“…In contrast, one observes a pronounced dependence on probe wavelength which is caused by the spectral narrowing already mentioned. The shape of the pressure dependence, however, does not change, indicating that the process involved in isomer formation might be closely related to vibrational energy relaxation, as has been suggested before. ,,− 6 Pressure dependence of the inverse absorbance rise times listed in Table . (□), sc CO 2 405 nm probe; (×), sc CO 2 370 nm probe; (▴), sc CHF 3 410 nm probe; (○), sc C 2 H 6 405 nm probe.…”
Section: Resultsmentioning
confidence: 65%
“…In this case, in-cage dynamics might be affected by internal energy of radical fragments, intramolecular multidimensional dynamics, or possibly competition between different in-cage reactions, vibrational relaxation, and cage escape. Recently, we reported on first steps in this direction by looking at the photochemical kinetics of diiodomethane (CH 2 I 2 ) in sc fluid solution. , This molecule is a promising candidate, because its photodissociation is sufficiently rapid not to be quenched to any large extent in liquid solution ,− and a fast in-cage reaction product was observed by femtosecond transient absorption spectroscopy which could be identified as iso-diiodomethane (CH 2 I−I) . This assignment was confirmed by a combination of time-resolved resonance Raman measurements with quantum-chemical density functional theory (DFT) calculations − and recent picosecond time-resolved X-ray diffraction measurements in methanol solution …”
Section: Introductionmentioning
confidence: 84%
“…We now turn to the density dependence of the rise time τ rise , which directly relates to that of the isomer formation rate, k iso ≃ τ rise - 1 (405 nm). , If we assume that collisional stabilization plays a crucial role in isomer formation 24,28 and that the change of collision rate controlled k f makes the dominant contribution to the observed k iso ( p ) dependence, then one would expect k iso to depend linearly on the collision rate in sc solution, if higher frequency solute modes (>200 cm -1 ) were mainly involved in vibrational cooling of the complexed radical . Such a linear dependence is not observed in our experiments (Figure ).…”
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.
“…In contrast, one observes a pronounced dependence on probe wavelength which is caused by the spectral narrowing already mentioned. The shape of the pressure dependence, however, does not change, indicating that the process involved in isomer formation might be closely related to vibrational energy relaxation, as has been suggested before. ,,− 6 Pressure dependence of the inverse absorbance rise times listed in Table . (□), sc CO 2 405 nm probe; (×), sc CO 2 370 nm probe; (▴), sc CHF 3 410 nm probe; (○), sc C 2 H 6 405 nm probe.…”
Section: Resultsmentioning
confidence: 65%
“…In this case, in-cage dynamics might be affected by internal energy of radical fragments, intramolecular multidimensional dynamics, or possibly competition between different in-cage reactions, vibrational relaxation, and cage escape. Recently, we reported on first steps in this direction by looking at the photochemical kinetics of diiodomethane (CH 2 I 2 ) in sc fluid solution. , This molecule is a promising candidate, because its photodissociation is sufficiently rapid not to be quenched to any large extent in liquid solution ,− and a fast in-cage reaction product was observed by femtosecond transient absorption spectroscopy which could be identified as iso-diiodomethane (CH 2 I−I) . This assignment was confirmed by a combination of time-resolved resonance Raman measurements with quantum-chemical density functional theory (DFT) calculations − and recent picosecond time-resolved X-ray diffraction measurements in methanol solution …”
Section: Introductionmentioning
confidence: 84%
“…We now turn to the density dependence of the rise time τ rise , which directly relates to that of the isomer formation rate, k iso ≃ τ rise - 1 (405 nm). , If we assume that collisional stabilization plays a crucial role in isomer formation 24,28 and that the change of collision rate controlled k f makes the dominant contribution to the observed k iso ( p ) dependence, then one would expect k iso to depend linearly on the collision rate in sc solution, if higher frequency solute modes (>200 cm -1 ) were mainly involved in vibrational cooling of the complexed radical . Such a linear dependence is not observed in our experiments (Figure ).…”
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.
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