Theoretical studies of CO_2(H_2O)20,24,28 clusters: stabilization of cages in hydrates by CO_2 guest molecules

摘要

Theoretical studies on dodecahedral (20-mer), tetrakaidecahedral (24-mer) and hexakaidecahedral (28-mer) water clusters containing CO_2 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 CO_2 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 CO_2-filled tetrakaidecahedral structures in hydrates.