Thomas Manz

New Mexico State University, Chemical & Materials Engineering, Faculty Member

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Papers by Thomas Manz

Seven confluence principles: a case study of standardized statistical analysis for 26 methods that assign net atomic charges in molecules

RSC Advances, 2020

This article studies two kinds of information extracted from statistical correlations between met... more This article studies two kinds of information extracted from statistical correlations between methods for assigning net atomic charges (NACs) in molecules. First, relative charge transfer magnitudes are quantified by performing instant least squares fitting (ILSF) on the NACs reported by Cho et al. (ChemPhysChem, 2020, 21, 688-696) across 26 methods applied to $2000 molecules. The Hirshfeld and Voronoi deformation density (VDD) methods had the smallest charge transfer magnitudes, while the quantum theory of atoms in molecules (QTAIM) method had the largest charge transfer magnitude. Methods optimized to reproduce the molecular dipole moment (e.g., ACP, ADCH, CM5) have smaller charge transfer magnitudes than methods optimized to reproduce the molecular electrostatic potential (e.g., CHELPG, HLY, MK, RESP). Several methods had charge transfer magnitudes even larger than the electrostatic potential fitting group. Second, confluence between different charge assignment methods is quantified to identify which charge assignment method produces the best NAC values for predicting via linear correlations the results of 20 charge assignment methods having a complete basis set limit across the dataset of $2000 molecules. The DDEC6 NACs were the best such predictor of the entire dataset. Seven confluence principles are introduced explaining why confluent quantitative descriptors offer predictive advantages for modeling a broad range of physical properties and target applications. These confluence principles can be applied in various fields of scientific inquiry. A theory is derived showing confluence is better revealed by standardized statistical analysis (e.g., principal components analysis of the correlation matrix and standardized reversible linear regression) than by unstandardized statistical analysis. These confluence principles were used together with other key principles and the scientific method to make assigning atom-in-material properties non-arbitrary. The N@C 60 system provides an unambiguous and non-arbitrary falsifiable test of atomic population analysis methods. The HLY, ISA, MK, and RESP methods failed for this material.

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Identifying misbonded atoms in the 2019 CoRE metal-organic framework database

RSC Advances, 2020

Databases of experimentally-derived metal-organic framework (MOF) crystal structures are useful f... more Databases of experimentally-derived metal-organic framework (MOF) crystal structures are useful for large-scale computational screening to identify which MOFs are best-suited for particular applications. However, these crystal structures must be cleaned to identify and/or correct various artifacts. The recently published 2019 CoRE MOF database (Chung et al., J. Chem. Eng. Data, 2019, 64, 5985-5998) reported thousands of experimentally-derived crystal structures that were partially cleaned to remove solvent molecules, to identify hundreds of disordered structures (approximately thirty of those were corrected), and to manually correct approximately 100 structures (e.g., adding missing hydrogen atoms). Herein, further cleaning of the 2019 CoRE MOF database is performed to identify structures with misbonded or isolated atoms: (i) structures containing an isolated atom, (ii) structures containing atoms too close together (i.e., overlapping atoms), (iii) structures containing a misplaced hydrogen atom, (iv) structures containing an under-bonded carbon atom (which might be caused by missing hydrogen atoms), and (v) structures containing an over-bonded carbon atom. This study should not be viewed as the final cleaning of this database, but rather as progress along the way towards the goal of someday achieving a completely cleaned set of experimentally-derived MOF crystal structures. We performed atom typing for all of the accepted structures to identify those structures that can be parameterized by previously reported forcefield precursors (Chen and Manz, RSC Adv., 2019, 9, 36492-36507). We report several forcefield precursors (e.g., net atomic charges, atom-in-material polarizabilities, atom-in-material dispersion coefficients, electron cloud parameters, etc.) for more than five thousand MOFs in the 2019 CoRE MOF database.

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Computationally designed tandem direct selective oxidation using molecular oxygen as oxidant without coreductant

RSC Advances, 2016

Developing greener technologies to produce chemicals has attracted much recent attention. For the... more Developing greener technologies to produce chemicals has attracted much recent attention. For the selective oxidation of organic compounds, direct selective oxidation in which molecular O 2 is utilized as oxidant without using a co-reductant or co-oxidant is desirable to avoid forming waste co-products; however, existing catalysts are limited in the types of substrates that can be used to achieve this. To address this challenge, we introduce a tandem direct selective oxidation process that separates the molecular oxygen activation step from the substrate oxidation step. Specifically, two reactions occur in separate reactors over two different catalysts: (1) molecular oxygen activation via reaction with an oxygen acceptor molecule to produce an oxygen transfer intermediate, and (2) substrate oxidation via reaction with the oxygen transfer intermediate to produce substrate oxide and regenerate the oxygen acceptor molecule. The oxygen acceptor molecule should be recycled back to the first reactor to achieve a net reaction of 2 substrate + O 2 / 2 substrate oxide. This separation of molecular oxygen activation and substrate oxidation steps reduces by-product formation by avoiding some side reactions. We use density functional theory (DFT) to study propene epoxidation as an important example. This reaction is of interest because: (a) propylene oxide (PO) is one of the leading commodity chemicals worldwide, (b) all current commercial PO production processes produce co-products, and (c) propene epoxidation illustrates the class of difficult terminal alkene epoxidations for substrates containing allylic hydrogen atoms. Using DFT calculations, we identify plausible candidates for the oxygen transfer intermediate and catalysts for the molecular oxygen activation and substrate oxidation reactions. The Zr(C 6 H 4-1,2-(N(C 6 H 3-2 0 ,6 0-(CH 3) 2)O) 2) 2 [DMZB] and the Ru(meso-tetrakis(2,6-dichlorophenyl)porphyrin) [RuTDCPP] catalysts were chosen for reactions (1) and (2), respectively. Several pyridine based N-oxides were tested as oxygen transfer intermediates. Our DFT computations indicate 2,6-dimethylpyridine N-oxide should perform well. The RuTDCPP catalyst was prior experimentally demonstrated to oxidize organic substrates (e.g., 1-octene) using aromatic N-oxides as oxidants with above 90% selectivity towards the desired product under mild conditions. For molecular O 2 activation (reaction (1)) and propene epoxidation (reaction (2)), our computed enthalpic energetic spans are 33.3 and 31.6 kcal/mol , respectively, predicting decent activities for both catalysts.

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A collection of forcefield precursors for metal- organic frameworks

RSC Advances, 2019

A host of important performance properties for metal-organic frameworks (MOFs) and other complex ... more A host of important performance properties for metal-organic frameworks (MOFs) and other complex materials can be calculated by modeling statistical ensembles. The principle challenge is to develop accurate and computationally efficient interaction models for these simulations. Two major approaches are (i) ab initio molecular dynamics in which the interaction model is provided by an exchange-correlation theory (e.g., DFT + dispersion functional) and (ii) molecular mechanics in which the interaction model is a parameterized classical force field. The first approach requires further development to improve computational speed. The second approach requires further development to automate accurate forcefield parameterization. Because of the extreme chemical diversity across thousands of MOF structures, this problem is still mostly unsolved today. For example, here we show structures in the 2014 CoRE MOF database contain more than 8 thousand different atom types based on first and second neighbors. Our results showed that atom types based on both first and second neighbors adequately capture the chemical environment, but atom types based on only first neighbors do not. For 3056 MOFs, we used density functional theory (DFT) followed by DDEC6 atomic population analysis to extract a host of important forcefield precursors: partial atomic charges; atom-in-material (AIM) C 6 , C 8 , and C 10 dispersion coefficients; AIM dipole and quadrupole moments; various AIM polarizabilities; quantum Drude oscillator parameters; AIM electron cloud parameters; etc. Electrostatic parameters were validated through comparisons to the DFT-computed electrostatic potential. These forcefield precursors should find widespread applications to developing MOF force fields.

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New scaling relations to compute atom-in-material polarizabilities and dispersion coefficients: part 2. Linear-scaling computational algorithms and parallelization

RSC Advances, 2019

We present two algorithms to compute system-specific polarizabilities and dispersion coefficients... more We present two algorithms to compute system-specific polarizabilities and dispersion coefficients such that required memory and computational time scale linearly with increasing number of atoms in the unit cell for large systems. The first algorithm computes the atom-in-material (AIM) static polarizability tensors, force-field polarizabilities, and C 6 , C 8 , C 9 , C 10 dispersion coefficients using the MCLF method. The second algorithm computes the AIM polarizability tensors and C 6 coefficients using the TS-SCS method. Linear-scaling computational cost is achieved using a dipole interaction cutoff length function combined with iterative methods that avoid large dense matrix multiplies and large matrix inversions. For MCLF, Richardson extrapolation of the screening increments is used. For TS-SCS, a failproof conjugate residual (FCR) algorithm is introduced that solves any linear equation system having Hermitian coefficients matrix. These algorithms have mathematically provable stable convergence that resists round-off errors. We parallelized these methods to provide rapid computation on multi-core computers. Excellent parallelization efficiencies were obtained, and adding parallel processors does not significantly increase memory requirements. This enables system-specific polarizabilities and dispersion coefficients to be readily computed for materials containing millions of atoms in the unit cell. The largest example studied herein is an ice crystal containing >2 million atoms in the unit cell. For this material, the FCR algorithm solved a linear equation system containing >6 million rows, 7.57 billion interacting atom pairs, 45.4 billion stored non-negligible matrix components used in each large matrix-vector multiplication, and $19 million unknowns per frequency point (>300 million total unknowns).

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New scaling relations to compute atom-in-material polarizabilities and dispersion coefficients: part 1. Theory and accuracy

RSC Advances, 2019

Polarizabilities and London dispersion forces are important to many chemical processes. Force fie... more Polarizabilities and London dispersion forces are important to many chemical processes. Force fields for classical atomistic simulations can be constructed using atom-in-material polarizabilities and C n (n ¼ 6, 8, 9, 10.) dispersion coefficients. This article addresses the key question of how to efficiently assign these parameters to constituent atoms in a material so that properties of the whole material are better reproduced. We develop a new set of scaling laws and computational algorithms (called MCLF) to do this in an accurate and computationally efficient manner across diverse material types. We introduce a conduction limit upper bound and m-scaling to describe the different behaviors of surface and buried atoms. We validate MCLF by comparing results to high-level benchmarks for isolated neutral and charged atoms, diverse diatomic molecules, various polyatomic molecules (e.g., polyacenes, fullerenes, and small organic and inorganic molecules), and dense solids (including metallic, covalent, and ionic). We also present results for the HIV reverse transcriptase enzyme complexed with an inhibitor molecule. MCLF provides the non-directionally screened polarizabilities required to construct force fields, the directionally-screened static polarizability tensor components and eigenvalues, and environmentally screened C 6 coefficients. Overall, MCLF has improved accuracy compared to the TS-SCS method. For TS-SCS, we compared charge partitioning methods and show DDEC6 partitioning yields more accurate results than Hirshfeld partitioning. MCLF also gives approximations for C 8 , C 9 , and C 10 dispersion coefficients and quantum Drude oscillator parameters. This method should find widespread applications to parameterize classical force fields and density functional theory (DFT) + dispersion methods.

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Synthesis and Characterization of N,N′-Bismesityl Phenanthrene- 9,10-diimine and Imine−Nitrone

by Thomas Manz and Diane A Dickie

ACS Omega, 2018

The sterically bulky compounds N,N′-bismesi-tyl phenanthrene-9,10-diimine [1] and imine−nitrone [... more The sterically bulky compounds N,N′-bismesi-tyl phenanthrene-9,10-diimine [1] and imine−nitrone [2] were synthesized. To the best of our knowledge, this is the first report of the synthesis of a bulky steric imine−nitrone accessed from the secondary ketimine using urea hydrogen peroxide over methyltrioxorhenium catalyst. Purified compounds were characterized using 1 H and 13 C NMR, high-resolution mass spectrometry, and infrared spectrometry. We report the first crystal structure of compound 1. Detailed IR bands of compounds 1 and 2 were assigned by comparing experimentally measured spectra to individually animated modes of quantum mechanically computed spectra. We believe these compounds may be of use as bidentate ligands in the synthesis of novel organometallic compounds. The asymmetric N and O coordination sites of compound 2 might impart interesting electronic effects to organometallic compounds compared to the symmetric N,N′-coordination sites of compound 1.

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Bond orders of the diatomic molecules

RSC Advances, 2019

Bond order quantifies the number of electrons dressed-exchanged between two atoms in a material a... more Bond order quantifies the number of electrons dressed-exchanged between two atoms in a material and is important for understanding many chemical properties. Diatomic molecules are the smallest molecules possessing chemical bonds and play key roles in atmospheric chemistry, biochemistry, lab chemistry, and chemical manufacturing. Here we quantum-mechanically calculate bond orders for 288 diatomic molecules and ions. For homodiatomics, we show bond orders correlate to bond energies for elements within the same chemical group. We quantify and discuss how semicore electrons weaken bond orders for elements having diffuse semicore electrons. Lots of chemistry is effected by this. We introduce a first-principles method to represent orbital-independent bond order as a sum of orbital-dependent bond order components. This bond order component analysis (BOCA) applies to any spin-orbitals that are unitary transformations of the natural spin-orbitals, with or without periodic boundary conditions, and to non-magnetic and (collinear or non-collinear) magnetic materials. We use this BOCA to study all period 2 homodiatomics plus Mo 2 , Cr 2 , ClO, ClO À , and Mo 2 (acetate) 4. Using Manz's bond order equation with DDEC6 partitioning, the Mo-Mo bond order was 4.12 in Mo 2 and 1.46 in Mo 2 (acetate) 4 with a sum of bond orders for each Mo atom of $4. Our study informs both chemistry research and education. As a learning aid, we introduce an analogy between bond orders in materials and message transmission in computer networks. We also introduce the first working quantitative heuristic model for all period 2 homodiatomic bond orders. This heuristic model incorporates s-p mixing to give heuristic bond orders of 3 4 (Be 2), 1 3 4 (B 2), 2 3 4 (C 2), and whole number bond orders for the remaining period 2 homodiatomics.

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Introducing DDEC6 atomic population analysis: part 4. Efficient parallel computation of net atomic charges, atomic spin moments, bond orders, and more

RSC Advances, 2018

The DDEC6 method is one of the most accurate and broadly applicable atomic population analysis me... more The DDEC6 method is one of the most accurate and broadly applicable atomic population analysis methods. It works for a broad range of periodic and non-periodic materials with no magnetism, collinear magnetism, and non-collinear magnetism irrespective of the basis set type. First, we show DDEC6 charge partitioning to assign net atomic charges corresponds to solving a series of 14 Lagrangians in order. Then, we provide flow diagrams for overall DDEC6 analysis, spin partitioning, and bond order calculations. We wrote an OpenMP parallelized Fortran code to provide efficient computations. We show that by storing large arrays as shared variables in cache line friendly order, memory requirements are independent of the number of parallel computing cores and false sharing is minimized. We show that both total memory required and the computational time scale linearly with increasing numbers of atoms in the unit cell. Using the presently chosen uniform grids, computational times of ~9 to 94 seconds per atom were required to perform DDEC6 analysis on a single computing core in an Intel Xeon E5 multi-processor unit. Parallelization efficiencies were usually >50% for computations performed on 2 to 16 cores of a cache coherent node. As examples we study a B-DNA decamer, nickel metal, supercells of hexagonal ice crystals, six X@C60 endohedral fullerene complexes, a water dimer, a Mn 12-acetate single molecule magnet exhibiting collinear magnetism, a Fe4O12N4C40H52 single molecule magnet exhibiting non-collinear magnetism, and several spin states of an ozone molecule. Efficient parallel computation was achieved for systems containing as few as one and as many as >8000 atoms in a unit cell. We varied many calculation factors (e.g., grid spacing, code design, thread arrangement, etc.) and report their effects on calculation speed and precision. We make recommendations for excellent performance.

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Introducing DDEC6 atomic population analysis: part 1. Charge partitioning theory and methodology

RSC Advances, 2016

Net atomic charges (NACs) are widely used in all chemical sciences to concisely summarize key inf... more Net atomic charges (NACs) are widely used in all chemical sciences to concisely summarize key information about the partitioning of electrons among atoms in materials. The objective of this article is to develop an atomic population analysis method that is suitable to be used as a default method in quantum chemistry programs irrespective of the kind of basis sets employed. To address this challenge, we introduce a new atoms-in-materials method with the following nine properties: (1) exactly one electron distribution is assigned to each atom, (2) core electrons are assigned to the correct host atom, (3) NACs are formally independent of the basis set type because they are functionals of the total electron distribution, (4) the assigned atomic electron distributions give an efficiently converging polyatomic multipole expansion, (5) the assigned NACs usually follow Pauling scale electronegativity trends, (6) NACs for a particular element have good transferability among different conformations that are equivalently bonded, (7) the assigned NACs are chemically consistent with the assigned atomic spin moments, (8) the method has predictably rapid and robust convergence to a unique solution, and (9) the computational cost of charge partitioning scales linearly with increasing system size. We study numerous materials as examples: (a) a series of endohedral C 60 complexes, (b) high-pressure compressed sodium chloride crystals with unusual stoichiometries, (c) metal–organic frameworks, (d) large and small molecules, (e) organometallic complexes, (f) various solids, and (g) solid surfaces. Due to non-nuclear attractors, Bader's quantum chemical topology could not assign NACs for some of these materials. We show for the first time that the Iterative Hirshfeld and DDEC3 methods do not always converge to a unique solution independent of the initial guess, and this sometimes causes those methods to assign dramatically different NACs on symmetry-equivalent atoms. By using a fixed number of charge partitioning steps with well-defined reference ion charges, the DDEC6 method avoids this problem by always converging to a unique solution. These characteristics make the DDEC6 method ideally suited for use as a default charge assignment method in quantum chemistry programs.

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Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials

RSC Advances, 2016

Net atomic charges (NACs) are widely used throughout the chemical sciences to concisely summarize... more Net atomic charges (NACs) are widely used throughout the chemical sciences to concisely summarize key information about charge transfer between atoms in materials. The vast majority of NAC definitions proposed to date are unsuitable for describing the wide range of material types encountered across the chemical sciences. In this article, we show the DDEC6 method reproduces important chemical, theoretical, and experimental properties across an extremely broad range of material types including small and large molecules, organometallics, nanoclusters, porous solids, nonporous solids, and solid surfaces. Some important comparisons we make are: (a) correlations between various NAC models and spectroscopically measured core-electron binding energy shifts for Ti-, Fe-, and Mo-containing solids, (b) comparisons between DDEC6 and experimentally extracted NACs for formamide and natrolite, (c) comparisons of accuracy of different NAC methods for reproducing the electrostatic potential surrounding a material across one and multiple system conformations, (d) comparisons between calculated and chemically expected electron transfer trends for atoms in numerous dense solids, solid surfaces, and molecules, (e) an assessment of NAC transferability between three crystal phases of the diisopropylammonium bromide organic ferroelectric, and (f) comparisons between DDEC6 and polarized neutron diffraction atomic spin moments for the Mn 12-acetate single-molecule magnet. We find the DDEC6 NACs are ideally suited for constructing flexible force-fields and give reasonable agreement with force-fields commonly used to simulate biomolecules and water. We find the DDEC6 method is more accurate than the DDEC3 method for analyzing a broad range of materials. This broad applicability to periodic and non-periodic materials irrespective of the basis set type makes the DDEC6 method suited for use as a default atomic population analysis method in quantum chemistry programs.

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Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders

RSC Advances, 2017

Developing a comprehensive method to compute bond orders is a problem that has eluded chemists si... more Developing a comprehensive method to compute bond orders is a problem that has eluded chemists since Lewis's pioneering work on chemical bonding a century ago. Here, a computationally efficient method solving this problem is introduced and demonstrated for diverse materials including elements from each chemical group and period. The method is applied to non-magnetic, collinear magnetic, and non-collinear magnetic materials with localized or delocalized bonding electrons. Examples studied include the stretched O 2 molecule, 26 diatomic molecules, 3d and 5d transition metal solids, periodic materials with 1 to 8748 atoms per unit cell, a biomolecule, a hypercoordinate molecule, an electron deficient molecule, hydrogen bound systems, transition states, Lewis acid–base complexes, aromatic compounds, magnetic systems, ionic materials, dispersion bound systems, nanostructures, and other materials. From near-zero to high-order bonds were studied. Both the bond orders and the sum of bond orders for each atom are accurate across various bonding types: metallic, covalent, polar-covalent, ionic, aromatic, dative, hypercoordinate, electron deficient multi-centered, agostic, and hydrogen bonding. The method yields similar results for correlated wavefunction and density functional theory inputs and for different S Z values of a spin multiplet. The method requires only the electron and spin magnetization density distributions as input and has a computational cost scaling linearly with increasing number of atoms in the unit cell. No prior approach is as general. The method does not apply to electrides, highly time-dependent states, some extremely high-energy excited states, and nuclear reactions.

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Computationally designed zirconium organometallic catalyst for direct epoxidation of alkenes without allylic H atoms: aromatic linkage eliminates formation of inert octahedral complexes

Theoretical Chemistry Accounts, 2016

Deactivation of Ti and Zr half-metallocene complexes activated with B(C6F5)3: a case study in constructing DFT-based QSARs to predict unimolecular rate constants

RSC Adv., 2015

ABSTRACT Two deactivation pathways of Ti and Zr half-metallocene complexes activated with B(C6F5)... more ABSTRACT Two deactivation pathways of Ti and Zr half-metallocene complexes activated with B(C6F5)3 in toluene solvent were studied using Density Functional Theory (DFT) with dispersion corrections: (a) H transfer from counterion to Me initiating group to release methane and (b) C6F5 transfer from counterion to metal. Transition state geometries and energies were computed for twenty-seven complexes, and the barrier height for the C6F5 transfer pathway was linearly correlated to the amount of steric congestion near the metal. Unimolecular rate constants for catalyst deactivation were predicted for all 27 catalysts by constructing a DFT-based quantitative structure activity relationship (QSAR). This QSAR was constructed by using the DFT-computed energy barrier (ΔV0) and vibrational frequency along the reaction coordinate (v‡) as chemical descriptors and fitting QSAR parameters to experimental data for reference systems. The computed rate constants were in excellent agreement with available experimental data. Specifically, the dominant deactivation pathway for each catalyst and the relative deactivation rates of different catalysts were correctly predicted. Of note, the IndTi(OC6H-2,3,5,6-Ph4)Me2/B(C6F5)3 system is predicted to have a good combination of slow deactivation and high olefin polymerization rates.

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Accurate Treatment of Electrostatics during Molecular Adsorption in Nanoporous Crystals without Assigning Point Charges to Framework Atoms

The Journal of Physical Chemistry C, Feb 27, 2011

ABSTRACT Molecular simulations have become an important complement to experiments for studying ga... more ABSTRACT Molecular simulations have become an important complement to experiments for studying gas adsorption and separation in crystalline nanoporous materials. Conventionally, these simulations use force fields that model adsorbate−pore interactions by assigning point charges to the atoms of the adsorbent. The assignment of framework charges always introduces ambiguity because there are many different choices for defining point charges, even when the true electron density of a material is known. We show how to completely avoid such ambiguity by using the electrostatic potential energy surface (EPES) calculated from plane wave density functional theory (DFT). We illustrate this approach by simulating CO2 adsorption in four metal−organic frameworks (MOFs): IRMOF-1, ZIF-8, ZIF-90, and Zn(nicotinate)2. The resulting CO2 adsorption isotherms are insensitive to the exchange-correlation functional used in the DFT calculation of the EPES but are sensitive to changes in the crystal structure and lattice parameters. Isotherms computed from the DFT EPES are compared to those computed from several point charge models. This comparison makes possible, for the first time, an unbiased assessment of the accuracy of these point charge models for describing adsorption in MOFs. We find an unusually high Henry’s constant (109 mmol/g·bar) and intermediate isosteric heat of adsorption (34.9 kJ/mol) for Zn(nicotinate)2, which makes it a potentially attractive material for CO2 adsorption applications.

Kinetic Modeling of Single-Site Olefin Polymerization with Multi-Response Data: Even Models with Many Parameters Cannot Fit An Elephant

Krista A. Novstrup 1, Grigori A. Medvedev 1, Nicholas E. Travia 2, Corneliu Stanciu 2, Jeffery M.... more

Selective oxidation passing through η3-ozone intermediates: applications to direct propene epoxidation using molecular oxygen oxidant

RSC Advances, 2014

Computation was used to design a new catalytic route for selective oxidation using molecular oxyg... more Computation was used to design a new catalytic route for selective oxidation using molecular oxygen as the oxidant without requiring a coreductant. Formation of η3-ozone intermediates is a key feature. Key steps in the catalytic cycle are: (a) the η3-ozone group adds an O atom to substrate (e.g., propene) to form substrate oxide (e.g., propylene oxide) plus a peroxo or adsorbed O2 group, (b) the peroxo or adsorbed O2 group adds an O atom to the substrate to form substrate oxide plus an oxo group, (c) an oxygen molecule adds to the oxo group to generate an η2-ozone group, and (d) the η2-ozone group rearranges to regenerate the η3- ozone group. Our Density Functional Theory (DFT) calculations reveal the first instances of this catalytic cycle for any material. We expect this catalytic cycle could be used to selectively oxidize a variety of
substrates. As a commercially important example, we focus on applications to direct propene epoxidation. Existing commercial manufacture of propylene oxide uses propene oxidation with one or more co-reactants and produces co-products/by-products. Direct propene epoxidation (i.e., without co-reactants) is a potentially greener process with economic and environmental benefits due to eliminating or reducing co-product/by-product formation. The grand challenge is to identify catalysts
that can efficiently activate an oxygen molecule and sequentially add the resulting O atoms to two propene molecules in a catalytic cycle. We use DFT to identify and study several catalysts. Our computations introduce two new classes of Zr organometallic complexes that have dinitrone and iminenitrone based bis-bidentate ligands, respectively. For these and bis-diimine ligated Zr complexes, we study the stability of different catalyst forms as a function of oxygen chemical potential and compute
complete catalytic cycles with transition states. A new homogeneous Zr catalyst is designed with a computed enthalpy energetic span (i.e., apparent activation energy for the entire catalytic cycle) of ~28.3 kcal/mol—the lowest reported for any direct propene epoxidation catalyst to date. We propose an electrochemical cell process for assembling these catalysts and a preliminary chemical process flow diagram for direct propene epoxidation.

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Hafnium catalysts for direct alkene epoxidation using molecular oxygen as oxidant

RSC Adv., 2015

ABSTRACT New zirconium (Zr) based organometallic catalysts for direct olefin epoxidation using O2... more ABSTRACT New zirconium (Zr) based organometallic catalysts for direct olefin epoxidation using O2 as oxidant without coreductant were introduced in a previous computational study (T. A. Manz and B. Yang, RSC Adv., 2014, 4, 27755–27774). In this paper, we use density functional theory (DFT) to study three Hf-based catalysts with the same bis(bidentate) ligands as the preceding Zr-based catalysts: (a) the diimine ligand N(Ar)–CH–CH–N(Ar) aka NCCN, (b) the imine–nitrone ligand N(Ar)–CH–CH–N(Ar)–O aka NCCNO, and (c) the dinitrone ligand O–N(Ar)–CH–CH–N(Ar)–O aka ONCCNO [Ar = –C6H3–2,6-iPr2]. Complete reaction cycles and energetic spans (i.e., effective activation energies for the entire catalytic cycle) are computed for propene epoxidation. For Hf_NCCNO and Hf_ONCCNO, the reaction cycles are similar to the Zr-based analogs and the formation of η3-ozone intermediates is still crucial. Hf_ONCCNO has a large enthalpic energetic span (60.4 kcal mol−1) due to forming inert octahedral complexes as the catalyst resting state. Our calculations predict an energetic span ≥40 kcal mol−1 for Hf_NCCN, which indicates it will not be a good catalyst. Computed enthalpic energetic spans of 30 kcal mol−1 are achieved for the Hf_NCCNO and Zr_NCCNO catalysts; however, transfer of allylic hydrogen from the reaction product forms a low energy deactivation product. Therefore, the Hf_NCCNO and Zr_NCCNO catalysts are only suitable for direct epoxidation of alkenes that do not have any allylic hydrogen atoms. As an example of an alkene with no allylic hydrogens, we computed enthalpic energetic spans (kcal mol−1) for direct ethylene epoxidation of (a) 25.0 for Hf_NCCNO, (b) 30.2 for Zr_NCCNO, and (c) 52.7 for Zr_NCCN.

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CHAPTER 5. Computing Accurate Net Atomic Charges, Atomic Spin Moments, and Effective Bond Orders in Complex Materials

RSC Catalysis Series, 2013

ABSTRACT The field of computational catalysis has existed in one form or another for at least 30 ... more ABSTRACT The field of computational catalysis has existed in one form or another for at least 30 years. Its ultimate goal - the design of a novel catalyst entirely from the computer. While this goal has not been reached yet, the 21st Century has already seen key advances in capturing the myriad complex phenomena that are critical to catalyst behaviour under reaction conditions. This book presents a comprehensive review of the methods and approaches being adopted to push forward the boundaries of computational catalysis. Each method is supported with applied examples selected by the author, proving to be a more substantial resource than the existing literature. Both existing a possible future high-impact techniques are presented. An essential reference to anyone working in the field, the book&#39;s editors share more than two decade&#39;s of experience in computational catalysis and have brought together an impressive array of contributors. The book is written to ensure postgraduates and professionals will benefit from this one-stop resource on the cutting-edge of the field.

388180 Atomistic Descriptors of Material Properties

Modeling and simulation of materials can be performed across various length scales. This presenta... more Modeling and simulation of materials can be performed across various length scales. This presentation focuses on modeling at the smallest scale using Density Functional Theory (DFT) and other ab initio methods to compute the properties of individual atoms in materials. Atomic population analysis techniques partition the total electron and spin density distributions of a material into individual atomic contributions. Integrating the electron density and spin density assigned to each atom gives the net atomic charges and atomic spin moments, respectively. Atomic spin moments quantify the magnetic structure of a material. The amount of electron density overlap between two nearby atoms can be used to compute effective bond orders that are important for understanding the nature of chemical bonds in materials. The effective volume of each atom (computed by the third order radial moment of the atomic electron density distribution) can be used to compute the C6 dispersion coefficients (Tkat...

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