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1.
Inorg Chem ; 2020 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-32678585

RESUMO

An analysis of how different density functionals, basis sets, and relativistic approximations affect the computed properties of lanthanide-containing molecules allows one to determine which method provides the highest accuracy. Historically, many different density functional methods have been employed to perform calculations on lanthanide complexes and so herein is a detailed analysis of how different methodological combinations change the computed properties of three different families of lanthanide-bearing species: lanthanide diatomic molecules (fluorides and oxides) and their dissociation energies; larger, molecular complexes and their geometries; and lanthanide bis(2-ethylhexyl)phosphate structures and their separation free energies among the lanthanide series. The B3LYP/Sapporo/Douglas-Kroll-Hess (DKH) method was shown to most accurately reproduce dissociation energies calculated at the CCSDT(Q) level of theory with a mean absolute deviation of 1.3 kcal/mol. For the calculations of larger, molecular complexes, the TPSSh/Sapporo/DKH method led to the smallest deviation from experimentally refined crystal structures. Finally, this same method led to calculated separation factors for lanthanide bis(2-ethylhexyl)phosphate structures that matched very closely with experimental values.

2.
Nat Commun ; 11(1): 3230, 2020 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-32591514

RESUMO

Machine learning and high-throughput computational screening have been valuable tools in accelerated first-principles screening for the discovery of the next generation of functionalized molecules and materials. The application of machine learning for chemical applications requires the conversion of molecular structures to a machine-readable format known as a molecular representation. The choice of such representations impacts the performance and outcomes of chemical machine learning methods. Herein, we present a new concise molecular representation derived from persistent homology, an applied branch of mathematics. We have demonstrated its applicability in a high-throughput computational screening of a large molecular database (GDB-9) with more than 133,000 organic molecules. Our target is to identify novel molecules that selectively interact with CO2. The methodology and performance of the novel molecular fingerprinting method is presented and the new chemically-driven persistence image representation is used to screen the GDB-9 database to suggest molecules and/or functional groups with enhanced properties.

4.
J Phys Chem A ; 123(46): 10116-10122, 2019 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-31670513

RESUMO

High carbon emissions have shown a strong correlation with rising global temperatures as the world's climate undergoes a dramatic shift. Work to mitigate the potential damage using materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and polymer membranes (PMs) has proven successful in small-scale approaches; however, research is still being performed to enhance the capabilities of these materials for use at an industrial scale. One strategy for increasing performance is to embed these materials with CO2-philic molecules, which enhance selective binding over other gases. Calixarenes are promising candidates due to their large chalice shape, which allows for the possibility to bind multiple CO2 molecules per site. In this study, a dataset including 40 functionalized calixarene structures and one unfunctionalized (bare) calixarene was constructed with an automated, high-throughput structure generation through directed modifications to a molecular scaffold. A conformational search based on molecular mechanics allowed the faster determination of optimal binding energies for a vast array of chemical functional groups with less computational effort. Density functional theory and symmetry-adapted perturbation theory calculations were performed for the exploration of their interactions with CO2. Our work has identified new organic cages with increased CO2-philicity. In four cases, CO2 binding is stronger than 9.0 kcal/mol and very close to the targets set by previous studies. The nature of the noncovalent interactions for these cases is analyzed and discussed. Conclusions from this study can aid synthetic efforts for the next generation of functional materials.

5.
Chem Sci ; 10(29): 7029-7042, 2019 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-31588270

RESUMO

Understanding H2 binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H2 with protons and electrons. This work reports the first thermodynamic and kinetic H2 binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al (1), Ga (2), and In (3), and L = [N(o-(NCH2PiPr2)C6H4)3]3-. Thermodynamic free energies (ΔG°) and free energies of activation (ΔG ‡) for binding equilibria were obtained via variable-temperature 31P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H2 and N2 binding to Ni, with ΔG° values for H2 binding found to span nearly the entire range of previous reports. The non-classical H2 adduct, (η2-H2)NiInL (3-H2), was structurally characterized by single-crystal neutron diffraction-the first such study for a Ni(η2-H2) complex or any d10 M(η2-H2) complex. UV-Vis studies and TD-DFT calculations identified specific electronic structure perturbations of the supporting metal which poise NiML complexes for small-molecule binding. ETS-NOCV calculations indicate that H2 binding primarily occurs via H-H σ-donation to the Ni 4p z -based LUMO, which is proposed to become energetically accessible as the Ni(0)→M(iii) dative interaction increases for the larger M(iii) ions. Linear free-energy relationships are discussed, with the activation barrier for H2 binding (ΔG ‡) found to decrease proportionally for more thermodynamically favorable equilibria. The ΔG° values for H2 and N2 binding to NiML complexes were also found to be more exergonic for the larger M(iii) ions.

6.
Angew Chem Int Ed Engl ; 58(46): 16533-16537, 2019 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-31529667

RESUMO

We report the first study of a gas-phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal-organic framework (MOF). Specifically, CuRhBTC (BTC3- =benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi-technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh2+ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh2+ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh2+ sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design.

7.
J Phys Chem Lett ; 10(14): 4129-4135, 2019 Jul 18.
Artigo em Inglês | MEDLINE | ID: mdl-31290671

RESUMO

Solving the coupled-cluster (CC) equations is a cost-prohibitive process that exhibits poor scaling with system size. These equations are solved by determining the set of amplitudes (t) that minimize the system energy with respect to the coupled-cluster equations at the selected level of truncation. Here, a novel approach to predict the converged coupled-cluster singles and doubles (CCSD) amplitudes, thus the coupled-cluster wave function, is explored by using machine learning and electronic structure properties inherent to the MP2 level. Features are collected from quantum chemical data, such as orbital energies, one-electron Hamiltonian, Coulomb, and exchange terms. The data-driven CCSD (DDCCSD) is not an alchemical method because the actual iterative coupled-cluster equations are solved. However, accurate energetics can also be obtained by bypassing solving the CC equations entirely. Our preliminary data show that it is possible to achieve remarkable speedups in solving the CCSD equations, especially when the correct physics are encoded and used for training of machine learning models.

8.
Angew Chem Int Ed Engl ; 58(24): 8115-8118, 2019 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-30974012

RESUMO

Three five-coordinate iron(IV) imide complexes have been synthesized and characterized. These novel structures have disparate spin states on the iron as a function of the R-group attached to the imide, with alkyl groups leading to low-spin diamagnetic (S=0) complexes and an aryl group leading to an intermediate-spin (S=1) complex. The different spin states lead to significant differences in the bonding about the iron center as well as the spectroscopic properties of these complexes. Mössbauer spectroscopy confirmed that all three imide complexes are in the iron(IV) oxidation state. The combination of diamagnetism and 15 N labeling allowed for the first 15 N NMR resonance recorded on an iron imide. Multi-reference calculations corroborate the experimental structural findings and suggest how the bonding is distinctly different on the imide ligand between the two spin states.

9.
J Phys Chem A ; 123(15): 3315-3325, 2019 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-30917281

RESUMO

The separation of nitrogen and oxygen gases is considered as a very challenging process, since both O2 and N2 are nonpolar molecules with similar kinetic diameters. Electronic structure theory can provide a fundamental understanding of effects that can lead to selective binding of nitrogen or oxygen gas for the development of novel separation processes. Boranes can bind dinitrogen through a dative bond, where the boron acts as a σ acceptor and back-donates through π orbitals. To better understand these interactions, we have performed highly accurate CCSD(F12)(T) and CCSDT(Q) computations for the BH3-N2 and BH3-O2 complexes. The coupled-cluster binding energies were used as reference for benchmarking different density functionals, and larger functionalized boranes were examined at the M05/def2-TZVPPD level. Symmetry adapted perturbation theory (SAPT) calculations were performed for the elucidation of the nature of the interaction between nitrogen and substituted boranes and how direct or distal functionalizations affect the strength of the weak dative bonds. By use of these methods, several boranes were found to bind N2 over O2. These molecular species are promising functional groups for incorporation into the next generation of advanced materials for efficient N2/O2 separations.

10.
Chem Rev ; 119(4): 2453-2523, 2019 02 27.
Artigo em Inglês | MEDLINE | ID: mdl-30376310

RESUMO

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

11.
Phys Chem Chem Phys ; 20(45): 28786-28795, 2018 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-30418434

RESUMO

High-valent Fe(iv)-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C-H bonds. Biomimetic Fe(iv)-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C-H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO2+ and a series of non-heme Fe(iv)-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe(iv)-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure-reactivity descriptors that can contribute to the development of the next generation of functional catalysts.

12.
J Phys Chem A ; 122(17): 4357-4365, 2018 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-29630381

RESUMO

We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion-molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 ± 1.4 and 67.4 ± 2.2 kJ mol-1 to the FeII or MnII centers of isolated tetra(4-sulfonatophenyl)metalloporphyrin tetraanions. In contrast, FeIII and MnIII trianion homologues were found to be much less reactive-indicating an upper bound to their dioxygen binding energies of 34 kJ mol-1. We modeled the corresponding O2 adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O2 binding on the FeII or MnII centers and suggested that O2 binds as a superoxide anion.

13.
J Chem Phys ; 147(18): 184111, 2017 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-29141437

RESUMO

A new large-scale parallel multiconfigurational self-consistent field (MCSCF) implementation in the open-source NWChem computational chemistry code is presented. The generalized active space approach is used to partition large configuration interaction (CI) vectors and generate a sufficient number of batches that can be distributed to the available cores. Massively parallel CI calculations with large active spaces can be performed. The new parallel MCSCF implementation is tested for the chromium trimer and for an active space of 20 electrons in 20 orbitals, which can now routinely be performed. Unprecedented CI calculations with an active space of 22 electrons in 22 orbitals for the pentacene systems were performed and a single CI iteration calculation with an active space of 24 electrons in 24 orbitals for the chromium tetramer was possible. The chromium tetramer corresponds to a CI expansion of one trillion Slater determinants (914 058 513 424) and is the largest conventional CI calculation attempted up to date.

14.
J Phys Chem C Nanomater Interfaces ; 121(40): 22295-22302, 2017 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-29051794

RESUMO

Identifying Cu-exchanged zeolites able to activate C-H bonds and selectively convert methane to methanol is a challenge in the field of biomimetic heterogeneous catalysis. Recent experiments point to the importance of trinuclear [Cu3(µ-O)3]2+ complexes inside the micropores of mordenite (MOR) zeolite for selective oxo-functionalization of methane. The electronic structures of these species, namely, the oxidation state of Cu ions and the reactive character of the oxygen centers, are not yet fully understood. In this study, we performed a detailed analysis of the electronic structure of the [Cu3(µ-O)3]2+ site using multiconfigurational wave-function-based methods and density functional theory. The calculations reveal that all Cu sites in the cluster are predominantly present in the Cu(II) formal oxidation state with a minor contribution from Cu(III), whereas two out of three oxygen anions possess a radical character. These electronic properties, along with the high accessibility of the out-of-plane oxygen center, make this oxygen the preferred site for the homolytic C-H activation of methane by [Cu3(µ-O)3]2+. These new insights aid in the construction of a theoretical framework for the design of novel catalysts for oxyfunctionalization of natural gas and suggest further spectroscopic examination.

15.
J Phys Chem A ; 121(8): 1726-1733, 2017 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-28128563

RESUMO

Multiconfigurational electronic structure theory calculations including spin-orbit coupling effects were performed on four uranium-based single-molecule-magnets. Several quartet and doublet states were computed and the energy gaps between spin-orbit states were then used to determine magnetic susceptibility curves. Trends in experimental magnetic susceptibility curves were well reproduced by the calculations, and key factors affecting performance were identified.

16.
J Am Chem Soc ; 138(22): 7161-70, 2016 06 08.
Artigo em Inglês | MEDLINE | ID: mdl-27180991

RESUMO

The air-free reaction of CoCl2 with 1,3,5-tri(1H-1,2,3-triazol-5-yl)benzene (H3BTTri) in N,N-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co3[(Co4Cl)3(BTTri)8]2·DMF), a sodalite-type metal-organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt(II) centers that exhibit a strong preference for binding O2 over N2, with isosteric heats of adsorption (Qst) of -34(1) and -12(1) kJ/mol, respectively. The low-spin (S = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O2 binds end-on to each framework cobalt center in a 1:1 ratio with a Co-O2 bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co3[(Co4Cl)3(BDTriP)8]2·DMF; H3BDTriP = 5,5'-(5-(1H-pyrazol-4-yl)-1,3-phenylene)bis(1H-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O2 affinities (Qst = -47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O2 adducts in Co-BTTri are best described as cobalt(II)-dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt(III)-superoxo moieties. The stability, selectivity, and high O2 adsorption capacity of these materials render them promising new adsorbents for air separation processes.


Assuntos
Cobalto/química , Complexos de Coordenação/química , Complexos de Coordenação/síntese química , Oxigênio/química , Pirazóis/química , Triazóis/química , Cristalografia por Raios X , Modelos Moleculares
17.
J Chem Theory Comput ; 11(7): 3010-21, 2015 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-26575738

RESUMO

The applicability and accuracy of the generalized active space self-consistent field, (GASSCF), and (SplitGAS) methods are presented. The GASSCF method enables the exploration of larger active spaces than with the conventional complete active space SCF, (CASSCF), by fragmentation of a large space into subspaces and by controlling the interspace excitations. In the SplitGAS method, the GAS configuration interaction, CI, expansion is further partitioned in two parts: the principal, which includes the most important configuration state functions, and an extended, containing less relevant but not negligible ones. An effective Hamiltonian is then generated, with the extended part acting as a perturbation to the principal space. Excitation energies of ozone, furan, pyrrole, nickel dioxide, and copper tetrachloride dianion are reported. Various partitioning schemes of the GASSCF and SplitGAS CI expansions are considered and compared with the complete active space followed by second-order perturbation theory, (CASPT2), and multireference CI method, (MRCI), or available experimental data. General guidelines for the optimum applicability of these methods are discussed together with their current limitations.

18.
J Am Chem Soc ; 137(49): 15354-7, 2015 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-26559923

RESUMO

A dicobalt(II) complex, [Co2(mBDCA-5t)](2-) (1), demonstrates a cofacial arrangement of trigonal monopyramidal Co(II) ions with an inter-metal separation of 6.2710(6) Å. Reaction of 1 with potassium superoxide generates an encapsulated Co-O-Co core in the dianionic complex, [Co2O(mBDCA-5t)](2-) (2); to form the linear Co-O-Co core, the inter-metal distance has diminished to 3.994(3) Å. Co K-edge X-ray absorption spectroscopy data are consistent with a +2 oxidation state assignment for Co in both 1 and 2. Multireference complete active space calculations followed by second-order perturbation theory support this assignment, with hole equivalents residing on the bridging O-atom and on the cryptand ligand for the case of 2. Complex 2 acts as a 2-e(-) oxidant toward substrates including CO and H2, in both cases efficiently regenerating 1 in what represent net oxygen-atom-transfer reactions. This dicobalt system also functions as a catalase upon treatment with H2O2.

19.
J Chem Theory Comput ; 11(4): 1574-84, 2015 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-26574367

RESUMO

The strength of interactions between CO2 and 23 building blocks of metal-organic frameworks are reported in this paper. This theoretical study is based on an incremental, explicitly correlated coupled-cluster scheme with interference effects. This scheme allows the accurate calculation of molecular complexes such as zinc acetate (32 non-hydrogen atoms) at the CCSD(T) level, close to the basis set limit. Higher CO2 affinity for complexes with nitrogen-containing heterocycles is predicted from the calculated interaction energies. The good agreement between the interaction energies obtained from the CCSD(T) scheme and DFT-D3 is discussed.

20.
Inorg Chem ; 54(17): 8251-63, 2015 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-26252363

RESUMO

The interactions of H2, CO, CO2, and H2O with the undercoordinated metal centers of the trimetallic oxo-centered M3(III)(µ3-O)(X) (COO)6 moiety are studied by means of wave function and density functional theory. This trimetallic oxo-centered cluster is a common building unit in several metal-organic frameworks (MOFs) such as MIL-100, MIL-101, and MIL-127 (also referred to as soc-MOF). A combinatorial computational screening is performed for a large variety of trimetallic oxo-centered units M3(III)O (M = Al(3+), Sc(3+), V(3+), Cr(3+), Fe(3+), Ga(3+), Rh(3+), In(3+), Ir(3+)) interacting with H2O, H2, CO, and CO2. The screening addresses interaction energies, adsorption enthalpies, and vibrational properties. The results show that the Rh and Ir analogues are very promising materials for gas storage and separations.

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