Electronically Excited Complex Formation in Magnesium Cluster–Halogen Atom Reactions

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 C_2v 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 C_2v and D_∞h isomers of the MgClMg structure slightly higher in energy.