Construction Principles of "Hyparenes": Families of Molecules with Planar Pentacoordinate Carbons

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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.