Affiliation:
1. Center for Computational Quantum Chemistry, Computational Chemistry Annex, University of Georgia, Athens, GA 30602–2525, USA.
Abstract
Density-functional theory calculations predict that three borocarbon units with planar pentacoordinate carbons −C
3
B
3
−, −C
2
B
4
−, and −CB
5
−, can replace the −(CH)
3
− subunits in aromatic or even in antiaromatic hydrocarbons to construct “hyparenes” (families of molecules with planar pentacoordinate carbons). These borocarbon units contribute two, one, and zero electrons, respectively, to the parent pi system. Depending on the choice of these units, the hyparenes (judging from computed proton and nucleus-independent chemical shifts), can maintain or can interconvert the aromatic or antiaromatic character of the parent compounds. The hyparenes are low-lying local minima with normal carbon-boron, boron-boron, and carbon-carbon bond lengths. The multicenter bonding in the hyparenes involves contributions of partial sigma and partial pi bonds to the planar pentacoordinate carbons; the octet rule is not violated. Borocarbon species, for which there is some mass spectrometric evidence, might be observed and identified, for example, in matrix isolation by vibrational spectroscopy.
Publisher
American Association for the Advancement of Science (AAAS)
Reference37 articles.
1. J. H. van't Hoff Arch. Neerl. Sci. Exactes Nat. 445 (1874). For a reproduction of this paper see
2. For J. H. van't Hoff's booklet The Arrangement of Atoms in Space see www.chem.yale.edu/∼chem125/125/history/vanthoff/tetrahedra.html
3. Web figures 1 through 4 and table 1 are available at Science Online at www.sciencemag.org/cgi/content/full/292/5526/2465/DC1.
4. All structures were first optimized at B3LYP/6-31G* and were shown to be minima by frequency calculations and to have stable wavefunctions The geometries then were refined at B3LYP/6-311+G**. Correlated ab initio methods (CCSD(T)/6-311+G** for 1 to 3 MP2/6-31G* and MP2/6-311+G** for 1 to 6 ) were employed to reoptimize some of the geometries as further checks. Frequency calculations at MP2/6-311+G** for 1 to 3 and MP2/6-31G* for 1 to 6 further confirm they are minima [Web fig. 2 (3)]. All calculations were carried out using Gaussian G98 (M. J. Frisch et al. Gaussian Pittsburgh PA 1998).
5. Typical reference bond length at B3LYP/6-311+G** are: r CC = 1.531 Å in ethane 1.396 Å in benzene and 1.329 Å in ethene; r CB = 1.554 Å in CH 3 BH 2 and 1.396 Å in CH 2 BH; and r BB = 1.629 Å in D 2d H 2 BBH 2 .
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