Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages.

Theoretical investigation of the H(CHB X ) (X=H, F, Cl, CN), H(CHB X Y ) (X,Y=F, Cl; n=1,5), and dimeric (H(CHB X )) (X=F, Cl) carborane superacids performed at the B3LYP/6-311++G(d,p) theory level revealed the similarity of their equilibrium structures and the possibility of nearly barrierless hydrogen atom migration among the substituents attached to one side of the icosahedral CB cage. The vertical electron detachment energies predicted at the OVGF/6-311++G(3df,2pd) theory level for the conjugate bases (CHB X ) were found to span the 5.82-9.00 ev range. The acid strengths (manifested by the Gibbs free deprotonation energies spanning the 213-266 kcal/mol range) predicted for the icosahedral H(CHB X ) carborane systems confirm their superacidic properties which might be increased even further by the attachment of the second carborane H(CHB X ) unit that leads to a dimeric structure mimicking a part of an experimentally observed H-bridged polymeric chain. The Gibbs free deprotonation energy of the dimeric (H(CHB Cl )) acid was predicted to be smaller by 17 kcal/mol than that of the corresponding monomeric H(CHB Cl ) acid.