Molecular Dynamics Simulations of Solvation and Solvent Reorganization Dynamics in CO 2 -Expanded Methanol and Acetone John L. Gohres, †,§,| Alexander V. Popov, ‡,| Rigoberto Hernandez,* ,‡,§,| Charles L. Liotta, †,‡,§ and Charles A. Eckert* ,†,‡,§ School of Chemical & Biomolecular Engineering, School of Chemistry & Biochemistry, Specialty Separations Center, and Center for Computational and Molecular Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332-0100 Received August 28, 2008 Abstract: Composition-dependent solvation dynamics around the probe coumarin 153 (C153) have been explored in CO 2 -expanded methanol and acetone with molecular dynamics (MD) simulations. Solvent response functions are biexponential with two distinct decay time scales: a rapid initial decay (∼0.1 ps) and a long relaxation process. Solvation times in both expanded solvent classes are nearly constant at partition compositions up to 80% CO 2 . The extent of solvation beyond this composition has the greatest tunability and sensitivity to bulk solvent composition. Solvent rotational correlation functions (RCFs) have also been used to explore rotational relaxation. Rotations have a larger range of time scales and are dependent on a number of factors including bulk composition, solvent-solvent interactions, particularly hydrogen bonding, and proximity to C153. The establishment of the solvation structure around a solute in a GXL is clearly a complex process. With respect to the local solvent domain around C153, it was seen to be primarily affected by a nonlinear combination of the rotational and diffusive transport dynamics. 1. Introduction Gas-expanded liquids (GXLs) are a leading candidate for next-generation tunable solvents and are formed by the dissolution of appreciable amounts of gas into an organic liquid. The resulting mixture is a volume-expanded liquid phase with tunable physical and solvation properties like dielectric constant and viscosity. 1,2 An inherent advantage of GXLs results from the increase in gas (H 2 ,O 2 ) solubility in the liquid phase. Relative to neat organic solvents, GXLs have improved yield and selectivity of homogeneously catalyzed oxidation reactions. 3 A significant amount of research has shown that GXLs are advantageous over organic solvents for a variety of reactions, 4-9 extractions, 10 and materials processing applications. 11-18 Consequently a great deal of effort has focused on understanding the molecular- scale properties of GXLs so to fully exploit their unique solvent properties. Electronic excitation of the laser dye Coumarin 153 (C153) creates an excited-state dipole moment nearly 9 Debye greater than the ground-state dipole moment. 19 Recent spectroscopic and molecular dynamics (MD) simulation results showed organic enrichment around C153 in CO 2 - expanded solvents. 20-22 Different solvation patterns between the ground and excited states of C153, specifically organic and CO 2 density enhancements relative to the ground-state cause a solvent relaxation process to solvate the excited C153. Solvent relaxation consists of electronic and nuclear rearrangements with time scales that are dependent on bulk solvent properties and molecular interactions. MD simula- tions provide a direct comparison to time-resolved fluores- cence and give molecular-level insight into solvation mecha- nisms that compose solvent reorganization. Many studies * Corresponding author e-mail: cae@gatech.edu. † School of Chemical & Biomolecular Engineering. § Specialty Separations Center. | Center for Computational and Molecular Science and Technology. ‡ School of Chemistry & Biochemistry. J. Chem. Theory Comput. 2009, 5, 267–275 267 10.1021/ct800353s CCC: $40.75  2009 American Chemical Society Published on Web 01/14/2009