The photochemistry of the trimetal cluster, Ru 3 (CO) 12 , was investigated on the ultrafast timescale using UV-vis pump, infrared probe spectroscopy in order to study the transient intermediates formed upon photoexcitation. The dynamics of these intermediates can only be unambiguously identified by monitoring the small but distinct infrared absorptions of bridging carbonyls. The nature and role of bridged carbonyl intermediates in the photochemistry of Ru 3 (CO) 12 in both coordinating and noncoordinating solvents is discussed. In an inert solvent such as cyclohexane, photoexcitation of Ru 3 (CO) 12 with 400 nm light produces two different species that have never been observed simultaneously. The first species is a carbonyl loss complex with a bridged carbonyl that forms in 134 ± 22 ps and survives beyond 800 ps; the second species is a bridged carbonyl complex with one metalmetal bond cleaved that forms in 23 ± 3 ps and has a lifetime of 60 ± 5 ps. When 266 nm light was used to photoexcite the cluster, both species exhibit similar dynamics. This is the first time multiple bridged carbonyl intermediates have been observed simultaneously for this cluster and this observation resolves an inconsistency in the literature. Interestingly, in neat solutions of THF only one feature was observed in the bridging carbonyl region, yet the dynamics of the feature and density functional theory (DFT) results indicate that there are, in fact, two bridging carbonyl complexes with overlapping bands. These results were surprising as it was previously unknown whether THF would block formation of bridging carbonyl complexes by solvating and stabilizing the coordinatively unsaturated metal.

The dynamic and equilibrium water vapor sorption properties of Sylgard-184, a commercially available poly(dimethylsiloxane) elastomer (PDMS), were determined via gravimetric analysis from 30 to 70 °C. Described here is a methodology for quantitatively assessing how water vapor diffuses and ad/absorbs into polymeric materials that are traditionally considered hydrophobic. PDMS materials are frequently chosen for their moisture barrier properties; our results, however, demonstrate that moisture is able to penetrate the material over a range of temperatures and humidities. The sorption values measured here ranged from ca. 0.1 to 1.4 cm(3) (STP) H(2)O/g Sylgard. The isotherms exhibited sigmoidal character and were fit to a triple mode sorption model. Asymptotic behavior at low water activities was characterized using a Langmuir type adsorption model, linear behavior was fit to a Henry's law type dependence, and the convex portion at higher activities was fit with good agreement to Park's equation for pooling or clustering. The thermal dependence of these sorption modes was also explored and reported. The dynamics of the sorption process were fit to a Fickian model and effective diffusivities are reported along with corresponding activation energies. The diffusivity values measured here ranged from ca. 0.5 to 3.5 × 10(-5) cm(2)/s depending on the temperature and relative humidity. The concentration dependence of the diffusivity showed a direct correlation with the three modes of uptake obtained from the isotherms. Corrections to the diffusivities were calculated using existing models that take into account adsorption and pooling.

ABSTRACT.

A high-fidelity model of kinetic and equilibrium sorption and diffusion is developed and exercised. The gas-diffusion model is coupled with a triple-sorption mechanism: Henry's law absorption, Langmuir adsorption, and pooling or clustering of molecules at higher partial pressures. Sorption experiments are conducted and span a range of relative humidities (0-95 %) and temperatures (30-60 °C). Kinetic and equilibrium sorption properties and effective diffusivity are determined by minimizing the absolute difference between measured and modeled uptakes. Uncertainty quantification and sensitivity analysis methods are described and exercised herein to demonstrate the capability of this modeling approach. Water uptake in silica-filled and unfilled poly(dimethylsiloxane) networks is investigated; however, the model is versatile enough to be used with a wide range of materials and vapors.