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1.
J Chem Phys ; 152(7): 074201, 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32087671

RESUMO

Cyanamides (NCN) have been shown to have a larger transition dipole strength than cyano-probes. In addition, they have similar structural characteristics and vibrational lifetimes to the azido-group, suggesting their utility as infrared (IR) spectroscopic reporters for structural dynamics in biomolecules. To access the efficacy of NCN as an IR probe to capture the changes in the local environment, several model systems were evaluated via 2D IR spectroscopy. Previous work by Cho [G. Lee, D. Kossowska, J. Lim, S. Kim, H. Han, K. Kwak, and M. Cho, J. Phys. Chem. B 122(14), 4035-4044 (2018)] showed that phenylalanine analogues containing NCN show strong anharmonic coupling that can complicate the interpretation of structural dynamics. However, when NCN is embedded in 5-membered ring scaffolds, as in N-cyanomaleimide and N-cyanosuccinimide, a unique band structure is observed in the 2D IR spectrum that is not predicted by simple anharmonic frequency calculations. Further investigation indicated that electron delocalization plays a role in the origins of the band structure. In particular, the origin of the lower frequency transitions is likely a result of direct interaction with the solvent.

2.
Inorg Chem ; 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-32049500

RESUMO

Organometallic uranium complexes that can activate small molecules are well-known. In contrast, there are no known organometallic trans-uranium species capable of small-molecule transformations. Using density functional theory, we previously showed that changing actinide-ligand bonds from U-O groups to Np-N- (amide/imido) bonds makes redox small-molecule activation more energetically favorable for Np species. Here, we determine how general this ligand-modulation strategy is for affecting small-molecule activation in Np species. We focus on two reactions, one involving redox transformation of the actinide(s) and the other involving no change in the oxidation state of the actinide(s). Specifically, we considered the hydrogen evolution reaction (HER) from H2O by actinide tris-aryloxide species. We also considered H2 capture and hydride transfer by actinide siloxide and silylamide complexes. For the HER, the barriers for Np(III) systems are much higher than those of U(III). The overall reaction energies are also much worse. An-O → An-N substitutions marginally improve the barriers by 1-4 kcal/mol and more substantially improve the reaction energies by 9-15 kcal/mol. For H2 capture and hydride transfer, the reaction energies for the U and Np species are similar. For both actinides, like-for-like An-O → An-N substitutions lead to improved reaction energies. Interestingly, in a recent report, it seemingly appears that U-O (siloxide) → U-N (silylamide) leads to complete shutdown of reactivity for H2 capture and hydride transfer. This observation is reproduced and explained with calculations. The ligand environments of the siloxide and silylamide that were compared are vastly different. The steric environment of the siloxide is conducive for reactivity while the particular silylamide is not. We conclude that small-molecule activation with organometallic neptunium species is achievable with a guided choice of ligands. Additional emphasis should be placed on ligands that can allow for improved transition state barriers.

3.
J Chem Phys ; 151(13): 134102, 2019 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-31594337

RESUMO

We have examined the performance of Multiconfiguration Pair-Density Functional Theory (MC-PDFT) for computing the ground-state properties of actinide species. Specifically, we focused on the properties of UN2 and various actinyl species. The properties obtained with MC-PDFT at the scalar-relativistic level are compared to Kohn-Sham DFT (KS-DFT); complete active space self-consistent field theory, CASSCF; coupled-cluster theory, CCSD(T) and CCSDT; as well as multireference perturbation theory (CASPT2). We examine the degree to which MC-PDFT improves over KS-DFT and CASSCF while aligning with CASPT2, CCSD(T), and CCSDT. All properties that we considered were for the CASPT2 electronic ground states. For structural parameters, MC-PDFT confers very little advantage over KS-DFT, especially the B3LYP density functional. For NpO2 3+, MC-PDFT and local KS-DFT functionals excessively favor the bent structure, whereas CCSDT and CASPT2 predict the bent and linear structures as isoenergetic. For this special case, hybrid KS-DFT functionals like PBE0 and B3LYP provide results closer to CASPT2 and CCSDT than MC-PDFT. On a more positive note, MC-PDFT is very close to CASPT2 and CCSD(T) for the redox potentials, energetics of redox chemical reactions, as well as ligand-binding energies. These are encouraging results since MC-PDFT is more affordable. The best MC-PDFT functional is ft-PBE. Our findings suggest that MC-PDFT can be used to study systems and excited states with larger strong electron correlation effects than were considered here. However, for the systems and properties considered here, KS-DFT functionals do well, justifying their usage as the bulwark of computational actinyl chemistry over the last two to three decades.

4.
J Org Chem ; 84(18): 11856-11862, 2019 09 20.
Artigo em Inglês | MEDLINE | ID: mdl-31438666

RESUMO

Dipyrrinones are nonfluorescent yellow-pigmented constituents of bilirubin that undergo Z to E isomerization when excited with UV/blue light. Mechanical restriction of the E/Z isomerization process results in highly fluorescent compounds such as N,N-methylene-bridged dipyrrinones and xanthoglows. This manuscript describes the first examples of dipyrrinone analogues, which exhibit fluorescence without covalently linking the pyrole-pyrrolidine nitrogen atoms. Instead these analogues restrict E/Z isomerization through intramolecular hydrogen bonding, resulting in mild to moderately fluorescent compounds (ΦF = 0.01-0.30). Further, in basic solutions (pH > 12), the dipyrrinone analogues readily deprotonate and absorption/emission profiles of the fluorophores red-shifts by 10-49 nm. Directly from commercial materials, 10 analogues were prepared in 41-96% yields in one step. To estimate the capacity of which intramolecular hydrogen bonding has upon restricting the E/Z isomerization process, conformational energies of all analogues, in both the protonated and deprotonated species, were explored by using quantum-mechanical density functional theory (DFT) and time-dependent DFT calculations. The computed strengths of the intramolecular hydrogen bonds are related to the barriers of rotation about the 5-6 bond and both correlate with the experimentally measured fluorescence quantum yields.

5.
Inorg Chem ; 58(10): 6731-6741, 2019 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-31050297

RESUMO

There is recent interest in organometallic complexes of the trans-uranium elements. However, preparation and characterization of such complexes are hampered by radioactivity and chemotoxicity issues as well as the air-sensitive and poorly understood behavior of existing compounds. As such, there are no examples of small-molecule activation via redox reactivity of organometallic trans-uranium complexes. This contrasts with the situation for uranium. Indeed, a multimetallic uranium(III) nitride complex was recently synthesized, characterized, and shown to be able to capture and functionalize molecular nitrogen (N2) through a four-electron reduction process, N2 → N24-. The bis-uranium nitride, U-N-U core of this complex is held in a potassium siloxide framework. Importantly, the N24- product could be further functionalized to yield ammonia (NH3) and other desirable species. Using the U-N-U potassium siloxide complex, K3U-N-U, and its cesium analogue, Cs3U-N-U, as starting points, we use scalar-relativistic and spin-orbit coupled density functional theory calculations to shed light on the energetics and mechanism for N2 capture and functionalization. The N2 → N24- reactivity depends on the redox potentials of the U(III) centers and crucially on the stability of the starting complex with respect to decomposition into the mixed oxidation U(IV)/U(III) K2U-N-U or Cs2U-N-U species. For the trans-uranium, Np and Pu analogues of K3U-N-U, the N2 → N24- process is endoergic and would not occur. Interestingly, modification of the Np-O and Pu-O bonds between the actinide cores and the coordinated siloxide framework to Np-NH, Pu-NH, Np-CH2, and Pu-CH2 bonds drastically improves the reaction free energies. The Np-NH species are stable and can reductively capture and reduce N2 to N24-. This is supported by analysis of the spin densities, molecular structure, long-range dispersion effects, as well as spin-orbit coupling effects. These findings chart a path for achieving small-molecule activation with organometallic neptunium analogues of existing uranium complexes.

6.
J Comput Chem ; 39(32): 2667-2678, 2018 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-30379335

RESUMO

Methane-to-methanol conversion (MMC) can be facilitated with high methanol selectivities by copper-exchanged zeolites. There are however two open questions regarding the use of these zeolites to facilitate the MMC process. The first concerns the possibility of operating the three cycles in the stepwise MMC process by these zeolites in an isothermal fashion. The second concerns the possibility of improving the methanol yields by systematic substitution of some copper centers in these active sites with other earth-abundant transition metals. Quantum-mechanical computations can be used to compare methane activation by copper oxide species and analogous mixed-metal systems. To carry out such screening, it is important that we use theoretical methods that are accurate and computationally affordable for describing the properties of the hetero-metallic catalytic species. We have examined the performance of 47 exchange-correlation density functionals for predicting the relative spin-state energies and chemical reactivities of six hetero-metallic [M-O-Cu]2+ and [M-O2 -Cu]2+ , (where MCo, Fe, and Ni), species by comparison with coupled cluster theory including iterative single, double excitations as well as perturbative treatment of triple excitations, CCSD(T). We also performed multireference calculations on some of these systems. We considered two types of reactions (hydrogen addition and oxygen addition) that are relevant to MMC. We recommend the use of τ-HCTH and OLYP to determine the spin-state energy splittings in the hetero-metallic motifs. ωB97, ωB97X, ωB97X-D3, and MN15 performed best for predicting the energies of the hydrogen and oxygen addition reactions. In contrast, local, and semilocal functionals do poorly for chemical reactivity. Using [Fe-O-Cu]2+ as a test, we see that the nonlocal functionals perform well for the methane CH activation barrier. In contrast, the semilocal functionals perform rather poorly. © 2018 Wiley Periodicals, Inc.

7.
J Chem Theory Comput ; 13(2): 616-626, 2017 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-28001390

RESUMO

We study the performance of multiconfiguration pair-density functional theory (MC-PDFT) and multireference perturbation theory for the computation of the bond dissociation energies in 12 transition-metal-containing diatomic molecules and three small transition-metal-containing polyatomic molecules and in two transition-metal dimers. The first step is a multiconfiguration self-consistent-field calculation, for which two choices must be made: (i) the active space and (ii) its partition into subspaces, if the generalized active space formulation is used. In the present work, the active space is chosen systematically by using three correlated-participating-orbitals (CPO) schemes, and the partition is chosen by using the separated-pair (SP) approximation. Our calculations show that MC-PDFT generally has similar accuracy to CASPT2, and the active-space dependence of MC-PDFT is not very great for transition-metal-ligand bond dissociation energies. We also find that the SP approximation works very well, and in particular SP with the fully translated BLYP functional SP-ftBLYP is more accurate than CASPT2. SP greatly reduces the number of configuration state functions relative to CASSCF. For the cases of FeO and NiO with extended-CPO active space, for which complete active space calculations are unaffordable, SP calculations are not only affordable but also of satisfactory accuracy. All of the MC-PDFT results are significantly better than the corresponding results with broken-symmetry spin-unrestricted Kohn-Sham density functional theory. Finally we test a perturbation theory method based on the SP reference and find that it performs slightly worse than CASPT2 calculations, and for most cases of the nominal-CPO active space, the approximate SP perturbation theory calculations are less accurate than the much less expensive SP-PDFT calculations.

8.
J Phys Chem B ; 120(50): 12883-12889, 2016 12 22.
Artigo em Inglês | MEDLINE | ID: mdl-27957849

RESUMO

Phosphatidylserine (PS) has previously been found to bind Cu2+ in a ratio of 1 Cu2+ ion per 2 PS lipids to form a complex with an apparent dissociation constant that can be as low as picomolar. While the affinity of Cu2+ for lipid membranes containing PS lipids has been well characterized, the structural details of the Cu-PS2 complex have not yet been reported. Coordinating to one amine and one carboxylate moiety on two separate PS lipids, the Cu-PS2 complex is unique among ion-lipid complexes in its ability to adopt both cis and trans conformations. Herein, we determine which stereoisomer of the Cu-PS2 complex is favored in lipid bilayers using density functional theory calculations and electron paramagnetic resonance experiments. It was determined that a conformation in which the nitrogen centers are cis to each other is the preferred binding geometry. This is in contrast to the complex formed when two glycine molecules bind to Cu2+ in bulk solution, where the cis and trans isomers exist in equilibrium, indicating that the lipid environment has a significant steric effect on the Cu2+ binding conformation. These findings are relevant for understanding lipid oxidation caused by Cu2+ binding to lipid membrane surfaces and will help us understand how ion binding to lipid membranes can affect their physical properties.


Assuntos
Cobre/química , Bicamadas Lipídicas/química , Fosfatidilcolinas/química , Fosfatidilserinas/química , Cátions Bivalentes , Espectroscopia de Ressonância de Spin Eletrônica , Concentração de Íons de Hidrogênio , Oxirredução , Teoria Quântica , Estereoisomerismo , Termodinâmica
9.
ACS Nano ; 10(4): 4372-83, 2016 04 26.
Artigo em Inglês | MEDLINE | ID: mdl-27017971

RESUMO

We report the synthesis, transport measurements, and electronic structure of conjugation-broken oligophenyleneimine (CB-OPI 6) molecular wires with lengths of ∼4 nm. The wires were grown from Au surfaces using stepwise aryl imine condensation reactions between 1,4-diaminobenzene and terephthalaldehyde (1,4-benzenedicarbaldehyde). Saturated spacers (conjugation breakers) were introduced into the molecular backbone by replacing the aromatic diamine with trans-1,4-diaminocyclohexane at specific steps during the growth processes. FT-IR and ellipsometry were used to follow the imination reactions on Au surfaces. Surface coverages (∼4 molecules/nm(2)) and electronic structures of the wires were determined by cyclic voltammetry and UV-vis spectroscopy, respectively. The current-voltage (I-V) characteristics of the wires were acquired using conducting probe atomic force microscopy (CP-AFM) in which an Au-coated AFM probe was brought into contact with the wires to form metal-molecule-metal junctions with contact areas of ∼50 nm(2). The low bias resistance increased with the number of saturated spacers, but was not sensitive to the position of the spacer within the wire. Temperature dependent measurements of resistance were consistent with a localized charge (polaron) hopping mechanism in all of the wires. Activation energies were in the range of 0.18-0.26 eV (4.2-6.0 kcal/mol) with the highest belonging to the fully conjugated OPI 6 wire and the lowest to the CB3,5-OPI 6 wire (the wire with two saturated spacers). For the two other wires with a single conjugation breaker, CB3-OPI 6 and CB5-OPI 6, activation energies of 0.20 eV (4.6 kcal/mol) and 0.21 eV (4.8 kcal/mol) were found, respectively. Computational studies using density functional theory confirmed the polaronic nature of charge carriers but predicted that the semiclassical activation energy of hopping should be higher for CB-OPI molecular wires than for the OPI 6 wire. To reconcile the experimental and computational results, we propose that the transport mechanism is thermally assisted polaron tunneling in the case of CB-OPI wires, which is consistent with their increased resistance.

10.
Inorg Chem ; 55(7): 3541-6, 2016 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-26974702

RESUMO

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during in situ mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO2)(O2)(H2O)2](H2O)2, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts with water to release O2 gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U2O7 conformer consists of two bent (UO2)(2+) uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a µ2-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do (1)H- and (17)O-nuclear magnetic resonance spectra. The reactivity of U2O7 in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle.

11.
Chem Sci ; 7(3): 2399-2413, 2016 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29997782

RESUMO

Multi-configuration pair-density functional theory (MC-PDFT) has proved to be a powerful way to combine the capabilities of multi-configuration self-consistent-field theory to represent the an electronic wave function with a highly efficient way to include dynamic correlation energy by density functional theory. All applications reported previously involved complete active space self-consistent-field (CASSCF) theory for the reference wave function. For treating large systems efficiently, it is necessary to ask whether good accuracy is retained when using less complete configuration interaction spaces. To answer this question, we present here calculations employing MC-PDFT with the separated pair (SP) approximation, which is a special case (defined in this article) of generalized active space self-consistent-field (GASSCF) theory in which no more than two orbitals are included in any GAS subspace and in which inter-subspace excitations are excluded. This special case of MC-PDFT will be called SP-PDFT. In SP-PDFT, the electronic kinetic energy and the classical Coulomb energy, the electronic density and its gradient, and the on-top pair density and its gradient are obtained from an SP approximation wave function; the electronic energy is then calculated from the first two of these quantities and an on-top density functional of the last four. The accuracy of the SP-PDFT method for predicting the structural properties and bond dissociation energies of twelve diatomic molecules and two triatomic molecules is compared to the SP approximation itself and to CASSCF, MC-PDFT based on CASSCF, CASSCF followed by second order perturbation theory (CASPT2), and Kohn-Sham density functional theory with the PBE exchange-correlation potential. We show that SP-PDFT reproduces the accuracy of MC-PDFT based on the corresponding CASSCF wave function for predicting C-H bond dissociation energies, the reaction barriers of pericyclic reactions and the properties of open-shell singlet systems, all at only a small fraction of the computational cost.

12.
J Am Chem Soc ; 137(50): 15732-41, 2015 Dec 23.
Artigo em Inglês | MEDLINE | ID: mdl-26575438

RESUMO

Self-assembled conjugated molecular wires containing thiophene up to 6 nm in length were grown layer-by-layer using click chemistry. Reflection-absorption infrared spectroscopy, ellipsometry and X-ray photoelectron spectroscopy were used to follow the stepwise growth. The electronic structure of the conjugated wires was studied with cyclic voltammetry and UV-vis spectroscopy as well as computationally with density functional theory (DFT). The current-voltage curves (±1 V) of the conjugated molecular wires were measured with conducting probe atomic force microscopy (CP-AFM) in which the molecular wire film bound to a gold substrate was contacted with a conductive AFM probe. By systematically measuring the low bias junction resistance as a function of length for molecules 1-4 nm long, we extracted the structure dependent tunneling attenuation factor (ß) of 3.4 nm(-1) and a contact resistance of 220 kΩ. The crossover from tunneling to hopping transport was observed at a molecular length of 4-5 nm with an activation energy of 0.35 eV extracted from Arrhenius plots of resistance versus temperature. DFT calculations revealed localizations of spin densities (polarons) on molecular wire radical cations. The calculations were employed to gauge transition state energies for hopping of polarons along wire segments. Individual estimated transition state energies were 0.2-0.4 eV, in good agreement with the experimental activation energy. The transition states correspond to flattening of dihedral angles about specific imine bonds. These results open up possibilities to further explore the influence of molecular architecture on hopping transport in molecular junctions, and highlight the utility of DFT to understand charge localization and associated hopping-based transport.

13.
J Chem Theory Comput ; 11(9): 4093-101, 2015 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-26575905

RESUMO

The electronic structure of a diiron (FeFe) complex with strong metal-metal interaction and those of analogous complexes (CoCo, CoMn, CoFe, and FeMn) with much weaker metal-metal bonding are investigated with wave function-based methods and density functional theory. The delocalization and bonding between the metal centers in the diiron complex is only fully captured after inclusion of the complete set of 3d and 4d orbitals in the active space, a situation best suited for restricted active space (RAS) approaches. Truncation of the included set of 4d orbitals results in inappropriate localization of some 3d orbitals, incorrect description of the ground spin state as well as wrong spin state energetics, as compared to experiment. Using density functional theory, some local functionals are able to predict the correct ground spin states, and describe the chemical bonding and structural properties of all the metal-metal complexes considered in this work. In contrast, the introduction of some exact exchange results in increased localization of 3d orbitals and wrong spin state energetics, a situation that is particularly troublesome for the diiron complex.

14.
Phys Chem Chem Phys ; 17(33): 21448-57, 2015 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-26219236

RESUMO

Diamine-appended metal-organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO2 molecules insert into the metal-nitrogen bonds to form ordered ammonium carbamate chains [McDonald et al., Nature, 2015, 519, 303]. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations. Distinct spectral changes at the N and O K-edges are apparent upon CO2 adsorption in both mmen-Mg2(dobpdc) and mmen-Mn2(dobpdc), and these are evaluated based upon computed spectra from three potential adsorption structures. The computations reveal that the observed spectral changes arise from specific electronic states that are signatures of a quasi-trigonal planar carbamate species that is hydrogen bonded to an ammonium cation. This eliminates two of the three structures studied, and confirms the insertion mechanism. We note the particular sensitivity of X-ray absorption spectra to the insertion step of this mechanism, underpinning the strength of the technique for examining subtle chemical changes upon gas adsorption.

15.
J Am Chem Soc ; 137(23): 7391-6, 2015 Jun 17.
Artigo em Inglês | MEDLINE | ID: mdl-25990021

RESUMO

Metal-organic frameworks with Zr6 nodes, UiO-66 and NU-1000, were investigated as supports for Ir(CO)2 and Ir(C2H4)2 complexes. A single bonding site for the iridium is identified on the nodes of NU-1000, whereas two sites are identified on UiO-66, although at low iridium loadings only one site is occupied. Density functional theory calculations provide structural results that are in good agreement with infrared and X-ray absorption fine-structure spectra. The reactivity of node-supported Ir(CO)2 with C2H4 and the catalytic activity and selectivity of the species initially present as Ir(C2H4)2 for ethylene hydrogenation and dimerization were investigated both experimentally and computationally and shown to be strongly influenced by the node.

16.
J Am Chem Soc ; 137(14): 4690-700, 2015 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-25830409

RESUMO

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, ((Mes)PDI(Me))UI3(THF) (1), ((Mes)PDI(Me))UI2(THF)2 (2), [((Mes)PDI(Me))UI]2 (3), and [((Mes)PDI(Me))U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f (2) configurations and reduced ligand frameworks, specifically [(Mes)PDI(Me)](•/-), [(Mes)PDI(Me)](2-), [(Mes)PDI(Me)](3-) and [(Mes)PDI(Me)](4-), respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of -116.0 and -144.4 kcal mol(-1) were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium-arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η(5)-pyridyl ring.


Assuntos
Dimerização , Elétrons , Compostos Organometálicos/química , Piridinas/química , Urânio/química , Ligantes , Fenômenos Magnéticos , Modelos Moleculares , Conformação Molecular , Oxirredução , Teoria Quântica
18.
Nature ; 519(7543): 303-8, 2015 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-25762144

RESUMO

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as 'phase-change' adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg(2+) within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.


Assuntos
Aminas/química , Dióxido de Carbono/química , Dióxido de Carbono/isolamento & purificação , Sequestro de Carbono , Adsorção , Efeito Estufa/prevenção & controle , Magnésio/metabolismo , Ribulose-Bifosfato Carboxilase/química , Ribulose-Bifosfato Carboxilase/metabolismo , Temperatura , Difração de Raios X
19.
Chem Sci ; 6(9): 5177-5185, 2015 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-28717499

RESUMO

Using a combination of density functional theory and lattice models, we study the effect of CO2 adsorption in an amine functionalized metal-organic framework. These materials exhibit a step in the adsorption isotherm indicative of a phase change. The pressure at which this step occurs is not only temperature dependent but is also metal center dependent. Likewise, the heats of adsorption vary depending on the metal center. Herein we demonstrate via quantum chemical calculations that the amines should not be considered firmly anchored to the framework and we explore the mechanism for CO2 adsorption. An ammonium carbamate species is formed via the insertion of CO2 into the M-Namine bonds. Furthermore, we translate the quantum chemical results into isotherms using a coarse grained Monte Carlo simulation technique and show that this adsorption mechanism can explain the characteristic step observed in the experimental isotherm while a previously proposed mechanism cannot. Furthermore, metal analogues have been explored and the CO2 binding energies show a strong metal dependence corresponding to the M-Namine bond strength. We show that this difference can be exploited to tune the pressure at which the step in the isotherm occurs. Additionally, the mmen-Ni2(dobpdc) framework shows Langmuir like behavior, and our simulations show how this can be explained by competitive adsorption between the new model and a previously proposed model.

20.
J Am Chem Soc ; 136(50): 17484-94, 2014 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-25411020

RESUMO

The capture of uranyl, UO2(2+), by a recently engineered protein (Zhou et al. Nat. Chem. 2014, 6, 236) with high selectivity and femtomolar sensitivity has been examined by a combination of density functional theory, molecular dynamics, and free-energy simulations. It was found that UO2(2+) is coordinated to five carboxylate oxygen atoms from four amino acid residues of the super uranyl binding protein (SUP). A network of hydrogen bonds between the amino acid residues coordinated to UO2(2+) and residues in its second coordination sphere also affects the protein's uranyl binding affinity. Free-energy simulations show how UO2(2+) capture is governed by the nature of the amino acid residues in the binding site, the integrity and strength of the second-sphere hydrogen bond network, and the number of water molecules in the first coordination sphere. Alteration of any of these three factors through mutations generally results in a reduction of the binding free energy of UO2(2+) to the aqueous protein as well as of the difference between the binding free energies of UO2(2+) and other ions (Ca(2+), Cu(2+), Mg(2+), and Zn(2+)), a proxy for the protein's selectivity over these ions. The results of our free-energy simulations confirmed the previously reported experimental results and allowed us to discover a mutant of SUP, specifically the GLU64ASP mutant, that not only binds UO2(2+) more strongly than SUP but that is also more selective for UO2(2+) over other ions. The predictions from the computations were confirmed experimentally.


Assuntos
Simulação de Dinâmica Molecular , Proteínas/química , Urânio/química , Desenho de Fármacos , Modelos Moleculares , Ligação Proteica
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