The Structure of the Strongest Brønsted Acid:  The Carborane Acid H(CHB11Cl11)

The strongest and most robust carborane acid, H(CHB11Cl11), has a monomeric structure in the gas phase. IR spectra show two νH−Cl bands at 2357(br) and 2066(br) cm-1 which, together with DFT calculations, indicate the coexistence of at least two isomers. The acidic proton bridges adjacent chlorine atoms with asymmetric Cl−H···Cl hydrogen bonding. The 12,7 isomer is more stable than the 7,8 isomer. These monomers can be condensed into an amorphous solid phase but are metastable. They quickly decay, first to an amorphous dimeric structure, then to a crystalline polymeric phase that has been characterized by X-ray crystallography. In the polymeric structure, the acidic proton bridges chlorine atoms from the 7−11 positions of carborane anions in linear chains. The dimeric phase (νCl−H···Cl = 1100−2200 cm-1) and polymeric phase (νasClHCl ca. 1100 cm-1, v broad) have more nearly symmetrical, low-barrier H-bonding. These findings have implications for the dependency of acid strength upon phase.