Showing papers by "Peter J. Rossky published in 2010"

Molecular origins of fluorocarbon hydrophobicity

Abstract: We have undertaken atomistic molecular simulations to systematically determine the structural contributions to the hydrophobicity of fluorinated solutes and surfaces compared to the corresponding hydrocarbon, yielding a unified explanation for these phenomena. We have transformed a short chain alkane, n-octane, to n-perfluorooctane in stages. The free-energy changes and the entropic components calculated for each transformation stage yield considerable insight into the relevant physics. To evaluate the effect of a surface, we have also conducted contact-angle simulations of water on self-assembled monolayers of hydrocarbon and fluorocarbon thiols. Our results, which are consistent with experimental observations, indicate that the hydrophobicity of the fluorocarbon, whether the interaction with water is as solute or as surface, is due to its “fatness.” In solution, the extra work of cavity formation to accommodate a fluorocarbon, compared to a hydrocarbon, is not offset by enhanced energetic interactions with water. The enhanced hydrophobicity of fluorinated surfaces arises because fluorocarbons pack less densely on surfaces leading to poorer van der Waals interactions with water. We find that interaction of water with a hydrophobic solute/surface is primarily a function of van der Waals interactions and is substantially independent of electrostatic interactions. This independence is primarily due to the strong tendency of water at room temperature to maintain its hydrogen bonding network structure at an interface lacking hydrophilic sites.

Water reorientation, hydrogen-bond dynamics and 2D-IR spectroscopy next to an extended hydrophobic surface

Abstract: The dynamics of water next to hydrophobic groups is critical for several fundamental biochemical processes such as protein folding and amyloid fiber aggregation. Some biomolecular systems, like melittin or other membrane-associated proteins, exhibit extended hydrophobic surfaces. Due to the strain these surfaces impose on the hydrogen (H)-bond network, the water molecules shift from the clathrate-like arrangement observed around small solutes to an anticlathrate-like geometry with some dangling OH bonds pointing toward the surface. Here we examine the water reorientation dynamics next to a model hydrophobic surface through molecular dynamics simulations and analytic modeling. We show that the water OH bonds lying next to the hydrophobic surface fall into two subensembles with distinct dynamical reorientation properties. The first is the OH bonds tangent to the surface; these exhibit a behavior similar to the water OHs around small hydrophobic solutes, i.e. with a moderate reorientational slowdown explained by an excluded volume effect due to the surface. The second is the dangling OHs pointing toward the surface: these are not engaged in any H-bond, reorient much faster than in the bulk, and exhibit an unusual anisotropy decay which becomes negative for delays of a few picoseconds. The H-bond dynamics, i.e. the exchanges between the different configurations, and the resulting anisotropy decays are analyzed within the analytic extended jump model. We also show that a recent spectroscopy technique, two-dimensional time resolved vibrational spectroscopy (2D-IR), can be used to selectively follow the dynamics of dangling OHs, since these are spectrally distinct from H-bonded ones. By computing the first 2D-IR spectra of water next to a hydrophobic surface, we establish a connection between the spectral dynamics and the dynamical properties that we obtain directly from the simulations.

Dependence of Electrochemical and Electrogenerated Chemiluminescence Properties on the Structure of BODIPY Dyes. Unusually Large Separation between Sequential Electron Transfers

Abstract: Electrochemistry and electrogenerated chemiluminescence (ECL) of selected substituted BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been studied. The location and nature of substituents on positions 1-8 are important in predicting the behavior, and especially the stability, of the radical ions formed on electron transfer. Dyes with unsubstituted positions 2, 6, and 8 show a kinetic contribution to both oxidation and reduction. Dyes with only unsubstituted positions 2 and 6 and a substituted 8 position show chemically reversible reduction but irreversible oxidation. Unsubstituted positions 2 and 6 tend to show dimer formation on oxidation. Completely substituted dyes show nernstian oxidation and reduction. Oxidation and reduction studies of simple BODIPY dyes show an unusually large separation between the first and second reduction peaks and also the first and second oxidation peaks, of about 1.1 V, which is very different from that observed for polycyclic hydrocarbons and other heteroaromatic compounds, where the spacing is usually about 0.5 V. Electronic structure calculations confirmed this behavior, and this effect is attributed to a greater electronic energy required to withdraw or add a second electron and a lower relative solvation energy for the dianion or dication compared with those of the polycyclic hydrocarbons. ECL was generated for all compounds either by annihilation or by using a co-reactant.

Exploring nanoscale hydrophobic hydration

Abstract: In this lecture, aspects of the hydration of hydrophobic interfaces that are emergent nanoscale properties of the interface chemical structure are discussed. General results inferred from systematic computational studies are emphasized, with a central theme focusing on the separate roles of surface topography and surface chemistry. The roles of surface curvature, polarity, and chemical heterogeneity, as well as the important role of solvent thermodynamic state are considered. The potential importance of understanding evolved natural biological interfaces on the same basis as model synthetic surfaces is pointed out, and progress in this direction is discussed.

Understanding the Effectiveness of Fluorocarbon Ligands in Dispersing Nanoparticles in Supercritical Carbon Dioxide

Abstract: To augment the experimental search for a nonfluorous capping ligand that is effective in dispersing nanoparticles in supercritical carbon dioxide, we have developed a simulation protocol, involving atomistic molecular dynamic simulations, which semiquantitatively reproduces empirical observations and hence can be used to gain insight into the relevant physical phenomena. We have used this protocol to examine the reasons behind the exceptional effectiveness of perfluoroalkanethiols, compared to alkanethiols, as capping ligands for nanoparticles in supercritical carbon dioxide. From these simulations, we infer that the principal reason for this enhanced effectiveness is that the C−F bond is much more polar than the C−H bond and hence the fluorocarbons can interact with the quadrupolar carbon dioxide in addition to interacting with its van der Waals centers. For the models studied, the effect of the electrostatic interaction offsets the fact that the dispersion forces exerted by a dense fluorocarbon layer ar...

Nonadiabatic simulations of exciton dissociation in poly-p-phenylenevinylene oligomers.

Abstract: We present simulations of exciton dissociation and charge separation processes in the prototypical conjugated polymer, poly-p-phenylenevinylene. Our mixed quantum/classical simulations focus on the nonadiabatic excited state dynamics of single and pi-stacked oligomers of varying length. By applying a constant external electric field, our simulations reveal the details and time scale for exciton dissociation and fluorescence quenching and suggest how those processes relate to charge carrier (polaron) formation in polymer systems. We find that, in such a polarizing environment, sufficiently long chromophores (either single or interacting chains) can form polaron pairs via a delayed exciton dissociation mechanism or nearly instantaneously following photoexcitation. However, we find that these processes are mechanically essentially the same, being highly nonadiabatic in character and requiring transitions through "gateway" states to reach the completely charge separated electronic states. Finally, we observe thermally driven polaron hopping dynamics between chains, similar to the energy transfer dynamics we had described previously (J. Phys. Chem. A 2009, 113 (15), 3427). Our results are consistent with a range of apparently conflicting experiments, resolving some controversies regarding the molecular mechanism for charge carrier photogeneration in conjugated polymers.

Response of observables for cold anionic water clusters to cluster thermal history.

Abstract: We have used mixed quantum-classical molecular dynamics simulations to explore the role of structural relaxation when binding an excess electron to neutral water clusters. The structural and spectral properties of the water cluster anions were investigated as a function of the size (n=45 and 104), nominal temperature (Tnom=50, 100 and 150 K) and the preparation method of the parent neutral clusters. In particular, we consider two different protocols for preparing the initial neutral clusters, which differ markedly in their thermal history. In the first, warm equilibrium neutral clusters are gradually quenched to increasingly lower temperature. In the second, neutral clusters are formed spontaneously at ~0 K and then warmed to the same target temperatures, yielding inherently metastable, non-equilibrium structures. Electron attachment to these alternative sets of clusters shows that below a critical temperature (~200K), the metastable water clusters bind a surface state excess electron significantly more strongly than the quenched, equilibrium clusters. The structural analysis indicates that these cluster anions with larger vertical detachment energies (VDE) more frequently stabilize the electron by double acceptor-type water molecules, and exhibit a weak temperature dependence of the VDE, compared to the quenched clusters. These results suggest that the alternative classes of cluster anions seen experimentally may reflect differences in thermal history of such clusters.

Understanding the Relative Effectiveness of Alkanethiol Ligands in Dispersing Nanoparticles in Supercritical Carbon Dioxide and Ethane

Abstract: There is considerable interest in developing a nonfluorous capping ligand that is effective in dispersing nanoparticles in supercritical carbon dioxide. To augment the experimental search for such a molecule, a simulation protocol is developed, involving atomistic molecular dynamics simulations, which captures the relevant physical phenomena related to the effectiveness of capping ligands in stabilizing nanoparticle dispersions, evidenced by a consistency with empirical observation. The method is used to determine why the cheap and benign alkanethiol ligands are effective in supercritical ethane and several organic solvents but not in supercritical CO2 at convenient conditions of temperature and pressure; n-dodecanethiol is used as the representative ligand. We conclude that the ineffectiveness of the alkanethiol ligands in CO2 is primarily because they cannot compensate for the quadrupolar interactions which account for a substantial portion of the cohesive energy of bulk CO2, which is lost to the CO2 mo...

Analysis of localization sites for an excess electron in neutral methanol clusters using approximate pseudopotential quantum-mechanical calculations

Abstract: We have used a recently developed electron-methanol molecule pseudopotential in approximate quantum mechanical calculations to evaluate and statistically analyze the physical properties of an excess electron in the field of equilibrated neutral methanol clusters ((CH3OH)n, n=50–500). The methanol clusters were generated in classical molecular dynamics simulations at nominal 100 and 200 K temperatures. Topological analysis of the neutral clusters indicates that methyl groups cover the surface of the clusters almost exclusively, while the associated hydroxyl groups point inside. Since the initial neutral clusters are lacking polarity on the surface and compact inside, the excess electron can barely attach to these structures. Nevertheless, most of the investigated cluster configurations do support weakly stabilized cluster anion states. We find that similarly to water clusters, the pre-existing instantaneous dipole moment of the neutral clusters binds the electron. The localizing electrons occupy diffuse, w...

Paul F. Barbara (1953–2010)

Abstract: Paul F. Barbara, 57, passed quietly on Sunday, 31 October 2010, from complications following cardiac arrest. He made deep contributions to understanding how molecular shapes and reactivities are induced by their environments. Paul's intense focus on understanding molecules at their most fundamental level had very early roots. After his election to the National Academy of Sciences, he was asked to comment on what brought him to science ([ 1 ][1]). He replied, “The first book I ever got was a book on molecules, and I think that was in fourth grade.” He went on to talk about his early fascination with molecules: “I constantly imagined that they were there and that if you had an ideal microscope, you'd be able to see them and they'd be everywhere around you.” Around 1996, Barbara realized that early imagination and, in fact, embarked on studying individual polymer molecules, a focus that would dominate his scientific curiosity from then on. In groundbreaking work, his group developed the means to observe polymers on very small length scales and detect the weak light needed to probe their molecular arrangements. They could pick out individual structural arrangements and see how these affected the observed molecular behavior in complex environments. They studied not only synthetic polymers, such as the candidates for components in plastic solar cells, but also biopolymers. Stimulated by his collaborator, biological chemist Karin Musier-Forsyth, his group performed elegant studies on the sequence of molecular events occurring during reverse transcription in HIV. Before studying single molecules, Paul had already made deep and lasting contributions to the understanding of condensed phase molecular dynamics in complex chemical contexts that are ubiquitous in both solution chemistry and biological chemistry. These included proton and electron transfer reactions that are the essential steps in electrochemical processes and that make up the elementary chemical steps underlying the basic processes of photosynthesis as well as the metabolism of essentially all living species. Paul also developed a detailed description of solvated electron dynamics, a fundamental process associated with radiation chemistry. He and his co-workers achieved this through the development and application of novel ultrafast spectroscopies that could resolve different steps in these exceedingly fast processes. ![Figure][2] CREDIT: MARSHA MILLER/UNIVERSITY OF TEXAS AT AUSTIN Paul called himself an “instrument jock,” and his mastery of state-of-the-art experimental tools was certainly an essential part of his impact. He was well known for casually disassembling finicky instruments in front of their horrified users whose careers depended on their instruments. They would then stare in disbelief as he casually put the apparatus together again in a new way that worked much better. But the vital feature that made Paul Barbara a unique force in everything he did was that Paul was a dynamo. A dynamo not only of energy, and there was ceaseless energy, but also of ideas. Ideas for experiments—and for theories—as well as for conceptual molecular interpretations of physical behavior that were never confined by conventional wisdom. He would call the lab from home into the wee hours, night after night, eager to discuss the newest data coming from those finicky instruments, encouraging his students, while reminding them of the control experiments that would make the labors worthwhile. Paul grew up in New York City and received his B.S. in chemistry at Hofstra University in Long Island, New York, in 1974. He completed his Ph.D. at Brown University in 1978 and pursued postdoctoral studies at Bell Laboratories until 1980. He was a faculty member for 18 years at the University of Minnesota, where he was named 3M-Alumni Distinguished Professor of Chemistry. Moving in 1998 to Austin, he held the Richard J. V. Johnson–Welch Regents Chair in Chemistry at the University of Texas. He received many awards and accolades throughout his career, beginning with a Presidential Young Investigator Award in 1984. In 2009, he was awarded the E. Bright Wilson Award in Spectroscopy by the American Chemical Society, recognizing his innovative experimental probes of the dynamics of chemical processes using both ultrafast and single-molecule spectroscopies. In 2006, he was elected to the National Academy of Sciences. During his career, he published more than 200 influential and widely cited journal articles. He was also a mentor to more than 100 graduate students and postdoctoral research fellows; 34 are now professors at universities in the United States, Asia, and Europe, carrying forward his legacy in their own labs. Paul's attitude toward science, while constantly focused on where careful measurements could resolve outstanding controversies, was playful and exuberant. When he had new data that inspired a new idea for how things worked, his excitement was child-like; infectious, it nucleated his colleagues to focus collectively on the grand problems that would change the way that people think. That excitement about new discoveries and the possibilities to upend the status quo with a new concept seemed to grow boundlessly over the many years that we knew and worked with Paul. He left us far too soon, and the scientific world will be too still without his presence. 1. [↵][3] 1. K. Mossman , Proc. Natl. Acad. Sci. U.S.A. 104, 17567 (2007). 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