Vibrational Spectroscopy and Computational Studies of Cubane-1,4-Dicarboxylic Acid

Cubane-1,4-dicarboxylic acid is a key intermediate in the synthesis of the Platonic solid, cubane. While cubane has been extensively studied, its precursor has not. Here, we provide a comprehensive characterization of the vibrational spectra (infrared, Raman, inelastic neutron scattering (INS)) of cubane-1,4-dicarboxylic acid and its isotopomer with the acidic hydrogens exchanged for deuterium. In combination with density functional theory studies of the complete unit cell, we show that the dynamics of the carboxylic acid and the cubane core are largely independent; the effect is mostly the result of the increased mass of the substituent at the 1,4 positions: 45 versus 1. The known crystal structure is unusual in that the carboxylic acid is present as two conformers: syn and anti. The calculations show that the in-plane and out-of-plane C-O-H bending modes have different transition energies in the two conformers. For all the other modes, both conformers contribute approximately equally.