1,721,017 research outputs found
Efficiency of pi-pi tunneling in [2]rotaxane molecular electronic switches
No full textWe perform large-scale density functional and matrix Green's function calculations, and study the coherent charge tunneling properties of molecular electronic devices based on the central part of [2]rotaxane molecules. We extract molecular core regions from realistic monolayer configurations with folded molecular structures and sandwich them between Au(111) electrodes to form device models. We show that the electrical switching behavior can be observed within the pi-pi stacked serial arrangement of redox-active components in the [2]rotaxane monolayer as with the parallel arrangement in the [2]catenane case. We thus demonstrate the effectiveness of the pi-pi electron tunneling and the universality of the switching mechanism based on the energetic movement of frontier orbitals accompanying the conformational switching. In addition, via considering the energetic ordering of highest-occupied molecular orbital (HOMO) and HOMO-1 levels that originated from tetrathiafulvalene and dioxynaphthalene in several ground-state conformation device models, we show that the molecule-electrode configurations critically affect the device functionality
Calculation of solvation free energies of charged solutes using mixed cluster/continuum models
No full textWe derive a consistent approach for predicting the solvation free energies of charged solutes in the presence of implicit and explicit solvents. We find that some published methodologies make systematic errors in the computed free energies because of the incorrect accounting of the standard state corrections for water molecules or water clusters present in the thermodynamic cycle. This problem can be avoided by using the same standard state for each species involved in the reaction under consideration. We analyze two different thermodynamic cycles for calculating the solvation free energies of ionic solutes: (1) the cluster cycle with an n water cluster as a reagent and (2) the monomer cycle with n distinct water molecules as reagents. The use of the cluster cycle gives solvation free energies that are in excellent agreement with the experimental values obtained from studies of ion-water clusters. The mean absolute errors are 0.8 kcal/mol for H+ and 2.0 kcal/mol for Cu2+. Conversely, calculations using the monomer cycle lead to mean absolute errors that are >10 kcal/mol for HI and >30 kcal/mol for Cu2+. The presence of hydrogen-bonded clusters of similar size on the left- and right-hand sides of the reaction cycle results in the cancelation of the systematic errors in the calculated free energies. Using the cluster cycle with 1 solvation shell leads to errors of 5 kcal/mol for H+ (6 waters) and 27 kcal/mol for Cu2+ (6 waters), whereas using 2 solvation shells leads to accuracies of 2 kcal/mol for Cu2+ (18 waters) and 1 kcal/mol for H+ (10 waters)
Investigations of heterogeneous and homogeneous transition metal catalysis using density functional theory.
No full textInvestigations of heterogeneous and homogeneous transition metal catalysis using density functional theory
Dendritic anion hosts: Perchlorate uptake by G5-NH2 poly(propyleneimine) dendrimer in water and model electrolyte solutions
No full textPerchlorate (ClO4-) has emerged as a major groundwater and surface water contaminant in the United States. Ion exchange (IX) is the most widely used technology for treating water containing lower concentrations of perchlorate (< 100 ppb). However, a major drawback of IX is the need for frequent regeneration or disposal of the perchlorate-laden resins. As a first step toward the development of high-capacity, selective and recyclable dendritic ligands for the recovery of perchlorate from aqueous solutions by dendrimer filtration, we tested the hypothesis that dendrimers with hydrophobic cavities and positively charged internal groups should selectively bind ClO4- over more hydrophilic anions such as Cl-, NO3, SO42-, and HCO3-. We measured the uptake Of ClO4- by the fifth generation (G5-NH2) poly(propyleneimine) (PPI) dendrimer with a diaminobutane core and terminal NH2 groups in deonized water and model electrolyte solutions as a function of (i) anion-dendrimer loading, (ii) solution pH, (iii) background electrolyte concentration, and (iv) reaction time. The ClO4- binding capacity of this dendrimer is comparable to those of perchlorate-selective IX resins. However, its ClO4- binding kinetics is faster and reaches equilibrium in similar to 1 h. Note also that only a high pH (similar to 9.0) aqueous solution is needed to release more than 90% of the bound C104- anions by deprotonation of the dendrimer tertiary amine groups. The overall results of this study suggest that dendritic macromolecules such as the G5-PPI NH2 dendrimer provide ideal building blocks for the development of high-capacity, selective and recyclable ligands for the recovery of anions such as perchlorate from aqueous solutions by dendrimer enhanced filtration
PAMAM Dendrimers Undergo pH Responsive Conformational Changes without Swelling
No full textAtomistic molecular dynamics (MD) simulations of a G4-NH(2) PAMAM dendrimer were carried out in aqueous solution using explicit water molecules and counterions (with the Dreiding III force field optimized using quantum mechanics). Our simulations predict that the radius of gyration (R(g)) of the dendrimer changes little with pH from 21.1 angstrom at pH similar to 10 (uncharged PAMAM) to 22.1 angstrom at pH similar to 5 (charged with 126 protons), which agrees quantitatively with recent small angle neutron scattering (SANS) experiments (from 21.4 angstrom at pH 10 to 21.5 angstrom at pH 5). Even so we predict a dramatic change in the conformation. The ion pairing in the low pH form leads to a locally compact dense shell with an internal surface area only 37% of the high pH form with a dense core. This transformation from "dense core" at high pH to "dense shell" at low pH could facilitate the encapsulation and release of guest molecules (e.g., drugs) using pH as the trigger, making dendrimers a unique drug delivery vehicle
Hydrogen storage in LiAlH4: Predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics
No full textWe use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH4. First, we explore the decomposition mechanism of monoclinic LiAlH4 into monoclinic Li3AlH6 plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH4 and Li3AlH6 agree well with experimental data. On the other hand, the alternative decomposition of LiAlH4 into LiAlH2 plus H-2 is predicted to be unstable with respect to that through Li3AlH6. Next, we investigate thermal decomposition of Li3AlH6 into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH6 to form the stable Li-H-Li-H complex. Then, two sequential H-2 desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li3AlH6(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H-2 is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+1/2H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl. (C) 2004 American Institute of Physics
Evaluation of B3LYP, X3LYP, and M06-Class Density Functionals for Predicting the Binding Energies of Neutral, Protonated, and Deprotonated Water Clusters
No full textIn this paper we assess the accuracy of the B3LYP, X3LYP, and newly developed M06-L, M06-2X, and M06 functionals to predict the binding energies of neutral and charged water clusters including (H(2)O)(n), n = 2-8, 20), H(3)O(+)(H(2)O)(n), n = 1-6, and OH(-)(H(2)O)(n), n = 1-6. We also compare the predicted energies of two ion hydration and neutralization reactions on the basis of the calculated binding energies. In all cases, we use as benchmarks calculated binding energies of water clusters extrapolated to the complete basis set limit of the second-order Moller-Plesset perturbation theory with the effects of higher order correlation estimated at the coupled-cluster theory with single, double, and perturbative triple excitations in the aug-cc-pVDZ basis set. We rank the accuracy of the functionals on the basis of the mean unsigned error (MUE) between calculated benchmark and density functional theory energies. The corresponding MUE (kcal/mol) for each functional is listed in parentheses. We find that M06-L (0.73) and M06 (0.84) give the most accurate binding energies using very extended basis sets such as aug-cc-pV5Z. For more affordable basis sets, the best methods for predicting the binding energies of water clusters are M06-L/aug-cc-pVTZ (1.24), B3LYP/6-311++G(2d,2p) (1.29), and M06/aug-cc-PVTZ (1.33). M06-L/aug-cc-pVTZ also gives more accurate energies for the neutralization reactions (1.38), whereas B3LYP/6-311++G(2d,2p) gives more accurate energies for the ion hydration reactions (1.69)
Dendritic Chelating agents. 2. U(VI) binding to poly(amidoamine) and poly(propyleneimine) dendrimers in aqueous solutions
No full textChelating agents are widely employed in many separation processes used to recover uranyl [U(VI)] from contaminated aqueous solutions. This article describes an experimental investigation of the binding of U(VI)to poly(amidoamine) [PAMAM] and poly(propyleneimine) [PPl] dendrimers in aqueous solutions. We combine fluorescence spectroscopy with bench scale ultrafiltration experiments to measure the extent of binding and fractional binding of U(VI) in aqueous solutions of these dendrimers as a function of (i) metal-ion dendrimer loading, (ii) dendrimer generation, (iii) dendrimer core and terminal group chemistry, and (iv) solution pH and competing ligands (NO3-, PO43-, CO32-, and Cl-). The overall results of this study suggest that uranyl binding to PAMAM and PPl dendrimers in aqueous solutions involves the coordination of the UO22+ ions with the dendrimer amine, amide, and carboxylic groups. We find significant binding of U(VI)to PAMAM dendrimers in (i) acidic solutions containing up to 1.0 M HNO3 and H3PO4 and (ii) in basic solutions containing up to 0.5 M Na2CO3. However, no binding of U(VI) by PAMAM dendrimers is observed in aqueous solutions containing 1.0 M NaCl at pH 3.0. These results strongly suggest that PAMAM and PPl dendrimers can serve as high capacity and selective chelating ligands for U(VI) in aqueous solutions
First-principles approach to the charge-transport characteristics of monolayer molecular-electronics devices: Application to hexanedithiolate devices
No full textWe report on the development of an accurate first-principles computational scheme for the charge transport characteristics of molecular monolayer junctions and its application to hexanedithiolate (C6DT) devices. Starting from the Gaussian basis set density-functional calculations of a junction model in the slab geometry and corresponding two bulk electrodes, we obtain the transmission function using the matrix Green's function method and analyze the nature of transmission channels via atomic projected density of states. Within the developed formalism, by treating isolated molecules with the supercell approach, we can investigate the current-voltage characteristics of single and parallel molecular wires in a consistent manner. For the case of single C6DT molecules stretched between Au(111) electrodes, we obtain reasonable quantitative agreement of computed conductance with a recent scanning tunneling microscope experiment result. Comparing the charge transport properties of C6DT single molecules and their monolayer counterparts in the stretched and tilted geometries, we find that the effect of intermolecular coupling and molecule tilting on the charge transport characteristics is negligible in these devices. We contrast this behavior to that of the pi-conjugated biphenyldithiolate devices we have previously considered and discuss the relative importance of molecular cores and molecule-electrode contacts for the charge transport in those devices
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