The Magnetic Properties Of Clathrate Hydrates Icy Crystal Materials And Its Magnetic Regulation Revealed By Theoretical Studies

Clathrate hydrates(CHs)have been among highlighted materials in the energy and environmental fields due to their impressive potential as energy storage media and application devices for shifting the paradigm of human society.Numerous studies have been performed to explore the inherent nature of inclusion phenomena occurring in the cage-stacked framework of CHs.Notable physicochemical characteristics of CHs such as unique magnetic properties,ionic conductivity enhancement,melting point increase much above room temperature or unusual structural transformation with many vacant cages might lead to CHs to be converted to quite promising hydrate-based functional materials.The potential applications of CHs are promising,but their inherent nature still adequately unanswered through both microscopic and macroscopic approaches.Thus,additional in-depth investigations are needed to understand the unique inclusion phenomena and unrevealed host-guest interactions occurring in icy materials.In this work,the unique inclusion and coupling phenomena of reactive species or magnetic guest molecules into CHs are performed by means of quantum chemical calculations and the ab initio molecular dynamics simulations.In addition,the nature and mobility mechanism of hydroxide ions in CHs is also discussed in detail.The innovations and primary rsearch results are as follows:(1)Unique Solvating Effect in Azabenzene Clathrate Hydrates.The water solvent is a well-known solvating medium which can stabilize unstable species in bulky water or molecular clusters,but little is known about the solvating performance of CHs cages and the changes of molecular properties of the solutes.In this work,based on the experimentally observed CHs structures,we theoretically explore their structures and properties,taking azabenzenes(pyridazine,pyrimidine,pyrazine,and pyridine)and benzene as the guests which have negative electron affinities,focusing on stability,electron affinity,vibrational shifts,proton transfer,and especially unique solvating effect of the CHs cavity,using the DFT calculations.Calculations indicate that inclusion of the guest azabenzene/benzene in the clathrate hydrate cages could considerably change the structures and properties including stability increase,the C-H stretching blue-shifts of the guests,electron affinity conversion from the negative to considerably large positive values,acting as better electron carriers.Further,electron capturing not only strengths the host-guest electrostatic and H-bonding interactions but also changes the H-bonding structures,and even induces the spontaneous or low-barrier transfer of the cage protons,which lead to two types of structural motifs(anion clathrate hydrates and the proton-transferred counterparts),depending on original electron and proton affinities,polarity and orientations of the neutral guests in the cages.Unique solvating effect of the cage can induce the IR spectra new strong absorptions,and considerably change other properties of azabenzene anions.This work characterizes a unique solvating model for stabilizing azabenzenes and their anions and provides novel insights into the guest-host interaction including the electrostatic,H-bonding,and confining interactions and the structure and property changes of the CHs.Clearly,this information is useful in designing novel icy materials through inclusion of some species and finding their practical applications.(2)Magnetic Dioxygen Clathrate Hydrates:A Type of Promising Building Blocks for Icy Crystalline Materials.Clathrate Hydrates have recently attracted considerable research interest in fundamental science and practical applications because guest molecules with special electronic properties can be selectively trapped in the hydrogen-bonded water cages,and thus such CHs could be a novel class of promising functional materials.However,information about the electronic properties of CHs containing paramagnetic guest molecules and the spin coupling mechanisms are quite scarce.In this work,at the ab initio molecular dynamics level,we firstly simulate the magnetic properties and spin-coupling mechanism of the structure 1-type double O2 CHs(O2@CHs)derived from the experimental crystal structure.The results show that these O2@CHs exhibit rich diversity of spin coupling characteristics.depending on the O2-occupation patterns of the nanocages and relati ve orientations of two O2 which govern the orbital overlaps between the spin carriers.O2@CHs presents the antiferromagnetic coupling when two guest O2 are encapsulated in two symmetric 51262 cages or one 51262 cages with parallel orientations,while it exhibits the paramagnetic state for all other population patterns.The direct O2…O2 spin couplings operate through the diffuse parts of the O2 orbitals in these systems.Further,the elastic strain is examined for tuning their magnetic properties,finding that spin arrangement and magnetic characteristics can reversibly convert between the antiferromagnetic[??…??…??…??]n and ferromagnetic[?…?…?…?]n mode with nonlinear responses under isotropic strain between-7%and 20%.Under the compression strain,the increased spin polarization from the guest O2 to the host cage supports the O2…O(host)…O2 superexchange coupling mediated by the hosting water network as a coexisting coupling pathway,which plays a key role in improving the magnetic coupling.These intriguing findings here are expected to provide helpful information for developing novel CHs-based icy crystal magnetic nanomaterials.(3)Ion-doped Clathrate Hydrate Host Assisted Superexchange Spin Coupling of Paramagnetic Guest.The guest-induced magnetic properties of CHs have a high application prospect as a new type of icy magnetic material,but it difficult for practice applications due to its weak magnetism.Using first-principles calculations,we propose a way to overcome this problem by embedding OH-ion into the host lattice of CHs.We show that due to ionic interaction between a cationic guest and the surrounding anionic host lattice,paramagnetic O2 is more stable in the ionic CHs.A systematic study of these materials shows fascinating magnetic properties.The results indicate that anionic host lattice not only greatly alter the magnetic coupling mode between the guest O2,but also enhances their magnetic coupling strength.The structure with AFM and greater magnetic exchange energy is O2·EMN+@CHs(Eex=-23 meV),which has significantly stronger magnetic coupling interaction than pure O2@CHs(PM,Eer=0 meV)and nonionic O2·MTHF@CHs(FM,Eex=2 meV).The presence of superexchange AFM coupling in O2·EMN+@CHs arising from the p-orbital mixing between paramagnetic guest O2 via OH-doped host water framework.We also examined the effect of applying strain on O2·X@CH(X=EMN+OH-,MTHF)and found that the increase of compressive leads to a significant enhancement of AFM coupling.whereas FM is more favorable with the increase of tension.This result revealed that OH-incorporated in the host lattice and applied strain will significantly improve performance.The present findings well explain the magnetic coupling mechanisms of unique phenomena occurring in ionic CHs with paramagnetic guests and may also open a promising way to explore CHs-based spintronics devices.(4)Transport Dynamics of Hydroxide Ions in Ionic Clathrate Hydrates.Porous crystalline CHs have been explored as a potential solid ionic conductor because of their relatively high conductivities even at low temperature.CHs exhibits excellent ionic-conducting performance originating from the interesting host H-bonding networks,where the conducting medium plays a crucial role.The influence of H-bonding networks upon the mobility of the OH-constitutes a crucial knowledge gap in our current understanding of the conductivity of ion CHs.Specifically,the relevant molecular mechanisms associated with OH-mobility and diffusion in CHs via a proton-transfer reaction remain open questions.Here we report insights into the local hydration and electronic structure of the OH-in Me4N+OH-@CHs from ab initio molecular dynamics and explore the mechanism of proton-transfer between OH-and H-bonding networks.The relatively small energy barrier observed(?1.99 kcal/mol)supports the hypothesis that proton transfer may be a very rapid process in Me4N+OH-@CHs and that this reaction can significantly promote the OH-migration in Me4N+OH-@CHs.It is particularly significant that OH-can form metastable transient covalent hydrogen bonding units(OH-…H+…OH-)with its adjacent H2O due to its dynamic characteristics,and the whole proton transfer process can be expressed as a low barrier three well proton transfer potential energy surface.Furthermore,according to the in-depth electronic analysis,direct links between OH-migration and microelectronic properties have been established:the migration activity of OH-is closely relevant to the degree of interaction between OH-and its H-bonding donor and the large capacity of "electron pocket" around Fermi level.Therefore,our study presents a microscopic understanding of OH-migration in ionic CHs and provides a theoretical basis for the design of high performance ionic solid conductor materials.In summary,in this paper,quantum chemistry calculation and ab initio molecular dynamics simulation are used to systematically investigate and accurately characterize the structure,state,magnetic properties and the transport dynamics of hydroxide ions in CHs,systematically discussing the unique solvation effect of CHs,the novel superexchange spin coupling mechanism of O2@CHs,the magnetic changes of O2@CHs caused by strain effect and the migration mechanism of hydroxide ions in ionic CHs.Functional materials are increasingly attracting the attention of scientific researchers.Conducting a deep research on the potential properties of CHs ice crystal materials will certainly provide an important theoretical basis and reference for the exploring the application of new ice crystal materials.