Experimental measurements of the decomposition of methyltrichlorosilane (MTS), a common silicon carbide precursor, in a high-temperature flow reactor are presented. The results indicate that methane and hydrogen chloride are major products of the decomposition. No chlorinated silane products were observed. Hydrogen carrier gas was found to increase the rate of MTS decomposition. The observations suggest a radical-chain mechanism for the decomposition. The implications for silicon carbide chemical vapor deposition are discussed.

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Calibrated by both experimental data and high-level coupled-cluster calculations, the BAC-MP4 methodology was applied to 51 SbL(n) (L = H, CH(3), C(2)H(5), Cl, and OH, n = 1-5) molecules, providing calculated heats of formation and associated thermodynamic parameters. These data identify a linear variation in heats of formation with ligand substitution, trends in bond dissociation energies (BDEs) with ligand identity [BDE(Sb-C(2)H(5)) < BDE(Sb-CH(3)) < BDE(Sb-H) < BDE(Sb-Cl) < BDE(Sb-OH)], and a monotonic decrease in BDE upon successive ligand elimination. The linear variation in BDE is consistent with the behavior of other group V elements, in contrast to the characteristic high-low-high trend of adjacent group III (In) and group IV (Sn) elements. Additionally, these data complement those of previous studies of metal-organic species and provide a foundation of thermochemical data that can aid in the selection of CVD precursors and deposition conditions for the growth of antimony-containing materials.

Understanding the reaction mechanisms for the decomposition of NO2-containing energetic materials in the condensed phase is critical to our development of detailed kinetic models of these energetic materials in propellant combustion. To date, the reaction mechanisms in the condensed phase have been represented by global reactions. The detailed elementary reactions subsequent to the initial NO2 bond scissioning are. not known. Using quantum chemical calculations, we have investigated the possible early steps in the decomposition of energetic materials that can occur in the condensed phase. We have used methylnitrate, methylnitramine, and nitroethane as prototypes for O-NO2, N-NO2, and C-NO2 nitro compounds. We find the energetic radicals formed from the initial NO2 bond scissioning can be converted to unsaturated non-radical intermediates as an alternative to the unzipping of the energetic radical. This reaction pathway is caused by the cage effect, which prevents the NO2 molecule from diffusing away before it can react with the energetic radical to form HONO. We propose a new, prompt oxidation mechanism in which the HONO, while still trapped within the cage, can add back onto the energetic molecule. This produces oxidation products in the condensed phase that normally would not be produced until much later in the flame. We propose that this prompt oxidation mechanism may be a general feature of both nitramines and nitrate esters. The resulting HONO formed by the H-atom abstraction will be strongly influenced by the physical properties of the condensed phase. The applicability of this mechanism is demonstrated for decomposition of ethylnitrate, illustrating the importance of the cage effect in enabling this mechanism to occur at low temperatures.

The micro-structure of an atmospheric pressure, opposed flow, methane diffusion flame has been studied using heated micro-probe sampling and chemical kinetic modeling. Mole fraction profiles of major products as well as trace aromatic, substituted aromatic, and polycyclic aromatic hydrocarbons (PAH up to C{sub 16}H{sub 10}, e.g. pyrene) were quantified by direct gas chromatography/mass spectrometry (GC/MS) analysis of samples withdrawn from within the flame without any pre-concentration. Mole fractions range from 0.8 to 1.0 {times} 10{sup {minus}7}. The experimental measurements are compared to results from a newly-developed chemical kinetic model that includes chemistry for the production and consumption of aromatics and PAH species. The model predictions are in reasonable agreement with the experimental data for the major species profiles and for the peak concentrations of many of the trace aromatics and PAH species. 36 refs.

We demonstrate the use of molecular dynamics and molecular mechanics methods to calculate properties and behavior of metal-chelate complexes that can be used as MRI contrast agents. Static and dynamic properties of several known agents were calculated and compared with experiment. We calculated the static properties such as the q-values (number of inner shell waters) and binding distances of chelate atoms to the metal ion for a set of chelates with known X-ray structure. The dynamic flexibility of the chelate arms was also calculated. These computations were extended to a series of exploratory chelate structures in order to estimate their potential as MRI contrast agents. We have also calculated for the first time the NMR relaxivity of an MRI contrast agent using a long (5 nsec) molecular dynamics simulation. Our predictions are promising enough that the method should prove useful for evaluating novel candidate compounds before they are synthesized. One novel static property, the projected area of chelate atoms onto a virtual surface centered on the metal ion (gnomonic projection), was found to give an effective measure of how well the chelate atoms use the free space around the metal ion.

The kinetics and product branching of the HCO + NO2 reaction have been studied at room temperature using laser flash kinetic spectroscopy and modeled with microcanonical variational RRKM theory. The rate constant for the disappearance of HCO radical at 296 K is (5.7 +/- 0.9) x 10(-11) cm(3) molecule(-1) s(-1), and it is independent of the pressure of SF6 buffer gas up to 700 Torr. The CO2 yield is 52 +/- 14%. Less than 10% of the reaction goes through the most exothermic product channel, HNO + CO2. The least exothermic product channel, H + CO2 + NO, is responsible for the remaining CO2. HONO has been observed, though not quantitatively, as a reaction product corresponding to the HONO + CO product channel. It must account for the 48 +/- 14% of reaction which does not yield CO2. The rates of formation of the collision complexes HCO(ONO) and HCO(NO2) are calculated by variational RRKM theory to be comparable and only weakly temperature dependent. Branching ratios for the decomposition of these complexes are also calculated. They are found to match most experimental observations from 300 to 1600 K.

The kinetics of gas-phase reactions occurring during the CVD of boron nitride (BN) from BCl3 and NH3 are investigated using an elementary reaction mechanism whose rate constants were obtained from theoretical predictions and literature sources. Plug-flow calculations using this mechanism predict that unimolecular decomposition of BCl3 is not significant under typical CVD conditions, but that some NH3 decomposition may occur, especially for deposition occurring at atmospheric pressure. Reaction of BCl3 with NH3 is rapid under CVD conditions and yields species containing both boron and nitrogen. One of these compounds, CI2BNH2, is predicted to be a key gas-phase precursor to BN.

The reaction of CN with NO has been studied between 2200 and 2810 K with a shock tube equipped with two stabilized cw CO lasers using BrCN as the CN radical source. The rates of CO formation and NO removal, measured simultaneously with the lasers, were kinetically modeled employing a detailed reaction mechanism constructed on the basis of the result of our quantum chemical (BAC-MP4) calculations. The kinetic modeling, aided by sensitivity analysis, suggests that the CN+NO reaction occurs primarily by the abstraction channel, producing NCO+N (1), via a triplet NCON intermediate. The rate constant for this reaction can be effectively represented by k 1 = 5.5 x 1012 exp(-15,410/T) cm3/mole.s. This is consistent with the result of the quantum calculations which, however, also indicates the presence of two other minor product channels: CO+N2 (2) and NCN+O (3). The latter is enthalpically limited, whereas the former is entropically controlled because of the tight cyclic transition state, -CNNO-‡.

Kinetic modeling of existing data on the reduction of NO by H2, initiated photolytically or thermally, with a comprehensive reaction mechanism established by means of ab initio quantum-chemical and statistical rate-constant calculations, allows us to identify several key elementary processes which are important in different temperature and NO-concentration regimes. At T < 900 K, the reduction of NO by H2 and other hydrides induced photolytically occurs primarily by the bimolecular reaction HNO + HNO → cis/trans-(HNO)2 under NO-lean conditions, and by the termolecular process HNO + 2NO → HN2O + NO2, followed by the fast redox reaction, HN2O + NO → HN2 + NO2 and N2 + HONO under NO-rich conditions. In the temperature range of 900 K < T < 1500 K, the reduction of NO by H2, initiated by the H2 + NO → H + HNO reaction, occurs readily and the global reduction rate is dominated by the HNO + NO → N2O + OH reaction. At temperatures higher than 1500 K, commonly heated by shock waves, the rate of NO reduction is controlled almost exclusively by the H + NO → N + OH reaction. The N atom thus formed generates efficiently the O atom by the fast N + NO → N2 + O process. These two NO reduction reactions are greatly enhanced in this temperature regime by the abundance of H atoms, produced by the fast chain processes: O + H2 → H + OH and OH + H2 → H + H2O. The rate constants as well as the mechanisms of these key elementary processes involving H/N/O-species have been interpreted in terms of the theoretical results derived from ab initio quantum-chemical and statistical-theory calculations.

In this work, three different ab initio methods are used to predict bond dissociation enthalpies (BDE) and atomization energies for TiCl{sub n} (n = 1-4) and Ti(NH{sub 2})n (n = 1-4) compounds, as well as for the complex TiCl{sub 4}:NH{sub 3}. There is considerable variation in the predicted BDES, even for highly electron-correlated methods. However, bond-additivity corrections applied to coupled-cluster calculations at the CCSD(T) level, expected to be the most reliable of the three methods, yield Ti-Cl BDEs in good agreement with experimental results. An experimental estimate of the TiCl{sub 4} BDE is also reported that is consistent with the ab initio results and recent experiments by others indicating that the TiCl{sub 3} heat of formation reported in the JANAF Tables is too low. Finally, the predicted BDEs indicate that the gas-phase reaction of TiCl{sub 4} and NH{sub 3} to form the complex Cl{sub 4}Ti:NH{sub 3} is exothermic by 17 kcal mol{sub {minus}1}. In addition, decomposition of the complex to form Cl{sub 3}TiNH{sub 2} and HCl is endothermic by 20 kcal mol{sup {minus}1}.

The chemical processes involved in the decomposition of energetic materials have been investigated theoretically using quantum chemical methods to determine the thermochemistry and reaction pathways. The Bond-Additivity-Corrected Moller-Plesset 4th order perturbation theory method (BAC-MP4) has been used to determine heats of formation and free energies of reaction intermediates of decomposition. In addition, the BAC-MP4 method has been used to determine action pathways involving these intermediates. A theoretical method for calculating solvation energies has been developed to treat the non-idealities of high pressure and the condensed phase. The resulting chemical processes involving decomposition and ignition are presented for nitrate compounds, nitramines, and nitromethane.

Ab initio electronic-structure calculations are combined with empirical bond-additivity corrections to yield thermochemical properties of gas-phase molecules. A self-consistent set of heats of formation for molecules in the Si-H, Si-H-Cl, Si-H-F, Si-N-H and Si-NH- F systems is presented, along with preliminary values for some Si-O-C-H species.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical study of the thermochemistry of molecules in the silicon-carbon-hydrogen systemMark D. Allendorf and Carl F. MeliusCite this: J. Phys. Chem. 1992, 96, 1, 428–437Publication Date (Print):January 1, 1992Publication History Published online1 May 2002Published inissue 1 January 1992https://pubs.acs.org/doi/10.1021/j100180a080https://doi.org/10.1021/j100180a080research-articleACS PublicationsRequest reuse permissionsArticle Views331Altmetric-Citations88LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

A self-consistent set of thermochemical parameters for about 100 molecules in the Si−B−H−Cl system is obtained using a combination of ab initio electronic structure calculations and empirical corrections (the BAC-MP4 method). The species include stable and radical species as well as a few transition states. Trends in calculated heats of formation, bond dissociation enthalpies, and heats of reaction for various molecular decomposition channels are discussed. Silylboranes are most likely to decompose via the elimination of H2, HCl, or silylenes. The presence of the B atom reduces the energy required for the 1,1 elimination of H2 from the Si atom, relative to the analogous reaction in disilane.

The unimolecular rearrangement processes of unsaturated aliphatic hydrocarbons representative of intermediates in rich hydrocarbon flames have been investigated using the quantum chemical BAC-MP4 method. In particular, we have investigated the unimolecular reaction mechanisms involving (1) allene-cyclopropene-propyne rearrangement, (2) vinylacetylene pyrolysis leading to acetylene and diacetylene, and (3) various C 6 H 6 compounds, including 1,5-hexadiyne, 1,2,4,5-hexatetraene, and 1,2-hexadien-5-yne leading to 3,4-dimethylenecyclobutene, fulvene, benzene, 2-ethynyl-1,3-butadiene, and other C 6 H 6 species. Rate constants for various reaction steps give good agreement with available experimental measurements. The reaction mechanisms are also consistent with various deuterium isotope labeling experiments. The results indicate that many different types of reaction mechanisms occur, including concerted pathways involving carbene and vinylidene intermediates and insertion reactions of vinylidenes into C−H bonds and C−C multiple bonds. New decomposition pathways are presented for vinylacetylene pyrolysis, for conversion of 1,2-hexadien-5-yne to fulvene, and for conversion of fulvene to benzene.

Using potential energy surface information from BAC-MP4 calculations and statistical-dynamical methods, we have calculated the branching fraction for the NH+NO reaction, NH+NO→N2+OH (1) →N2O+H. (2) We find that reaction (2) dominates over the entire temperature range considered, 300 K

Phosphoramide mustard (PM) and isophosphoramide mustard (IPM) are the active metabolites of two important alkylating anticancer drugs cyclophosphamide and ifosfamid. Although PM and IPM are structurally similar they have very different reactivities. In order to elucidate the origin of these differences, we have performed ab initio quantum chemical calculations on all plausible protonation states of PM and IPM. These calculations involved 6-31g* self-consistent field structure optimizations followed by single point 6-31g** second-order Moller Plesset energy calculations. Additionally a polarizable continuum model was used to estimate the aqueous-phase energetics. Both PM and IPM are predicted by this study to have a zwitterionic aqueous-phase ground state. It is possible that protonation of the reactive sites may underlie the observed differences in reaction rate. The effects of protonation on the structure and electronic distribution of the phosphoramide moeity are also discussed.