Following an overview on the nature of bonding of such hypervalent molecules as Li
nA (Li
6C, Li
3O, Li
4O, Li
3S, Li
4S, Li
4P) and M
2CN (M=Li, Na, K), the present paper deals with the molecular and electronic structures of newly found lithium-rich Li
2F and Li
2OH molecules as well as Li
2F
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 Li
2F, 4.32±0.2 eV for Li
3F
2, and 4.30±0.2 eV for Li
4F
3. Agreements of thhse IEs with theoretical ones calculated by ab initio MO methods support that Li
2F is in a hyperlithiated configuration (HLC) in which the excess electron delocalizes over the two lithiums, while Li
3F
2 and Li
4F
3 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 Li
2OH in HLC, 3.687±0.003 eV for Li
3 (OH)
2 in HLC, 4.133±0.003 eV for Li
3 (OH)
2 in SC, and 3.418±0.009 eV for Li
4 (OH)
3 in SC, and 3.60±0.11 eV for Li
5 (OH)
4 in SC. Also, IE of Li
3O was determined as 3.59±0.02 eV from reinvestigation with the photoionization technique. Furthermore, Li
3O 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 Li
3O. This is the first experimental evidence for “electronomers” or electronic isomers of Li
3O, 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.
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