Are Cyclogallenes [M2(GaH)3] (M = Li, Na, K) Aromatic?

Abstract

Ab initio quantum mechanical methods were applied to examine M2(GaH)3 (M = Li, Na, K) compounds, which are models for the newly synthesized class of Ga3 three-membered-ring cyclogallene compounds. Basis sets of triple-ζ 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 Na2(GaH)3 possesses C3 symmetry (the deviation from C3h symmetry is very small), the remaining two compounds are C3h 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 curren...