Raúl A. Taccone

Kinetic Study of OH Radical Reactions with CF3CClCCl2, CF3CClCClCF3 and CF3CF=CFCF3

The relative rate technique has been used to determine the rate constants of the reactions of OH radicals with CF(3)CCl=CCl(2) (k(1)), CF(3)CCl=CClCF(3) (k(2)) and CF(3)CF=CFCF(3) (k(3)). Experiments were carried out at (298±2) K and atmospheric pressure using ultrapure nitrogen as gas bath. The decay rates of the organic species were measured relative to those of ethane, methanol, acetone, chloroethane and 2-butanone. The following rate constants were derived in units of cm(3) molecule(-1) s(-1): k(1)= (10±1)×10(-13), k(2)=(2.1±0.2)×10(-13) and k(3)=(3.7±0.2)×10(-13). This is the first experimental determination of k(1) and k(2). The rate constants obtained are compared with previous literature data to establish reactivity trends and are used to estimate the atmospheric lifetimes of the studied perhaloalkenes. From the calculated lifetimes, using an average global concentration of hydroxyl radicals, the atmospheric loss of these compounds by the OH-initiated oxidation was determined. Also, estimations have been made of the ozone depletion potential (ODP), the radiative forcing efficiency (RE), the halocarbon global warming potential (HGWP) and the global warming potential (GWP) of the perhaloalkenes. The approximate nature of these values is stressed considering that these are short-lived compounds for which these atmospheric parameters may vary according to latitude and season.

Theoretical study of the relative reactivity of chloroethenes with atmospheric oxidants (OH, NO3, O(3P), Cl(2P) and Br(2P))

The reactivity of a series of chloroethenes with different electrophiles of tropospheric and stratospheric interest is analyzed by frontier molecular orbital theory and a correlation with calculated orbital energies is investigated. The reactions of CH2CHCl; CH2CCl2; (Z)-CHClCHCl; (E)-CHClCHCl; CHClCCl2 and CCl2CCl2 with O(3P), Cl(2P), Br(2P) atoms and with OH and NO3 radicals were studied using semiempirical methods (AM1 and PM3) and ab initio calculations at the HF and B3LYP levels of theory with the 6-31G** basis set, using the Gaussian 98 suite of programs. In contrast to the majority of reaction series of small radicals and molecules with alkenes and alkanes, the rate constants for the reactions with halogenated ethenes do not correlate with the ionization potential of the halogenated ethene. The energy and the carbon–carbon π-bonding form of the HOMO change on addition of chlorine atoms as substituents to the carbon–carbon σ-bonding framework of the alkenes. For the reactions studied the complete interaction HOMO–SOMO was considered, taking into account the contribution of the different atomic orbitals to the HOMO of the chloroethene through the atomic orbital coefficients, and a good correlation with the experimental values was obtained.

Water Catalysis of the Reaction between Methanol and OH at 294 K and the Atmospheric Implications

The rate coefficient for the reaction CH3 OH+OH was determined by means of a relative method in a simulation chamber under quasi-real atmospheric conditions (294 K, 1 atm of air) and variable humidity or water concentration. Under these conditions, a quadratic dependence of the rate coefficient for the reaction CH3 OH+OH on the water concentration was found. Thus the catalytic effect of water is not only important at low temperatures, but also at room temperature. The detailed mechanism responsible of the reaction acceleration is still unknown. However, this dependence should be included in the atmospheric global models since it is expected to be important in humid regions as in the tropics. Additionally, it could explain several differences regarding the global and local atmospheric concentration of methanol in tropical areas, for which many speculations about the sinks and sources of methanol have been reported.

Kinetic study of the OH and Cl-initiated oxidation, lifetimes and atmospheric acceptability indices of three halogenated ethenes

The gas-phase kinetics for the reactions of OH radicals and Cl atoms with (E/Z)-CHClCHF, (E/Z)-CFClCFCl, and CCl2CF2 were investigated at room-temperature and atmospheric pressure. A conventional relative-rate technique was used to determine the rate coefficients k(OH + (E/Z)-CHClCHF) = (6.3 ± 1.2) × 10−12, k(OH + (E/Z)-CFClCFCl) = (1.6 ± 0.2) × 10−12, k(OH + CCl2CF2) = (5.0 ± 0.7) × 10−12, k(Cl + (E/Z)-CHClCHF) = (11 ± 2) × 10−11, k(Cl + (E/Z)-CFClCFCl) = (5.4 ± 1.3) × 10−11, and k(Cl + CCl2CF2) = (6.3 ± 1.5) × 10−11 cm3 per molecule per s. These rate coefficients were compared with previous literature data to analyze the effect of halogen substitution in ethenes on the reactivity towards OH and Cl, and used to estimate the global atmospheric lifetimes for the studied haloethenes. The calculated lifetimes, using average global concentrations of OH radicals and Cl atoms, indicate that the atmospheric loss of these compounds is determined by the OH-initiated oxidation. Also, the atmospheric implications of the halogenated ethenes studied were evaluated by estimating acceptability indices such as the global warming potential (GWP) and the ozone depletion potential (ODP). From these potentials, the contribution of (E/Z)-CHClCHF, (E/Z)-CFClCFCl, and CCl2CF2 to radiative forcing of climate change and to ozone layer depletion is expected to be negligible.

Water Catalysis of the Reaction Between the Hydroxyl Radicals and Linear Saturated Alcohols (ethanol and n-propanol) at 294 K

The rate coefficients for the reactions of OH with ethanol and n-propanol were determined by a relative method in a smog chamber at 294 K, 1 atm of air or N2 and a wide range of humidity. The rate coefficients for both reactions show a quadratic dependence on the water concentration as in the case of the reaction of OH with methanol (Jara-Toro et al. Angew. Chem., Int. Ed., 2017, 56, 2166). The detailed mechanism responsible for the reaction acceleration was studied theoretically at the uMP2/aug-cc-pVDZ level of theory while the electronic energies of all the structures were refined at the uCCSD(T)/aug-cc-pVDZ level. From these results it is suggested that the catalytic effect of two water molecules is due to two cooperative effects in the reactions between the ROH(H2O) and OH(H2O) equilibrium complexes: (1) an enhanced capture cross-section as a consequence of the larger dipolar moment of the ROH(H2O) and OH(H2O) complexes as compared to those of the free reactants ROH and OH and (2) a strong stabilization of the TSs below the energy of the reactants that leads to a very fast decomposition of the pre-reactive complexes to products with an extremely low probability of dissociation back to the reactants. The tropospheric lifetime of these alcohols is also shown to strongly depend on the humidity, suggesting the need to incorporate this dependence in global atmospheric models.