Theoretical Studies for the Supercritical CO2 Solubility of Organophosphorous Molecules: Lewis Acid-Base Interactions and C?H?|O Weak Hydrogen Bonding

摘要

Exploring the basic concepts for the design of CO₂-philic molecules is important due to the possibility for “green” chemistry in supercritical CO₂ as substitute solvent systems. The Lewis acid-base interactions and C?HˇO weak hydrogen bonding were suggested as two key factors for the solubility of CO₂-philic molecules. We have performed high level quantum mechanical calculations for the van der Waals complexes of CO₂ with trimethylphosphate and trimethylphosphine oxide, which have long been used for metal extractants in supercritical CO2 fluid. Structures and energies were calculated using the MP2/6-31+G(d) and recently developed multilevel methods. These studies indicate that the Lewis acid-base interactions have larger impact on the stability of structure than the C?HˇO weak hydrogen bonding. The weak hydrogen bonds in trimethylphosphine oxide have an important role to the large supercritical CO2 solubility when a metal is bound to the oxygen atom of the P=O group. Trimethylphosphate has many Lewis acid-base interaction sites so that it can be dissolved into supercritical CO₂ easily even when it has metal ion on the oxygen atom of the P=O group, which is indispensable for a good extractant.