Electronically Excited Complex Formation in Magnesium Cluster-Halogen Atom Reactions.

Abstract

A near ultraviolet transition of Mg2F has been observed in emission from the reaction between magnesium clusters, most likely Mg3, and fluorine atoms. Because there is little evidence for upper-state internal excitation, the spectrum is assigned assuming that the upper state is quenched to its lowest vibrational levels. Two of possibly three ground-state vibrational frequencies, υ1 = 516 ± 10 cm-1 and υ2 = 104 ± 10 cm-1, have been established. Dispersed laser-induced fluorescence studies extrapolating on the observed chemiluminescence indicate an excited-state symmetric stretch frequency of order 370 ± 30 cm-1. Electronic structure calculations at the CCSD(T)/CBS level predict that the ground state of Mg2F has C2v symmetry and can be described as an Mg2+F- ion pair with two Mg-F bonds. Like the MgF A-X transition that is largely a transition between Mg orbitals, the observed transition in Mg2F is largely between orbitals on the magnesium dimer ion. The asymmetric C∞v Mg2+F- complex is also a minimum and is predicted to be 6.7 kcal/mol higher in energy. Calculated structures for the Mg2Cl isomers are also presented and used to further interpret the experimental results for the reaction of Mg clusters with Cl atoms. In contrast to Mg2F, the ground state of Mg2Cl is a linear C∞v MgMgCl structure with the C2v and D∞h isomers of the MgClMg structure slightly higher in energy.