2 research outputs found
Multicenter Bonding in Ditetracyanoethylene Dianion: A Simple Aromatic Picture in Terms of Three-Electron Bonds
The nature of the multicenter, long
bond in ditetracyanoethylene
dianion complex [TCNE]<sub>2</sub><sup>2–</sup> is elucidated
using high level <i>ab initio</i> Valence Bond (VB) theory
coupled with Quantum Monte Carlo (QMC) methods. This dimer is the
prototype of the general family of pancake-bonded dimers with large
interplanar separations. Quantitative results obtained with a compact
wave function in terms of only six VB structures match the reference
CCSDÂ(T) bonding energies. Analysis of the VB wave function shows that
the weights of the VB structures are not compatible with a covalent
bond between the π* orbitals of the fragments. On the other
hand, these weights are consistent with a simple picture in terms
of two resonating bonding schemes, one displaying a pair of interfragment
three-electron σ bonds and the other displaying intrafragment
three-electron π bonds. This simple picture explains at once
(1) the long interfragment bond length, which is independent of the
countercations but typical of three-electron (3-e) CC σ bonds,
(2) the interfragment orbital overlaps which are very close to the
theoretical optimal overlap of 1/6 for a 3-e σ bond, and (3)
the unusual importance of dynamic correlation, which is precisely
the main bonding component of 3-e bonds. Moreover, it is shown that
the [TCNE]<sub>2</sub><sup>2–</sup> system is topologically
equivalent to the square C<sub>4</sub>H<sub>4</sub><sup>2–</sup> dianion, a well-established aromatic system. To better understand
the role of the cyano substituents, the unsubstituted diethylenic
Na<sup>+</sup><sub>2</sub>[C<sub>2</sub>H<sub>4</sub>]<sub>2</sub><sup>2–</sup> complex is studied and shown to be only metastable
and topologically equivalent to a rectangular C<sub>4</sub>H<sub>4</sub><sup>2–</sup> dianion, devoid of aromaticity
Exploring Lanthanide Doping in UiO-66: A Combined Experimental and Computational Study of the Electronic Structure
Lanthanide-based
metal–organic frameworks show very limited
stabilities, which impedes their use in applications exploiting their
extraordinary electronic properties, such as luminescence and photocatalysis.
This study demonstrates a fast and easy microwave procedure to dope
UiO-66, an exceptionally stable and tunable Zr-based metal–organic
framework. The generally applicable synthesis methodology is used
to incorporate different transition metal and lanthanide ions. Selected
experiments on these newly synthesized materials allow us to construct
an energy scheme of lanthanide energy levels with respect to the UiO-66
host. The model is confirmed via absolute intensity measurements and
provides an intuitive way to understand charge transfer mechanisms
in these doped UiO-66 materials. Density functional theory calculations
on a subset of materials moreover improve our understanding of the
electronic changes in doped UiO-66 and corroborate our empirical model