An MRD-CI study of the electronic spectrum of Si3C3

2008

Inorg. Chem.

The aromatic, bonding, and structural characteristics of the Si 4C 2H 2-C 2B 4H 6, Si 2C 4H 4-C 4B 2H 6, and other Si n C 2H 2-C 2B n H n+2 ( n = 1, 2, 3, 5) isovalent pairs are studied using density functional theory (DFT) and coupled cluster methods to fully illustrate the homology of the two species. This homology, which is based on the replacement of the carborane B-H units by isovalent Si atoms, is extended to all three characteristics (structural, electronic, and aromatic) and includes all three lowest-energy structures of the isovalent pairs. This type of "boron connection", which has been tested for silicon clusters recently, seems to be a valid and extremely useful concept. For the aromatic properties of the Si n C 2H 2-C 2B n H n+2 species, expressed through the nucleus independent chemical shifts (NICS), a strange odd-even effect with respect to the number of Si atoms is observed which seems rather difficult to explain. To help possible future identification and characterization of the Si n C 2H 2 clusters, their infrared, Raman, and optical excitation spectra are calculated within the framework of DFT, using the 6-311+G(2d, p) basis set. It is expected that the present results would facilitate the exploitation of the well-known carborane and metallacarborane chemical properties and applications for the design and development of novel silicon-carbon-based composite materials.

Section: Results and Discussion For The Si N C2h2−c2b N H N+2 N = 1−5...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

The Boron Connection: A Parallel Description of Aromatic, Bonding, and Structural Characteristics of Hydrogenated Silicon−Carbon Clusters and Isovalent Carboranes

2008

Inorg. Chem.

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Their investigation allow, among others, further understanding of the differences between carbon and silicon elemental clusters and the bonds they form. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Their investigation allow, among others, further understanding of the differences between carbon and silicon elemental clusters and the bonds they form.…”

Section: Introductionmentioning

confidence: 99%

A new class of silicon-carbon clusters: A full study of the hydrogenated SinC2H2, n=3,4,5, clusters in comparison with their isoelectronic carboranes C2BnHn+2

The Journal of Chemical Physics

2008

The structural and electronic characteristics of the Si(n)C(2)H(2), n=3,4,5, clusters are studied by ab initio calculations based on coupled cluster and density functional theory using the hybrid B3LYP functional. It is demonstrated that all three clusters are structurally and electronically homologous to the corresponding isoelectronic organometallic carboranes C(2)B(n)H(n+2). This homology, which is in full agreement with the analogy of Si(6) (2-) and B(6)H(6) (2-) demonstrated recently by the author [J. Chem. Phys. 127, 014314 (2007)], includes not only the ground states but also the lower-lying isomers as well. These lowest lying isomers can be obtained by ortho, para, and meta substitutions from the corresponding Si(n) (2-), n=3,4,5, dianions. The energetic ordering of the low-lying isomers is in full agreement with the known valence and topological charge stability rules developed for carboranes. The hydrogenated clusters are much more stable than their nonhydrogenated counterparts. It is suggested that Si(3)C(2)H(2), Si(4)C(2)H(2), and Si(5)C(2)H(2), which can be probably found in interstellar space, are special examples of a general class of silicon-carbon clusters of the form Si(n)C(2)H(2), with analogous properties and similarities to the corresponding carboranes C(2)B(n)H(n+2). It is furthermore illustrated that the lowest energy structures of the Si(n)C(2) clusters can be obtained through a systematic and straightforward procedure from the Si(n)C(2)H(2) clusters. The present results could hopefully make possible the exploitation of the rich borane and carborane chemistry for the design and development of novel silicon and silicon-carbon composite nanomaterials.

Success and pitfalls of the Sin−2C2H2–C2Bn−2Hn isolobal analogy: Depth and breadth of the boron connection

The Journal of Chemical Physics

2009

The extent and depth of the so-called boron connection suggested recently by the present author [J. Chem. Phys. 128, 184305 (2008)] for the Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) isovalent species [following similar connection of (Si(n))(2-)-(B(n)H(n))(2-) dianions] are further investigated by considering larger species up to n=12 and additional isovalent moieties. Here we consider, using density functional and coupled clusters theory, isovalent Si(n-2)C(2)H(2) and Ge(n-2)C(2)H(2) clusters, in comparison to the corresponding C(2)B(n-2)H(n) carboranes and to each other for n=3-8, 12. Special attention is given to the n=8 species, where the corresponding carborane is highly fluxional, and to n=12, where the "parent structures" of the corresponding (Si(12))(2-) and (B(12)H(12))(2-) dianions have drastically different symmetries. The structures generated by substitutions on (Si(12))(2-), as well as (B(12)H(12))(2-), are compared and interrelated for both C(2)B(10)H(12) and Si(10)C(2)H(12). The carborane generated from the C(s) (Si(12))(2-) dianion provides an illustrative example of carborane rearrangements, reverting after geometry optimization to the second lowest C(2)B(10)H(12) meta isomer of C(2v) symmetry, 0.12 eV above the lowest energy para isomer. This demonstrates the amphidirectional character of the boron connection. It is found that n=8 is the upper limit of the n range in which the isolobal Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) analogy is fully operative not only for the lowest but also for the second and third lowest energy states. For n8) the boron connection, although not fully isolobal, is still valid in a broader and more general sense, still providing deeper and broader fundamental understanding and insight for both species. It was also found that the Ge(n-2)C(2)H(2) clusters are not fully homologous (isolobal) neither to the isovalent Si(n-2)C(2)H(2) clusters nor to the corresponding C(2)B(n-2)H(n) carboranes, preferring structures in which the two carbon atoms are always in adjacent positions. This is attributed to the relative weakness of the Ge-Ge and Ge-C bonds relative to Si-Si and Si-C bonds and the "inert pair effect."