ALUMINUM, GALLIUM AND INDIUM DIHYDRIDES - AN IR MATRIX-ISOLATION AND AB-INITIO STUDY

Aluminium, gallium and indium dihydrides have been synthesized by reaction of molecular hydrogen with excited 2S and 2D metal atoms (M) isolated in solid argon. Narrow band excitations show that insertion occurs efficiently from the 2S and 2D excited states as previously reponed by Parnis and Ozin for aluminium, but also takes place equally well with gallium and indium. Monohydride formation MH2 --> MH+ H. is evidenced when the dihydrides are subsequently excited with more energetic light (4.8 to 4.3 eV). Complete sets of isotopic substitutions (MH2, MHD and MD2) are reponed for each metal for all three normal modes, and the Ga-69/Ga-71 isotopic structure can be observed for gallium dihydride. Subsequent force field calculations demonstrate nearly identical GaH2 and AlH2 bond force constants, while the InH2 value is substantially smaller. These findings are indeed reflected in the bond distances which have been calculated using UHF, CCSD and DFT methods. However reproduction of the spectral characteristics provides a crucial test for the quality of the basis set as well as for the calculation method. Only DFT and CCSD methods are capable of reliably predicting the frequencies (less than 5% average error).