On the kinetics of the Al(13) (-)+Cl(2) reaction: Cluster degradation in consecutive steps

The kinetics of the reaction system initiated by the Al(13) (-)+Cl(2) reaction was experimentally studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The Al(13) (-) clusters were produced by laser desorption/ionization of LiAlH(4), then transferred into the ICR cell, cooled by collisions with Ar, and exposed to an excess of Cl(2) with a concentration of approximately 10(8) cm(-3). Relative concentration-time profiles of Al(n) (-) clusters with n=13, 11, 9, and 7 as well as profiles of Cl(-) ions have been recorded. Other ionic species, besides traces of Al(12)Cl(-), were not found, which indicates a double-step degradation mechanism via the odd-numbered Al(n) (-) clusters. From a kinetic analysis of the experimental results, a rate coefficient of (5+/-2)x10(-10) cm(3) s(-1) for the Al(13) (-)+Cl(2) reaction was obtained. Furthermore, it is inferred from a simultaneous fit of all concentration-time profiles that the Al(n) (-)+Cl(2) reactions for n=13, 11, 9, and 7 occur with rate coefficients near the Langevin limit in the range k(bim) approximately (5+/-4)x10(-10) cm(3) s(-1). The branching ratios between the Al(n-2) (-)-producing and Cl(-)-producing channels of a given cluster Al(n)Cl(2) (-) indicate an increasing contribution of the Cl(-)-producing channels with decreasing cluster size. Statistical rate theory calculations on the basis of molecular data from quantum chemical calculations show that the experimental Al(n) (-) profiles are compatible with a sequence of association-elimination reactions proceeding via the formation of highly excited Al(n)Cl(2) (-) adducts followed by a sequential elimination of two AlCl molecules. Rate coefficients for these reactions were calculated, and the production of Cl(-) was shown probably not to proceed via these Al(n)Cl(2) (-) intermediates.