RESUMO
The pressure and temperature dependence of the thermal decomposition of 1-bromo-3-chloropropane has been theoretically investigated. The reaction takes place majorly through the elimination of HBr. Molecular properties of 1-bromo-3-chloropropane and transition states were derived from MN15/6-311++G(3df,3pd) and G4 quantum-chemical calculations. The resulting rate constants obtained from the unimolecular reaction rate theory for the high- and low-pressure limits of reaction BrCH2CH2CH2Cl â CH2CHCH2Cl + HBr at 400-1000 K were k∞ = 6.1 × 1013 exp(-57.2 kcal mol-1/RT) s-1 and k0 = [BrCH2CH2CH2Cl] 1.45 × 10-1 (T/1000 K)-7.9 exp(-55.9 kcal mol-1/RT) cm3 molecule-1 s-1. A value of -26.3 ± 1.0 kcal mol-1 for the standard enthalpy of formation of 1-bromo-3-chloropropane at 298 K was derived.
RESUMO
This work reports the experimental study of the ozonolysis of indene in the presence of SO2 and the reaction conditions leading to the formation of secondary aerosols. The reactions have been carried out in a Teflon chamber filled with synthetic air mixtures at atmospheric pressure and room temperature. As in the case of styrene, SO2 plays a key role in the oxidation of the Criegee intermediates and enhances the formation of particulate matter. Thus, for the ozonolysis of indene, nucleation was observed for reacted indene concentrations above (4.5 ± 0.8) × 1011 molecule cm-3 in the absence of SO2 while new particle formation was observed for concentrations one order of magnitude lower, (3 ± 1) × 1010 molecule cm-3, in the presence of SO2. Within the detection limit of the system, SO2 concentrations remained constant during the experiments. The formation of secondary aerosols in the smog chamber was inhibited by H2O and so the potential formation of secondary aerosols under atmospheric conditions depends on the concentration of SO2 and relative humidity. Computational calculations have been performed for the ozonolysis of both indene and styrene in the presence of SO2 and water to identify the reaction channels and species responsible for new particle formation. The release of SO3 and its subsequent conversion into H2SO4 from the reaction of the Criegee intermediate H2COO in the ozonolysis of styrene makes this aromatic have a high potential of aerosol formation in the atmosphere. On the other hand, quantitative conversion of SO2 into SO3 does not occur following the ozonolysis of indene.
RESUMO
The kinetics of the gas phase recombination reaction HO + HO2 + He â HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k∞ = 3.55 × 10-12 (T/300)0.20 cm3 molecule-1 s-1 and k0 = [He] 1.55 × 10-31 (T/300)-3.2 cm3 molecule-1 s-1. The rate coefficients calculated over the 6 × 10-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO2 â H2O + O2, indicating that the recombination pathway is irrelevant.
RESUMO
A detailed kinetic study of the gas-phase thermal decomposition of 3-bromopropene over wide temperature and pressure ranges was performed. Quantum chemical calculations employing the density functional theory methods B3LYP, BMK, and M06-2X and the CBS-QB3 and G4 ab initio composite models provide the relevant part of the potential energy surfaces and the molecular properties of the species involved in the CH2âCH-CH2Br â CH2âCâCH2 + HBr (1) and CH2âCH-CH2Br â CH2âCH-CH2 + Br (2) reaction channels. Transition-state theory and unimolecular reaction rate theory calculations show that the simple bond fission reaction ( 2 ) is the predominant decomposition channel and that all reported experimental studies are very close to the high-pressure limit of this process. Over the 500-1400 K range a rate constant for the primary dissociation of k2,∞ = 4.8 × 10(14) exp(-55.0 kcal mol(-1)/RT) s(-1) is predicted at the G4 level. The calculated k1,∞ values lie between 50 to 260 times smaller. A value of 10.6 ± 1.5 kcal mol(-1) for the standard enthalpy of formation of 3-bromopropene at 298 K was estimated from G4 thermochemical calculations.
RESUMO
A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.