Photoinduced Isomerization Kinetics of Diiodomethane in Supercritical Fluid Solution:  Local Density Effects

Abstract

The density dependence of diiodomethane photoinduced isomerization in supercritical (sc) CO2, CHF3, and C2H6 was investigated by transient absorption spectroscopy, covering a fluid density range from 0.7 to 2.5 (in reduced units). The solvent-caged photoproduct iso-diiodomethane is formed even at the lowest density, and its yield increases about 4-fold over the whole range. At the same time, isomer formation rate constants increase by roughly an order of magnitude and show little variation between CO2, C2H6, and CHF3. Furthermore, the formation rate constant decreases significantly with increasing excitation energy. We propose an isomer formation mechanism involving a rapidly established preequilibrium between a solvent-caged iodine atom-methyliodide radical pair and a loosely bound iodine-methyliodide radical complex, from which the reaction subsequently proceeds to the isomer. The latter step seems to be controlled by collisional stabilization of the initially hot radical moiety, as the formation rate constant increases linearly with sc solvent viscosity. The model predicts a quadratic dependence of relative isomer yield on fluid density. A corresponding correlation is found with the local fluid density, calculated via solute-solvent radial distribution functions obtained from molecular dynamics (MD) simulations.

Characterization of iso-CF2I2 in frequency and ultrafast time domains

“…It was previously demonstrated that condensed phase environments promote a new photochemical path leading to the formation of metastable iso-polyhalomethane species. [2][3][4][5][6][7][8][9][10][11][12][13][14] The known functions of these reaction intermediates include the production of ions in the ocean 15 and photocyclopropanation reactions of olefins in solution. [16][17][18] In this regard, one of the most extensively investigated compounds is diiodomethane ͑CH 2 I 2 ͒.…”

Molecular models for 267 binary mixtures validated by vapor–liquid equilibria: A systematic approach

“…Several authors used them as solute models for predictions of the Henry's law constant: Boutard et al [15] for O 2 in ethanol, Krishnamurthy et al [16] for N 2 and O 2 in ethylene oxide and Shah and Maginn [17] for C 2 H 6 and C 2 H 4 in an ionic liquid. Grimm et al [18] used the CH 2 I 2 model to investigate local density effects on photoinduced isomerization kinetics of this substance in supercritical CO 2 . Müller et al [19,20] used several models (C 2 H 6 , C 2 H 4 , N 2 and C 2 F 6 ) for simulations on adsorption regarding micro-porous carbon.…”

Modeling Solvation in Supercritical CO2

“…[71] As equential molecular dynamics (MD) andq uantum mechanics approach was used by Millot et al to compute the UV/Vis spectrum of organic dyes, [72] and more recently Born-Oppenheimer MD simulations were performed by Canuto's group to study the electronic spectrum of para-nitroaniline. [73,74] The organization of the solvent around the solute and its influence on the solution structural and dynamic properties were investigated by means of MD for small solutes, [75][76][77][78][79][80][81][82][83][84] polymers [85,86] and proteins, [87] and passivated gold nanoparticles. [88] Some work based on Car-Parrinello MD was also performed.…”