Laser photolysis study in supercritical fluids

A high-pressure optical cell was designed that achieves an effective separation between the chemical sample and the pressurizing medium and system. This design limits possible contamination and catalytic effects under supercritical fluid sample conditions. Laser flash photolysis experiments were carried out with molybdenum hexacarbonyl dissolved in supercritical fluid CO2 which clearly demonstrated the effectiveness of this new design for kinetic investigations in supercritical fluids.; Ring-closure reaction of W(CO)5L (L = 1,10-phenanthroline) was studied in supercritical fluid ethane and CO2, and its large activation volume (as high as 7000 cm3/mol) was used to interpret the extremely large pressure effect on the rate constant in a highly compressible near-critical supercritical state. The van der Waals model used this work was found to be much more suitable than a conventional model used to described liquid solvents. The results demonstrate that the repulsive (or intrinsic) part of the activation volume can be thousands of cm3/mol even when the attractive contribution to the activation volume is small. Similar ring-closure reactions were also investigated in liquid CO2 and in several conventional organic solvents. The van der Waals model was also used to interpret the enhancement of the repulsive part of the activation volume reported in a supercritical fluid with a co-solvent, thus demonstrating a direct link between the activation volume and the compressibility of the reaction medium. This study also demonstrated the solubility enhancement of the reaction species resulting from adding small amounts of co-solvent.; Myoglobin was encapsulated in a sol-gel material for spectroscopic study in supercritical fluid CO2. The Soret absorption spectrum of carbondioxy-myoglobin, obtained by immersion of sol-gel-immobilized ferrimyoglobin in supercritical CO2, displays a 10 nm blue shift. This is attributed to the existence of the CO2 coordinated heme as an axial ligand observed here for the first time. Laser photolysis (532 nm) of this adduct results in release of bound CO2, followed by a relatively slow process with a rate of 321 +/- 11 s--1, probably ascribable to a gated ligand reentry into the distal pocket of metMb.