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Absorption and emission spectral characteristics, quantum yields, radiative and nonradiative rates, and rotation times of trans, trans-1,4-diphenyl-1,3-butadiene (DPB) in perfluoroalkane and alkane solvents are reported. The emission spectra and radiative rates show a continuous change in the character of the emitting state of DPB with solvent polarizability, presumably due to solvent-induced changes in the relative energies of the 2A(sub g) and 1B(sub u) states.

A 2-site model of fluoroform (CHF3) is developed for use in simulations of solvation in the near critical regime. The model is optimized to reproduce the critical point and the liquid-vapor coexistence curve of the real fluid.

Emission shifts is used to measure effective local densities surrounding anthracene and three anthracene derivatives in supercritical ethane, carbondioxide and fluoroform. To establish a relationship between emission frequency and local density, the sensitivity of each solute to the electronic and nuclear polarizability of its surroundings is calibrated using dielectric continuum models of solvatochromism together with data in the gas phase and in liquid solutions.

Research is done on data from prior simulation and experimental studies, which are collected and analyzed in an attempt to relate the extent of local density augmentation in supercritical fluids to the strength of intermolecular interactions. It is found that simulated and experimental augmentation data apparently follow distinct correlations with various free energies, indicating the presence of a widespread error in either the measurement or the interpretation of density augmentation in supercritical solvents.

The role of hydroxylic solvents in the excited-state tautomerization of 7-azaindole and 1-azacarbazole was determined by classical Monte Carlo and molecular dynamics computer simulations. The solvation structure and reactive fractions for a series of eight hydroxylic solvents: methanol, ethanol, 1-propanol, 2,2,2-tr-fluoroethanol, 2-propanol, tert-butyl alchol, ethylene glycol and water. The results showed the reaction rates in 7-azaindole and 1-azacarbazole systems were controlled by the fraction of solutes that were solvated by the hydroxylic solvents.

A data is presented that shows the rotation times of the two solutes to be approximately equal and linear functions of (bulk) solvent density, consistent with the simulations. It is stated that the rate of isomerization of HMS decreases approximately linearly with solvent density, whereas the isomerization of trans, trans-1,4-diphenylbutadiene (DPB) is remarkably insensitive to density.

We have tested the ability of the OPLS–AA models (optimized potentials for liquid simulations) of alkanes and perfluoroalkanes recently developed by Jorgensen and co-workers to represent the unusual mixing behavior of alkane+perfluoroalkane systems. We find that these all-atom Lennard-Jones (6-12)+Coulomb representations, together with the usual Lorentz–Berthelot combining rules, fail to reproduce the weaker-than-anticipated interactions between these two classes of molecules. Systematic disagreements with experiment are found in the case of second pressure virial coefficients, gas solubilities, and liquid–liquid mixing properties. These discrepancies are not specific to the choice of OPLS–AA potentials, but are rather linked to the failure of the geometric mean combining rule for relating unlike atom interactions. In all cases examined, a reduction in the strength of cross H+F interactions by ∼25% relative to the geometric mean is required in order to achieve reasonable agreement with experiment. Several less commonly used combining rules were also examined. Although some of these rules are able to provide a reasonable description of the interactions among perfluoroalkane and alkane species, they fail to provide a consistent treatment when atoms other than C, H, and F are considered, as is necessary for modeling the interaction of the former molecules with rare-gas atoms. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Response of solvent molecules to the electronic rearrangement of solute molecules influences the reaction rate. A slow response results in the reduction of reaction rate since the resulting products recross the energy barrier. In aqueous charge-transfer processes, more than half of the solvation response for atomic solutes is inertial, occurring for 20 femtoseconds. This ultrafast solvation component enhances the rates of electron transfer between metallocenes in aqueous solutions.

A study reports steady-state absorption and emission spectra and emission decay kinetics for three (alkylamino)benzonitriles, together with 4-aminobenzonitirle (ABN), in 24 room-temperature solvents. The study aims to learn how to model the solution-phase reaction potentials of the solutes to understand more fully how charge transfer is controlled by static and dynamic solvent effects.

A study was conducted, using the time-correlated single-photon counting technique; to examine solvation dynamics and rotation of the solvatochromic probe coumarin 153 in alkylphosphonium ionic liquids. The result reveals the distinct solvation dynamics of ionic liquids based on the large hydrophobic cation.

The steady-state spectra, rotation times, and time-resolved emission spectra of the probe 4-aminophthalimide (4-AP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim(super +)][PF6(super-)]) were measured over the temperature range 298-355 K. The steady-state spectroscopy indicated that the solvation energetics of 4-AP in [bmim(super +)][PF6(super-)] are comparable to those of 4-AP in highly polar but aprotic solvents such as dimethylformamide and acetonitrile.

Investigates the molecular basis for the solvent effects on the isolated C-H stretching bands observed in the Raman spectrum of cyclohexane-d[sub 11] using molecular-dynamics simulations. Line shape theory; Modeling Omega(t); Potential functions and simulation methods; Performance of the various Omega(t) models.

This paper describes results of simulations of solvation dynamics of a variety of solutes in two reference solvents, acetonitrile and methanol. Part of these studies involve attempts to realistically model the solvation dynamics observed experimentally with the fluorescence probe coumarin 153 (C153). After showing that linear response simulations afford a reliable route to the dynamics of interest, experimental and simulation results for C153 are compared. Agreement between the observed and calculated dynamics is found to be satisfactory in the case of acetonitrile but poor in the case of methanol. The latter failure is traced to a lack of realism in the dielectric properties of the methanol model employed. A number of further simulations are then reported for solvation of a number of atomic, diatomic, and benzenelike solutes which are used to elucidate what features of the solute are important for determining the time dependence of the solvation response. As far as large polyatomic solutes like C153 are concerned, the solute attribute of foremost importance is shown to be the ‘‘effective moment’’ of its charge distribution (actually the difference between the S1 and S0 charge distributions). This effective moment, determined from consideration of continuum electrostatics, provides a simple measure of how rapidly the solute’s electric field varies spatially in the important regions of the solvent. Simulations of fictitious excitations in a benzene solute show that this single quantity is able to correlate the dynamics observed in widely different solutes.Also explored is the effect of solute motion on its solvation dynamics. While of minor relevance for large solutes like C153, in small solutes of the size of benzene, solute motion can dramatically enhance the rate of solvation. A model based on independent solvent dynamics and solute rotational motion is able to account for the bulk of the observed effects. Finally, the... [ABSTRACT FROM AUTHOR]

Simulations of a simplified model system are used to test analytical theories of dielectric friction and explore its connection to dipole solvation dynamics. The simulation model consists of a point dipole solute interacting with a finite collection of dipolar solvent molecules, all situated on a simple cubic lattice and undergoing rotational Brownian motion in the pure diffusion limit. An extensive set of simulations are reported in which four model properties, the solute dipole moment and charge, and the solvent polarity and relaxation time, have been systematically varied. Static and dynamic aspects of dipole solvation observed in these systems are compared to the predictions of the simple continuum and dynamical mean spherical approximation (MSA) theories. Within the linear solvation regime the MSA theory is found to yield essentially quantitative predictions for both static and dynamic solvation properties. The simple continuum model, on the other hand, provides a poor description of either the static or the dynamic behavior. Solute rotational correlation functions of various rank and the dielectric friction functions calculated from them are compared to a variety of theories of rotational dielectric friction. Since all of the analytical theories examined rely on simple continuum descriptions of dipole solvation, they all fail to yield quantitatively accurate results. However, the more sophisticated theories do generally provide useful guides for understanding the trends observed in the data. The one instance where all of the theories fail in a qualitative manner is in predicting the rotational dynamics in the slow solvent limit. Reasons for this failure are discussed and a semiempirical approach for understanding the actual behavior in this limit is presented. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Small hydrogen bonded complexes of HCN and DCN have been examined in the gas phase using FTIR and photoacoustic Raman spectroscopy (PARS). Four distinct bands are seen and assigned to CN stretching modes of the HCN dimer (D) and trimer (T). Previous microwave studies have shown the HCN dimer to be linear and analysis of the IR trimer band shape unambiguously demonstrates that the HCN trimer is also linear. The pressure dependent intensity of the observed transitions yields IR and Raman cross sections relative to the monomer of: (σD/σM)IR =30±10, (σT‘/σM)IR =500±200, (σD/σM)Ram =0.5±0.1, (σT/σM)Ram =3.3±0.7. The dramatic enhancements seen in the IR cross sections are due to small but significant (∼2×) changes in the CH bond dipole derivative. CARS spectroscopy has been used to examine the complexes formed in supersonic expansions of pure HCN (DCN) and HCN diluted in Ar and He carriers. Due to the cooling and collision-free conditions in the jet, this method yields higher effective resolution than is possible with equilibrium samples. The added ability to ‘‘tune’’ the size of complexes by varying expansion conditions has allowed us to observe a number of other modes in HCN and DCN dimer and trimer as well as in higher polymers. These data have been used to partially refine a harmonic valence force field for the stretching modes of the dimer and trimer complexes. [ABSTRACT FROM AUTHOR]

The distributions of conformational defects that exist in the high-temperature phase II (also referred to as the hexagonal or rotator phase) of the crystalline n-alkanes C21 and C29 have been measured by an infrared CD2-substitution technique and have been accounted for in terms of a lattice model that provides freedom for longitudinal displacement of the chains. The defects consist almost entirely of gtg’ kinks distributed nonuniformly along the chain. The uneven distribution is indicated in the variation in the concentration of gauche bonds measured at various sites along the chain. The highest concentration is at the chain ends, and the concentrations at interior sites decrease exponentially in going toward the middle. To explain the distribution we used a modification of a lattice model that had been successfully applied to the lipid bilayer. Comparison of observed distributions with those computed from the model indicates that the factors that determine the shape of the distribution are quite different in the n-alkane and bilayer cases. For the bilayer, the dominant factor is the variation in the lateral density of chains; for the n-alkane, the dominant factor is associated with longitudinal displacement of the chains. [ABSTRACT FROM AUTHOR]

Papazyan and Maroncelli [J. Chem. Phys. 95, 9219 (1991)] recently reported computer simulations of solvation dynamics of an ion in a Brownian dipole lattice solvent. In the present article we compare these results to predictions of a number of theories of solvation dynamics in the diffusive limit. The frequency-dependent dielectric response functions needed as input to many of the theories are derived from further simulations of the lattice solvent [H. X. Zhou and B. Bagchi, J. Chem. Phys. 97, 3610 (1992)]. When properly applied, all of the currently popular molecular theories yield reasonable predictions for the time scale of the solvation response. The dynamical MSA model [P. G. Wolynes, J. Chem. Phys. 86, 5133 (1987)] and the memory function theory of Fried and Mukamel [J. Chem. Phys. 93, 932 (1990)] both provide nearly quantitative agreement with all aspects of the solvation dynamics observed in these simulations. [ABSTRACT FROM AUTHOR]

A study was performed by using computer simulations of realistic models of diphenlypolyene/CO2 systems. The results indicate that the density dependence of the simulated friction on both rotational and translational motions is insensitive to the density augmentation present in these systems.

Monte Carlo computer simulations were used to investigate the solvation and solvatochromic shifts of betaine-30 in 12 solvents.

The solute coumarin 153 was used to compare the effectiveness of the single-wavelength and spectral-reconstruction methods for determining the solvation/spectral response functions. Results showed that the linear-single-wavelength technique can predict solvation times with an accuracy of about 30% to 40%. However, the accuracy did not increase when the a nonlinear version of single-wavelength method was applied.