The Nature of Bonding of Hyperlithiated Molecules Beyond the Octet Rule

Abstract

Following an overview on the nature of bonding of such hypervalent molecules as Li_nA (Li₆C, Li₃O, Li₄O, Li₃S, Li₄S, Li₄P) and M₂CN (M=Li, Na, K), the present paper deals with the molecular and electronic structures of newly found lithium-rich Li₂F and Li₂OH molecules as well as Li₂F_n-1 (n=3, 4) and Li_n (OH) _n-1 (n=3-5) clusters which have been detected in supersonic beams effusing from a laser ablation source. The ionization energies (IEs) determined by photoionization were 3.78±0.2 eV for Li₂F, 4.32±0.2 eV for Li₃F₂, and 4.30±0.2 eV for Li₄F₃. Agreements of thhse IEs with theoretical ones calculated by ab initio MO methods support that Li₂F is in a hyperlithiated configuration (HLC) in which the excess electron delocalizes over the two lithiums, while Li₃F₂ and Li₄F₃ are in a segregated configuration (SC) comprising ionic and non-ionic lithiums resulting from localization of the excess valence electron. Ionization efficiency curves (IECs) measured for Li_n (OH) _n-1 (n=2-5) are well reproduced with a simulation involving Franck-Condon factors, and this enabled us to identify the global-minimum structure of these species predicted by theoretical calculations with the DFT method. The IEs determined were 4.053±0.003 eV for Li₂OH in HLC, 3.687±0.003 eV for Li₃ (OH) ₂ in HLC, 4.133±0.003 eV for Li₃ (OH) ₂ in SC, and 3.418±0.009 eV for Li₄ (OH) ₃ in SC, and 3.60±0.11 eV for Li₅ (OH) ₄ in SC. Also, IE of Li₃O was determined as 3.59±0.02 eV from reinvestigation with the photoionization technique. Furthermore, Li₃O was found to be a floppy molecule sharing both the D_3h and C_2v structures from a precise analysis of the observed IEC taking account of the potential energy surface for both neutral and cationic Li₃O. This is the first experimental evidence for “electronomers” or electronic isomers of Li₃O, which have nearly the same stability but are different in localization of the SOMO. It is eventually stressed that delocalization of the excess valence electron over all of the lithium atoms in a molecule is essential to afford hyperlithiated molecules and that the shape of SOMO or HOMO, which accommodates the excess valence electron or electrons, plays a key role in determining the stability of hyperlithiated molecules.