Theoretical studies of CO₂(H₂O)_20,24,28 clusters: Stabilization of cages in hydrates by CO₂ guest molecules

Theoretical studies on dodecahedral (20-mer), tetrakaidecahedral (24-mer) and hexakaidecahedral (28-mer) water clusters containing CO₂ guest molecules are carried out by optimizing geometry at the Hartree-Fock (HF) level with 6-31G^* basis set followed by single point energy calculations with 6-311++G** basis set and applying the Becke-3-parameter density functional theory (DFT) and Lee-Yang-Parr correlation functional (B3LYP). While the filled tetrakaidecahedral and hexakaidecahedral cage clusters are stabilized by 7.79 and 3.42 kcal/mol, respectively (relative to unfilled cage and separated CO₂ molecule, SEC), the filled dodecahedral cage shows no such stabilization. The largest SEC value for tetrakaidecahedral cluster, resulting from two H-bonds between the guest and the host, explains the dominance of CO₂-filled tetrakaidecahedral structures in hydrates.