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The standard gas-phase enthalpies of formation of chlorinated benzenes, phenols and dibenzo-p-dioxins have been predicted using G3X and/or G3XMP2 model chemistries coupled with isodesmic reactions and compared to the previous theoretical and experimental values. A set of values for chlorinated benzenes are first suggested based on experimental measurements and the closed agreed G3X calculations with different isodesmic reactions. The results on polychlorinated dibenzo-p-dioxins (PCDDs) show a large difference between G3XMP2 and previous experimental measurements and predictions using group additivity methods, semiempirical quantum chemistry, and DFT calculations, especially for highly chlorinated species. Using the well-balanced isodesmic reactions (IR3 and IR5), the discrepancies between G3XMP2 and DFT predictions on PCDDs can be reduced to within 16 kJ/mol. The relative stability of PCDD isomers can be rationalized by the positional interactions, and the overestimation by DFT with less balanced isodesmic reactions is due to the overestimation of the ortho-Cl−Cl repulsive interactions when comparing with G3XMP2. Our calculations suggest further experimental measurements, especially on highly chlorinated phenols and PCDDs. [ABSTRACT FROM AUTHOR]

Measurements of the acetyl yield from acetone photolysis have been made using laser flash photolysis/laser induced fluorescence. Φtotal(λ,p,T) was determined over the ranges:  266 ≤ λ/nm ≤ 327.5, 0.3 ≤ p/Torr ≤ 400 and 218 ≤ T/K ≤ 295. The acetyl yield was determined relative to that at 248 nm by conversion to OH by reaction with O2. Linear Stern−Volmer plots (1/[OH] vs [M]) describe the data for λ < 300 nm, but for λ > 300 nm, nonlinear Stern−Volmer plots were observed. This behavior is interpreted as evidence for dissociation from two excited states of acetone:  S1 when the Stern−Volmer plots are linear and both S1 and T1 when Stern−Volmer plots are nonlinear. A model for acetone photolysis is proposed that can adequately describe both the present and literature data. Barriers to dissociation are invoked in order to explain the dependence of pressure quenching of the acetone photolysis yields as a function of wavelength and temperature. This pressure quenching was observed to become more efficient with increasing wavelength, but it was only above ∼300 nm that a significant T dependence was observed, which became more pronounced at longer wavelengths. This is the first study to observe a T-dependent Φtotal(λ,p,T). A parametrized expression for Φtotal(λ,p,T) has been developed and is compared against the recommended literature data by running box model simulations of the atmosphere. These simulations show that acetone photolysis occurs more slowly at the top of the troposphere. [ABSTRACT FROM AUTHOR]