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

Ab initio quantum mechanical methods were applied to examine M(2)(GaH) (3) (M = Li, Na, K) compounds, which are models for the newly synthesi zed class of Ga-3 three-membered-ring cyclogallene compounds. Basis se ts of triple-zeta plus double polarization augmented with diffuse func tions (TZ2P+diffuse) quality were employed at the self-consistent fiel d (SCF) and density functional theory (B3LYP) levels of theory. Comput ed equilibrium geometries, harmonic vibrational frequencies, and chemi cal shifts are reported. The experimental (X-ray) structures of cyclog allenes are in close agreement with theory. While Na-2(GaH)(3) possess es C-3 symmetry (the deviation from C-3h symmetry is very small), the remaining two compounds are C-3h symmetric. The parent neutral three-m embered-ring structure (GaH)(3) is not stable as the imaginary vibrati onal frequencies of (GaH)(3) lead to Ga-Ga bond breaking. Upfield chan ges in the chemical shifts for the alkali metal atoms over and under t he ring plane are taken as strong evidence for ring currents in the cy clogallene moiety. This finding and the large negative nucleus indepen dent chemical shifts (NICS) clearly support the proposed aromatic char acter of the title compounds.