Theoretical study of aromaticity in small hydrogen and metal cation clusters X3+ (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-infinity h ), have been investigated using four methods: B3LYP, B3PW91, MP2, CCSD(T) and basis set 6-311 + G(3df). The calculations show that only the regular trigonal structure (D-3h) is stable. The related neutral clusters X3Cl (X=H, Li, Na, K, and Cu) are also investigated using two methods: B3LYP, MP2, and basis set 6-311 + G(3df). For H3Cl species, there is no a stable structure to be found. For other four X3Cl (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 sigma-aromaticity originating from s orbitals. For the Cu-3(+) ring the d orbitals do not play a significant role in the electron delocalization effects. (C) 2006 Wiley Periodicals, Inc.