An FT-IR Study of Crown Ether−Water Complexation in Supercritical CO2

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2003-12, Vol.107 (50), p.11239-11244
• Main Authors: Rustenholtz, Anne, Fulton, John L, Wai, Chien M
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp035798y

In the presence of 18-crown-6, D2O forms a 1:1 complex with the macrocyclic molecule in supercritical fluid CO2 with two different configurations. The D2O molecule can be bonded to two oxygen atoms of the crown cavity in a bridged configuration that is characterized by a broad peak at 2590 cm-1. The D2O molecule can also form one hydrogen bond with an oxygen atom of the crown cavity that can be characterized by two peaks at 2679 and 2733 cm-1, with the former assigned to the hydrogen-bonded O−D stretching and the latter the unbonded O−D stretching. The equilibrium constants of the two configurations in supercritical CO2 have been calculated. The enthalpy of formation is −12 ± 2 kJ mol-1 for the single-hydrogen-bond complex and −38 ± 3 kJ mol-1 for the bridged configuration complex. At high 18-crown-6 to D2O ratios, the formation of another complex in supercritical CO2 that involves one D2O molecule hydrogen bonded to two 18-crown-6 molecules becomes possible.