Influence of Local Structural Correlations on Free-Radical Reactions in Supercritical Fluids:  A Hierarchical Approach

Abstract

We present a general methodology that can be used to study the structural effects of supercritical solvents on fast reactions. Using a hierarchy of theoretical, computational, and spectroscopic methods, we investigate the effect of local solvent structure on a model free-radical reaction. Our results from theory and experiments indicate the existence of locally high solvent densities around reactants at subcritical bulk densities. By comparing simulations and experiments, we show how these local structural correlations enhance the rate constant for Heisenberg spin exchange in supercritical fluids.

Solvent Density Inhomogeneities in Supercritical Fluids

“…Carlier and Randolph 45 used electron paramagnetic resonance (EPR) to examine the hyperfine constant of di-tert-butyl nitroxide in SC ethane at T r ≈ 1.01 (Figure 6a) and 1.08. Later, Ganapathy et al 89 extended this work to SC CHF 3 and CO 2 , obtaining much the same behavior as in the earlier work, 45 discussed below. The theory of McRae, 85 eq 1, becomes applicable to the hyperfine splitting constant given an appropriate choice of the constants A, B, and C. Carlier, Randolph, and co-workers thus used a comparison of the measured splitting constants with those predicted by this theory on the basis of the bulk solvent properties (as outlined in section II) to extract the bulk density dependence of the local density enhancements, Figure 6b.…”

“…However, it must be noted that the experimental data extends only down to F r ) 0.5, whereas the maximum in F ˜l observed from simulation occurs at F r ≈ 0.3. In contrast, Randolph, O'Brien and co-workers 62,89,182 compared local density enhancements extracted from spectral shifts of the EPR hyperfine splitting constant for DTBN 62 in SC CO 2 , ethane (Figure 18), and CHF 3 , with those computed from simulations using appropriate Lennard-Jones parameters. In all cases, the EPR results substantially exceeded the simulated values of F ˜l for the same reduced temperature isotherm, with maximum EPR values attaining 200% to 500% enhancements and the simulated values attaining enhancements of only 20% to 50%.…”

“…In all cases, the EPR results substantially exceeded the simulated values of F ˜l for the same reduced temperature isotherm, with maximum EPR values attaining 200% to 500% enhancements and the simulated values attaining enhancements of only 20% to 50%. While most of the simulations used a cutoff radius including approximately two solvent shells, 89 use of a smaller radius incorporating just the first shell only brought the simulation value up to a 70% enhancement, Figure 18. 62 Additionally, the experimental F ˜l (F r ) curves exhibit sharp maxima falling in the range 0.35 < F r < 0.6, while the simulation curves are all relatively flat at low reduced densities, with no discernible peak.…”

The world as viewed by and with unpaired electrons

“…To provide a glimpse into the fascinating breadth of applications of EPR, we list additional areas with a leading reference, usually a book or other review: atmospheric pollution [187,188], carbon chars [189–191], catalysis [192,193], electrochemistry [194–196], even spin systems [112], free radical reactions [197–199], fullerenes [200], gas phase radicals [201–203], gemstones [204–206], magnetic field effects on reactions [207], molecular magnets [208], magnetic resonance force microscopy [209], multiphoton transitions [210], optically detected magnetic resonance (ODMR) [211–213], polymer chemistry [136,214], quantum dots [215,216], quantum computing [217], radiation damage and color centers [218] [219], sculpture limestone [220,221], hydrogenated silicon [222], spin exchange [208,223], spintronics [224], and supercritical fluids [225]. …”

Solute Diffusion into Polymer Swollen by Supercritical CO2 by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography

“…EPR makes it possible to evaluate the mobility of a probe on multiple spatial scales [ 50 ]: rotational mobility is readily obtained from the shape of the EPR spectrum, molecular-scale translational mobility (<10 nm path) can be obtained by exchange interaction rate measurement, and, finally, EPR imaging and probe uptake measurement deliver macroscopic diffusivity. High-pressure/in situ EPR spectroscopy uses equipment similar to that of high-pressure NMR [ 51 ], and has been used so far to evaluate density enhancement and exchange reactions in SCFs [ 52 , 53 , 54 , 55 , 56 , 57 ], clustering of the solute in SCFs [ 58 , 59 , 60 , 61 , 62 ], microemulsion formation [ 63 , 64 ], and glass transition pressure in polylactide and polylactide-co-glycolide subjected to subcritical CO 2 [ 48 ]. In addition, it has been reported that the mobility of spin probes can reflect the distribution of free volume in polymers plasticized by subcritical CO 2 , similar to a conventional plasticizer [ 49 ].…”