bluuebunny/crossref_metadata_2025 huggingface dataset:
Fiber tracking: principles and strategies – a technical review
Authors: Mori Susumu , van Zijl Peter C. M.
Published: December 2002

The state of the art of reconstruction of the axonal tracts in the central nervous system (CNS) using diffusion tensor imaging (DTI) is reviewed. This relatively new technique has generated much enthusiasm and high expectations because it presently is the only approach available to non‐invasively study the three‐dimensional architecture of white matter tracts. While there is no doubt that DTI fiber tracking is providing exciting new opportunities to study CNS anatomy, it is very important to understand its limitations. In this review we therefore assess the basic principles and the assumptions that need to be made for each step of the study, including both data acquisition and the elaborate fiber reconstruction algorithms. Special attention is paid to situations where complications may arise, and possible solutions are reviewed. Validation issues and potential future directions and improvements are also discussed. Copyright © 2002 John Wiley & Sons, Ltd.
The basis of anisotropic water diffusion in the nervous system – a technical review
Authors: Beaulieu Christian
Published: December 2002

Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system. Copyright © 2002 John Wiley & Sons, Ltd.
Diffusion‐tensor MRI: theory, experimental design and data analysis – a technical review
Authors: Basser Peter J. , Jones Derek K.
Published: December 2002

This article treats the theoretical underpinnings of diffusion‐tensor magnetic resonance imaging (DT‐MRI), as well as experimental design and data analysis issues. We review the mathematical model underlying DT‐MRI, discuss the quantitative parameters that are derived from the measured effective diffusion tensor, and describe artifacts thet arise in typical DT‐MRI acquisitions. We also discuss difficulties in identifying appropriate models to describe water diffusion in heterogeneous tissues, as well as in interpreting experimental data obtained in such issues. Finally, we describe new statistical methods that have been developed to analyse DT‐MRI data, and their potential uses in clinical and multi‐site studies. Copyright © 2002 John Wiley & Sons, Ltd.
Diffusion tensor imaging of normal and injured developing human brain ‐ a technical review
Authors: Neil J. , Miller J. , Mukherjee P. , Hüppi P. S.
Published: December 2002

The application of diffusion tensor imaging (DTI) to the evaluation of developing brain remains an area of active investigation. This review focuses on the changes in DTI parameters which accompany both brain maturation and injury. The two primary pieces of information available from DTI studies—water apparent diffusion coefficient and diffusion anisotropy measures—change dramatically during development, reflecting underlying changes in tissue water content and cytoarchitecture. DTI parameters also change in response to brain injury. In this context, not only does DTI offer the possibility of detecting injury earlier than conventional imaging methods, but also appears more sensitive to disruption of white matter than any other imaging method. DTI offers unique insight into brain injury and maturation, and does so in a fashion that can be readily applied in a clinical setting. Copyright ­© 2002 John Wiley & Sons, Ltd.
Diffusion tensor imaging and aging – a review
Authors: Moseley Michael
Published: December 2002

Diffusion‐tensor imaging (DTI) non‐invasively provides maps of microscopic structural information of oriented tissue in vivo, which is finding utility in studies of the aging population. In contrast to the white matter maturation process, investigators have observed significant declines in the white matter ordering in normal as well as in abnormal aging. These studies suggest that water proton non‐random, anisotropic diffusion measured by DTI is highly sensitive to otherwise subtle disease processes not normally seen with conventional MRI tissue contrast mechanisms. Copyright © 2002 John Wiley & Sons, Ltd.
Applications of diffusion‐weighted and diffusion tensor MRI to white matter diseases – a review
Authors: Horsfield Mark A. , Jones Derek K.
Published: December 2002

This paper reviews the current applications of diffusion‐weighted and diffusion tensor MRI in diseases of the brain white matter. The contribution that diffusion‐weighted imaging has made to our understanding of white matter diseases is critically appraised. The quantitative nature of diffusion MRI is one of its major attractions; however, this is offset by the more advanced hardware required to collect diffusion‐weighted images reliably, and the more complex processing to produce quantitative parametric diffusion images. With the now common availability of scanners equipped to perform echo‐planar imaging, the acquisition of diffusion tensor images is sure to become more widespread and routine. Copyright © 2002 John Wiley & Sons, Ltd.
Diffusion tensor imaging in spinal cord: methods and applications – a review
Authors: Clark Chris A. , Werring David J.
Published: December 2002

The spinal cord is a clinically eloquent site within the central nervous system, containing important sensorimotor tracts confined within a small cross‐sectional area. Damage to the spinal cord may be caused by a wide range of pathologies, and can result in profound functional disability. Characterization of the structural integrity of the spinal cord can be assessed using diffusion tensor imaging methods. Development and application of this technique may improve our understanding of the nature and evolution of structural damage in spinal cord disease. Possible developments include improved detection of ischaemic lesions, clarification of the relationship between clinical disability and structural damage to the cord and monitoring of anti‐inflammatory or neuroprotective therapies. In this review current technical aspects, clinical applications and the suggested future development of spinal cord diffusion imaging are discussed. Copyright © 2002 John Wiley & Sons, Ltd.
Neuropsychiatric applications of DTI – a review
Authors: Lim K. O. , Helpern J. A.
Published: December 2002

Psychiatric disorders are common throughout the world and are a leading cause of disability. There is a growing appreciation of the importance of connectivity to brain function. Disruption of this connectivity can result in brain dysfunction manifested in impaired cognitive functioning and the development of clinical symptoms. White matter forms the basis of anatomical connectivity. Diffusion tensor imaging (DTI) is a useful tool for examining and quantifying white matter microstructure. Clinical research studies in alcoholism, HIV‐1 infection, geriatric depression and schizophrenia using DTI have revealed abnormalities in white matter microstructure. The use of complementary imaging methods may be helpful in further characterizing these abnormalities. Other psychiatric disorders may also have white matter involvement amenable to study with DTI. Advances in acquisition and analysis methods will be necessary to further advance work in this field. The study of animal models and postmortem tissue may be helpful in elucidating the neurobiological underpinnings of abnormalities observed with DTI. Copyright © 2002 John Wiley & Sons, Ltd.
From the diffusion coefficient to the diffusion tensor
Authors: Le Bihan Denis , van Zijl Peter
Published: December 2002

With diffusion tensor MRI one has access to the organization in space of tissue microstructural components. This outstanding potential adds, however, another layer of complexity to the diffusion MRI data acquisition and analysis processes. Over the last few years many articles have been published dealing with those matters. This special issue is thus timely to provide readers with the synthesis and the overall viewpoints from leading contributors to the field. Copyright © 2002 John Wiley & Sons, Ltd.
Metal fingers on grain boundaries in multicrystalline silicon solar cells
Authors: Ebner Rita , Radike Michael , Schlosser Viktor , Summhammer Johann
Published: December 2002

We have developed a method of applying a net‐like finger grid to the front of multicrystalline (mc) silicon solar cells, which lies mainly on the grain boundaries (grain‐boundary‐oriented‐finger GBOF grid). This net has no busbars. It is drawn by a plotter, using screen printing paste dispensed through a fine tube. The power output of cells contacted in this manner has been tested in a statistical study of pairs and triplets of cells of size 100 × 100 mm2 (Bayer) and 103 × 103 mm2 (Eurosolare). In the pairs study, pairs of neighbouring wafers of the original ingot were processed into solar cells. One wafer received a GBOF grid, the other got the same grid rotated by 90°, and so had little coverage of grain boundaries. In the study of triplets the third wafer of each triplet was equipped with a standard H‐pattern of the same shading as the GBOF grid. In the pairs study, we find that under approximately standard conditions there is an 89% chance that the GBOF grid increases power output over cells with an identical, but 90° rotated, grid, the most probable increase being 2.6%. The triplets study shows that there is an 87% chance that the GBOF grid increases power output over cells with the standard H‐pattern, the most probable increase being 2.5%. Copyright © 2002 John Wiley & Sons, Ltd.
Comparative study of maximum power point tracking algorithms
Authors: Hohm D. P. , Ropp M. E.
Published: December 2002

Maximum power point trackers (MPPTs) play an important role in photovoltaic (PV) power systems because they maximize the power output from a PV system for a given set of conditions, and therefore maximize the array efficiency. Thus, an MPPT can minimize the overall system cost. MPPTs find and maintain operation at the maximum power point, using an MPPT algorithm. Many such algorithms have been proposed. However, one particular algorithm, the perturb‐and‐observe (P&O) method, claimed by many in the literature to be inferior to others, continues to be by far the most widely used method in commercial PV MPPTs. Part of the reason for this is that the published comparisons between methods do not include an experimental comparison between multiple algorithms with all algorithms optimized and a standardized MPPT hardware. This paper provides such a comparison. MPPT algorithm performance is quantified through the MPPT efficiency. In this work, results are obtained for three optimized algorithms, using a microprocessor‐controlled MPPT operating from a PV array and also a PV array simulator. It is found that the P&O method, when properly optimized, can have MPPT efficiencies well in excess of 97%, and is highly competitive against other MPPT algorithms. Copyright © 2002 John Wiley & Sons, Ltd.
Analysis of light scattering in amorphous Si:H solar cells by a one‐dimensional semi‐coherent optical model
Authors: Krč Janez , Smole Franc , Topič Marko
Published: December 2002

A one‐dimensional semi‐coherent optical model for thin‐film solar cells is presented. The optical circumstances at flat interfaces are addressed and the situation at rough interfaces in the model is described for the case of direct (coherent) incident and scattered (incoherent) incident light. After the model has been experimentally verified, analysis of the light scattering process in hydrogenated amorphous silicon (a‐Si:H) p–i–n solar cells is carried out. The influence of the interface root‐mean‐square roughness and the effect of different angular distribution functions of diffused light on quantum efficiency and short‐circuit current are investigated by the optical model. Copyright © 2002 John Wiley & Sons, Ltd.
Evaluation of the Algerian photovoltaic market potential
Authors: Labed S. , Lorenzo E.
Published: December 2002

The Algerian energy sector will certainly face a crisis this century with the gradual diminution of oil and gas reserves estimated at 4.2 billion Toe (Tons of oil equivalent), which can last for only a few decades at today's production levels. In the meantime the population and its energy needs will have doubled. This situation is complicated by the fact that the energy sector is Algeria's major source of revenue, and finances the national economy at a level of 66%. Many decision‐makers now agree that time has come to implement a new energy policy, based on energy diversification, and to set up new mechanisms. The Algerian energy sector is still dominated by hydrocarbons, and has promoted a major effort to reach a mean national electrification rate of 96%. Nevertheless, the remaining zones are very isolated and widespread. The conventional electrification solutions or grid extensions are no longer economically viable. The recourse to renewable energies in general and PV solar energy in particular has become a reality. This source represents a good alternative for 85% of Algerian territory, which is composed of arid and semi‐arid zones, characterized by low population density and limited energy needs. These regions represent a strategic alternative in order to reverse the population migration and to encourage a new migration to the south, since northern Algeria is less and less welcoming (high population density, unemployment, shrinkage of agricultural land, inflation). In this paper we try to evaluate the potential of the Algerian PV market in order to inform decision‐makers and future investors. To this end, and after identifying the PV solar energy segments, we attempt to estimate them under different scenarios. Copyright © 2002 John Wiley & Sons, Ltd.
TiO2DLAR coatings for planar silicon solar cells
Authors: Richards B. S. , Rowlands S. F. , Honsberg C. B. , Cotter J. E.
Published: December 2002

In this paper we demonstrate that a double‐layer anti‐reflection (DLAR) coating can be fabricated using only titanium dioxide (TiO2). Two TiO2thin films were deposited onto planar silicon wafers using a simple atmospheric pressure chemical vapour deposition (APCVD) system under different deposition conditions. Weighted average reflectances of 6.5% (measured) and 7.0% (calculated) were achieved for TiO2DLAR coatings in air and under glass, respectively. An increase in the short‐circuit current density of ΔJsc = 2.5 mA/cm2can be expected for an optimised TiO2DLAR coating when compared with a commercial TiO2single‐layer anti‐reflection coating. Copyright © 2003 John Wiley & Sons, Ltd.
Solar cell efficiency tables (version 21)
Authors: Green Martin A. , Emery Keith , King David L. , Igari Sanekazu , Warta Wilhelm
Published: December 2002

Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and new entries since July, 2002 are reviewed. Copyright © 2003 John Wiley & Sons, Ltd.
17.6% efficient tricrystalline silicon solar cells with spatially uniform texture
Authors: Schmiga Christian , Schmidt Jan , Metz Axel , Endrös Arthur , Hezel Rudolf
Published: December 2002

Up to now solar cells fabricated on tricrystalline Czochralski‐grown silicon (tri‐Si) have shown relatively low short‐circuit current densities of about 31–33 mA/cm2because the three {110}‐oriented grains cannot effectively be textured by commonly used anisotropic etching solutions. In this work, we have optimised a novel chemical texturing step for tri‐Si and integrated it successfully into our solar cell process. Metal/insulator/semiconductor‐contacted phosphorus‐diffused n+p junction silicon solar cells with a silicon‐dioxide‐passivated rear surface and evaporated aluminium contacts were manufactured, featuring a spatially uniform surface texture over all three grains on both cell sides. Despite the simple processing sequence and cell structure, an independently confirmed record efficiency of 17.6% has been achieved. This excellent efficiency is mainly due to an increased short‐circuit current density of 37 mA/cm2obtained by substantially reduced reflection and enhanced light trapping. Copyright © 2003 John Wiley & Sons, Ltd.
Synthesis, spectroscopic, theoretical and structural studies of new trihalomethyl sulphenyl derivatives of 5‐methyl‐1,3,4‐thiadiazole‐2‐thiol
Authors: Coyanis E. M. , Boese R. , Autino J. C. , Romano R. M. , Della Védova C. O.
Published: December 2002

Although 2‐mercapto‐5‐methyl‐1,3,4‐thiadiazole (mmtd) is commonly thought of as a thione tautomer, electrophilic substitution occurs on the thiol moiety. The tautomeric ability of mmtd allows a substitution reaction to take place at the sulphur; this is shown by reaction with Cl3−nF
nCSCI compounds (n = 0–2) to give perhalomethyldithio thiadiazole derivatives. Three novel perhalomethylsulphenyl compounds, which exhibit a wide range of potentially interesting applications, were obtained and characterized by x‐ray crystal diffraction, mass spectrometry, IR and Raman spectroscopy and density functional theory calculations. Copyright © 2002 John Wiley & Sons, Ltd.
Information theory description of synthetic strategies. A new similarity index
Authors: Barone R. , Petitjean M. , Baralotto C. , Piras P. , Chanon M.
Published: December 2002

Information theory was used to analyse and compare organic syntheses leading to the targets, daucene, longifolene and estrone. This paper expands the work of Bertz, who analysed syntheses from the complexity of molecular structures. Herein, a more complete model involving similarity was evaluated. We published previously a study in this direction limited to a skeletal level. In order to improve on this initial approach, we attempted to analyse some syntheses not only limited to the skeleton by including different definitions of similarity. Copyright © 2002 John Wiley & Sons, Ltd.
Thermal transformation of monoterpenes within thionin‐supported zeolite Na‐Y. Acid‐catalyzed or electron transfer‐induced?
Authors: Stratakis Manolis , Stavroulakis Manolis , Sofikiti Nikoletta
Published: December 2002

Several monoterpenes (monocyclic, bicyclic or acyclic) isomerize and finally transform to p‐cymene in the dark upon loading within thionin‐supported zeolite Na‐Y. The same reactions occur in Na‐Y dried under the same conditions as thionin/Na‐Y. It is postulated that the thermal treatment of Na‐Y generates ‘electron holes’ (probably acidic sites). The transformation of monoterpenes occurs more likely via an electron transfer‐induced reaction subordinated to the occurrence of the acidic sites. The radical cation of the more thermodynamically stable monoterpene, α‐terpinene, eventually dehydrogenates to p‐cymene. For comparison, the same reactions were performed within methyl viologen‐supported Na‐Y. Copyright © 2002 John Wiley & Sons, Ltd.
Cyclic hydrogen bond arrays: vibrational spectroscopic study of aminomethyl‐substituted resorcinarene‐based cavitands and their clathrates with THF
Authors: Dormann Jörg , Ruoff Andreas , Schatz Jürgen , Middel Oskar , Verboom Willem , Reinhoudt David N.
Published: December 2002

Mono‐ and tetrakis(aminomethyl)‐substituted resorcinarene‐based cavitands 1 and 2 and their clathrates 1
2·THF and 2·(THF)2 were investigated by FTIR spectroscopy and thermogravimetric analysis. Both hosts form stable clathrates with THF both in solution and in the solid state. Major intra‐ and intermolecular interactions could be identified and located by the use of vibrational spectroscopy. In all cases, dynamic cyclic intramolecular hydrogen bonds between the amino groups and neighbouring ether bridges are observable. Copyright © 2002 John Wiley & Sons, Ltd.
Conformational analysis of trans‐2‐halocyclohexanols and their methyl ethers: a 1H NMR, theoretical and solvation approach
Authors: Freitas M. P. , Tormena C. F. , Rittner R. , Abraham R. J.
Published: December 2002

The conformational equilibria of trans‐1‐methoxy‐2‐chloro‐ (1), trans‐1‐methoxy‐2‐bromo‐ (2) and trans‐1‐methoxy‐2‐iodocyclohexane (3), and their corresponding alcohols (4–6), were studied through a combined method of NMR, theoretical calculations and solvation theory. They can be described in terms of the axial–axial and equatorial–equatorial conformations, taking into account the main rotamers of each of these conformations. From the NMR experiments at 183 K in CD2Cl2–CS2, it was possible to observe proton H2 in the ax–ax and eq–eq conformers separately for 1 and 2, but not for 3, which gave directly their populations and conformer energies. In the alcohols the proportion of the ax–ax conformer was too low to be detected by NMR under these conditions. Those HH couplings together with the values at room temperature, in a variety of solvents allowed the determination of the solvent dependence of the conformer energies and hence the vapor state energy difference. The ΔE (Eax–Eeq) values in the vapor state for 1, 2 and 3 are −0.05, 0.20 and 0.55 kcal mol−1, respectively, increasing to 1.10, 1.22 and 1.41 kcal mol−1 in CD3CN solution (1 kcal = 4.184 kJ). For 4–6 the eq–eq conformation is always much more stable in both non‐polar and polar solvents, with energy differences ranging from 1.78, 1.94 and 1.86 kcal mol−1 (in CCl4) to 1.27, 1.49 and 1.54 kcal mol−1 (in DMSO), respectively. Comparison of the hydroxy and methoxy compounds gives the intramolecular hydrogen bonding energy for the alcohols as 1.40, 1.36 and 1.00 kcal mol−1 (in CCl4) for 4, 5 and 6, respectively. Copyright © 2002 John Wiley & Sons, Ltd.
Ab initio studies of the bromination of benzene
Authors: Smith William B.
Published: December 2002

The reaction of elemental bromine with benzene was followed using ab initio modeling. Two mechanisms were considered as possibilities based in part on tradition and in part on more current experimental evidence. Consideration of transition state energy rules against the conventional picture involving direct formation of the Wheland intermediate via electrophilic attack by bromine. The preferred pathway is calculated to be through the cation–anion radical pair which then goes to the Wheland intermediate. Copyright © 2002 John Wiley & Sons, Ltd.
Kinetics and mechanisms of the gas‐phase elimination of arylethyl N,N‐dimethylcarbamates and ω‐phenylalkyl N,N‐dimethylcarbamates
Authors: Chuchani Gabriel , Dominguez Rosa M. , Rotinov Alexandra , Herize Armando
Published: December 2002

The gas‐phase elimination kinetics of several arylethyl N,N‐dimethylcarbamates and ω‐phenylalkyl N,N‐dimethylcarbamates were determined in the temperature range 299.6–399.9 °C and pressure range 18–95 Torr. The reactions in a static system, seasoned with allyl bromide, and in the presence of a free radical suppressor are homogeneous and unimolecular and follow a first‐order rate law. The rate coefficients are given by the Arrhenius equations: for 4‐phenethyl N,N‐dimethylcarbamate, log[k1 (s−1)] = (11.32 ± 0.22) −(166.9 ± 2.5) kJ mol−1 (2.303RT)−1; for methylphenethyl N,N‐dimethylcarbamate, log[k1 (s−1)] = (12.07 ± 0.36) −(178.6 ± 4.3) kJ mol−1 (2.303RT)−1; for 4‐methoxyphenethyl N,N‐dimethylcarbamate, log[k1 (s−1)] = (11.03 ± 0.60) −(167.3 ± 7.1)­kJ mol−1 (2.303RT)−1; for 4‐nitrophenethyl N,N‐dimethylcarbamate, log[k1 (s−1)] = (11.31 ± 0.54) −(163.7 ±­6.1) kJ mol−1 (2.303RT)−1; for 3‐(4‐methoxyphenyl)propyl N,N‐dimethylcarbamate, log[k1 (s−1)] =­(13.52 ± 0.54) −(208.4 ± 6.8) kJ mol−1 (2.303 RT)−1; for 4‐phenyl‐1‐butyl N,N‐dimethylcarbamate, log[k1 (s−1)] = (12.00 ± 0.34) −(185.2 ± 4.2) kJ mol−1 (2.303 RT)−1; and for 5‐phenyl‐1‐pentyl N,N‐dimethyl carbamate, log[k1 (s−1)] = (11.79 ± 0.31) −(182.2 ± 3.9) kJ mol−1 (2.303RT)−1. The results imply the absence of anchimeric assistance of the phenyl group, while the acidity of the benzylic β‐hydrogen appears to be responsible for a small but significant rate augmentation in these eliminations. Copyright © 2002 John Wiley & Sons, Ltd.
Mechanism of elimination of amino acid derivatives in the gas phase. Pyrolysis kinetics of ethyl picolinate, ethyl 1‐methylpipecolinate and picolinic acid
Authors: Lafont Jennifer , Ensuncho Adolfo , Domínguez Rosa M. , Rotinov Alexandra , Herize Armando , Quijano Jairo , Chuchani Gabriel
Published: December 2002

The kinetics of the gas‐phase elimination of the title compounds were determined in a static reaction system over the temperature range 180.8–419.4 °C and the pressure range 15–86 Torr (1 Torr = 133.3 Pa). The reactions are homogeneous and unimolecular and obey a first‐order rate law. The observed rate coefficients are represented by the following Arrhenius equations: for ethyl picolinate, log[k1(s−1)] = (11.30 ± 0.24) −(180.9 ±­3.0) kJ mol−1 (2.303 RT)−1, for ethyl 1‐methylpipecolinate, log[k1(s−1)] = (13.36 ± 0.31) −(209.5 ± 3.9) kJ mol−1 (2.303 RT)−1 and for picolinic acid, log[k1(s−1)] = (12.05 ± 0.10) −(135.7 ± 0.9) kJ mol−1 (2.303 RT)−1. The data from this work together with those reported in the literature confirm previous considerations that amino acids or α‐nitrogen substituents of carboxylic acids undergo an extremely rapid decarboxylation process. The pyrolysis kinetics of picolinic acid, which is an intermediate of ethyl picolinate elimination, showed a dramatic fast decomposition into pyridine and CO2 gas. The decarboxylation process of α‐amino or α‐nitrogen substituents of carboxylic acids differs from the decarbonylation elimination of several known α‐substituted carboxylic acids in the gas phase. Copyright © 2002 John Wiley & Sons, Ltd.
Intramolecular proton transfer in tautomeric 2‐Imidazolone and 2‐thioimidazolone
Authors: Contreras J. Guillermo , Madariaga Sandra T.
Published: December 2002

The enol ⇌ keto and thiol ⇌ thione prototropic tautomerisms in 2‐imidazolone and 2‐thioimidazolone were studied by ab initio methods in the frame of MO theory. The energetics of these reactions were derived using various basis sets and levels including both polarization and diffuse functions. To account for electron correlation, second‐, third‐ and fourth‐order Møller–Plesset perturbation theory was applied. The thermodynamics for both reactions indicate that the keto and thione are the predominant species in the gas phase, whereas in solution the enol and keto forms are present in comparable concentrations. For both tautomerization reactions the transition states for the intramolecular mechanism were found and fully characterized. The calculated energetic barrier for the enol–keto system is 41.6 kcal mol−1, whereas that for the thiol–thione system is 32.5 kcal mol−1 (1 kcal = 4.184 kJ). The study of the solvent effect, using the PCM model, on Ea reveals that solvents lower the barrier by ca 11 and 7 kcal mol−1 for the enol–keto and thiol–thione systems, respectively. Copyright © 2002 John Wiley & Sons, Ltd.
Structural properties and 13C chemical shifts of bis‐ and tris(2‐thienyl)methinium and related cations: a combined theoretical and experimental study
Authors: Fabian Jürgen , Hartmann Horst , Noack Antje
Published: December 2002

The molecular structure of the parent bis‐ and tris(2‐thienyl)methinium ions were studied by first‐principles methods (second‐order Møller–Plesset many‐body perturbation theory and density functional theory). In contrast to bis(2‐thienyl)methinium, tris(2‐thienyl)methinium is non‐planar with torsion angles between the thienyl groups and the central plane of about 30°. The calculations on tris(2‐thienyl)methinium resulted in two almost isoenergetic conformers with barriers to isomerization less than 10 kcal mol−1 (1 kcal = 4.184 kJ). In order to discuss the 13C chemical shifts of Crystal Violet and of heteroanalogous compounds with thienyl‐2‐, with 5‐amino‐thienyl‐2‐, with 5‐methylmercapto‐thienyl‐2‐ and with 5‐amino‐1,3‐thiazolyl‐2‐heteroaryl groups, the structure was optimized at the DFT B3‐LYP/6–31G(d) level and the chemical shifts were calculated at the DFT‐GIAO/6–311 + G(2d,p) level of theory. In general, the experimental chemical shifts correlate very well with the experimental values. The lowest energy electronic transitions in the UV–VIS region were calculated by time‐dependent density functional response theory (TD‐DFRT) using the DFT B3‐LYP/6–31 + G(d) basis set. For cationic dyes the agreement between the results of TD‐DFRT calculations and experiment was less good than for neutral sulfur‐containing compounds. Semiempirical calculations (PPP, ZINDO/S) are better in that case and are more efficient. Copyright © 2002 Wiley & Sons, Ltd.
Quantitative approach to the Menschutkin reaction of benzylic systems
Authors: Yoh Soo‐Dong , Cheong Duk‐Young , Lee Oh‐Seuk
Published: December 2002

The quaternization reactions of substituted (Z)‐benzyl (X)‐benzenesulfonates with substituted (Y)‐pyridines were investigated in acetonitrile at 35 °C. The magnitudes of the Hammett reaction constants ρX, ρY and ρZ indicate that a stronger nucleophile leads to a lesser degree of bond breaking. In addition, a better leaving group is accompanied by a lesser degree of bond formation. Application of multi‐Hammett interactions, |ρYZ | >|ρXY | >|ρXZ |, predicts that these Menschutkin‐type reactions are dissociative SN2 reactions. In particular, the reaction of strongly activated benzyl derivatives with tertiary amines in acetonitrile reveals a more advanced bond breaking like SN1 reactions. The predicted mechanism for the benzylation of pyridines with benzylic systems is evident from More O'Ferrall–Jencks diagram and the semi‐empirical MO calculations with the AM1 method. Copyright © 2002 John Wiley & Sons, Ltd.
Photophysical and photochemical properties of 5,7‐dimethoxycoumarin under one‐ and two‐photon excitation
Authors: Belfield Kevin D. , Bondar Mykhailo V. , Liu Yong , Przhonska Olga V.
Published: December 2002

The spectroscopic and photochemical properties of 5,7‐dimethoxycoumarin (DMC) in acetonitrile (ACN) were investigated at room temperature under one‐ and two‐photon excitation. Two‐photon induced [2 + 2] photocycloaddition of DMC was investigated with 650 nm laser excitation (120 fs pulse width). GC and GC–MS analysis confirmed dimer formation, while HPLC analysis allowed specific photodimer identification through comparison with samples prepared via one photon irradiation. Photodimer products formed via two‐photon excitation correlated with those formed via UV irradiation. Syn head‐to‐head and syn head‐to‐tail photodimers were formed in 1:3.0 and 1:2.8 ratios under one‐photon (broadband UV) and two‐photon (650 nm, 120 fs) irradiation in anisole, respectively. The quantum yield for the photoreaction of DMC solution (10−4 M) under broadband UV irradiation was of the order of 1 × 10−3 and increased dramatically to 1 × 10−2 under two‐photon excitation. The spectral investigation of the photochemical products of DMC revealed efficient fluorescence emission in the spectral region 290–340 nm, attributed to the syn‐head‐to‐tail dimers of DMC. Evidence for an additional manifold for the photochemical reaction of DMC was found, resulting in the production of photochemical products that exhibited low fluorescence emission intensity along with dependences on excitation wavelength. Copyright © 2002 John Wiley & Sons, Ltd.
Gas‐phase Diels–Alder cycloaddition reaction in the presence of methanol and water vapor
Authors: Gholami M. R. , Talebi B. A.
Published: December 2002

Hydrogen bonding effects of protic solvents, apart from bulk properties, on the reaction rate of the cycloaddition of cylopentadiene and vinyl acetate in the presence of water and methanol in the gas phase were investigated. The results showed that methanol increases the reaction rate in the gas phase more than the water. This is attributed to the stronger hydrogen bonding effect of methanol in this phase. Ab initio and semi‐empirical calculations show that methanol stabilizes the transition state of the reaction more than water. This arises from two different origins, distribution of charge and geometry of the hydrogen bond. Copyright © 2002 John Wiley & Sons, Ltd.
RAFTing down under: Tales of missing radicals, fancy architectures, and mysterious holes
Authors: Barner‐Kowollik Christopher , Davis Thomas P. , Heuts Johan P. A. , Stenzel Martina H. , Vana Philipp , Whittaker Michael
Published: December 2002

This highlight describes recent developments in reversible addition–fragmentation transfer (RAFT) polymerization. Succinct coverage of the RAFT mechanism is supplemented by details of synthetic methodologies for making a wide range of architectures ranging from stars to combs, microgels, and blocks. In addition, RAFT reactions in different media such as emulsion and ionic liquids receive attention. Finally, a specific example of a novel material design is briefly introduced, whereas polymers prepared via RAFT are adopted for microporous/honeycomb membrane design. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 365–375, 2003
Kinetics of surface‐initiated atom transfer radical polymerization
Authors: Kim Jong‐Bum , Huang Wenxi , Miller Matthew D. , Baker Gregory L. , Bruening Merlin L.
Published: December 2002

Although atom transfer radical polymerization (ATRP) is often a controlled/living process, the growth rate of polymer films during surface‐initiated ATRP frequently decreases with time. This article investigates the mechanism behind the termination of film growth. Studies of methyl methacrylate and methyl acrylate polymerization with a Cu/tris[2‐(dimethylamino)ethyl]amine catalyst system show a constant but slow growth rate at low catalyst concentrations and rapid growth followed by early termination at higher catalyst concentrations. For a given polymerization time, there is, therefore, an optimum intermediate catalyst concentration for achieving maximum film thickness. Simulations of polymerization that consider activation, deactivation, and termination show trends similar to those of the experimental data, and the addition of Cu(II) to polymerization solutions results in a more constant rate of film growth by decreasing the concentration of radicals on the surface. Taken together, these studies suggest that at high concentrations of radicals, termination of polymerization by radical recombination limits film growth. Interestingly, stirring of polymerization solutions decreases film thickness in some cases, presumably because chain motion facilitates radical recombination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 386–394, 2003
Using diethylphosphites as thermally latent curing agents for epoxy compounds
Authors: Chiu Yie‐Shun , Liu Ying‐Ling , Wei Wen‐Lung , Chen Wen‐Yu
Published: December 2002

Diethylphosphite (DEP) and its derivative exhibited thermally latent properties for epoxy curing reactions through the formation of phosphonic acid as an active species from a reaction of ethanol elimination, which was observed with 1H NMR and pyrolysis gas chromatography/mass spectrometry measurements. The thermally latent properties and curing reaction kinetics of the curing reaction of DEPs with diglycidyl ether of bisphenol A were studied with differential scanning calorimetry. The cured epoxy resins possessed a phosphorous element coming from the DEP derivatives, exhibiting improved flame retardancy. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 432–440, 2003
Cobalt‐catalyzed alternating and nonalternating copolymerization of carbon monoxide with aziridine
Authors: Zhao Jiquan , Ding Errun , Allgeier Alan M. , Jia Li
Published: December 2002

Catalysts CH3COCo(CO)3PPh3 (1) and HCo(CO)3PPh3 (2) catalyze the copolymerization of aziridine and carbon monoxide. Catalyst 2 can be conveniently generated in
situ via reaction of Na+Co(CO)4, N,N‐dimethylanilinium chloride, and PPh3. The copolymerization alternates at high catalyst loadings to produce poly(β‐alanine). The end groups of the poly(β‐alanine) product are characterized by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry and by comparison of the 1H NMR spectra of the polymer and a stepwise synthesized model compound. At low catalyst loadings, the polymer product contains both the β‐alanine units and amine units because of nonalternating enchainment of the comonomers. The characterization of the amine units is again supported by comparison of the 1H NMR spectra of the polymers and the stepwise synthesized model compounds. The molecular weights of the polymers are determined by gel permeation chromatography. The thermal stability of the polymers is probed by differential scanning calorimetry and thermogravimetric analysis. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 376–385, 2003
Hydrophilic and hydrophobic copolymer systems based on acrylic derivatives of pyrrolidone and pyrrolidine
Authors: González Nieves , Elvira Carlos , San Román Julio
Published: December 2002

This article deals with the synthesis of hydrophilic methacrylic monomers derived from ethyl pyrrolidone [2‐ethyl‐(2‐pyrrolidone) methacrylate (EPM)] and ethyl pyrrolidine [2‐ethyl‐(2‐pyrrolidine) methacrylate (EPyM)] and their respective homopolymers. For the determination of their reactivity in radical copolymerization reactions, both monomers were copolymerized with methyl methacrylate (MMA), the reactivity ratios being calculated by the application of linear and nonlinear mathematical methods. EPM and MMA had ratios of rEPM = 1.11 and rMMA = 0.76, and this indicated that EPM with MMA had a higher reactivity in radical copolymerization processes than vinyl pyrrolidone (VP; rVP = 0.005 and rMMA = 4.7). EPyM and MMA had reactivity ratios of rEPyM = 1.31 and rMMA = 0.92, and this implied, as for the EPM–MMA copolymers, a tendency to form random or Bernoullian copolymers. The glass‐transition temperatures of the prepared copolymers were determined by differential scanning calorimetry (DSC) and were found to adjust to the Fox equation. Total‐conversion copolymers were prepared, and their behavior in aqueous media was found to be dependent on the copolymer composition. The swelling kinetics of the copolymers followed water transport mechanism case II, which is the most desirable kinetic behavior for a swelling controlled‐release material. Finally, the different states of water in the hydrogels—nonfreezing water, freezing bound water, and unbound freezing water—were determined by DSC and found to be dependent on the hydrophilic and hydrophobic units of the copolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 395–407, 2003
Syntheses and properties of novel fluorinated polyamides based on a bis(ether‐carboxylic acid) or a bis(ether amine) extended from bis(4‐hydroxyphenyl)phenyl‐2,2,2‐trifluoroethane
Authors: Hsiao Sheng‐Huei , Chen Wei‐Tsun
Published: December 2002

Two series of novel fluorinated aromatic polyamides were prepared from 1,1‐bis[4‐(4‐carboxyphenoxy)phenyl]‐1‐phenyl‐2,2,2‐trifluoroethane with various aromatic diamines or from 1,1‐bis[4‐(4‐aminophenoxy)phenyl]‐1‐phenyl‐2,2,2‐trifluoroethane with various aromatic dicarboxylic acids with the phosphorylation polyamidation technique. These polyamides had inherent viscosities ranging from 0.51 to 1.54 dL/g that corresponded to weight‐average and number‐average molecular weights (by gel permeation chromatography) of 36,200–80,000 and 17,200–64,300, respectively. All polymers were highly soluble in aprotic polar solvents, such as N‐methyl‐2‐pyrrolidone and N,N‐dimethylacetamide, and some could even be dissolved in less‐polar solvents like tetrahydrofuran. The flexible and tough films cast from the polymer solutions possessed tensile strengths of 76–94 MPa and initial moduli of 1.70–2.22 GPa. Glass‐transition temperatures (Tg's) and softening temperatures of these polyamides were observed in the range of 185–268 °C by differential scanning calorimetry or thermomechanical analysis. Decomposition temperatures (Td's) for 10% weight loss all occurred above 500 °C in both nitrogen and air atmospheres. Almost all the fluorinated polyamides displayed relatively higher Tg and Td values than the corresponding nonfluorinated analogues. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 420–431, 2003
Composition distribution of ethylene or propylene–norbornene copolymers obtained with zirconocene catalysts
Authors: Naga Naofumi , Imanishi Yukio
Published: December 2002

Copolymerization of ethylene or propylene and norbornene (NB) was carried out with stereospecific zirconocene catalysts rac‐ethylenebis(indenyl)zirconium dichloride, rac‐dimethylsilylenebis(indenyl)zirconium dichloride (2), rac‐dimethylsilylenebis(2‐methylindenyl)zirconium dichloride, and diphenylmethylene(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride combined with cocatalysts at 40 °C. Temperature‐rising elution fractionation of the copolymers was carried out with cross‐fractionation chromatography with o‐dichlorobenzene as a solvent, and a broad distribution of the copolymer composition was detected. The fraction eluted at lower temperature contained higher NB. The effect of the polymerization time was examined in the ethylene–NB copolymerization with catalyst 2, and the higher‐temperature elution fraction increased with increasing polymerization time. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 441–448, 2003
Performance of the Cr[CH(SiMe3)2]3/SiO2 catalyst for ethylene polymerization compared with the performance of the Phillips catalyst
Authors: Ikeda Haruhiko , Monoi Takashi , Sasaki Yasuaki
Published: December 2002

The catalysis of a silica‐supported chromium system {Cr[CH(SiMe3)2]3/SiO2} was compared with a silica‐supported chromium oxide catalyst, the Phillips catalyst (CrO3/SiO2). This catalyst was prepared by the calcining of the typical silica support used for the Phillips catalyst at 600 °C and by the support of tris[bis(trimethylsilyl)methyl]chromium(III) {Cr[CH(SiMe3)2]3} on the silica. In the slurry‐phase polymerization, this catalyst conducted the polymerization of ethylene at a high activity without organoaluminum compounds as cocatalysts or scavengers. The activity per Cr was about 6–7 times higher than that of the Phillips catalyst. Upon the introduction of hydrogen to the system, the molecular weight of polyethylene did not change with the Phillips catalyst, but it decreased with the Cr[CH(SiMe3)2]3/SiO2 catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 413–419, 2003
Novel route to poly(p‐phenylene vinylene) polymers
Authors: Wu Xufeng , Shi Gaoquan , Qu Liangti , Zhang Jiaxin , Chen Feng'en
Published: December 2002

Poly(p‐phenylene vinylene) (PPV), poly(2,5‐dioctyl‐p‐phenylene vinylene) (PDOPPV), and poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene] (MEHPPV) were synthesized by a liquid–solid two‐phase reaction. The liquid phase was tetrahydrofuran containing 1,4‐bis(bromomethyl)benzene, 1,4‐bis(chloromethyl)‐2,5‐dioctylbenzene, or 1,4‐bis(chloromethyl)‐2‐methoxyl‐5‐(2′‐ethylhexyloxy)benzene as the monomer and a certain amount of tetrabutylammonium bromide as a phase‐transfer catalyst. The solid phase consisted of potassium hydroxide particles with diameters smaller than 2 mm. The experimental results demonstrated that the reaction conversions of PPV and PDOPPV were fairly high (∼65%), but the conversion of MEHPPV was only 45%. Moreover, gelation was found in the polymerization processes. As a result, PPV was insoluble and PDOPPV and MEHPPV were partially soluble in the usual organic solvents, such as tetrahydrofuran and chloroform. Soluble PDOPPV and MEHPPV were obtained with chloromethylbenzene or bromomethylbenzene as a retardant regent. The molar mass of soluble PDOPPV was measured to be 2 × 104 g mol−1, and that of MEHPPV was 6 × 104 g mol−1. A thin, compact film of MEHPPV was formed via spin coating, and it emitted a yellow light. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 449–455, 2003
A complex defect studied through DFT: The Fe3–O2− pair in KMgF3
Authors: Aramburu J. A. , García‐Lastra J. M. , Barriuso M. T. , Moreno M.
Published: December 2002

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies performed by different authors in KMgF3, KZnF3, and RbCdF3 have detected a strongly axial Fe3+ center that was tentatively assigned to a FeF5O4− complex. The current work is devoted to exploring the properties of the FeF5O4− center (C4V symmetry) in KMgF3 by means of density functional calculations. The equilibrium distance between Fe3+ and O2− (1.83 Å) is found to be only ∼10% higher than that computed for the Fe3+–O2− pair in vacuo (1.63 Å), but ∼25 pm smaller than that corresponding to FeO units. These data together with the charge transferred onto the oxygen in the antibonding a (≈3z2 − r2) orbital reveals the formation of a Fe3+–O2− pair inside the KMgF3 lattice. In comparison with FeF the substitution of a F− ion by an O2− one is found to produce an increase of distances between Fe3+ and the four equatorial (Req) and apical (Rax) F− ligands equal to 10 pm and 17 pm, respectively. The current calculations reasonably account for the experimental fingerprint of this center, where the isotropic superhyperfine constant for the equatorial fluorines (As(eq)) is found to be about twice that for the apical fluorine (As(ax)). This huge difference between As(eq) and As(ax) cannot be explained on the basis of the equilibrium Req and Rax values and the law relating As to the metal–ligand distance, R, for the octahedral FeF unit. This salient feature stresses that laws valid for a given kind of complex cannot be transferred to complexes with lower symmetry. Along this line, the current results cast doubts on the conclusions previously reached using the empirical superposition model. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003
First‐principles elastic properties of BAs
Authors: Herrera‐Cabrera M. J. , Rodríguez‐Hernández P. , Muñoz A.
Published: December 2002

We used an ab initio total energy pseudopotential technique within the density functional theory in the local‐density approximation to determine the full set of first‐order elastic constants of BAs, which have not been established experimentally. We also calculated the bulk modulus, the optical phonon frequency at the Γ point, and we present a study of the electronic band structure. Finally we also determined the pressure dependence of the elastic constants Cij. Due to the absence of the experimental data our results can be considered as reliable predictions of the elastic moduli and the pressure derivatives. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003