Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P32: Gas Phase Clusters - Experiment and Theory in Concert (C): Other Clusters (GPC3)Focus
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Sponsoring Units: DCP Chair: Knut Asmis, Leipzig University Room: BCEC 204A |
Wednesday, March 6, 2019 2:30PM - 3:06PM |
P32.00001: Finite temperature anharmonic gas phase vibrational spectroscopy with DFT-based MD simulations Invited Speaker: Marie-Pierre Gaigeot We will review our works on gas phase vibrational spectroscopy of gas phase molecules and clusters by finite temperature DFT-based molecular dynamics simulations. The anharmonic dynamical spectra have been used in order to make systematic links to the most advanced action spectroscopy experiments. |
Wednesday, March 6, 2019 3:06PM - 3:42PM |
P32.00002: Deconstructing the curious behavior of carboxylate at the air-water interface with cluster ion spectroscopy Invited Speaker: Mark Johnson The transport of divalent metal ions (e.g., Mg2+ and Ca2+) into the troposphere is thought to arise from preferential complexation with the anionic head groups of fatty acids at the surfaces of sea-spray aerosols. Attempts to quantify the formation of these ionic complexes by monitoring the vibrational frequencies of the CO stretching vibrations has proven to be difficult, however, because the ion-driven spectral response is surprisingly similar to that of the hydrated anion. We trace the origin of this effect by studying the stepwise hydration behavior of the isolated carboxylate and contact ion pair in the gas-phase. The results reveal the critical importance of solvent coordination in the structural interpretation of surface-sensitive spectral signatures of ion complexation at the interface. Surprisingly, not only does stepwise hydration of the RCO2- anion and the [Ca2+-RCO2-]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the two (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that the two CO bands evolve toward their interfacial asymptotes from opposite directions. Theoretical simulations of the [Ca2+-RCO2-]+-(H2O)n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the tail as the key parameter driving the observed trends. |
Wednesday, March 6, 2019 3:42PM - 4:18PM |
P32.00003: Accurate spectroscopic characterization of molecular complexes as a first step toward the understanding of intermolecular interactions Invited Speaker: Cristina Puzzarini Noncovalent interactions play a key role in several biological and technological processes, yet their characterization and interpretation are still far from being satisfactory. Integrated experimental and computational investigations of molecular complexes can play an invaluable to understand the properties of intermolecular interactions. |
Wednesday, March 6, 2019 4:18PM - 4:54PM |
P32.00004: A cluster approach to solvation effects in flexible peptides Invited Speaker: Etienne Garand Utilizing a method based upon a unique combination of mass spectrometry and laser spectroscopy, our research focuses on capturing and characterizing complexes of interest to provide a molecular-level detailed examination of their structures and interactions. To access microsolvated charged complexes, we have developed new capabilities to perform controlled gas-phase clustering in cryogenic ion trap. This versatile approach enables us to form clusters with >30 solvent molecules surrounding any bare ion obtained by electrospray ionization. The structures of these carefully isolated and prepared complexes are then directly probed in the mass spectrometer using infrared predissociation spectroscopy, yielding well-resolved spectra that contain a wealth of information. Recent studies on the solvation-dependent structures of flexible model peptidic systems, (Gly)nH+ and (Ala)nH+ will be presented. In addition, this talk will highlight two methodologies necessary for deconvoluting the complex vibrational spectra of these species: IR-IR conformer-specific spectroscopy and solvent-specific isotope labeling. This combination of spectroscopic characterizations yields unambiguous spectral assignments and clearly reveals the structural evolution of the flexible peptides in response to increasing solvation. The results also provide insights on the isomerization pathways and energetics involved in these structural changes. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P32.00005: Observation of H2O and D2O water dimers in CCl4 at room temperature via Matrix Isolation Spectroscopy Doseok Kim, Adam H Turner, Sung Hyun Huh, Soo Ryeon Ryu The vibrational spectrum of a single molecule is typically investigated in the gas phase, however, the spectrum is crowded by combination bands with rotational transitions and not all molecules can be prepared in the gas phase. Room-temperature matrix isolation spectroscopy (MIS) is a tool that utilizes a low solubility of the solute molecule as well as weak interactions between solute and solvent molecules to obtain a spectrum of isolated solute molecules. Water (H2O and D2O) was loaded on top of the nonpolar solvent CCl4, and the in situ inclusion process was monitored in the CCl4 phase by infrared absorption spectroscopy. These in situ spectra showed non-linear deviations from the monomer spectrum over time attributed to formation of water dimers. The water dimer spectrum was extracted from the time-series of the spectra using 2D correlation spectroscopy and principal component analysis (PCA). These dimer peaks from the 2D correlation and PCA analysis compared to those obtained from DFT calculations using the CC-VSCF approximation showed good agreement and allowed the assignment of the dimer modes. Changes from the monomer to the dimer spectra as well as the difference between H2O and D2O dimer spectra were discussed in terms of hydrogen bonding. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P32.00006: Exploring molecular formation and growth upon ionization of van der Waals clusters using ab-initio molecular dynamics Tamar Stein, Partha Bera, Timothy J Lee, Martin Head-Gordon Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM)1. The mechanism for their formation in the low temperature environment of the ISM is, as of yet, nonetheless a mystery. Understanding the mechanism of formation of complex molecules such as PAHs and nitrogen-based PAHs (PAHN) is a long-standing challenge which has been drawing much attention for the past several decades. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P32.00007: Investigation of effect of temperature and size on the ionization potential of semiconductor clusters using effective stochastic potential method Shafi Ali, Jeremy Scher, Arindam Chakraborty
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