Essential Chemistry Research Articles

Computer Generation of Reaction Mechanisms Using Quantitative Rate Information: Application to Long-Chain Hydrocarbon Pyrolysis

In recent years, the use of mechanistic modeling to identify the underlying kinetics of complex systems has increased greatly. One of the challenges to kinetic modeling is the construction of a model which can capture the essential chemistry of a system while a manageable size is retained. The rate-based generation of mechanistic models is an attractive approach because kinetically significant species can be determined and selectively included in the final mechanism. An algorithm for the rate-based generation of reaction mechanisms developed previously1 was improved and used to construct a compact mechanistic model for low-pressure tetradecane pyrolysis. Though thousands of species and reactions were generated, only a small portion of these (2% of species and 20% of reactions) was deemed necessary and incorporated into the final model. Experimental data were used to determine frequency factors for a subset of the reaction families, while all other kinetic parameters were set on the basis of literature values. With no adjustment to the optimized frequency factors, the mechanistic model was able to accurately predict reactant conversions and product yields for varying reaction conditions and initial reactant loadings. It was also observed that increasing the quantity of species initially seeded resulted in a smaller mechanism that had comparable fitting and predicting abilities as those of the models seeding only the reactant. Subsequent regeneration of the reaction mechanisms using the optimized values for the frequency factors resulted in smaller models with comparable capabilities.

New functions for the ancient globin family: bacterial responses to nitric oxide and nitrosative stress.

Globin-like oxygen-binding proteins occur in bacteria, yeasts and other fungi, and protozoa. The simplest contain protohaem as sole prosthetic group, but show considerable variation in their similarity to the classical animal globins and plant globins. Flavohaemoglobins comprise a haem domain homologous to classical globins and a ferredoxin-NADP+ reductase (FNR)-like domain that converts the globin into an NAD(P)H-oxidizing protein with diverse reductase activities. In Escherichia coli, the prototype flavohaemoglobin (Hmp) is clearly involved in responses to nitric oxide (NO) and nitrosative stress: (i) the structural gene hmp is upregulated by NO and nitrosating agents; (ii) purified Hmp binds NO avidly, but also converts it to nitrate (aerobically) or nitrous oxide (anaerobically); (iii) hmp mutants are hypersensitive to NO and nitrosative stresses. Here, we review recent advances in E. coli and the growing number of microbes in which globins are known, draw particular attention to the essential chemistry of NO and related reactive species and their interactions with globins, and suggest that microbial globins have additional functions unrelated to 'NO' stresses.

Essential Chemistry References from Academic Press

Essential Chemistry References from Academic Press

Chemical reaction engineering in the design of CVD reactors

The current status of modeling on CVD processes to produce thin films is summarized in this review. The experimental methodologies for extracting the essential chemistry in CVD reactors had been developed to efficiently design CVD reactors. Tubular wall deposition method provides information on the rate limiting step of the deposition process and the rate constants. Molecular size of film forming species can be estimated from the deposition rate profile in the tubular reactor if the process is limited by the gas phase diffusion. Microcavity-deposition method provides the tool to analyze the surface reactions. The microcavity-deposition method can be combined with macrocavity-deposition method to extract the important reaction pathways like as the intermediate deposition. The use of computer chemistry is also useful in detecting key reactions and it may be used to construct new deposition chemistry. The computer fluid dynamics coupled with the chemistries obtained from these approaches will be a powerful tool to design CVD reactors.

Variability in essential oil chemistry and plant morphology within a Leptospermum scoparium population

Essential oil composition and plant morphology were observed over four years in individual plants raised from seed of a wild population of Leptospermum scoparium (Myrtaceae) collected at a single site in New Zealand. Principal component analyses of data from young and mature plants showed no significant grouping of plants on the basis of oil composition, but identified differences between the essential oil components contributing most to variation in oil composition in both young and mature plants. The dominant variables were six sesquiterpene components in young plants, and three monoterpenes and two sesquiterpenes in mature plants. Levels of these components differed significantly at the population level between young and mature plants and also within and between seasons. Levels of all these components varied markedly within and between individual plants at all sample times. The habit, leaf size and density, and stem and foliage colour also varied markedly between individual plants. The variation observed indicates the need for more extensive sampling and statistical analysis over more than one growing season if sufficiently reliable data on essential oil compositions in individual plants or populations are to be obtained for chemotaxonomic or plant selection purposes.

A proposal for water oxidation in photosystem II

Abstract

Integrated dynamic simulation of rapid thermal chemical vapor deposition of polysilicon

A physically-based dynamic simulator has been constructed to investigate the time-dependent behavior of equipment, process, sensor, and control systems for rapid thermal chemical vapor deposition (RTCVD) of polysilicon from SiH/sub 4/. The simulator captures the essential physics and chemistry of mass transport, heat transfer, and chemical kinetics of the RTCVD process as embodied in equipment. In order to complete the system-level description, reduced-order models are also employed to represent processes involving high complexity of physics. Integration of individual simulator elements for equipment, process, sensors, and control systems enables the evaluation of not only the deposition rate and film thickness, but also of a broad range of dynamic system properties such as equipment performance, gas flow conditions, wafer temperature variation, wafer optical properties (absorptivity/emissivity), reaction gas composition, total process cycle time, consumables volume, and reactant utilization. This makes the simulator directly applicable to the optimization of process recipes and equipment design, to process control strategy, and to fault classification. This case study of polysilicon RTCVD demonstrates (1) that integrated dynamic simulation is a versatile tool for representing system-level dynamics and (2) that such representation is pivotal in successful applications of modeling and simulation for manufacturing optimization and control.

On the oscillatory laser-augmented combustion of HMX

The steady and unsteady combustion of cyclotetramethylene-tetranitramine (HMX) is examined experimentally and theoretically. New experimental data are presented for laser-augmented combustion, both macroscopically steady and unsteady (laser-recoil). The one-dimensional approximation is investigated and found to be reasonably valid when containment rings are used to direct the combustion gases normal to the burning surface. The experimental results are interpreted within the framework of the classical, linearized quasi-steady (QS) theory of unsteady combustion of homogeneous solids. The Ward-Son-Brewster (WSB) model, which has recently been shown to give surprisingly good predictions of steady-state burning behavior of HMX, is also found to agree well with oscillatory laser-augmented combustion (the laser-recoil response function). In this model, condensed-phase chemistry is represented by single-step, zero-order, high-activation energy thermal decomposition that is, overall, relatively energetically neutral. Gas-phase chemistry is represented by an exothermic, single-step, second-order (overall) process with negligible energy barrier. A decomposition energy barrier of 42 kcal/mol is found, which is consistent with the mechanism of homolytic N−NO 2 bond scission or HONO elimination. The vanishingly small gas-phase energy barrier and second-order behavior are consistent with a bimolecular chain reaction mechanism involving NO 2 , HONO, and so on and a steady-state pool of chain carriers near the surface to form NO In short, new experimental data verify that the essential physics and chemistry of HMX combustion are represented at a simple level by the WSB model. The model is based on highly simplified but rational chemistry, matches both steady and unsteady burning data, and is generally consistent with known detailed chemistry in both the gas and condensed phases.

Mechanism reduction during computer generation of compact reaction models

AbstractOne of the challenges in building reaction mechanisms using algorithms for automated model construction is to describe the essential chemistry and enable prediction of experimental data over wide ranges of reaction conditions while maintaining a manageable model size. Two complementary methodologies for building compact reaction mechanisms were developed and combined with existing algorithms based on graph theory and bond–electron matrix operations. Each strategy was developed using pentadecylbenzene pyrolysis as an illustrative example. The first approach used a radical lumping strategy to group radicals according to their reactivity. The mechanism was reduced from 719 to 215 species and successfully predicted the experimentally observed initial reactivity. However, implementation of the radical lumping criteria alone was insufficient to allow for secondary reactions to higher‐rank products. Therefore, on‐the‐fly sensitivity analysis was incorporated to identify the important and necessary species as the mechanism was generated to guide the mechanism building process. The generic algorithms developed can be applied to generate compact reaction mechanisms for a wide array of higher molecular‐weight reactants.

Essential Chemistry (Chang, Raymond)

Essential Chemistry offers material that would be appropriate as an introduction to chemistry. I would recommend it for students in education and majors in biology, chemistry, and physics, and other programs requiring only one year of chemistry

Linear pendant anhydrides: novel building blocks for high solids, high performance coatings

Increasing sensitivity to environmental issues worldwide has led to unique new driving forces that are shaping the development of coatings technology for the future. While lower VOC (volatile organic compound) levels in paints and coatings offer compliance with legislative mandates, they do not necessarily offer appearance and/or performance benefits consistent with demands from manufacturers and consumers. Therefore, it is incumbent upon materials suppliers and coatings formulators to improve the overall performance of coatings from primers to clearcoats while operating in the higher solids realm which is now necessary. Recent work at DOW has led to the development of unique raw materials for solventborne coatings which allow for medium to very high solids (2.6 lbs VOC/gal or less) formulations that handle and perform like low solids/high VOC formulations. These new raw materials have been coined ‘linear pendant anhydride’ (LPA) resins — a broad term which applies to materials from low molecular weight oligomers to high molecular weight polymers. When formulated with epoxy resins, very high solids levels can be obtained with superior overall performance to other ‘state-of-the-art’ coatings technologies (i.e. 2-component polyurethane enamels). Discussed here are details of the essential resin chemistry, formulation and testing of LPA-based coatings for both ambient and thermal cure applications. Particular emphasis is on the remarkable durability of these coatings.

Chemistry of Sol-Gel Synthesis of Aluminum Oxides with In Situ Water Formation: Control of the Morphology and Texture

Sol-gel synthesis of aluminum oxide from mixtures containing Al(O-s-Bu)_3, acetic acid, and sec-butyl alcohol, whereby water was produced in situ, was characterized by infrared, ^1H NMR, ^(13)C NMR, and ^(13)C and ^(27)Al magic angle spinning NMR spectroscopies. The molar ratio (R) of acetic acid to Al(O-s-Bu)_3 strongly influenced the nature of the product. When R was < 2, the product was an opaque gelatinous precipitate, and when R was ≥ 2, transparent gels were formed. Acetic acid played crucial roles in both the hydrolysis and condensation reactions that constitute the essential chemistry of the sol-gel synthesis. Acetic acid influenced the rate of hydrolysis by (1) reacting rapidly with Al(O-s-Bu)_3 to give Al(O-s-Bu_2-OCOCH_3), which is much less reactive than Al(O-S-BU)_3 in hydrolysis, (2) catalyzing condensation reactions, and (3) catalyzing the dehydration of sec-butyl alcohol to give water. Acetic acid is involved in all the reactions of the sol-gel synthesis and thus provides a delicate control of the chemistry. The ratio R also influenced the morphology of the products formed as a result of drying at temperatures in the range 50-300 °C; these products were characterized by infrared spectroscopy, scanning electron microscopy, and surface area/pore volume measurements. Upon heating, the gelatinous precipitate was transformed into a dense material with a low surface area and low pore volume. Prior to drying, this material contained principally oligomeric species with few OH ligands and little unreacted acetic acid, thus favoring intermolecular condensation reactions and collapse of the structure during drying. In contrast, gels produced at values of R ≥ 2 gave materials with higher surface areas and higher pore volumes. Prior to drying, these materials contained principally polymeric species incorporating acetate and OH ligands located close to each other, thus favoring intramolecular condensation reactions during drying. Removal of unreacted acetic acid from the gel led to high-surface-area and high-pore-volume materials.

The chemistry of interstellar CH +: The contribution of Bates and Spitzer (1951)

We review the present understanding of interstellar CH +. We show that the essential chemistry was correctly identified by Bates and Spitzer in 1951. More recent work has been concerned with defining appropriate venues within which this chemistry can function efficiently. This process had led to a much fuller understanding of the nature of the interstellar medium.

Deuterium tracing studies and microkinetic analysis of ethylene hydrogenation over platinum

Deuterium tracing experiments of ethylene hydrogenation over platinum were conducted at temperatures from 248 to 333 K and D 2/C 2H 4 pressure ratios between 50:75 and 150:25 Torr. Information about the rates of adsorption/de sorption and hydrogenation processes on the surface was extracted by kinetic analysis of the deuterium tracing data combined with results from steady-state kinetic studies and temperature programmed desorption experiments. This analysis suggests that the essential surface chemistry can be described by a Horiuti-Polanyi mechanism that includes competitive and noncompetitive hydrogen adsorption pathways, each modified by a hydrogen activation step. The importance in these analyses of utilizing data from diverse experimental measurements is illustrated.

Microkinetic analysis of diverse experimental data for ethylene hydrogenation on platinum

Kinetic analysis employing a mechanism that captures the essential surface chemistry of the reaction allows quantitative interpretation of diverse experimental data. This approach is used with a Horiuti-Polanyi mechanism, modified by hydrogen activation steps, to describe the surface chemistry for ethylene hydrogenation over platinum catalysts. In this investigation, kinetic analysis provides a quantitative means of comparing, contrasting, and consolidating results from steady-state kinetic studies, deuterium tracing measurements, vibrational spectroscopy, and temperature programmed desorption

Chirospecific analysis in flavor and essential oil chemistry Part A. Filbertone ? the character impact compound of hazel-nuts

The enantiomers of (E)-5-methyl-2-hepten-4-one (filbertone) have been directly separated by capillary gas chromatography on fused silica capillaries with heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin as stationary phase. The enantiomeric composition of this compound in raw and roasted hazel-nuts of different origins was determined by this method. Additionally, the odor quality of both enantiomers of (E)-5-methyl-2-hepten-4-one and (Z)-5-methyl-2-hepten-4-one were described by sniffing synthetic (racemic) filbertone on a sniffing port at the end of aβ-cyclodextrin capillary. Besides the Chromatographie separation, a filbertone sample (100 μg), isolated by a combination of MPLC/preparative capillary gas chromatography from roasted hazel-nuts, was investigated for its enantiomeric composition by1H-NMR, adding the chiral shift reagent Eu(hfc)3. The taste quality of the two filbertone enantiomers was determined by a test panel using the pure (+)-(E)- and (−)-(R)-isomers isolated by preparative HPLC.

Chirospecific analysis in flavor and essential oil chemistry Part B. Direct enantiomer resolution of frans-?-ionone and trans-?-damascone by inclusion gas chromatography

The direct capillary gas Chromatographic separation oftrans-α-ionone andtrans-α-damascone enantiomers is reported using heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin in polysiloxane as a suitable chiral stationary phase. The method described has been applied to determine the naturally occurring enantiomeric composition oftrans-α-ionone in the absolutes of the flowers ofBoronia megastigma Nees andOsmanthus fragrance Lour., Costus root oil (Saussurea lappa Clarke) as well as in odor concentrates of violet flowers (Viola odorata L.), fresh raspberries, carrots and vanilla pods. Furthermore, the optical purity oftrans-α-ionone in the solvent extracts of distilled alcoholic raspberry beverage, raspberry fruit juice concentrate and black tea was investigated.Trans-α-ionone was either isolated by headspace stripping in vacuo or by organic solvent extraction and subsequently enriched by multidimensional preparative Chromatographic techniques.Trans-α-damascone was isolated from Virginia tobacco extract and from black tea aroma and was enriched for direct chirospecific analysis by medium pressure liquid chromatography followed by multidimensional preparative capillary gas chromatography. The importance of chirality as an essential criterion for the discrimination of natural and nature-identical aromas is discussed briefly.

Essential Oil Polymorphism in FinnishThymusSpecies

Chemical polymorphism concerning the essential oils of the genus THYMUS is a widespread phenomenon, especially in the northern species. The two Finnish species, T. SERPYLLUM ssp. SERPYLLUM and T. SERPYLLUM ssp. TANAENIS, turned out to form four different chemotypes each, with hedycaryol, germacra-1(10),5-dien-4-ol, germacra-1(10),4-dien-6-ol, linalool, and linalyl acetate as type-characterizing compounds. Otherwise the oils of the two subspecies were similar containing myrcene, TRANS-beta-ocimene, beta-caryophyllene, and germacrene D as the main terpene hydrocarbons. 1,8-Cineol and camphor represented another great portion in both oils. If Finland is regarded as an area of T. SERPYLLUM (s.l.), a total of six types of plants can be defined with regard to the essential oil chemistry only. Including the frequency of these six types at the four areas investigated, a certain gradient from the south to the north can be seen. A most interesting aspect is the fact that the most frequent, linalyl acetate containing chemotype of the northern Lapland has nearly the same oil composition as T. PRAECOX ssp. ARCTICUS in Island, Norway, and Greenland.

Onion essential oil chemistry. Cis-and trans-2-mercapto-3,4-dimethyl 2,3-dihydrothiophene from pyrolysis of bis(1-propenyl) disulfide

Heating the onion oil component bis(1-propenyl) disulfide gives cis/-and trans-2-mercapto-3,4-dimethyl-2,3-dihydrothiophene which can lose H 2S upon further heating giving 3,4-dimethylthiophene. A dithio-Claisen rearrangement is proposed for the key step.

Isotopes: Essential chemistry and applications II. Edited by J R Jones Published by The Royal Society of Chemistry, London 1988, pp262 ISBN: 0–85186‐746‐4 Price: £39.50

Isotopes: Essential chemistry and applications II. Edited by J R Jones Published by The Royal Society of Chemistry, London 1988, pp262 ISBN: 0–85186‐746‐4 Price: £39.50