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High-level electronic structure calculations are carried out to obtain optimized geometries and excitation energies of neutral lithium, sodium, and potassium complexes with two ethylenediamine and one or two crown ether molecules. Three different sizes of crowns are employed (12-crown-4, 15-crown-5, 18-crown-6). The ground state of all complexes contains an electron in an s-type orbital. For the mono-crown ether complexes, this orbital is the polarized valence s-orbital of the metal, but for the other systems this orbital is a peripheral diffuse orbital. The nature of the low-lying electronic states is found to be different for each of these species. Specifically, the metal ethylenediamine complexes follow the previously discovered shell model of metal ammonia complexes (1s, 1p, 1d, 2s, 1f), but both mono- and sandwich di-crown ether complexes bear a different shell model partially due to their lower (cylindrical) symmetry and the stabilization of the 2s-type orbital. Li(15-crown-5) is the only complex with the metal in the middle of the crown ether and adopts closely the shell model of metal ammonia complexes. Our findings suggest that the electronic band structure of electrides (metal crown ether sandwich aggregates) and expanded metals (metal ammonia aggregates) should be different despite the similar nature of these systems (bearing diffuse electrons around a metal complex).
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The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors
Beryllium ammonia complexes Be(NH 3 ) 4 are known to bear two diffuse electrons in the periphery of a Be(NH 3 ) 4 2+ skeleton. The replacement of one ammonia with a methyl group forms CH 3 Be(NH 3 ) 3 with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH 3 ) 4 . Two CH 3 Be(NH 3 ) 3 monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH 3 ) 3 Be(CH 2 ) n Be(NH 3 ) 3 , n = 1–6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H 2 in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H 2 . The biradical character of the system is analyzed, and the singlet–triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH 3 ) 4 complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.
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- Award ID(s):
- 1940456
- PAR ID:
- 10329654
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 156
- Issue:
- 19
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- 194302
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0089815
