We have developed a computer code for an IBM PC/XT/AT or compatible which can be used to estimate, edit, or enter thermodynamic property data for gas phase radicals and molecules using Benson's group additivity method. The computer code is called THERM (THermo Estimation for Radicals and Molecules). All group contributions considered for a species are recorded and thermodynamic properties are generated in old NASA polynomial format for compatibility with the CHEMKIN reaction modeling code. In addition, listings are created in a format more convenient for thermodynamic, kinetic, and equilibrium calculations. Polynomial coefficients are valid from 300-5000 K using extrapolation methods based upon the harmonic oscillator model, an exponential function, or the Wilhoit polynomials. Properties for radical and biradical species are calculated by applying bond dissociation increments to a stable parent molecule to reflect loss of H atom. THERM contains a chemical reaction interpreter to calculate thermodynamic property changes for chemical reactions as functions of temperature. These include equilibrium constant, heat release (required heat, AHr), entropy change (ASr), Gibbs free energy change (AG,), and the ratio of forward to reverse Arrhenius A-factors (for elementary reactions). This interpreter can also process CHEMKIN input files. A recalculation procedure is incorporated for rapid updating of a database of chemical species to reflect changes in estimated bond dissociation energies, heats of formation, or other group values. All input and output files are in ASCII so that they can be easily edited, expanded, or updated.

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The kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen are analyzed using quantum Rice-Ramsperger-Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation. Thermodynamic properties of species and transition states are determined by ab initio methods at the G2 and CBS-Q//B3LYP/6-31G(d,p) levels of theory and isodesmic reaction analysis. Rate coefficients for reactions of the energized adducts are obtained from canonical transition state theory. The reaction of C 2 H 5 with O 2 forms an energized peroxy adduct with a calculated well depth of 35.3 kcal mol -1 at the CBS-Q//B3LYP/6-31G(d,p) level of theory. The calculated (VTST) high-pressure limit bimolecular addition reaction rate constant for C 2 H 5 + O 2 is 2.94 × 10 13 T -0.44 . Predictions of the chemically activated branching ratios using both collisional deactivation models are similar. All of the product formation pathways of ethyl radical with O 2 , except the direct HO 2 elimination from the CH 3 CH 2 OO • adduct, involve barriers that are above the energy of the reactants. As a result, formation of the stabilized CH 3 CH 2 OO • adduct is important at low to moderate temperatures; subsequent reactions of this adduct should be included in kinetic mechanisms. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adducts and the thermally activated reactions of the stabilized adducts are assembled into a reaction mechanism. Comparisons of predictions using this mechanism to experiment demonstrate the necessity of including dissociation of the stabilized ethylperoxy adduct. Two channels are particularly important, direct HO 2 elimination and reverse reaction to C 2 H 5 + O 2 , where the ratio of these rates is a function of temperature and pressure. The predictions, using unadjusted rate coefficients, are consistent with literature observations over extended temperature and pressure ranges. Comparison of a mechanism using 7 × 3 Chebyshev polynomials to represent k(T,P) to a conventional mechanism which used k(T) only (different values for k(T) at different pressures) showed good agreement. The kinetic implications for low-temperature ignition due to the direct formation of ethylene and HO 2 from ethylperoxy are discussed.

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A method to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions is described. A three-frequency version of QRRK theory is developed and collisional stabilization is estimated using the modified strong-collision approximation. The methodology permits analysis of reaction systems with an arbitrary degree of complexity in terms of the number of isomer or product channels. Specification of both high and low pressure limits is also provided. The chemically activated reaction of vinyl radical with molecular oxygen is used to demonstrate the approach. Subsequent dissociation of the stabilized vinyl peroxy radical is used to illustrate prediction of dissociation rate coefficients. These calculations confirm earlier results that the vinoxy + O channel is dominant under combustion conditions. The results are also consistent with RRKM results using the same input conditions. This approach provides a means to provide reasonably accurate predictions of the rate coefficients that are required in many detailed mechanisms. The major advantage is the ability to provide reasonable estimates of rate coefficients for many complex systems where detailed information about the transition states is not available. It is also shown that a simpler 1-frequency model appears adequate for high temperature conditions.

We show that the benzyl radical decomposes to the C7H6 fragment fulvenallene (+H), by first principles/RRKM study. Calculations using G3X heats of formation and B3LYP/6-31G(2df,p) structural and vibrational parameters reveal that the reaction proceeds predominantly via a cyclopentenyl-allene radical intermediate, with an overall activation enthalpy of ca. 85 kcal mol(-1). Elementary rate constants are evaluated using Eckart tunneling corrections, with variational transition state theory for barrierless C-H bond dissociation in the cyclopentenyl-allene radical. Apparent rate constants are obtained as a function of temperature and pressure from a time-dependent RRKM study of the multichannel multiwell reaction mechanism. At atmospheric pressure we calculate the decomposition rate constant to be k [s(-1)] = 5.93 x 10(35)T(-6.099) exp(-49,180/T); this is in good agreement with experiment, supporting the assertion that fulvenallene is the C7H6 product of benzyl decomposition. The benzyl heat of formation is evaluated as 50.4 to 52.2 kcal mol(-1), using isodesmic work reactions with the G3X theoretical method. Some novel pathways are presented to the cyclopentadienyl radical (C5H5) + acetylene (C2H2), which may constitute a minor product channel in benzyl decomposition.

Ab initio calculations were performed on CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH molecules using the Gaussian92 system of programs. Geometries of stable rotational conformers and transition states for internal rotation were optimized at the RHF/6-31G* and MP2/6-31G* levels of theory. Harmonic vibrational frequencies were computed at the RHF/6-31G* level of theory. Potential barriers for internal rotations were calculated at the MP2/6-31G**/HF/6-31G* level. Parameters of the Fourier expansion of the hindrance potentials have been tabulated. Standard entropies (S°298) and heat capacities (C p (T)'s, 300 ≤ T/K ≤ 1500) were calculated using the rigid-rotor−harmonic-oscillator approximation based on the information obtained from the ab initio studies. Contributions from hindered rotors were calculated by summation over the energy levels obtained by direct diagonalization of the Hamiltonian matrix of hindered internal rotations. Enthalpies of formation for these three molecules were calculated using isodesmic reactions. Enthalpies of formation were calculated to be ΔH f°298(CH3CH2OOH) = −41.5 ± 1.5 kcal mol-1, ΔH f°298(CH3CHClOOH) = −50.9 ± 3.4 kcal mol-1, and ΔH f°298(CH3CCl2OOH) = −55.3 ± 2.2 kcal mol-1. Entropies (S°298) are calculated to be 76.1, 79.2 and 86.6 cal mol-1 K-1 for CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH, respectively.

A better understanding of the formation of polycyclic aromatic hydrocarbons (PAH) is of great practical interest because of their potential hazardous health effects and their role as intermediates in soot and fullerene formation. The potential surfaces of the reactions C 6 H 5 + C 2 H 2 and 1-C 10 H 7 + C 2 H 2 were explored by densityfunctional theory using BLYP and B3LYP functionals. Vibrational analysis allowed the determination of thermodynamic data and deduction of high-pressure-limit rate constants via transition state theory. The pressure and temperature dependences of these chemically activated reactions were computed using the modified strong collision approximation. The comparison of the predictions for the C 6 H 5 + C 2 H 2 system with experimental data showed good agreement in particular at high temperatures relevant for a combustion environment. The dominant product from acetylene addition to 1-naphthyl at low pressures is the five-membered ring species acenaphthylene, consistent with the more pronounced formation of fullerenes under such conditions. High pressure favors formation of stabilized initial adducts, i.e., phenylvinyl and 1-naphthylvinyl. Some products not considered previously, such as 1-acenaphthenyl, 1-naphthylacetylene, 2-vinylphenyl, and 1-vinyl-2-phenyl, are found to be important under some pressure and temperature conditions. All of our results are consistent with known free-radical chemistry. Rate constants describing the formation of phenylacetylene, phenylvinyl, 1-vinyl-2-phenyl, 1-naphthylvinyl, 1-vinyl-8-naphthyl, 1-naphthylacetylene, acenaphthylene, and 1-acenaphthenyl are given at 20 and 40 Torr as well as at 1 and 10 atm for the temperature range from 300 to 2100 K.