Theoretical study on the aromaticity from d-AOs in cationic X ₃+ (X = Sc, Y, La) clusters

摘要

The stable structures and aromatic characters for three cationic X₃+ (X = Sc, Y, and La) and three relevant neutral X₃Cl (X = Sc, Y, La) clusters are investigated at the DFT and post HF level of theory. The calculated results show that the X₃+ cations each has two stable structures: the regular trigon (D_3h) and the line (D_¥h {{hbox{D}}_{infty {rm{h}}}} ) with the regular trigon (D_3h) being the ground state, while for three neutral X₃Cl clusters, Sc₃Cl has three stable isomers: the trigon-pyramidal (C_3v), bidentate (C_2v-1), and C_2v-2 structures, Y₃Cl and La₃Cl each has only two stable isomers: the trigon-pyramidal (C_3v) and bidentate (C_2v-1) structures. The ground states for three X₃Cl species are all the bidentate (C_2v-1) isomers. The calculations of the resonance energy (RE) and NICS show that trigonal X₃+ isomers exhibit higher degree of aromaticity. The detailed molecular orbital analyzes reveal that the isolated trigonal Sc₃+ and Y₃+ cations each has one delocalized π-type MO and shows single π-aromaticity, while the isolated trigonal La₃+ cation has one delocalized σ-type MO and shows single σ-aromaticity. The single π- or σ-aromaticity for X₃+ are attributed to the contributions mainly from the d AOs of the corresponding transition metal X atoms. However, when a singly negatively charged counterion Cl- is added to Sc₃+, Y₃+, and La₃+ cations respectively, the aromatic type for the two Sc₃+, Y₃+ units in the corresponding neutral Sc₃Cl, Y₃Cl complexes are changed from π-aromaticity into σ-aromaticity, whereas the σ-aromaticity of the La₃+ units in the La₃Cl complex keeps unchanged in this process. Thus three Sc₃+, Y₃+, La₃+ units in the corresponding X₃Cl complexes all have only one σ-type MO and exhibit single σ-aromaticity.