Juergen Eckert

On Demand: The Singular rht Net, an Ideal Blueprint for the Construction of a Metal–Organic Framework (MOF) Platform†

The need for tunable functional solid-state materials is ever increasing because of the growing demand to address persisting challenges in global energy issues, environmental sustainability, and others. [1] It is practical and preferable for such materials to be pre-designed and constructed to contain the desired properties and specific functionalities for a given targeted application. An emerging unique class of solid-state materials, namely metal–organic frameworks (MOFs), has the desired attributes and offers great promise to unveil superior materials for many lasting challenges [2] since desired functionality can be introduced pre- and/or post-synthesis. [3] A remarkable feature of MOFs is the ability to build periodic structures with in-built functional properties using the molecular building block (MBB) approach, which utilizes pre-selected organic and inorganic MBBs, with desired function, that are judiciously chosen to possess the proper geometry, shape, and directionality required to target given underlying nets. [4]

Simulations of hydrogen sorption in rht-MOF-1: identifying the binding sites through explicit polarization and quantum rotation calculations

Grand canonical Monte Carlo (GCMC) simulations of hydrogen sorption were performed in rht-MOF-1, a metal–organic framework (MOF) that consists of isophthalate groups joined by copper paddlewheel clusters and Cu3O trimers through tetrazolate moeities. This is a charged rht-MOF that contains extra-framework nitrate counterions within the material. For the simulations performed herein, excellent agreement with experiment was achieved for the simulated hydrogen sorption isotherms and calculated isosteric heat of adsorption, Qst, values only when using a polarizable potential. Thermodynamic agreement is demonstrated via comparing to experimental isotherms and binding sites are revealed by combining simulation and inelastic neutron scattering (INS) data. Simulations involving explicit many-body polarization interactions assisted in the determination of the binding sites in rht-MOF-1 through the distribution of the induced dipoles that led to strong adsorbate interactions. Four distinct hydrogen sorption sites were determined from the polarization distribution: the nitrate ions located in the corners of the truncated tetrahedral cages, the Cu2+ ions of the paddlewheels that project into the truncated tetrahedral and truncated octahedral cages (Cu1 ions), the Cu2+ ions of the Cu3O trimers (Cu3 ions), and the sides of the paddlewheels in the cuboctahedral cage. The simulations revealed that the initial sorption sites for hydrogen in rht-MOF-1 are the nitrate ions; this site corresponds to the high initial Qst value for hydrogen (9.5 kJ mol−1) in the MOF. The radial distribution functions, g(r), about the Cu2+ ions at various loadings revealed that the Cu1 ions are the preferred open-metal sorption sites for hydrogen at low loading, while the Cu3 ions become occupied at higher loadings. The validation of the aforementioned sorption sites in rht-MOF-1 was confirmed by calculating the two-dimensional quantum rotational levels about each site and comparing the levels to the transitions that were observed in the experimental INS spectra for hydrogen in the compound. For each binding site, the rotational transitions from j = 0 to j = 1 were in good agreement to certain transitions that were observed in the INS spectra. From these calculations, the assignment of the peaks in the INS spectra for hydrogen in rht-MOF-1 has been made.

Dramatic effect of pore size reduction on the dynamics of hydrogen adsorbed in metal–organic materials

The effects of pore size reduction on the dynamics of hydrogen sorption in metal–organic materials (MOMs) were elucidated by studying SIFSIX-2-Cu and its doubly interpenetrated polymorph SIFSIX-2-Cu-i by means of sorption, inelastic neutron scattering (INS), and computational modeling. SIFSIX-2-Cu-i exhibits much smaller pore sizes, which possess high H2 sorption affinity at low loadings. Experimental H2 sorption measurements revealed that the isosteric heat of adsorption (Qst) for H2 in SIFSIX-2-Cu-i is nearly two times higher than that for SIFSIX-2-Cu (8.6 vs. 4.6 kJ mol−1). The INS spectrum for H2 in SIFSIX-2-Cu-i is rather unique for a porous material, as only one broad peak appears at low energies near 6 meV, which simply increases in intensity with loading until the pores are filled. The value for this rotational transition is lower than that in most neutral metal–organic frameworks (MOFs), including those with open Cu sites (8–9 meV), which is indicative of a higher barrier to rotation and stronger interaction in the channels of SIFSIX-2-Cu-i than the open Cu sites in MOFs. Simulations of H2 sorption in SIFSIX-2-Cu-i revealed two hydrogen sorption sites in the MOM: direct interaction with the equatorial fluorine atom (site 1) and between two equatorial fluorine atoms on opposite walls (site 2). The calculated rotational energy levels and rotational barriers for the two sites in SIFSIX-2-Cu-i are in good agreement with INS data. Furthermore, the rotational barriers and binding energies for site 2 are slightly higher than that for site 1, which is consistent with INS results. The lowest calculated transition for the primary site in SIFSIX-2-Cu is also in good agreement with INS data. In addition, this transition in the non-interpenetrating material is higher than any of the sites in SIFSIX-2-Cu-i, which indicates a significantly weaker interaction with the host as a result of the larger pore size.

Probing the dynamics of complexed local anesthetics via neutron scattering spectroscopy and DFT calculations

Since potential changes in the dynamics and mobility of drugs upon complexation for delivery may affect their ultimate efficacy, we have investigated the dynamics of two local anesthetic molecules, bupivacaine (BVC, C18H28N2O) and ropivacaine (RVC, C17H26N2O), in both their crystalline forms and complexed with water-soluble oligosaccharide 2-hydroxypropyl-β-cyclodextrin (HP-β-CD). The study was carried out by neutron scattering spectroscopy, along with thermal analysis, and density functional theory computation. Mean square displacements suggest that RVC may be less flexible in crystalline form than BVC, but both molecules exhibit very similar dynamics when confined in HP-β-CD. The use of vibrational analysis by density functional theory (DFT) made possible the identification of molecular modes that are most affected in both molecules by insertion into HP-β-CD, namely those of the piperidine rings and methyl groups. Nonetheless, the somewhat greater structure in the vibrational spectrum at room temperature of complexed RVC than that of BVC, suggests that the effects of complexation are more severe for the latter. This unique approach to the molecular level study of encapsulated drugs should lead to deeper understanding of their mobility and the respective release dynamics.

Computational study of inelastic neutron scattering vibrational spectra of water clusters and their relevance to hydration water in proteins.

BACKGROUNDnInelastic neutron scattering (INS) vibrational spectra for hydration water in proteins can be obtained from spectral differences, but their interpretation has mainly been limited to comparisons with various forms of ice at high hydration levels without making use of available structural information from neutron protein crystallography.nnnMETHODSnThe INS vibrational spectra of free and partially constrained water clusters (up to n=17) were calculated with DFT methods using published energy-minimized structures.nnnRESULTSnReference is made to neutron diffraction studies of hydrated proteins, which contain a wealth of structural information both on individual water molecules and small clusters in the inner shell in order to select representative clusters to serve as models for bound, rather than free clusters as they would occur in a protein.nnnCONCLUSIONSnINS spectra of the water librational region calculated for a combination of model bound clusters provide a qualitative account of the essentially featureless experimental spectra on water in proteins at very low hydration levels, but do indicate that the well-known rise in intensity near 500cm-1 is connected to increasing numbers of four-coordinate water molecules in larger clusters.nnnGENERAL SIGNIFICANCEnThe combination of structural information of hydration water from neutron protein crystallography with much more sophisticated computational methods than used herein should lead to a much more detailed picture of the hydration of proteins. This article is part of a Special Issue entitled Science for Life Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers

Neutron powder diffraction from d-alanine, a d-form amino acid present in higher animals including humans, shows subtle structural differences when compared with its enantiomer, l-alanine. These dissimilarities are due to different rearrangements of the NH3 + group as revealed by Raman scattering.

Hydrogen Storage Materials

An eventual realization of a Hydrogen Economy requires working solutions in three fundamental areas, namely hydrogen production, hydrogen storage, and fuel cells, in addition to the development of an extensive, new infrastructure. While neutron scattering experiments and the associated techniques of analysis have been of utility in all three of these research areas, they have had by far the most significant impact on the development and understanding of materials for hydrogen storage applications. This chapter examines some of these contributions.

Design and Synthesis of Novel Porous Metal-Organic Frameworks (MOFs) Toward High Hydrogen Storage Capacity

Statement of Objectives: 1. Synthesize viable porous MOFs for high H2 storage at ambient conditions to be assessed by measuring H2 uptake. 2. Develop a better understanding of the operative interactions of the sorbed H2 with the organic and inorganic constituents of the sorbent MOF by means of inelastic neutron scattering (INS, to characterize the H2-MOF interactions) and computational studies (to interpret the data and predict novel materials suitable for high H2 uptake at moderate temperatures and relatively low pressures). 3. Synergistically combine the outcomes of objectives 1 and 2 to construct a made-to-order inexpensive MOF that is suitable for super H2 storage and meets the DOE targets - 6% H2 per weight (2kWh/kg) by 2010 and 9% H2 per weight (3kWh/kg) by 2015. The ongoing research is a collaborative experimental and computational effort focused on assessing H2 storage and interactions with pre-selected metal-organic frameworks (MOFs) and zeolite-like MOFs (ZMOFs), with the eventual goal of synthesizing made-to-order high H2 storage materials to achieve the DOE targets for mobile applications. We proposed in this funded research to increase the amount of H2 uptake, as well as tune the interactions (i.e. isosteric heats of adsorption), by targeting readily tunable MOFs:

CCDC 893528: Experimental Crystal Structure Determination

Related Article: Marianne B. Lalonde, Rachel B. Getman, Jeong Yong Lee, John M. Roberts, Amy A. Sarjeant, Karl A. Scheidt, Peter A. Georgiev, Jan P. Embs, Juergen Eckert, Omar K. Farha, Randall Q. Snurr, Joseph T. Hupp|2013|CrystEngComm|15|9408|doi:10.1039/C3CE40198G