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]


  1. Henry Rzepa (talk) 08:17, 19 July 2017 (ST)
  2. Eduard Matito (talk) 18:25, 20 July 2017 (CEST)

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?

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

Characterization and Identification of Molecular Electrides

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.

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.

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