The spontaneous and photoactivated reactions between Ga(2) and H(2) 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 Ga(2)H(2). 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 Ga(2) dimer toward H(2) are evaluated. The matrix isolation experiments have shown that Ga(2) reacts spontaneously with H(2,) at 12 K, to give the cyclic subvalent hydride Ga(micro-H)(2)Ga (D(2h) symmetry), which can be transformed into two other isomers of Ga(2)H(2) 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 Ga(2), Ga atoms do not react spontaneously with H(2); on photoactivation they instead yield the radical species GaH(2). 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 Ga(2).

Matrix isolation experiments give evidence for the formation of the loosely bonded metal-silane complex M.SiH(4) by the spontaneous reaction of Al or Ga atoms (M) with silane in a solid Ar matrix at 12 K; however, Ga(2) appears to insert spontaneously into an Si--H bond to form HGaGaSiH(3), probably with the structure HGa(micro-SiH(3))Ga. In M.SiH(4) the metal atom is eta(2)-coordinated by the silane, resulting in a species with C(2v) symmetry. The complex has a distinctive photochemistry: it can be converted on photolysis at lambda approximately 410 or approximately 254 nm to its tautomer, HMSiH(3), which also has a doublet ground electronic state and from which it can be regenerated with lambda approximately 580 nm radiation. Broadband UV-visible photolysis (lambda=200-800 nm) results in decomposition of HMSiH(3), the univalent species MSiH(3) being the only detectable product. The experimental data collected for several silane isotopomers (SiH(4), SiD(4), and SiD(3)H) and different reagent concentrations, together with the results of sophisticated quantum chemical calculations, are used to explore in detail the properties of the detected species and the reaction pathways compassing their formation.

Isolated Ga2 dimers were characterized in an argon matrix with the aid of resonance Raman and UV/Vis spectroscopy. The resonance Raman spectra gave evidence of not only the nu(Ga-Ga) fundamental, but also four overtones. Each of the signals exhibits 69Ga/71Ga isotopic splitting leading to the triplet pattern characteristic of two equivalent Ga atoms. On the basis of the experimental data, a harmonic frequency and anharmonicity constant have been determined for Ga2. An estimate of the dissociation energy on the assumption of a Morse-type potential energy curve results in a De value (upper limit) of about 145 kJ mol(-1). The force constant (64.8+/-0.3 N m(-1)) and dissociation energy of Ga2 are compared with those of other diatomics and those of molecules featuring Ga-Ga bonds.

In this work, the spontaneous and photolytically activated reactions of Ga and In atoms (M) with O2 (in Ar and solid O2) are studied with the aid of the matrix-isolation technique and the use of IR, Raman, and UV/Vis spectroscopy in combination with detailed quantum-chemical calculations. Vibrational spectra were recorded for several different isotopomers (69Ga, 71Ga, 16O2, 18O2, 16O18O). The results show that the spontaneously formed cyclic MO2 molecules photoisomerize to give the linear OMO molecules. The collected vibrational data were then used to characterize the bond properties of the linear OMO molecules in detail. The results are compared to those obtained for CO2(+) and neutral OEO compounds, where E is an element of Group 14. Quantum-chemical calculations were carried out at various levels of theory for GaO2. These calculations indicate that linear OMO is slightly more stable than its cyclic isomer. These calculations were also used to obtain information about the reaction mechanism, and show that the formation of the cyclic isomer from Ga atoms and O2 occurs without a significant barrier. Abrupt changes of the dipole moment and the O-O bond length during the approach of the O2 molecule toward the Ga atom mark the point on the potential energy surface at which one electron jumps from the Ga atom onto the O2 unit. The isomerization of cyclic GaO2 to the linear global minimum structure is accompanied by a significant barrier, which explains why this reaction requires photoactivation.

Matrix isolation experiments have enabled the first characterization of the aluminum–silane complex Al⋅SiH4 (η2 coordination) which can be converted reversibly by selective photolysis into its tautomer HAlSiH3 [Eq. (1)]. Broadband UV/Vis photolysis causes decomposition of HAlSiH3 to give the aluminum(I) species AlSiH3.