ARE CYCLOGALLENES [M(2)(GAH)(3)] (M=LI, NA, K) AROMATIC

摘要

Ab initio quantum mechanical methods were applied to examine M(2)(GaH)(3) (M = Li, Na, K) compounds, which are models for the newly synthesized class of Ga-3 three-membered-ring cyclogallene compounds. Basis sets of triple-zeta plus double polarization augmented with diffuse functions (TZ2P+diffuse) quality were employed at the self-consistent field (SCF) and density functional theory (B3LYP) levels of theory. Computed equilibrium geometries, harmonic vibrational frequencies, and chemical shifts are reported. The experimental (X-ray) structures of cyclogallenes are in close agreement with theory. While Na-2(GaH)(3) possesses C-3 symmetry (the deviation from C-3h symmetry is very small), the remaining two compounds are C-3h symmetric. The parent neutral three-membered-ring structure (GaH)(3) is not stable as the imaginary vibrational frequencies of (GaH)(3) lead to Ga-Ga bond breaking. Upfield changes in the chemical shifts for the alkali metal atoms over and under the ring plane are taken as strong evidence for ring currents in the cyclogallene moiety. This finding and the large negative nucleus independent chemical shifts (NICS) clearly support the proposed aromatic character of the title compounds.