Theoretical study of aromaticity in small hydrogen and metal cation clusters X + 3 (X=H, Li, Na, K, and Cu)

Five cation clusters X   + 3(X=H, Li, Na, K, and Cu) with two possible isomers, i.e., regular trigonal structure (D 3h ) and linear structure (D ∞h ), have been investigated using four methods: B3LYP, B3PW91, MP2, CCSD(T) and basis set 6‐311+G(3 df ). The calculations show that only the regular trigonal structure (D 3h ) is stable. The related neutral clusters X 3 Cl (X=H, Li, Na, K, and Cu) are also investigated using two methods: B3LYP, MP2, and basis set 6‐311+G(3 df ). For H 3 Cl species, there is no a stable structure to be found. For other four X 3 Cl (X=Li, Na, K, and Cu) species, there are two stable isomers, for which the bidentate structures (C 2v ‐1) [see Fig. 1(d)] are global minima. According to the general criteria for aromaticity including resonance energy (RE) and nucleus‐independent chemical shift (NICS), the five trigonal isomers exhibit a higher degree of aromaticity. Molecular orbital analysis reveals that the five trigonal X   + 3 (X=H, Li, Na, K, and Cu) isomers possess only σ‐aromaticity originating from s orbitals. For the Cu   + 3ring the d orbitals do not play a significant role in the electron delocalization effects. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007