Aromaticity of regular heptagonal P73- trianion in the MP72- (M = Li, Na, K, Rb, and Cs) species

Geometries, electronic structures and vibrational frequencies of alkali metal-P73- compounds MP72- (M = Li, Na, K, Rb, and Cs) were examined using density functional theory (DFT) methods. To our knowledge, these compounds are first reported here. Calculation results show that regular heptagonal P73- trianion can coordinate with alkali metal atoms to form the pyramidal structures MP72- species that maintain the planar cyclo-P73- trianion structure. On the basis of the molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) analysis, we revealed that regular heptagonal P73- trianion exhibits characteristic of π aromaticity with 10 delocalized π electrons and maintains its structural and electronic integrity inside the pyramidal MP72- (M = Li, Na, K, Rb, and Cs) clusters.