yseujans trifena ellishiah
Orestes Hardy
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Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. Here, we combine infrared spectroscopy and many-body molecular dynamics simulations to probe the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with ordered cobalt open coordination sites. Building upon the agreement between experimental and theoretical spectra, we demonstrate that water at low relative humidity binds initially to open metal sites and subsequently forms disconnected one-dimensional chains of hydrogen-bonded water molecules bridging between cobalt atoms. With increasing relative humidity, these water chains nucleate pore filling, and water molecules occupy the entire pore interior before the relative humidity reaches 30%. Systematic analysis of rotational and translational dynamics indicates heterogeneity in this pore-confined water, with water molecules displaying variable mobility as a function of distance from the interface.
Due to the formation of frustrated hydrogen-bond (H-bond) networks, water confined within pores or at interfaces exhibits significantly altered physical properties compared to bulk water, with important implications for different fields, including chemistry1,2, biology3,4,5, and atmospheric science6,7. As a consequence of the unique structural and dynamical properties of frustrated H-bond networks, confinement of water gives rise to anomalous behavior, as inferred from measurements of various quantities, such as the dielectric constant8 and diffusion coefficient9. Significant progress has recently been made in developing a more complete picture of the water H-bonding structure10, especially due to the introduction of accurate many-body molecular models11. Nevertheless, a precise prediction of the properties of water across different phases and in different environments remains a challenge due to the dynamic nature of the H-bond network which results from the subtle balance between energetic, entropic, and nuclear quantum effects11,12,13.
Experimental and Theoretical Infrared Spectra of Water in Co2Cl2BTDD. a Difference Diffuse-Reflectance IR spectra of the water OH-stretch region in Co2Cl2BTDD under variable RH conditions. b Calculated IR intensity using the MB-pol model of water in Co2Cl2BTDD ranging from one water molecule per cobalt (1) to twelve water molecules per cobalt (12)
At higher water loadings, the 1-D water chains bridging the hydrophilic open Co2+ sites act as nucleators for the pore filling process, templating the formation of concentric cylindrical shells that extend along the hydrophobic pore channels. As the water loading increases, the MB-MD simulations indicate that the water molecules become, on average, more mobile (Fig. 5a). Because the orientational correlation functions reflect the extent of molecular rotation over time, this suggests the emergence of liquid-like behavior. However, at the experimental maximum loading of 12 H2O/Co2+, the average orientational mobility of the water molecules in the Co2Cl2BTDD pores remains intermediate between that calculated for ice and bulk water. A similar slowdown was predicted for water adsorbed in MIL-5345.
Water confined in Co2Cl2BTDD pores exhibits similarities and differences with water adsorbed on surfaces. For example, water at metal surfaces tends to display long-range order51. Although at low RH, water inside Co2Cl2BTDD displays a similar order due to coordination to the open cobalt sites, higher RH disrupts the long-range order, and water molecules display progressively liquid-like behavior as they approach the center of the MOF pores. This is reflected in the broadening of the OH-stretch vibrational lineshapes towards lower frequencies characteristic of H-bond networks.
In summary, water adsorbed in Co2Cl2BTDD displays heterogeneous structural and dynamical behavior which varies as a function of both RH and distance from the pore surface. By directly connecting adsorption isotherms with the evolution of IR spectra of water inside MOF pores as a function of RH, the foregoing combined experimental and theoretical approach provides detailed insights into the molecular mechanisms that determine water adsorption in porous materials exhibiting both hydrophilic and hydrophobic regions. These mechanistic insights can contribute to the design of next-generation porous materials for water harvesting. Fundamentally, our approach advances the understanding of water structure and dynamics within amphipathic confined and interfacial environments which are widespread in biology, atmospheric science, and chemistry.
The MB-pol water model used in this study is available in OpenMM ( _openmm.html) and i-PI ( ). All computer codes used in the analysis presented in this study are available from the authors upon request.
Studies of small molecule interactions with metal nodes in MOFs are supported through a CAREER grant from the National Science Foundation to M.D. (DMR-1452612). A.J.R. is supported by the Martin Family Fellowship for Sustainability. A.J.R. and M.D. thank the Abdul Latif Jameel World Water and Food Security Lab for seed funding for water capture. Theoretical research is supported by the Department of Energy through grant no. DE-SC0019333 to F.P. and used computational resources of the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation through grant no. ACI-1053575 under allocation TG-CHE110009, as well as of the Air Force Office of Scientific Research through grant no. FA9550-16-1-0327.
The paper was written through contributions of all authors. A.J.R. and K.M.H. contributed equally to this work. A.J.R. synthesized materials and designed and performed infrared experiments. K.M.H. designed and performed theoretical calculations. M.D. and F.P. designed and supervised research.
As well as this overarching challenge, there are a lot of difficult concepts in this topic, and much scope for students to develop misconceptions. Some of the most prevalent challenges and misconceptions which seemed to crop up year after year in my teaching have been:
I have found that explicitly teaching the electrostatic attraction between the shared pair of electrons and the atomic nuclei as the reason that sharing electrons results in strong bonds, helps students to grasp the structure-property links, and to differentiate between covalent bonds and intermolecular forces in molecular structures.
So, having Identified what I think are the key challenges in teaching this topic well, the remainder of this blog series will describe how I now teach Structure and Bonding to try to overcome some of these challenges and prevent these misconceptions from arising.
Structure and Bonding is a publication which uniquely bridges the journal and book format. Organized into topical volumes, the series publishes in depth and critical reviews on all topics concerning structure and bonding. With over 50 years of history, the series has developed from covering theoretical methods for simple molecules to more complex systems.
Topics addressed in the series now include the design and engineering of molecular solids such as molecular machines, surfaces, two dimensional materials, metal clusters and supramolecular species based either on complementary hydrogen bonding networks or metal coordination centers in metal-organic framework materials (MOFs). Also of interest is the study of reaction coordinates of organometallic transformations and catalytic processes, and the electronic properties of metal ions involved in important biochemical enzymatic reactions.
Volumes on physical and spectroscopic techniques used to provide insights into structural and bonding problems, as well as experimental studies associated with the development of bonding models, reactivity pathways and rates of chemical processes are also relevant for the series.
Structure and Bonding is able to contribute to the challenges of communicating the enormous amount of data now produced in contemporary research by producing volumes which summarize important developments in selected areas of current interest and provide the conceptual framework necessary to use and interpret mega-databases.
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Knowledge of structure and bonding can help explain the properties of materials. All the properties of a particular substance depend upon the elements present and how they are bonded to each other. This programme is designed to develop students understanding of structure and bonding as well as developing thinking and research skills.
He was one of the leading figures in British chemistry and his aim was to understand the mechanism of organic reactions. He also contributed to the Cahn-Ingold-Prelog system, which is used to name the stereoisomers of a molecule.
Chemical structure refers to the way atoms are arranged within molecules. Butlerov realised that chemical compounds are not a random cluster of atoms and functional groups, but structures with definite order.
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