Estudo teórico da ozonólise do canfeno em fase gasosa

Abstract

In this work, a chemical model for camphene ozonolysis, leading to carbonyl final products, is proposed and discussed on the basis of the thermochemical properties and kinetic data obtained at DFT levels of calculation. The mechanism is initiated by the electrophilic attack of ozone to the double bond in camphene leading to a 1,2,3-trioxolane intermediate, which decomposes to peroxy radicals and carbonyl compounds in a total of 10 elementary reactions. The thermodynamic properties (enthalpy and entropies differences) are calculated at 298K. For the thermochemical evaluation, theoretical calculations are performed with the B3LYP, MPW1PW91 and mPW1K density functionals and the basis sets 6-31G(d), 6-31G(2d,2p), 6-31+G(d,p) and 6-31+G(2d,2p). Eventually, single point calculations adopting the 6-311++G(2d,2p) basis set are performed in order to improve the electronic energies. The enthalpy profiles suggest highly exothermic reactions for the individual steps, with a global enthalpy difference of -179.18 kcalmol-1, determined at the B3LYP/6-31+G(2d,2p) level. The Gibbs free energy differences for each step, at 298K, calculated at the B3LYP/6-311++G(2d,2p)//B3LYP/6-31+G(2d,2p) level, are used to estimate the composition of a final product mixture under equilibrium conditions as 58% of camphenilone and 42% of 6,6-dimethyl--caprolactone-2,5-methylene. For the reaction kinetics, the bimolecular O3 + camphene step is assumed to be rate determining in the global mechanism. A saddle point for the ozone addition to the double bond is located and rate constants are determined on the basis of the transition state theory. This saddle point is well represented by a loosely bound structure and corrections for the basis set superposition error (BSSE) are calculated, either by considering the effect over the geometry optimization procedure (here referred as CP1 procedure), or the effect of the BSSE over the electronic energy of a previously optimized geometry, included a posteriori (here referred as CP2). Optimized geometries are located for the pre-barrier complex. However, the investigation of the energetic profiles of these structures suggests that they do not play any important role in the kinetics of the ozonolysis of camphene. The rate coefficients, calculated at 298K from the data obtained at the mPW1K/6-31+G(d,p), CP1/B3LYP//6-31+G(2d,2p) and CP2/B3LYP//6-31+G(2d,2p) levels (3.62x10-18, 1.12x10-18 and 1.39x10-18 cm3.molecule-1.s-1), are found in good agreement with the available experimental data at the same temperature, 0.9x10-18 cm3.molecule-1.s-1 (R. Atkinson, S. M. Aschmann, J. Arey. Atmos. Environ. 24, 2647 (1990)). The observed variational effect was found to be very low and the ratios between conventional and variational rate constants are less than 1.06. In the range from 100 to 1000K the Arrhenius behavior is observed. The importance of the BSSE corrections for the final rate constants must be pointed out. Furthermore, this work will contribute to a better understanding of the chemistry of monoterpenes in the atmosphere, as well as the implications for the phenomena of pollution.