The kinetics and mechanism for the thermal decomposition of diketene have been studied in the temperature range 510-603 K using highly diluted mixtures with Ar as a diluent. The concentrations of diketene, ketene, and CO2 were measured by FTIR spectrometry using calibrated standard mixtures. Two reaction channels were identified. The rate constants for the formation of ketene (k(1)) and CO2 (k(2)) have been determined and compared with the values predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for the branching reaction. The first-order rate constants, k(1) (s(-1)) = 10(15.74 +/- 0.72) exp(-49.29 (kcal mol(-1)) (+/- 1.84)/RT) and k(2) (s(-1)) = 10(14.65 +/- 0.87) exp(-49.01 (kcal mol(-1)) (+/- 2.22)/RT); the bulk of experimental data agree well with predicted results. The heats of formation of ketene, diketene, cyclobuta-1,3-dione, and cyclobuta-1,2-dione at 298 K computed from the G2M scheme are -11.1, -45.3, -43.6, and -40.3 kcal mol(-1), respectively. (C) 2007 Wiley Periodicals, Inc.

We have developed bond additivity correction (BAC) procedures for the G3-based quantum chemistry methods, G3B3 and G3MP2B3. We denote these procedures as BAC-G3B3 and BAC-G3MP2B3. We apply the procedures to compounds containing atoms from the first three rows of the periodic table including H, B, C, N, O, F, Al, Si, P, S, and Cl atoms. The BAC procedure applies atomic, molecular, and pairwise bond corrections to theoretical heats of formation of molecules. The BAC-G3B3 and BAC-G3MP2B3 procedures require parameters for each atom type but not for each bond type. These parameters have been obtained by minimizing the error between the BAC-G3B3 and BAC-G3MP2B3 predictions and the experimental heats of formation for a 155 molecule reference set, containing open and closed shell molecules representing various functional groups, multireference configurations, isomers, and degrees of saturation. As compared to former BAC-MP4, BAC-G2, and BAC-hybrid methods, BAC-G3B3 provides better agreement with experiment for a wider range of chemical moieties, including highly oxidized species involving SOx s, NOx s, POx s, and halogens. The BAC-G3B3 and BAC-G3MP2B3 procedures are applied to an extended test suite involving 273 compounds. We assess the overall quality of BAC-G3B3 with experiments and other theoretical approaches. For the reference set, the average error for the BAC-G3B3 results is 0.44 kcal/mol as compared to 0.82 kcal/mol for the raw G3B3. For the extended test set, the average error for the BAC-G3B3 results is 0.91 kcal/mol as compared to 1.38 kcal/mol for the raw G3B3. As compared to the other BAC procedures, the improved predictive capability of BAC-G3B3 and BAC-G3MP2B3 procedures is, to a large extent, due to the improved quality of G3-based methods resulting in much smaller BAC correction terms.

Chemical inhibition of laminar propane flames by organophosphorus compounds has been studied experimentally and computationally using a detailed chemical kinetic reaction mechanism. Both fuel-lean and fuel-rich propane flames were studied to examine the role of equivalence ratio in flame inhibition. The experiments examined a wide variety of organophosphorus compounds. We report on experimental species flame profiles for tri-methyl phosphate (TMP) and compare them with modeled species flame profile results of TMP and di-methyl methyl phosphonate (DMMP). Both experiments and kinetic modeling indicate that inhibition efficiency is effectively the same for all of the organophosphorus compounds examined, independent of the molecular structure of the initial inhibitor molecule. Chemical inhibition is due to reactions involving small P-bearing species HOPO2 and HOPO produced by the organophosphorus compounds (OPCs). Ratios of HOPO2 and HOPO concentrations differ between lean and rich flames, with HOPO2 dominant in lean flames while HOPO dominates in rich flames. Resulting HOPO2 and HOPO species profiles do not significantly depend on the initial source of the HOPO2 and HOPO, and thus are relatively insensitive to the initial OPC inhibitor. A more generalized form of the Twarowski mechanism is developed to account for the results observed, and new theoretical values are determined for heats of formation of the important P-containing species, using the BAC-G2 method.

In this work, the BAC-MP4 method is extended for the first time to compounds in the fourth row of the periodic table, resulting in a self-consistent set of thermochemical data for 56 tin-containing molecules in the Sn-H-C-Cl system. The BAC-MP4 method combines ab initio electronic structure calculations with empirical corrections to obtain accurate heats of formation. To obtain electronic energies for tin-containing species, the standard 6-31G(d,p) basis set used in BAC-MP4 calculations is augmented with a relativistic effective core potential to describe the electronic structure of the tin atom. Both stable compounds and radical species are included in this study. Trends within homologous series and calculated bond dissociation energies are consistent with previous BAC-MP4 predictions for group 14 compounds and the limited data available from the literature, indicating that the method is performing well for these compounds.

There is much interest in the combustion mechanism of organophosphorus compounds (OPCs) due to their role as potential halon replacements in fire suppression. A continuing investigation of the inhibition activity of organophosphorus compounds under a range of equivalence ratios was performed experimentally and computationally, as measured by the burning velocity. Updates to a previous mechanism were made by the addition and modification of reactions in the mechanism for a more complete description of the inhibition reactions. Reaction pathways for HOPO2 + H and HOPO + H are analyzed using the BAC-G2 approach. A new reaction pathway for HOPO2 + H = PO2 + H2O has been identified which results in a higher rate constant than that reported in the literature. In this work, the laminar flame speed is measured experimentally and calculated numerically for a premixed propane/air flame at 1 atm, under a range of equivalence ratios, undoped and doped with dimethyl methylphosphonate (DMMP). A detailed investigation of the catalytic cycles involved in the recombination of key flame radicals is made for two equivalence ratios, fuel lean and fuel rich. From this, the importance of different catalytic cycles involved in the lean versus rich case is discussed. The chemical kinetic model indicates that the HOPO2 ⇔ PO2 inhibition cycle is more important in the lean flame than the rich. The OPCs are similarly effective across the range, demonstrating the robustness of OPCs as flame suppressants. In addition, it is shown that the phosphorus compounds are most active in the high-temperature region of the flame. This may, in part, explain their high level of inhibition effectiveness.

We have developed a detailed kinetic model to follow the decomposition of formic acid at modestly high pressures (1-10 GPa) and temperature (500-1000K) and further include our refinement of a fluid exponential-6 equation of state for formic acid and corresponding reaction species. We also include the effects of bimolecular and water catalyzed reactions, calculated from ab initio molecular orbital calculations. We present a comparison between our simulations and experimental observations made using near- near-simultaneous high-pressure FTIR and Raman spectroscopy. We discuss, in detail, the simultaneous implications our experimental observations provide in relation to computed reaction timescales and dominant species employed in our model.

A self-consistent set of thermochemical data for 55 molecules in the Al-H-C-O-F-Cl system are obtained from ab initio quantum-chemistry calculations using the BAC-G2 method. Calculations were performed for both stable and radical species. Good agreement is found between the calculations and experimental heats of formation in most cases where data are available for comparison. Electronic energies, molecular geometries, moments of inertia, and vibrational frequencies are provided in the Supporting Information, as are polynomial fits of the thermodynamic data (heat of formation, entropy, and heat capacity) over the 300--3000 K temperature range.

In information theory, a signature is characterized by the information content as well as noise statistics of the communication channel. Biosignatures have analogous properties. A biosignature can be associated with a particular attribute of a pathogen or a host. However, the signature may be lost in backgrounds of similar or even identical signals from other sources. In this paper, we highlight statistical and signal processing challenges associated with identifying good biosignatures for pathogens in host and other environments. In some cases it may be possible to identify useful signatures of pathogens through indirect but amplified signals from the host. Discovery of these signatures requires new approaches to modeling and data interpretation. For environmental biosignal collections, it is possible to use signal processing techniques from other applications (e.g., synthetic aperture radar) to track the natural progression of microbes over large areas. We also present a computer-assisted approach to identify unique nucleic-acid based microbial signatures. Finally, an understanding of host-pathogen interactions will result in better detectors as well as opportunities in vaccines and therapeutics.

New bond additivity correction (BAC) methods have been developed li,I the G2 method, BAC-G?, as well as for a hybrid density functional theory (DFT) Moller-Plesset (MP)2 method, BAG-hybrid. These BAC methods use a new form of BAC corrections, involving atomic, molecular, and bond-wise additive terms. These terms enable one to treat positive and negative ions as well as neutrals. The BAC-G2 method reduces errors in the G2 method due to nearest-neighbor bonds. The parameters within the BAC-G2 method only depend on atom types. Thus the BAC-G2 method can be used to determine the parameters needed by BAG methods involving lower levels of theory, such as BAC-hybrid and BAC-MP4. The BAG-hybrid method is expected to scale well for large molecules. The BAC-hybrid method uses the differences between the DFT and MP2 predictions as an indication of the method's accuracy, whereas the BAC-G2 method uses its internal methods (G1 and G2MP2) to accomplish this. A statistical analysis of the error in each of the methods is presented on the basis of calculations performed for large sets (more than 120) of molecules.

We present heats of formation and bond energies for Group-III compounds obtained from calculations of molecular ground-state electronic energies. Data for compounds of the form MXn are presented, where M = B, Al, Ga, and In, X = H, CI, and CH3, and n = 1-3. Energies for the B, Al, and Ga compounds are obtained from G2 predictions, while those for the In compounds are obtained from CCSD(T)/CBS calculations; these are the most accurate calculations for indium-containing compounds published to date. In most cases, the calculated thermochemistry is in good agreement with published values derived from experiments for those species that have well-established heats of formation. Bond energies obtained from the heats of formation follow the expected trend (C1 >> CH3 similar to H). However. the CH3M-(CH3)(2) bond energies obtained for trimethylgallium and trimethylindium are considerably stronger (> 15 kcal mol(-1)) than currently accepted values.

The modeling of the growth of silicon carbide from the vapor phase in the Si-C-H system requires a good understanding of the gas-phase chemistry. The object of this paper is to complement the previous studies on the kinetic modeling of the gas-phase in the system SiH4 / C3H8. To date, kinetic approaches to modeling the gas-phase chemistry have not been fully developed Previous kinetic models have only dealt with the pyrolysis of individual precursors (silane and propane) without allowing for the formation of organosilicon species. This study provides a progress report on our efforts to develop a full gas-phase mechanism that includes organosilicon compounds. Rate constants for this mechanism are determined where possible from experimental data available in the literature. However, for several important reactions, experimental data are not available. Consequently, we are performing ab initio calculations to determine activation energies and are using RRKM calculations to estimate pressure fall-off effects for unimolecular reactions. In this contribution, we focus on the formation of methylsilane H3SiCH3 and discuss the importance ol: such species in the gas-phase chemistry of SiC deposition.

Understanding the role of gas-phase reactions is an important step in the development of useful CVD process models. In this article, we review the general types of gas-phase reactions that can occur and discuss quantum-chemistry techniques for predicting their thermochemistry and kinetics. We also describe the use of high-temperature flow reactors to measure gas-phase reaction kinetics. Coupling these theoretical and experimental methods is a powerful approach to the characterization of CVD precursor chemistry. We illustrate this in a discussion of the reaction between BC1 3 and NH 3 , which is important in the deposition of hexagonal boron nitride coatings.

The G2 atomization energies of fluorine and oxygen containing Ga compounds are greatly in error. This arises from an inversion of the Ga 3d core orbital and the F 2s or O 2s valence orbitals. Adding the Ga 3d orbital to the correlation treatment or removing the F 2s orbitals from the correlation treatment are shown to eliminate the problem. Removing the O 2s orbital from the correlation treatment reduces the error, but it can still be more than 6 kcal/mol. It is concluded that the experimental atomization energy of GaF2 is too large.

Kinetics and mechanism for the bimolecular decomposition of HONO have been studied by ab initio molecular orbital (G2M) and transition-state theory calculations. The reaction can take place by the interaction of a cis and a trans isomer or two cis or two trans isomers, via four-, five-, and six-member ring transition states, with decreasing reaction barriers as the size of the ring increases. The lowest energy path with a 13.7 kcal/mol barrier was found to occur by the six-member ring TS1 formed by the reaction of cis- and trans-HONO. A similar six-member ring TS (TS2) formed by two cis isomers has a barrier height of 15.1 kcal/mol, which is very close to the 5-ring TS formed by two trans isomers, 15.7 kcal/mol. The total rate constant computed with the ab initio MO results, including the three reaction channels mentioned above and an additional channel involving a five-member ring TS formed by a cis-and a trans isomer with a 17.7 kcal/mol barrier, can be represented by the three-parameter expression for the 300-5000 K temperature range: k = 5.8 x 10(-25)T(3.64) exp(-6109/T) cm(3)/(molecule.s), which includes the Boltzmann-averaged contribution of the cis isomer. The theoretical value was found to be considerably lower than the available experimental results (which are believed to have suffered from deleterious surface effects).

Using stationary-point information from a BAC-MP4 potential-energy surface and statistical-dynamical methods, we have calculated the total rate coefficient for the two-channel reaction,[GRAPHICS]in the temperature range 1000 K greater than or equal to T greater than or equal to 2500 K. The result obtained,k(T) = 3.0 x 10(-1)T(3.52)exp(-3950/RT) cm(3)/mole s,is in excellent agreement with recent shock-tube measurements of k(T) by Braun-Unkhoff, et al. and Hennig and Wagner. Qualitative considerations suggest that the radical channel (R2) is dominant in this temperature range. The analysis and the results are discussed in some detail. (C) 1998 John Wiley & Sons, Inc.

This article provides a short summary of the theoretical approaches to understanding gas-phase reactions. In particular, the quantum-chemical bond-additivity correction (BAC) method for predicting molecular thermochemistry of gas-phase molecules is described. A brief discussion of the use of RRKM methods for predicting the rates of unimolecular reactions and of ab initio methods for predicting the rates of bimolecular reactions is also presented. Finally, the question of when gas-phase reactions are likely to be important in CVD is discussed. Criteria are proposed for performing a first-order evaluation of the extent of precursor pyrolysis in the CVD of hard coatings. Examples from the CVD of titanium-containing species, boron nitride, and silicon carbide will be used for illustration.

We have performed ab initio restricted Hartree–Fock (RHF) and restricted second-order Møller–Plesset perturbation theory (RMP2) calculations of the relative energies of all C20H12, C18H12 and C14H10 polycyclic aromatic hydrocarbons (PAHs). Since all of these PAH wavefunctions exhibit large unrestricted Hartree–Fock (UHF) instabilities, the C14H10 PAHs were also studied using higher levels of electron correlation. Additionally, the role of UHF instabilities was investigated for the the linear conjugated polyenes from C2H4 to C20H22. Our results suggest that the RHF and RMP2 methods yield energies in relatively good agreement with highly electron-correlated methods, but the UHF instabilities indicate a fundamental shortcoming of the single determinant wavefunction description of these systems.