Using a variety of theoretical methods, such as fluid model computations, molecular dynamics simulations and ab initio calculations, we showed that it is possible to obtain aromatic behavior with simple, but extremely stable hydrogenated silicon molecules, without the need of multiple bonds - and therefore without the incorporation of bulky molecules of substitution. In this sense, we are following a completely new approach that seems to be in opposition to present strategies for achieving silicon aromaticity.
Due to its aromatic character, the discovery of benzene has certainly revolutionized the whole development of organic chemistry - both for the fundamental understanding of life at the biochemical level as for the manufacturing of industrial and pharmaceutical products. Therefore, scientists have been searching for a similar, but silicon-based molecule for more than a century to imitate the aromaticity of benzene since silicon is in the same chemical group as carbon. To pursue this goal, researchers have thus explored the formation of multiple bonds between silicon atoms. So far, however, the formation of those multiple bonds has only been possible if the hydrogen atoms were replaced by large and complex stabilization substitutes. Unfortunately, the resulting molecules become so cumbersome that it appears impossible to use them as "building blocks" for the construction of a new set of molecules that is as rich and various as the counterpart in organic chemistry.
As an alternative approach, we have therefore exploited the natural tendency of silicon to overcoordination for the construction of molecules with electron-deficient hexagonal ring geometries. More specifically, we added a central atom in silicon molecules composed of atoms having already four neighbors each.
Figure 1: Although the addition of a silicon atom in the center of one hexagon gives rise to an unstable molecule (a), we find a quasi-stable structure at low temperatures when the atom is inserted between two hexagons (b) and an extremely stable nanostructure (even at temperatures exceeding 1200K) when three hexagons are used (c).
The resulting structures are more stable than any other known silicon nanocrystals and exhibit strongly aromatic properties because of extensive electron delocalization.
Figure 2: Cyclic electron delocalization between the three hexagons and the central atom is at the origin of the high stability of the proposed nanostructure; the indicated stabilization energies are given in kcal / mol.
Since aromaticity is related to induced ring currents, magnetic properties are particularly important for its detection and evaluation. Among several indexes, nucleus-independent chemical shifts (NICS) have become the most widely used aromaticity probe due to its simplicity and efficiency. In Figure 3, we display the NICS values for the proposed Si19H12 cluster evaluated in two perpendicular directions. Since σ contributions usually fall off faster than π ones normal to a ring, we show in Figure 3(a) a scan of NICS values determined along the longitudinal direction in the center of the nanoparticle. As can be seen, the shielding never drops below the maximum values of benzene over the entire length inside of the finite nanotube. Even outside of the nanotube, the shielding diminishes surprisingly slowly leading to NICS(1) values of -7.1 ppm and -9.4 ppm at a distance of 1Å below (see Figure 3b) and above (see Figure 3d) the external hexagons of the finite nanotube, respectively. In the orthogonal direction, each of the three hexagons exhibits a magnetic shielding that also largely exceeds the one of benzene which is coherent with the extremely high stabilization energy of about 5.1 eV. As for benzene, the shielding has a local minimum in the center of both external hexagons which can be understood by the fact that the principal delocalization routes pass trough the twelve bonds connecting the external hexagons to the central one.
Figure 3: Chemical shifts determined in two perpendicular directions. This quantity is regularly employed to evaluate the degree of aromaticity. The corresponding maximum value for benzene is only about -10 ppm. Therefore, the nanocrystal proposed here appears to be more aromatic than the best known aromatic representative in organic chemistry.
As shown above, the resulting optical and electronic characteristics are comparable to those currently
obtained only by nanostructures containing expensive or toxic metal atoms.
Figure 4: Adding a central silicon atom increases the stability by 5.1eV although this value should only be around 3.8eV for clusters of this size and 4.6eV for crystalline bulk silicon. This stability is the result of the highly aromatic character induced by the central atom. Besides, the non-toxicity of the proposed nanocrystal, it exhibits outstanding optical properties in a very wide range of wavelengths: Until now, the incorporation of metal atoms, such as cadmium and lead, was necessary in order to make such a small nanostructure absorb light in the visible and infrared.
In summary, we propose for the first time the concept of electron delocalization in electron-deficient hydrogenated silicon nanocrystals leading to aromatic-like behavior. This novel phenomenon seems to be inherently connected to the unique intermediate position that nanoscale objects take between structures of molecular and microscopic dimensions. Because of the absence of formal multiple bonds, we define “electron-deficiency aromaticity” as a chemical property in which the addition of one atom to an already saturated ring structure causes an increase of stability that exceeds the corresponding cohesive energy as a result of the induced electron delocalization.
Electron-Deficiency Aromaticity in Silicon Nanoclusters
J. Chem. Theory Comput. 8
, 2088-2094 (2012).See also :
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