Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H32: The Journal of Chemical Physics Editors’ Choice LecturesInvited
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Sponsoring Units: DCP Chair: Laura Gagliardi, University of Minnesota Room: BCEC 204A |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H32.00001: Revisiting Glassy Behavior in Polystyrene with Single Molecule Approaches Invited Speaker: Laura Kaufman Polystyrene in the rubbery regime displays phenomenology also seen in small molecule supercooled liquids, and dynamic heterogeneity in this system has been well studied. We have revisited several aspects of glassy behavior and dynamic heterogeneity in polystyrene using single molecule approaches. In particular, we have characterized exchange time (τex, the timescale a particular dynamical environment maintains a given dynamics) at a number of temperatures down to and including the glass transition temperature. These studies reveal that the ratio of τex to the alpha relaxation time (τex/τα) is independent of temperature in the range probed. More recently, we have characterized dynamic heterogeneity in polystyrene as a function of fragility as controlled by molecular weight. Over a broad range of molecular weights, we find a similar degree of dynamic heterogeneity and ratio of τex to the alpha relaxation time in polystyrene. Finally, aiming to resolve long-standing questions regarding the origins of rotational-translational decoupling, we have combined rotational and translational measurements of single molecule probes in polystyrene. Initial results suggest slowly rotating molecules are typically also slowly translating molecules but continue to suggest overall translational diffusion is faster than would be expected given system viscosity. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H32.00002: Nucleation of the short-chain n-alkanes from the vapor phase: Experiments and Monte Carlo simulations Invited Speaker: Barbara Wyslouzil The n-alkanes do not nucleate easily from the vapor phase, and, when rapidly cooled in a supersonic expansion, saturation levels on the order of 10,000 are typical. [1, 2] The nucleation rates measured for n-pentane through n-heptane, at temperatures ranging from ~109 K to 168 K, are on the order of 1017 cm-3s-1, increasing slightly with decreasing temperature. Despite the high degree of supercooling, Monte Carlo (MC) simulations suggest that for n-pentane through n-heptane the critical clusters remain liquid like under experimental conditions, whereas n-octane and n-nonane adopt more ordered structures. For all three alkanes, the scaled experimental and simulated nucleation rates are offset by ~3 orders of magnitude. Explicitly accounting for the surface tension difference between the real and model substances, however, increases the offset to ~ 6 orders of magnitude, equivalent to a a formation free energy difference of ~13kBT . This difference could be reduced by changing the cutoff criterion or by using model potentials that better describe n-alkane behavior. |
Tuesday, March 5, 2019 3:42PM - 4:18PM |
H32.00003: Inferring properties of intrinsically disordered proteins from single-molecule FRET and small-angle X-ray scattering data Invited Speaker: Robert Best Intrinsically disordered proteins are a class of polypeptides in which a significant portion of the chain is not folded into a specific three-dimensional structure. Because of their abundance and importance in many biological contexts, it is important to characterize the extent of structure formation in these proteins. However, as a result of their extensive disorder, conventional structure-determination methods used for folded proteins are not appropriate, and the broad distribution of structures in such a disordered system needs to be considered. I will describe methods for using molecular simulations to account for the diversity of structures in intrinsically disordered proteins while fitting to the experimental data, which help to reconcile differences between different experimental methods. Methods based on molecular simulation are more computationally demanding, but also the most general approach. I will show how in many cases, if the IDP is sufficiently disordered, it is also possible to use simpler approximations which we have developed. |
Tuesday, March 5, 2019 4:18PM - 4:54PM |
H32.00004: Accurate modeling of liquid water and proton transfer in water Invited Speaker: Xifan Wu Water is of the utmost importance for life and proton transfer via hydronium and hydroxide ions in water is also ubiquitous. A genuinely predictive ab initio model of water is the key to understanding the proton transfer effect in water. However, accurate prediction of water requires to climb up the Jacob's ladder within density functional theory to include the treatment of van der Waals interactions and the mitigation of self-interaction error. We demonstrate that a fully ab initio approach, relying on the strongly constrained and appropriately normed (SCAN) density functional, provides such a description of water. SCAN accurately describes the balance among covalent bonds, hydrogen bonds, and van der Waals interactions that dictates the structure and dynamics of liquid water. At the similar level of theory, we then show that structural diffusion of hydronium preserves the previously recognized concerted behavior. However, by contrast, proton transfer via hydroxide is dominated by stepwise events, arising from a stabilized hyper-coordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron scattering results. Asymmetry in the temporal correlation of proton transfer enables hydronium to diffuse faster than hydroxide and may underlie observed isotope anomalies. |
Tuesday, March 5, 2019 4:54PM - 5:30PM |
H32.00005: Imaging excited states of nanomaterials Invited Speaker: Duc Nguyen Nanomaterials are promising for applications in photocatalysis, photosensitization, photodetection, photovoltaics and optoelectronics. In these applications, nanomaterials are first photoexcited, then the generated energy or charge is either used to break chemical bonds or transferred to nearby electrodes, particles or molecules of interest. Understanding photoexcited nanomaterials at their characteristic length scale is critical for performance optimization, however, is challenging given their small size and fast relaxation. We develop and use single-molecule adsorption scanning tunneling microscopy (SMA-STM), a powerful technique capable of imaging photoexcited nanomaterials with sub-nanometer spatial resolution, to investigate photoexcited quantum dots (QDs), carbon nanotubes (CNTs) and their interactions. Images of individual photoexcited QDs (absorption images) vary significantly from dot-to-dot. For a single QD, different excited states are probed by changing the applied electric field. Using the STM tip to nudge and roll the QDs on the surface, different images of the excited state at different angles are obtained. Energy transfer in arrays of QDs, QD-CNT interactions are also imaged and manipulated at individual nanoparticle level. Finally, I will discuss my recent work on probing molecular-scale catalytic interactions of oxygen with an oxygen reduction molecular catalyst using another optical STM technique, STM tip-enhanced Raman spectroscopy (STM-TERS). |
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