Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U43: Molecules, Clusters, and Complexes |
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Sponsoring Units: DCP Chair: Jeff Cina, University of Oregon Room: Hilton Baltimore Holiday Ballroom 2 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U43.00001: Terahertz Spectroscopy of Water Vapors, Chemical Vapors and Ionized Air Benjamin Graber, Rongjia Tao, Dong Ho Wu In the past, a few research groups have demonstrated that terahertz spectroscopy could be a useful tool for the identification of chemicals. However most of those demonstrations have been done with solid-phase or liquid-phase chemicals. There are little demonstrations for the detection and identification of chemicals in the gas-phase, as it is very difficult in part due to the presence of water-absorption lines in the terahertz frequency range. As the water absorption lines predominate in the 0.1 - 2THz spectral range, and can interfere with already weak terahertz signatures generated by chemical vapors, it is often very hard to obtain meaningful terahertz spectrum of chemical vapor. Regardless we recently have been able to obtain some terahertz spectra of chemical vapors and ionized air produced by several different ionization sources, including corona discharge and nuclear isotopes. Throughout data analysis we learned that water molecules, nitrogen and oxygen molecules play very important roles in these terahertz spectra. In this presentation we will discuss our experiments and the roles of these molecules. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U43.00002: Radiative electron attachment to molecules of astrophysical interest. Benchmark study of CN$^-$ Viatcheslav Kokoouline, Nicolas Douguet, Samantha Fonseca dos Santos, Olivier Dulieu, Maurice Raoult, Ann Orel We have developed a first-principles approach to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest Mol $+e^-\to$ Mol$^- + \hbar\omega$. (Mol$^-$ = C$_n$H$^-$, C$_n$N- ). The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. The benchmark calculation for the formation of the simplest observed ion, CN-, by REA has produced a low rate coefficient, $5\times 10^{-17}$cm$^3/$s at 30 K. We will present also a preliminary result for the C$_4$H$^-$ formation by REA. For this molecule, the REA rate coefficient is expected is larger by about a factor of 10 due to a larger transition dipole moment. This study suggests that the negative molecular ions, recently observed in the interstellar medium, can hardly be formed by the process of radiative electron attachment. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U43.00003: Stability and Meta-stability of Clusters in a Reactive Atmosphere: Theoretical Evidence for Unexpected Stoichiometries of Mg$_M$O$_x$ Saswata Bhattacharya, Sergey V. Levchenko, Luca M. Ghiringhelli, Matthias Scheffler Applying genetic algorithm and replica exchange molecular dynamics in a cascade approach we calculate structure and composition of Mg$_M$O$_x$ clusters at realistic temperatures and oxygen pressures. The cascade starts with force field and goes up to density functional theory with exact exchange plus correlation in the random phase approximation\footnote{X. Ren, P. Rinke, C. Joas, and M. Scheffler, Invited Review: Random-phase approximation and its applications in computational chemistry and materials science. J. Mater. Sci. {\bf 47}, 21 (2012).}. The stable compositions are identified using \textit{ab initio} atomistic thermodynamics. We find that at realistic environmental conditions small clusters ($M$ = 1-5) are in thermodynamic equilibrium when $x > M$. Non-stoichiometric clusters are found to have in general higher spin multiplicity than stoichiometric ones. This suggests a possibility of tuning magnetic properties by changing environmental conditions. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U43.00004: First Observation of the $2^1\Pi$ state of NaH Chin-Chun Tsai, Hsien-Yu Huang, Tsai-Lien Lu, Thou-Jen Whang The upper levels (to the last bound vibrational level) of NaH $2^1\Pi$ state have been observed for the first time by using pulsed optical-optical double resonance fluorescence depletion spectroscopy. About 20 rovibrational energy levels, $\textit{v}$=2-5 and $\textit{J}$=1-9, were assigned to this electronic state by comparing the successive rotational spectra through selected intermediate levels of the $A^1\Sigma^+$ state. A decreased background fluorescence on the recorded spectra near the atomic asymptotic of Na(3$\textit{d}$)+H(1$\textit{s}$) indicates that the dissociation limit of $2^1\Pi$ state is approaching. Compared with the eigenvalues solved from the potential of Aymer's $\textit{ab}$ initial calculations, the vibrational quantum numbers were assigned. Un-observed lower levels ($\textit{v}$=0 and 1) are due to the lack of Franck-Condon factor under accessible intermediate levels of the $A^1\Sigma^+$ state. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U43.00005: Probing the Electronic Structure of Small Metal-Nitride Clusters using Anion Photoelectron Spectroscopy Cuneyt Berkdemir, K.D. Dasitha Gunaratne, Shibo Cheng, A.W. Castleman, Jr. Gas-phase spectroscopic studies have greatly enhanced our understanding of the electronic structure and chemical bonding in metal-nitrides and oxides as well as metal-halides. While photoelectron spectroscopy of negatively charged clusters is a useful technique, spectroscopic investigations concerning metal-nitrides are still scarce. To gain insights into the electronic structures of select metal-nitrides, we have investigated the structures, ground electronic states and electron affinities of niobium and tantalum mono and dinitrides by obtaining their electron binding energies and photoelectron angular distributions via a Wiley-McLaren time-of-flight mass spectrometer coupled with a velocity map imaging apparatus. The metal-nitride anions are formed by laser ablation of niobium and tantalum metal rods with a buffer gas consisting of N2 in excess argon. The formation of anionic NbNx and TaNx (x=1,2) species have been confirmed via isotopic distributions of the respective molecules. DFT calculations are performed to predict the structures, vibrational frequencies and electron affinities of the observed anions and their neutral counterparts. As an analogy, we compared the electronic properties of NbN/ZrO and TaN/WC diatomics because they have the same number of valence electron. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U43.00006: Ab Initio Study of KCl and NaCl Clusters Clifton Brownrigg, Ajit Hira, Jose Pacheco, Justin Salazar We continue our interest in the theoretical study of molecular clusters to examine the chemical properties of small K$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ and Na$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ clusters (n $=$ 2 - 15). The potentially important role of these molecular species in biochemical and medicinal processes is well known. This work applies the hybrid ab initio methods of quantum chemistry to derive the different alkali-halide (M$_{\mathrm{n}}$H$_{\mathrm{n}})$ geometries. Of particular interest is the competition between hexagonal ring geometries and rock salt structures. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Magic numbers for cluster stability are identified and are related to the property of cluster compactness. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Calculations have been performed to examine the interactions of these clusters with some atoms and molecules of biological interest, including O, O2, and Fe. The potential for design of new medicinal drugs is explored. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U43.00007: Structural and thermodynamic properties of Au$_{2-58}$ clusters Yi Dong, Michael Springborg, Ingolf Warnke The geometries and electronic properties of the isolated neutral Au$_{2-58}$ are studied theoretically using a parametrized density-functional tight-binding method combined with genetic algorithms. Various descriptors are used in analyzing the structural and electronic properties. In addition, the temperature dependence of the vibrational heat capacities of the optimized clusters will be presented, which allow to study the low temperature properties of the clusters. We find that the vibrational heat capacity of the Au clusters is strongly size dependent in particular at low temperatures. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U43.00008: Quantum Theoretical Study of Palladium and Silver Clusters Ajit Hira, Justin Salazar, Jose Pacheco We continue our interest on the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{\mathrm{n}}$ and Ag$_{\mathrm{n}}$ clusters (n $=$ 2 thru 12). Transition-metal clusters can be useful for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E) sites, hollow sites (H) site and on-top (T) sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals will also be investigated. The implications for the molecular dissociation of the H$_2$ and O$_2$ species will be considered. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U43.00009: First principles NEXAFS simulations of N-donor Uranyl complexes C.D. Pemmaraju, R. Duan, R. Copping, B. Jeon, S.J. Teat, M. Janousch, T. Tyliszczak, A. Canning, N. Gr{\O}nbech-Jensen, D.K. Shuh, D. Prendergast The synthesis and study of soft-donor uranyl complexes can provide new insights into the coordination chemistry of non-aqueous [UO]2$^{+}$ Recently, the tunable N-donor ligand 2,6-Bis(2-benzimidazyl)pyridine (BBP) was employed to produce novel uranyl complexes in which the [UO]2$^{+}$ cation is ligated by anionic and covalent groups with discrete chemical differences. In this work we investigate the electronic structure of the three such uranyl-BBP complexes via near-edge X-ray absorption fine structure (NEXAFS) experiments and simulations using the eXcited electron and Core-Hole (XCH) approach [1]. The evolution of the structural as well as electronic properties across the three complexes is studied systematically. Computed N K-edge and O K-edge NEXAFS spectra are compared with experiment and spectral features assigned to specific electronic transitions in these complexes. Studying the variations in spectral features arising from N K-edge absorption provides a clear picture of ligand-uranyl bonding in these systems. References: [1] D. Prendergast and G. Galli, X-ray absorption spectra of water from first-principles calculations, Phys. Rev. Lett., 215502 (2006). [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U43.00010: Electronic structure and charge transfer states of a multichromophoric heptad Luis Basurto, Rajendra Zope, Tunna Baruah A multichromophoric Heptad molecule containing Zn-tetraphenyl porphyrin, BDPY dye, bisphenyl anthracene, and C$_{60}$ attached to a hexaphenyl -benzene core was synthesized by Gust et al. (J. Phys. Chem. B, 113, 7147 (2009)). The snowflake like molecule behaves like an antenna capturing photons at different wavelengths and transferring the energy to the porphyrin. We present a DFT based study on the ground state of the complex and also on the lowest two charge transfer (CT) states of the complex carried out using a perturbative delta-SCF method. The calculations, done using a mixed all-electron and pseudo-potential approach, show that the ionization potential of porphyrin and the electron affinity of C$_{60}$ in the complex changes significantly from isolated molecules. Our calculated value of the lowest CT state is within 0.2 eV of the experimental estimate. This CT state contains a hole on porphyrin HOMO and a particle on the C$_{60}$ LUMO. A comparison of the energetics with experiment indicates that the process probably involves excitation from the HOMO-1 of porphyrin to the porphyrin LUMO followed by electron transfer and hole bubbling up resulting in a CT state with the hole on porphyrin HOMO and particle on C$_{60}$ LUMO. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U43.00011: The effect of structural conformations and solvent effects in a light-harvesting Carotenoid-diaryl-Porphyrin-C$_{60}$ (CPC$_{60}$) molecular triad on the charge transfer excitation energies Tunna Baruah, Marco Olguin, Rajendra Zope We present a detailed study of charge transfer (CT) excited states for a large number of structural conformations in a light-harvesting Carotenoid-diaryl-Porphyrin-C$_{60}$ (CPC$_{60}$) molecular triad. The molecular triad undergoes a photinduced charge transfer state exhibiting a large excited state dipole moment, making it suitable for application as a molecular-scale optoelectronic device. One important consideration is that the conformational flexibility of the CPC$_{60}$ triad impacts its dynamics in solvents. Since many experimental photochemical measurements for the traid are made in solution, studying the effect of conformational changes on the CT energy furthers the understanding of its photoconversion properties. We have calculated a few low lying CT excited state energies for a series of triad conformers, where the conformers were generated by incrementally scanning a 360 degree torsional (dihedral) twist at the C$_{60}$-porhyrin linkage and the porphyrin-cartotenoid linkage. The CT excitation energy was calculated at each 45 degree dihedral increment. Addtionally, several different CPC$_{60}$ conformations were taken from molecular dynamics simulations of the triad in water and other solvents of varying polarity. Our calculations show that structural change [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U43.00012: Chemical Nonlinearities and Radical Pair Lifetime Estimation Gregory Robinson Much attention has recently developed around chemical reactions that depend on applied static magnetic fields as weak as earth's. This interest is largely motivated by experiments that implicate the role of spin-selective radical pair recombination in biological magnetic sensing. Existing literature uses a straightforward calculation to approximate the expected lifetime of coherent radical pairs as a function of the minimum RF amplitude that is observed to disrupt magnetic navigation, apparently by decohering the radical pair via electronic Zeeman excitations. But we show that chemical nonlinearities can preclude direct computation of coherent pair lifetime without considering the cellular signalling mechanisms involved, and discuss whether it can explain the surprising fragility of some animals' compass sense. In particular, we demonstrate that an autocatalytic cycle can introduce threshold effects on the disruption sensitivity to applied oscillatory magnetic fields. We will show examples in the mean-field limit and consider the consequences of noise and fluctuations in the Freidlin-Wentzell picture of perturbed dynamical systems. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U43.00013: Analysis of direct and indirect phonon-mediated bond excitation in the explosive RDX Brent Kraczek, Peter W. Chung Understanding detonation pathways is essential to controlling the sensitivity of high energy explosives. Central to these pathways is initiation, the initial chemical reactions that lead to detonation. Phonons play an active role in initiation caused by compressive wave energy, such as those caused by shock loading, by converting the wave energy to thermal energy that causes bond-breaking. In the conventional model for phonon-mediated initiation energy follows an indirect route, in which the wave energy excites low-frequency phonons which in turn excite higher-frequency vibrons that break the key initial bonds in the chemical decomposition pathways. Using lattice dynamics calculations of $\alpha$-RDX (the crystalline $\alpha$-phase of cyclotrimethylene trinitramine), we find that a direct route of energy transfer is more likely. We have calculated the total energy available to different phonon modes and the fractions of the mode energies that go into the bonds of the material. This enabled approximation of the maximum and minimum energy exciting the bonds due to different phonon modes throughout thermal relaxation. We find that low-frequency modes provide significantly more energy than high-frequency modes to the key bonds, implying that the direct pathway is responsible. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U43.00014: The first-principles study on the electronic and optical properties of (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ from many-body perturbation theory Hiroki Kawai, Giacomo Giorgi, Maurizia Palummo, Koichi Yamashita Gallium zinc oxynitride (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ is one of the promising candidates as overall water-splitting photocatalyst under visible light. In 2005, the high photocatalytic activity was reported on the GaN-rich alloys$^{\mathrm{[1]}}$ and nowadays, the ZnO-rich ones with the higher visible-light absorption were also synthesized by some groups$^{\mathrm{[2,\thinspace 3]}}$. Thus the further improvement of the photocatalytic water splitting is being expected. In spite of such a huge potential of this material, the origin of the visible-light absorption is not well understood. The first-principles methods based on many-body perturbation theory (MBPT), GW approximation and Bethe-Salpether equation, combining with density functional theory, enable us to do reliable analysis of the electronic and optical properties. On this meeting, we will discuss the origin of visible-light absorption of (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ by MBPT results focusing on the non-isovalent character. [1]K. Maeda. et al. \textit{J.Am.Chem.Soc}. 127, 8286 (2005), [2]H. Chen. et al. \textit{J.Phys.Chem.C}, 114, 1809 (2010), [3]K. Lee. et al. \textit{Nano Lett}, 12, 3268 (2012) [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U43.00015: ABSTRACT WITHDRAWN |
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