaction scheme.     

A new method of plutonium speciation in sea water

A new method of plutonium speciation in large volume of sea water was developed by using adsorption of Pu(IV)-Xylenol Orange chelate and Pu-Arsenazo chelate on XAD-2 resin, respectively. The tetravalent plutonium ion reacts selectively with Xylenol Orange in acid solution and that adsorbed on XAD-2 resin. Total plutonium can be collected onto the resin in the form of its Arsenazo-III complex. The determination of plutonium then was carried out by alpha-ray spectrometric method after decomposition of organic complexes and ion exchange separation. The present method is confirmed for convenient and rapid preconcentration procedure for plutonium shipboard chemistry.     

Computational modeling of the optical rotation of amino acids: a new look at an old rule for pH dependence of optical rotation

The molar rotation of a solution of a natural alpha amino acid is changed in the positive direction by addition of a strong acid. Three decades ago, an attempt to rationalize this old rule, named for Clough, Lutz, and Jirgensons (CLJ), was made by assigning circular dichroism octants for overlapping carbonyl n to pi* transitions. Modern quantum chemical methods allow us to take a new look at this phenomenon. Time-dependent density functional theory was used to model the electronic structure and transitions responsible for CLJ. We show that sector rules originally developed for circular dichroism (CD) can be applied to the optical rotation in this case, but with some restrictions, and with great caution, due to the change of the overall charge of the acids upon protonation and the distortion of the COO- chromophore in the zwitterions. We have prepared sector maps based on first-principles computations to study the correspondence between CD and optical rotation for zwitterionic and protonated l-amino acid chromophores. The CLJ effect is correctly obtained from the computations for all 12 amino acids studied in this work.     

The forgotten liquid crystals of Daniel Vorlander: A new look at mesogenic 4-pyridones

Three classes of 4-pyridone (1H-pyridin-4-one) terminated liquid crystals were synthesized and their mesogenic properties examined. These liquid crystal classes differ in the composition of the aromatic core group which vary among azobenzene, stilbene or tolane groups. The synthesis of the 4-pyridones was accomplished by aromatic nucleophilic substitution reaction of the appropriate activated aryl fluoride with 4-hydroxypyridine. These pyridone liquid crystals possess broad enantiotropic nematic and smectic A phases and may possess useful properties such as high birefringence and polarity including hydrogen bonding capability. The original report of a 4-pyridone containing liquid crystal due to Vorlander has been substantiated.     

A new look at the crystallization of polyethylene. II. Crystallization from the melt at low supercoolings

This article is part of the general project laid out in Part I (ref. 9) and is concerned with obtaining information on primary (unthickened) crystals of polyethylene formed at low supercoolings. For this, a technique had to be devised by which crystallization could be speeded up so as to eliminate or at least reduce lamellar thickening. Indeed we were able to increase the rate of crystallization by an order of magnitude using a technique which we have called enhanced self-nucleation. Using this technique we find that when viewed under an optical microscope, spherulites crystallize uniformly over the field of view, and not, as is usual, by a radial growth process. Isothermal crystallization in bulk linear polyethylene has been studied by means of the enhanced self-nucleation technique as a function of crystallization time by using Raman LAM and melting points to assess variations of fold length Data have been obtained at very much shorter times than before. At short times, we find a constant fold length; at longer times the crystals thicken linearly with the logarithm of time. Values of the initial fold length for crystallization temperatures between 118 and 130°C are presented. Associated with the thickening at short times we find an induction time which increases with temperature.     

A combined method for the determination of the isotopic vector of plutonium isotopes in environmental samples

A combination of alpha-spectrometry, liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) was used for the determination of plutonium isotopes. ²³⁸Pu and ^239+240Pu were measured by alpha-spectrometry after separation of Pu by anion-exchange using ²³⁶Pu tracer as recovery monitor. After alpha-measurement, one part of the sample was dissolved for determining ²⁴¹Pu by LSC. Another part was used for the measurement of the ²⁴⁰Pu/²³⁹Pu atom ratio by AMS at VERA. Thus, it was possible to obtain complete information on the Pu isotopic composition of the samples. This method was applied to environmental reference samples and samples contaminated from nuclear reprocessing.     

A new look at the crystallization of polyethylene. III. Crystallization from the melt at high supercoolings

Polyethylene melts normally cannot be crystallized in an isothermally controlled manner below ca. 110°C as crystallization is too fast owing to the presence of preexisting nuclei. We here present a new simple method for producing droplets enabling crystallization to be conducted isothermally down to 75°C and allowing measurements to be performed on them, together with the results of these measurements. We conclude that in contrast to the earlier claims the homogeneous nucleation regime has not been attained even at these low temperatures, the crystal nucleation being dominated by the polymer-substrate interface. The droplets crystallized at these low temperatures have a lamellar morphology as revealed by electron microscopy and grow at rates exceeding 1 m s^?1 [i.e., six orders of magnitude faster than growth at more usual crystallization temperatures previously considered as “low” (110–120°C)]. We have measured the lamellar thickness l as a function of growth temperature, thus extending the fundamentally important l vs. ΔT (supercooling) relation beyond the previously realizable limits towards high ΔT values. The implications of all these results for the existing ideas of polymer crystallization are likely to be far reaching. Further, the new method for achieving high supercoolings opens up new possibilities for the study of annealing phenomena and the effect of nucleating agents.     

A new look at multiphase Rachford–Rice equations for negative flashes

The elegant method proposed by Michelsen [M.L. Michelsen, Calculation of multiphase equilibrium, Comput. Chem. Eng. 18 (1994) 545–550] for solving multiphase Rachford–Rice (RR) equations as a bound constrained minimization is extended to the negative flash region; this method is definitely much more robust than any method based on equation solving. The problem is reformulated as an unbounded linearly constrained minimization. The feasible domain is limited by (only few of the) hyperplanes defined by equilibrium constants. The proposed method is applied to several three-phase cases.