CEQUINOR   05415
Unidad Ejecutora - UE
congresos y reuniones científicas
Photochemical study of ClSO2NCO: comparison between gas phase and matrix isolation results
Congreso; XXI Congreso Interamericano de Fotoqu¨ªmica (21st I-APS Conference); 2011
The photochemical study of species isolated in noble gases matrixes is a powerful tool for understanding photoevolutive processes in single molecules. The inert environment reduces the possibility of molecular interactions, and the vibro-rotational transitions are negligible due to the low temperatures.. In addition, the photochemistry of matrix isolated species can be assumed as a suitable model for understanding photodegradation of compounds in the atmosphere, since the dilution mimic very well the conditions to which the substances are exposed when reached by solar irradiation. We present here the results obtained for the photochemical study of ClSO2NCO in gaseous phase and isolated in Ar matrix, stressing the differences observed using both techniques. For the gas phase experience 5 Torr of the compound were loaded in vacuo, into a cross-shaped glass cell equipped with quartz and CsI windows, to allow the measurement of the IR spectra during the UV-vis irradiation. On the other hand, a gas mixture of chlorosulfonyl isocyanate and Argon, in a 1:1000 proportion, was prepared by standard manometric methods. The mixture was deposited on a CsI window cooled to ca. 10, using the pulse deposition technique. The matrix isolated FTIR spectra were recorded with resolutions of 0.5 and 0.125 cm-1. The samples were exposed to broad-band UV¨C visible radiation (200 ¡Ü ¦Ë ¡Ü 800 nm) and the output from the lamp was limited by a water filter to absorb IR radiation and so minimize heating effects. The results were complement by quantum chemical calculations at the B3LYP and MP2 levels of theory and employing the 6-31+G(d) and aug-cc-pVDZ basis sets. The IR spectra taken before and after irradiation has revealed the decay of the bands belonging to the parent molecule, and the appearance and growth of new absorptions corresponding to different products. To identify these bands and help to establish the photochemical mechanisms, the integrated intensities of the new absorptions have been plotted as a function of the irradiation time. In the gas phase experiences, the photoproducts were destilled from the cell, and IR spectra of compounds of different volatility were taken to identify the species. As it was expected, in the gaseous phase only stable moelcules such as SO2 and CO were observed as volatile products. The IR spectra of the non-volatile species were interpreted in terms of dimeric and trimeric forms. In the matrix experiment, not only SO2 and CO were identified but also the presence of ClNCO and ClCO¡¤ radical. The comparison of our experimental vibrational spectra with data reported for SO2 as a monomer isolated in argon matrixes indicates a certain degree of perturbation for the expected vibrational frequencies. Such perturbations were attributed to molecular complexation. Quantum chemical calculations for feasible ClNCO¡¤¡¤¡¤SO2 complexes support this assumption. The extreme dilution conditions do not favor the existence of more than one isolated molecule on a matrix site. For these reason only unimolecular processes are observed in matrix conditions. In the gas phase, the presence of bands in the C=O region suggests the formation of molecular aggregates, like dimers or trimers.