Solvent Density Dependence of Translational Diffusion of Transient Radicals in the Medium-Density Region of Trifluoromethane and Carbon Dioxide

Application of Fokker-Planck-Kramers equation treatment for short-time dynamics of diffusion-controlled reaction in supercritical Lennard-Jones fluids over a wide density range

The validity of a Fokker-Planck-Kramers equation (FPKE) treatment of the rate of diffusion-controlled reaction at short times [K. Ibuki and M. Ueno, J. Chem. Phys. 119, 7054 (2003)] is tested in a supercritical Lennard-Jones fluid over a wide density range by comparing it with the Langevin dynamics and molecular dynamics simulations and other theories. The density n range studied is 0.323n(c)< or =n< or =2.58n(c) and the temperature 1.52T(c), where n(c) and T(c) are the critical density and temperature, respectively. For the rate of bimolecular reactions, the transition between the collision-limited and diffusion-limited regimes is expected to take place in this density range. The simulations show that the rate constant decays with time extensively at high densities, and that the magnitude of decay decreases gradually with decreasing density. The decay profiles of the rate constants obtained by the simulations are reproduced reasonably well by the FPKE treatment in the whole density range studied if a continuous velocity distribution is used in solving the FPKE approximately. If a discontinuous velocity distribution is used instead of the continuous one, the FPKE treatment leads to a rate constant much larger than the simulation results at medium and low densities. The rate constants calculated from the Smoluchowski-Collins-Kimball (SCK) theory based on the diffusion equation are somewhat smaller than the simulation results in medium and low densities when the intrinsic rate constant is chosen to adjust the steady state rate constant in the low density limit to that derived by the kinetic collision theory. The discrepancy is relatively small, so that the SCK theory provides a useful guideline for a qualitative discussion of the density effect on the rate constant.

The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.

Flexible Biradicals in Liquid and Supercritical Carbon Dioxide: The Exchange Interaction, the Chain Dynamics, and a Comparison with Conventional Solvents

X-band time-resolved electron paramagnetic resonance (TREPR) spectra of three flexible biradicals of varying chain length and structure were obtained in liquid and supercritical carbon dioxide (CO2) solutions and compared to conventional solvents. For C16 acyl-alkyl biradical 1a, an average spin exchange interaction between the radical centers, J(avg), was obtained by spectral simulation using a simple model for spin-correlated radical pairs (SCRPs) and a small amount of T2 relaxation from a previously established J modulation mechanism. A large solvent effect on J(avg) was observed for the first time, varying by almost 1 order of magnitude from CO2 (J(avg) = -120 +/- 10 MHz) to heavy mineral oil (-11 +/- 3 MHz) for 1a. For C15 bis(alkyl) biradical 1b, spectra obtained under supercritical conditions are only slightly different from those detected in liquid CO2 but differ from spectra taken in benzene. For C10 acyl-alkyl biradical 2a, more emissive spin polarization due to S-T- mixing is observed in CO2 than in benzene. These results are discussed in terms of solvent properties such as dielectric constant, viscosity, and specific interactions. Both chain dynamics and changes to the equilibrium distribution of end-to-end distances can alter J(avg) and the observed ratio of S-T0 to S-T- mixing; however, faster chain dynamics is concluded to be the most likely cause of the observed effects in these systems.

Diffusion coefficients as a monitor of reaction kinetics of biological molecules

For revealing spectrally silent dynamics in chemical reactions, a new method, the time-dependent diffusion coefficient, is presented. Principles and typical examples of this method, in particular applications to biologically related reactions, are reviewed. The pulsed laser induced transient grating signal of the photo-decomposition reaction of caged ATP showed that the diffusion coefficient increases gradually with time reflecting the molecular size decrease by the dissociation. Hence, this rate should be a direct measurement of the photo-dissociation rate of ATP from the caged state. In an application to a protein folding reaction, the time-development of the diffusion coefficient was observed during the folding reaction. This time dependence was interpreted in terms of the intermolecular interaction change; i.e., conversion from the intermolecular hydrogen bonding to intramolecular one. It was found that the change of the hydrogen bonding network occurred by the two state manner in entire refolding process of cytochrome c. The unique feature of this time-dependent diffusion coefficient method is discussed.

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

Photothermal methods are currently being employed in a variety of research areas, ranging from materials science to environmental monitoring. Despite the common term which they are collected under, the implementations of these techniques are as diverse as the fields of application. In this review, we concentrate on the recent applications of time-resolved methods in photochemistry and photobiology.

Is the translational diffusion of organic radicals different from that of closed-shell molecules?

The translational diffusion of photochemically created intermediate radicals is measured by the transient grating technique. The diffusional behavior of these intermediates is different from that of stable molecules, which have already been investigated extensively. The investigation of the diffusion of these species will provide an opportunity to reveal the unique intermolecular interaction between the intermediates and matrix. This information will be valuable for understanding photochemistry in solutions.