Molecular Electrides

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Electride.png

INTRODUCTION[edit]

--Eduard (talk) 17:13, 17 August 2017 (CEST)

Electrides are among the most intriguing materials lately discovered. These ionic compounds have electrons occupying the anionic positions of solid compounds. Electrons in electrides act as separated individual entities, constituting the smallest possible anions in a molecule (actually these anions’ mass is 1820 times smaller than the smallest anion (H – ) reported thus far). All materials present defects at any temperature, due to misalignments or absences of atoms, the latter giving rise to vacancies that can be occupied by other particles. Farbe centers (from the German word Farben —color), commonly known as F centers, are vacancies occupied by electrons randomly displaced around the solid. The energy in the vacancy can undergo energetic excitations (in the UV-Vis range) that give rise to colors in e.g. gems. Electrides are actually stoichiometric F centers, i.e., isolated electrons are replicated in the solid structure according to the symmetry of the crystal. Therefore, unlike regular F centers, electride’s isolated electrons do feel each other’s presence. Electrides remind of alkaline metal solutions of ammonia, that form gold-blue colored materials that consist of positively charge alkaline metals and free electrons solvated by ammonia molecules. An important difference with electrides is that the latter occurs in the liquid (disordered) state, whereas electrides are solid (ordered) structures.

LIST OF PARTICIPANTS[edit]

Henry Rzepa, Imperial College London[edit]

slides: DOI:b9r9

Eduard Matito (talk) 18:25, 20 July 2017 (CEST)[edit]

Ampendas (talk) 20:04, 24 August 2017 (CEST)[edit]

Maximum probability domains have also been used to characterize molecular electrides. Slides

Jcontreras (talk) 09:05, 28 August 2017 (CEST)[edit]

--Jcontreras (talk) 23:08, 2 September 2017 (CEST) Slides

DISCUSSION[edit]


Henry (talk) 12:26, 19 July 2017 (CEST)[edit]

Computed charge density (eÅ-3) of Na2He at 300 GPa, plotted in the [110] plane of the conventional cell.
Recently the comples Na2He was reported as a stable compound of helium and sodium at high pressure.[1] This compound really does stress-test what the molecular community considers to be a "bond". An analysis of the wavefunction leads to the following (non comprehensive) conclusions

ELF basin centroids for Na2He (pink spheres)

  1. The high applied pressure results in increasing Pauli repulsions between the He and Na atoms,
  2. with the effect that the Na ionises to Na+
  3. The ionised electron now occupies the interstitial cavities of the solid, thus reducing the Pauli repulsions with He or indeed of any electronic reorganisation of He (such as ionisation).
  4. to form a molecular electride.
  5. So does the He actually form any "bond" as such, or is it a compound of He without a bond to He?
  6. To investigate this aspect a discrete molecule (i.e. non-periodic) B3LYP/Def2-SVPD calculation[2] was performed, from which these properties emerged
    • the ionised electrons are located in discrete diatropic ELF basins (population ~ 1.9e)
    • QTAIM analysis reveals that the centroids of the ELF basins correspond to non-nuclear [3,-3] attractors (NNNAs) in the topology[3]). These NNAs are typical of electrides.
    • The calculated Wiberg bond index for the Na is 2.48, whilst that for He is only 0.15.

So the challenge is this:

  1. Can this be a true compound of He, even if no conventional bonds/ionization of any type can be associated with the He atom?
  2. Might examples under ambient pressures exist?

--CarlosMF (talk) 10:03, 7 September 2017 (CEST) In the Charge-shift bonds part, Julia Contreras uploaded an example of an As-He bond under pressure that was revealed through the NCI technique. During the live slam it was mentioned that probably the He in this Na compound are not really bonded, but just happen to be "trapped" by the Na electride that would form also without He... Perhaps it is interesting to confirm/refuse this argument through similar calculations to those used for the arsenolite-He complex under pressure.

Eduard (talk) 17:14, 17 August 2017 (CEST)[edit]

Characterization and Identification of Molecular Electrides[4]

The density due to the free electrons of electrides is not large enough to be located in the X-ray of the crystal structure. As a consequence, the evidence for these species is indirect and it comes from: (i) the similarity of this structure with analogue alkalide (i.e., the cationic structure) (ii) the chemical shift of the corresponding cation (133Cs), (iii) EPR studies, (iv) Density topology and ELF computational studies (v) Atomic-resolution scanning tunneling microscopy.[5]

Electrides show particular magnetic (exalted susceptibilities that correlates with the channel area), chemical (organic synthesis, preparation of nanoscale metal and alloy particles), electric (an ideal electride should be a (Mott*) insulator) and optical properties (low optical spectra peaks as compared to alkali anions; huge nonlinear optical properties (NLOP), like large static first hyperpolarizabilities which make them of high interest due to their potential utilization in optical and optoelectronic devices).

Electrides are ionic compounds thus far occurring in the solid state, where the anionic part is constituted by isolated electrons. We show herein that molecular electrides in the gas phase can exist, provide an unambiguous computational means to characterize and identify these species as well as a recipe to design new electrides. We herein provide an unambiguous computational means to distinguish electrides from similar species, proving the existence of some electrides in the gas phase. We also put forward a recipe to design new electrides.

--Ampendas (talk) 20:03, 24 August 2017 (CEST) I would say that synchrotron facilities should be used to examine the electron density of such systems. They are clearly able to locate hydrogen atoms in organic and inorganic compounds. I am not aware whether this has been attempted or not.

Jcontreras (talk) 09:05, 28 August 2017 (CEST)[edit]

--Julia Contreras Within the world of electrides I believe that turning to solid state holds the clue to understand what new properties the localization of electrons in a core-like manner can bring.

The Electron Localization Function can show how alkali metals become electrides under pressure (Figure 1). Na.png

Valence electrons localize in the interstices bringing extremely astonishing new properties, easily visualized from the new electronic structure (Figure 2). More specifically, it has been (theoretically and experimentally) shown that at pressures around 500GPa become insulators!

K.png

This type of electrides are fundamental in order to develop theories of electrides, since their experimental characterization associated with macroscopic behavior sheds no doubt on their electride nature.

--CarlosMF (talk) 19:36, 2 October 2017 (CEST)[edit]

Within the world of silicon-based materials, a very interesting class of compounds is that of silsesquioxanes (https://en.wikipedia.org/wiki/Silsesquioxane). It was recently called to my attention that, in the simplest octasilsesquioxane (H8Si8O12), the LUMO orbital is located in the center of the cage.

It is known that these cages have a very large σ-hole in the center, and the binding of F can be used as a kind of template effect to build this structures[6].

So I ran some simple (R)B3LYP/6-311+G(d,p) calculations and we can clearly see that in the neutral compound the LUMO is located in the middle of the cage. Also, for the singlet dianion compound, this orbital will now be the HOMO. I assume that this compound could then be considered as an electride? Or is this orbital view too over-simplistic?

Lumo neutral h8si8o12.jpg
Homo dianion h8si8o12.jpg
LUMO of H8Si8O12 HOMO of H8Si8O122−

I am currently exploring what happens by adding two Li atoms to see whether the electrons will jump from the Li atoms into the cage...

Also, it might be interesting to know whether silica-based electrides exist in the solid state for instance, considering that the Si-O-Si motif is present in many solids, such as silicates or zeolites.

RELEVANT REFERENCES[edit]


  1. X. Dong, A.R. Oganov, A.F. Goncharov, E. Stavrou, S. Lobanov, G. Saleh, G. Qian, Q. Zhu, C. Gatti, V.L. Deringer, R. Dronskowski, X. Zhou, V.B. Prakapenka, Z. Konôpková, I.A. Popov, A.I. Boldyrev, and H. Wang, Nature Chemistry, 2017, 9, 440-445, 2017. DOI:10.1038/nchem.2716
  2. For calculation details see DOI:10.14469/hpc/2156
  3. See J. A. Platts, J. Overgaard, C. Jones, B. B. Iversen and A. Stasch, J. Phys. Chem. A, 2011, 115, 194–200. DOI:10.1021/jp109547w for experimental detection of a non-nuclear attractor
  4. Postils V., Garcia-Borràs M., Solà M., Luis J.M., Matito E.; On the existence and characterization of molecular electrides. Chem. Comm. 51, 4865-4868 (2015). DOI:10.1039/C5CC00215J
  5. Y. Toda, Y. Kubota, M. Hirano, H. Hirayama and H. Hosono, ACS nano, 2011, 5, 1907–1914. DOI:10.1021/nn102839k
  6. Bauzá, A.; Mooibroek, T. J.; Frontera, A. Tetrel-bonding: a rediscovered supramolecular force? Angew. Chem. Int. Ed. 52, 12317–12321 (2013). DOI:10.1002/anie.201306501