Rotational Diffusion and Trans-cis Isomerization of Substituted Diphenyl Polyenes in the Compressible Region of Supercritical Fluids.

Abstract

We examine rotational diffusion and trans-cis isomerization of two Biphenyl polyenes, diphenylbutadiene (DPB) and hydroxymethyl-stilbene (HMS) in supercritical fluid CO2 and ethane. The results of density dependent rotational diffusion measurements are interpreted using a model which we have recently developed for rotational diffusion in compressible fluids. Comparison of DPB and HMS fluorescence lifetimes indicate that the rate of the isomerization reaction of DPB is nearly independent of density while HMS exhibits a strong density dependence in CO2. Analysis of the HMS data using Kramers theory indicates that the density dependence is the result of solvent-solute frictional interactions.

Quadrupolar Solvent Effects on Solvation and Reactivity of Solutes Dissolved in Supercritical CO2

“…A comparison of HMS and DPB in supercritical CO 2 demonstrates the influence of a polar functional group on solvent−solute friction. A comparison of HMS in supercritical ethane and CO 2 (Figure ) emphasizes the specificity of the polar solute interaction with CO 2 . Interestingly, the HMS−ethane results exhibit the same modest increase with increasing density that we have observed in the DPB−CO 2 system.…”

Solvation and Friction in Supercritical Fluids: Simulation−Experiment Comparisons in Diphenyl Polyene/CO2Systems

“…First, the free rotor times used in their analysis are seen to substantially overestimate the true free rotor times of these solutes. As noted by Anderton and Kauffman, , within their modeling scheme, changes in these times have a dramatic impact on the extent of density augmentation deduced from rotation times. If the values of the free rotor times determined here (2.1 ps for DPB and 1.9 ps for HMS) were employed, significant and comparable density augmentation in the DPB/CO 2 and HMS/CO 2 systems would be implied by their analysis.…”

“…They concluded that the local density in the vicinity of DPB differs little from that of the bulk fluid, whereas the local solvent density around HMS is ∼40% greater than the bulk solvent density. By analogy to the pronounced differences in rotational times observed for HMS in liquid alkane versus alcohol solvents, Kauffmann and co-workers attributed this difference in augmentation to the ability of HMS to hydrogen bond to CO 2 . ,, …”

“…For this reason, Kauffmann and co-workers have performed parallel studies of both solutes in a number of liquid solvents. , As a result of these and other studies, the liquid-state rotational behavior of DPB and HMS are reasonably well established. Finally, we note that measurement of rotation times in these solutes is facilitated by the fact that both of them undergo rapid excited-state isomerizations in liquid solvents − and in supercritical fluids. ,− Although we do not address this reactive dynamics in the present work, understanding the rotational friction on these solutes will be useful in future studies of their isomerization reactions.

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“…Although this model has proven successful in explaining the rotation of molecules such as DPB in some liquid solvents ,− it is far from universally applicable, and it would not be surprising to see it yield inaccurate predictions at supercritical densities. In addition, as already discussed by Kauffman, , relating rotation times to solvent friction also depends sensitively on how the free rotor limit is treated, and rather different conclusions concerning the extent of density augmentation can be drawn from the same data depending on one's approach . Given these uncertainties, simulations are valuable both for examining the extent of augmentation in these systems as well as for providing guidance on how to interpret rotation times in terms of solvent friction.…”