The Crystal Structure of Carbonic Acid


S. Benz1, D. Chen2,1, A. Möller1, M. Hofmann3, D. Schnieders1, R. Dronskowski1,2


1 Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, D-52056 Aachen, Germany. 2 Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 518055 Shenzhen, China. 3 Research Neutron Source Heinz Maier-Leibnitz (FRM II), Technical University Munich, D-85748 Garching, Germany.



Abstract:

Ubiquitous carbonic acid, H2CO3, a key molecule in biochemistry, geochemistry, and also extraterrestrial chemistry, has been covered in a plethora of physicochemical studies but its sheer existence is not accurately reflected from chemical textbook knowledge, even up to the present day. To provide undisputable proof of carbonic acid\ís existence, its crystal structure has now been determined from neutron-diffraction data on a deuterated sample in a specially built hybrid clamped cell. At 1.85 GPa, D2CO3 crystallizes in the monoclinic space group P21/c with a = 5.392(2), b = 6.661(4), c = 5.690(1) Å, β = 92.66(3) °, Z = 4, with one symmetry-inequivalent anti-anti shaped D2CO3 molecule forming dimers, as previously predicted. The theoretical free energy of formation including zero-point energies is −684 kJ mol−1 at 2 GPa and 300 K. As regards quantum chemistry and chemical bonding, solid-state density-functional theory based on plane waves but unitarily transformed to atomic orbitals evidences π interactions within the CO3 molecular core, the presence of ionic valence-bond mesomeric structures, very strong hydrogen bonding between the molecules, and a massive influence of the crystal field on all bonds; even though the crystal consists of molecular units, its energetics resembles a typical inorganic salt. Theory also suggests other thinkable, energetically even slightly lower polymorphs but there is no experimental evidence as yet. In addition, extended phonon calculations emphasize the locality of the vibrations, being rather insensitive to the extended structure.