CHLOROGALLANES (GACLH2, GACL2H, AND GACL3) AND THEIR DIMER ISOMERS

The dimers of the three chlorides of gallium(III) have been studied by using ab initio quantum-mechanical methods at high levels of theory for these relatively large molecules. The problem of accurately predicting the geometry of the bridge region is investigated. Large basis sets (including f functions) and electron correlation are required to obtain the Ga-Cl(b) distance correctly (b denotes bridging). Theoretical bond angles converge much more quickly with respect to level of theory, leading us to suggest that the experimental Ga-Cl(b)-Ga angle of 87.2-degrees for Ga2Cl2H4 be revised slightly upward to 90-degrees. The H-Ga-H angle is predicted to be 132-degrees. The geometry and energy of all possible isomers of the monochloro- and dichlorogallane dimers are also reported, as well as those for Ga2Cl6. Agreement between theory and experiment for Ga2Cl6 is significantly better than for Ga2Cl2H4. For each Cl on a bridging site, the dimerization energy (relative to an analogous molecule with a H at that site) is increased by 6-8 kcal mol-1, and for each terminally bonded Cl the dimerization energy is decreased by 1-2 kcal mol-1 relative to the analogous molecule with a H bonded at that position. Harmonic vibrational frequencies for the monochlorogallane dimer are predicted and compare well with the experimental infrared and Raman fundamentals.