Simulations of rotational dynamics and electronic spectroscopy in supercritical fluids.

摘要

Molecular dynamics simulations of diphenylbutadiene (DPB) and hydroxymethylstilbene (HMS) in supercritical CO₂ were performed in order to understand the role of solute-solvent interactions in determining solvation structure and rotational dynamics in supercritical solvents. A characteristic feature of solvation in supercritical solvents is the buildup of solvent density in the neighborhood of the solute—a phenomenon known as local density augmentation. Effects of density augmentation can be found in many solute-centered observables such as electronic spectral shifts and solvent-induced friction. Experimental measures of local densities derived from absorption shifts were compared to simulated shifts based on two simple models. Although neither accurately reproduced the magnitude of the absorption shifts measured in experiment, the density dependence of the simulated shifts was close to experiment. The augmentation deduced from these experimental data was close to the found in simulation. The simulations slightly underestimated the extent of density augmentation seen in experiment, following the pattern observed in prior studies. Following the examination of the solvation structure, the rotational dynamics of both solutes were analyzed and compared with two experimental studies performed by different researchers. Whereas both sets of experimental data showed a linear relationship between the rotation times of DPB and the bulk density, the data sets reported quantitatively different density dependent behaviors of the HMS rotation times. Succinctly, although both data sets implied that the rotation times of DPB were ignorant of the local density augmentation, they didn't agree on whether or not the same was true for HMS. In order to try to understand these differences, the rotational friction was examined in simulation, as it characterizes the observed rotation times. Surprisingly, it was found that integral friction was linear in the bulk density, and therefore, was ignorant of the local density augmentation. However, a subsequent detailed examination of the time-dependent rotational friction in simulation showed why this is the case.; In order to obtain more quantitative agreement between simulated and experimental spectral shifts, simulation studies of anthracene in a series of representative liquid solvents were performed. Anthracene was chosen as it represents a simple case, where the solute-solvent interactions are dominated by dispersion interactions. (Abstract shortened by UMI.)