This article presents a comprehensive treatment of the polycyclic aromatic hydrocarbon (PAH) hypothesis. The interstellar, infrared spectral features which have been attributed to emission from highly vibrationally excited PAHs are discussed in detail. These include major (most intense) bands at 3040, 1615, "1310," 1150, and 885 cm-1 (3.29, 6.2 "7.7," 8.7, and 11.3 micrometers), minor bands and broad features in the 3200-2700 cm-1 [correction of 3200-2700-1] (3.1-3.7 micrometers), 1600-1100 cm-1 (6.0-9 micrometers) and 910-770 cm-1 (11-13 micrometers) regions, as well as the vibrational quasi-continuum spanning the entire mid-IR and the electronic transitions which contribute to the high-frequency IR continuum. All the major and minor bands, as well as the quasi-continuum, can be attributed to vibrational transitions in molecular-sized PAHs. The latter two broad features probably arise from very large PAHs, PAH clusters, and amorphous carbon particles. A precise match of the interstellar spectra with laboratory spectra is not yet possible because laboratory spectra are not available of PAHs in the forms probably present in the interstellar medium (completely isolated, ionized, some completely dehydrogenated, and containing between about 20 and 40 carbon atoms). The method with which one can calculate the IR fluorescence spectrum from a vibrationally excited molecule is also described in detail. Fluorescence band intensities, relaxation rates, and dependence on molecule size and energy content are treated explicitly. Analysis of the interstellar spectra indicates that the PAHs which dominate the infrared spectra contain between about 20 and 40 carbon atoms. The results obtained with this method are compared with the results obtained using a thermal approximation. It is shown that for high levels of vibrational excitation and emission from low-frequency modes, the two methods give similar results. However, at low levels of vibrational excitation and for the high-frequency modes (for example, the 3040 cm-1, 3.3 micrometers band), the thermal approach overestimates the emission intensities. For calculations of molecular reactions (such as H-loss, deuterium enrichment, and carbon skeleton rearrangement) a thermal approximation is invalid. The relationship between PAH molecules and amorphous carbon particles is presented and their production in circumstellar shells is described. The most likely interstellar PAH molecular structures are discussed and the possibility of destructive reactions with interstellar oxygen and hydrogen atoms is considered in detailed and found to be unimportant. Interstellar PAH size and abundance estimates are made. On the order of a few percent of the available interstellar carbon is tied up in the small (20-40 carbon atom) PAHs which are responsible for the sharp features, and a similar amount is tied up in the larger (200-500 carbon atom) PAHs or PAH clusters and amorphous carbon particles which are responsible for the broad components underlying the 1600-1100 and 900-770 cm...

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Unimolecular reaction systems in which multiple isomers undergo simultaneous reactions via multiple decomposition reactions and multiple isomerization reactions are of fundamental interest in chemical kinetics. The computer program suite described here can be used to treat such coupled systems, including the effects of collisional energy transfer (weak collisions). The program suite consists of MultiWell, which solves the internal energy master equation for complex unimolecular reactions systems; DenSum, which calculates sums and densities of states by an exact-count method; MomInert, which calculates external principal moments of inertia and internal rotation reduced moments of inertia; and Thermo, which calculates equilibrium constants and other thermodynamics quantities. MultiWell utilizes a hybrid master equation approach, which performs like an energy-grained master equation at low energies and a continuum master equation in the vibrational quasicontinuum. An adaptation of Gillespie's exact stochastic method is used for the solution. The codes are designed for ease of use. Details are presented of various methods for treating weak collisions with virtually any desired collision step-size distribution and for utilizing RRKM theory for specific unimolecular rate constants.

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Collisional energy transfer plays a key role in recombination, unimolecular, and chemical activation reactions. For master equation simulations of such reaction systems, it is conventionally assumed that the rate constant for inelastic energy transfer collisions is independent of the excitation energy. However, numerical instabilities and nonphysical results are encountered when normalizing the collision step-size distribution in the sparse density of states regime at low energies. It is argued here that the conventional assumption is not correct, and it is shown that the numerical problems and nonphysical results are eliminated by making a plausible assumption about the energy dependence of the rate coefficient for inelastic collisions. The new assumption produces a model that is more physically realistic for any reasonable choice of collision step-size distribution, but more work remains to be done. The resulting numerical algorithm is stable and noniterative. Testing shows that overall accuracy in master equation simulations is better with this new approach than with the conventional one. This new approach is appropriate for all energy-grained master equation formulations.

The potential energy surface and chemical kinetics for the reaction of HO with CO, which is an important process in both combustion and atmospheric chemistry, were computed using high-level ab initio quantum chemistry in conjunction with semiclassical transition state theory under the limiting cases of high and zero pressure. The reaction rate constants calculated from first principles agree extremely well with all available experimental data, which range in temperature over a domain that covers both combustion and terrestrial atmospheric chemistry. The role of quantum tunneling is confirmed to be extremely important, which supports recent work by Continetti and collaborators regarding the loss of hydrogen atoms from vibrationally excited states of HOCO. A sensitivity analysis has been carried out and serves as the basis for a plausible estimate of uncertainty in the calculations.

A new algorithm [Nguyen, T. L.; Stanton, J. F.; Barker, J. R. Chem. Phys. Lett. 2010, 9, 499] for the semiclassical transition-state theory (SCTST) formulated by W. H. Miller and co-workers is used to compute rate constants for the isotopologues of the title reaction, with no empirical adjustments. The SCTST and relevant results from second-order vibrational perturbation theory (VPT2) are summarized. VPT2 is used at the CCSD(T) level of electronic structure theory to compute the anharmonicities of the fully coupled vibrational modes (including the reaction coordinate) of the transition structure. The anharmonicities are used in SCTST to compute the rate constants over the temperature range from 200 to 2500 K. The computed rate constants are compared to experimental data and theoretical calculations from the literature. The SCTST results for absolute rate constants and for both primary and secondary isotope effects are in excellent agreement with the experimental data for this reaction over the entire temperature range. The sensitivity of SCTST to various parameters is investigated by using a set of simplified models. The results show that multidimensional tunneling along the curved reaction path is important at low temperatures and the anharmonic coupling among the vibrational modes is important at high temperatures. The theoretical kinetics data are also presented as fitted empirical algebraic expressions.

Equilibrium constants and rate constants involving Cl•(aq), Cl2 •-(aq), H2O, and H2O2(aq) are measured at 297 ± 2 K by analyzing the kinetics of formation and decay of Cl2 •-. The new results are in good agreement with previous studies, when available. The reaction between solvated chlorine atoms and hydrogen peroxide is reported for the first time:  Cl•(aq) + H2O2(aq) → H+(aq) + Cl-(aq) + HO2 •(aq), k 10 = (2.0 ± 0.3) × 109 M-1 s-1 (±σ). The new results are used along with evaluations of literature data to arrive at recommendations for several key rate constants and equilibrium constants. Of particular interest, the recommended equilibrium constant for the reaction Cl•(aq) + Cl-(aq) ↔ Cl2 •-(aq) is K 5 = (1.4 ± 0.2) × 105 M-1(±σ).