Why Does a Ga2 Dimer React Spontaneously with H2, but a Ga Atom Does Not?-A Detailed Quantum Chemical Investigation of the Differences in Reactivity Between Ga Atoms and Ga2 Dimers, in Combination with Experimental Results

The spontaneous and photoactivated reactions between Ga2 and H2 in a matrix of solid Ar at 12 K have been followed by using IR spectroscopy and have been shown to give access to several isomers of the subvalent hydride Ga2H2. We now present Raman spectra for this system, to complete its characterization on the basis of vibrational spectra. In addition, the differences between the reactivity of a Ga atom and a Ga2 dimer toward H2 are evaluated. The matrix isolation experiments have shown that Ga2 reacts spontaneously with H2, at 12 K, to give the cyclic subvalent hydride Ga(μ‐H)2Ga (D2h symmetry), which can be transformed into two other isomers of Ga2H2 by selective photoactivation. Interestingly, the spontaneous reaction is subject to a marked isotopic effect. In total, the experimental results provide detailed information about the reaction mechanism. In contrast to Ga2, Ga atoms do not react spontaneously with H2; on photoactivation they instead yield the radical species GaH2. The quantum chemical calculations presented herein start with an analysis of the structures and relative energies of the relevant species at the MP2 level, by using extended basis sets, and lead on to a discussion of the correlation diagrams for both reactions. Finally, CASSCF and MRCI methods, in combination with moderate‐sized basis sets, were employed to analyze in detail the mechanisms of the two reactions. It will be shown that the computational results, in concert with the experimental findings, provide a satisfying explanation of the contrasting reactivities of Ga and Ga2. As a first step toward the understanding of the unusually high reactivity of clusters, the mechanism for the reaction between Ga2 and H2 was analyzed in detailed quantum chemical calculations in combination with experimental studies using the matrix isolation technique. The figure shows the definition of the reaction coordinates r for the reaction of H2 with Ga2 to give Ga(μ‐H)2Ga.