27 research outputs found
potential energy surfaces and reaction dynamics studies of small triatomic systems: o+h2, oh+h and oh+d
This dissertation is focused on the interaction of open-shell atoms and molecules. We have studied the systems in the title by means of electronic structure and statistical reaction dynamics methods.
We present an ab initio study of the O(3P) + H2 system. In particular, we have calculated potential energy surfaces for the van der Waals region of the interaction and derived and calculated the spin-orbit coupling matrix in the diabatic representation.
The rest of the dissertation is comprised of statistical, coupled-states dynamics studies. Cross sections are calculated by the coupled-states (CS) statistical method including the full open-shell character of the systems. All electronic and spin-orbit couplings are included.
We report state-to-state and overall thermal rate constants for the isotope exchange D(2S) + OH(2Pi) -> OD(2Pi) + H(2S) for 0 K < T < 500 K.
We predict a reaction rate constant of 14.22x10-11 cm3 molecule-1 s-1 at T=100 K and 10.78x10-11 cm3 molecule-1 s-1 at T=300 K.
At lower temperatures, (T ~ 50 K), the value rises to k(T)=15x10-11 cm3 molecule-1 s-1. A negative temperature dependence in the rate constant is observed. The state-resolved cross sections and rate-constants predict a significant propensity toward formation of the OD Pi(A') \Lambda-doublet level and the ground spin-orbit manifold, F1.
This dissertation is also concerned with the study of vibrational and rotational relaxation of OH(^2\Pi) by collision with H atoms. Four potential energy surfaces (PESs) ({1,3}A' and {1,3}A")
describe the interaction of OH(X2Pi) with H atoms. Of these, three are repulsive, while one (1A') correlates with
the deep H2O well. Consequently, rotationally- and ro-vibrationally-inelastic
scattering of OH in collisions with H can occur by scattering
on the repulsive PESs, in a manner similar to the inelastic scattering
of OH by noble gas atoms, or by collisions which enter the H2O
well and then re-emerge. We report state-to-state cross sections and thermal rate constants for the collisions. At 300 K, we predict large (~1x10-10 cm3 molecule-1 s-1) vibrational relaxation rates out of both v=2 and v=1, comparable to earlier experimental observations.
This surprisingly fast relaxation results from capture into the H2O complex. There also exists a significant propensity toward formation of OH in the Pi(A') Lambda-doublet level. We also report state-resolved cross sections and rate constants for rotational excitation within the OH v=0 manifold. Collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted Lambda-doublet population
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Multiple Coherent States for First-Principles Semiclassical Initial Value Representation Molecular Dynamics
A multiple coherent states implementation of the semiclassical approximation is introduced and employed to obtain the power spectra with a few classical trajectories. The method is integrated with the time-averaging semiclassical initial value representation to successfully reproduce anharmonicity and Fermi resonance splittings at a level of accuracy comparable to semiclassical simulations of thousands of trajectories. The method is tested on two different model systems with analytical potentials and implemented in conjunction with the first-principles molecular dynamics scheme to obtain the power spectrum for the carbon dioxide molecule.Chemistry and Chemical Biolog
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Theoretical Characterization of the Air-Stable, High-Mobility Dinaphtho[2,3-b:2'3'-f]-thiophene Organic Semiconductor
Recently, an optimum mobility of has been measured for single-crystal organic field-effect transistors based on the dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT) molecule. Here, on the basis of quantum chemistry calculations and molecular dynamics simulations, we investigate the microscopic charge transport parameters of the DNTT molecule and crystal. Our findings confirm that the moderate anisotropy of the hole mobility in DNTT is highly dependent on the presence of in-plane herringbonelike intermolecular interactions with large electronic coupling (transfer integral) values (ca. 0.1 eV). Also, we demonstrate that the π-extended heteroaromatic structure leads to strong electronic coupling interactions among neighboring molecules and to a decrease of the intramolecular reorganization energy. In DNTT, thermal modulations of the electronic couplings at 300 K remain small when compared to those exhibited by the pentacene single crystal. This theoretical study suggests that heteroacenes are a promising route toward high-mobility organic semiconductor materials. Charge transport is discussed in the framework of both band and hopping models.Chemistry and Chemical Biolog
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Accelerated Computational Discovery of High-Performance Materials for Organic Photovoltaics by Means of Cheminformatics
In this perspective we explore the use of strategies from drug discovery, pattern recognition, and machine learning in the context of computational materials science. We focus our discussion on the development of donor materials for organic photovoltaics by means of a cheminformatics approach. These methods enable the development of models based on molecular descriptors that can be correlated to the important characteristics of the materials. Particularly, we formulate empirical models, parametrized using a training set of donor polymers with available experimental data, for the important current–voltage and efficiency characteristics of candidate molecules. The descriptors are readily computed which allows us to rapidly assess key quantities related to the performance of organic photovoltaics for many candidate molecules. As part of the Harvard Clean Energy Project, we use this approach to quickly obtain an initial ranking of its molecular library with 2.6 million candidate compounds. Our method reveals molecular motifs of particular interest, such as the benzothiadiazole and thienopyrrole moieties, which are present in the most promising set of molecules.Chemistry and Chemical Biolog
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The Harvard Clean Energy Project: Large-Scale Computational Screening and Design of Organic Photovoltaics on the World Community Grid
This Perspective introduces the Harvard Clean Energy Project (CEP), a theory-driven search for the next generation of organic solar cell materials. We give a broad overview of its setup and infrastructure, present first results, and outline upcoming developments. CEP has established an automated, high-throughput, in silico framework to study potential candidate structures for organic photovoltaics. The current project phase is concerned with the characterization of millions of molecular motifs using first-principles quantum chemistry. The scale of this study requires a correspondingly large computational resource, which is provided by distributed volunteer computing on IBM’s World Community Grid. The results are compiled and analyzed in a reference database and will be made available for public use. In addition to finding specific candidates with certain properties, it is the goal of CEP to illuminate and understand the structure–property relations in the domain of organic electronics. Such insights can open the door to a rational and systematic design of future high-performance materials. The computational work in CEP is tightly embedded in a collaboration with experimentalists, who provide valuable input and feedback to the project.Chemistry and Chemical Biolog
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Tuning Charge Transport in Solution-Sheared Organic Semiconductors Using Lattice Strain
Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33Å to 3.08 Å. We believe that 3.08Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04Å has been achieved through intramolecular bonding). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from for unstrained films to a high mobility of for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.Chemistry and Chemical Biolog
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Lead candidates for high-performance organic photovoltaics from high-throughput quantum chemistry – the Harvard Clean Energy Project
The virtual high-throughput screening framework of the Harvard Clean Energy Project allows for the computational assessment of candidate structures for organic electronic materials – in particular photovoltaics – at an unprecedented scale. We report the most promising compounds that have emerged after studying 2.3 million molecular motifs by means of 150 million density functional theory calculations. Our top candidates are analyzed with respect to their structural makeup in order to identify important building blocks and extract design rules for efficient materials. An online database of the results is made available to the community.Engineering and Applied Science
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Effects of Odd–Even Side Chain Length of Alkyl-Substituted Diphenylbithiophenes on First Monolayer Thin Film Packing Structure
Because of their preferential two-dimensional layer-by-layer growth in thin films, 5,5′bis(4-alkylphenyl)-2,2′-bithiophenes (P2TPs) are model compounds for studying the effects of systematic chemical structure variations on thin-film structure and morphology, which in turn, impact the charge transport in organic field-effect transistors. For the first time, we observed, by grazing incidence X-ray diffraction (GIXD), a strong change in molecular tilt angle in a monolayer of P2TP, depending on whether the alkyl chain on the P2TP molecules was of odd or even length. The monolayers were deposited on densely packed ultrasmooth self-assembled alkane silane modified surfaces. Our work shows that a subtle change in molecular structure can have a significant impact on the molecular packing structure in thin film, which in turn, will have a strong impact on charge transport of organic semiconductors. This was verified by quantum-chemical calculations that predict a corresponding odd–even effect in the strength of the intermolecular electronic coupling.Chemistry and Chemical BiologyEngineering and Applied Science
First-Principles Semiclassical Initial Value Representation Molecular Dynamics
A method for carrying out semiclassical initial value representation
calculations using first-principles molecular dynamics (FP-SC-IVR) is
presented. This method can extract the full vibrational power spectrum of
carbon dioxide from a single trajectory providing numerical results that agree
with experiment even for Fermi resonant states. The computational demands of
the method are comparable to those of classical single-trajectory calculations,
while describing uniquely quantum features such as the zero-point energy and
Fermi resonances. By propagating the nuclear degrees of freedom using
first-principles Born-Oppenheimer molecular dynamics, the stability of the
method presented is improved considerably when compared to dynamics carried out
using fitted potential energy surfaces and numerical derivatives.Comment: 5 pages, 2 figures, made stylistic and clarity change
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Confined organization of fullerene units along high polymer chains
Conductive fullerene (C_60) units were designed to be arranged in one dimensional close contact by locally organizing them with covalent bonds in a spatially constrained manner. Combined molecular dynamics and quantum chemical calculations predicted that the intramolecular electronic interactions (i.e. charge transport) between the pendant C_60 units could be controlled by the length of the spacers linking the C_60 units and the polymer main chain. In this context, C_60 side-chain polymers with high relative degrees of polymerization up to 1220 and fullerene compositions up to 53% were synthesized by ruthenium catalyzed ring-opening metathesis polymerization of the corresponding norbornene-functionalized monomers. UV/vis absorption and photothermal deflection spectra corroborated the enhanced inter-fullerene interactions along the polymer chains. The electron mobility measured for the thin film field-effect transistor devices from the polymers was more than an order of magnitude higher than that from the monomers, as a result of the stronger electronic coupling between the adjacent fullerene units within the long polymer chains. This molecular design strategy represents a general approach to the enhancement of charge transport properties of organic materials via covalent bond-based organization