Construction principles of "hyparenes": Families of molecules with planar pentacoordinate carbons

Density-functional theory calculations predict thai. three borocarbon units with planar pentacoordinate carbons - C3B3-, - C2B4-, and - CB5-, can repl ace the -(CH)(3)- subunits in aromatic or even in antiaromatic hydrocarbons to construct "hyparenes" (families of molecules with planar pentacoordinat e carbons). These borocarbon units contribute two, one, and zero electrons, respectively, to the parent pi system. Depending on the choice of these un its, the hyparenes (judging from computed proton and nucleus-independent ch emical shifts), can maintain or can interconvert the aromatic or antiaromat ic character of the parent compounds. The hyparenes are low-lying Local min ima 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 spe ctrometric evidence, might be observed and identified, for example, in matr ix isolation by vibrational spectroscopy.