Dielectron Clathrate Hydrates with Unique Superexchange Spin

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Dielectron Clathrate Hydrates with Unique Superexchange Spin Couplings Qi Luo, Changzhe Zhang, and Yuxiang Bu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01801 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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The Journal of Physical Chemistry

Dielectron Clathrate Hydrates with Unique Superexchange Spin Couplings Qi Luo, Changzhe Zhang, Yuxiang Bu* School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, People’s Republic of China ABSTRACT: Clathrate hydrates have exhibited prospective applications not only in energy and environmental fields but also in icy magnetic materials because of the possibility of inclusion of radical species and their unique structures.

In particular, the existence of

dielectron clathrate hydrates have been experimentally reported.

However, their structure

and electronic or spintronic properties have not been explored yet.

Here, we investigate four

kinds of dielectron clathrate hydrates ((H2O)n2−, n=36, 44, 48, and 52) and their spintronic properties using the density functional theory method, and find that all dielectron hydrates feature stable bipolarons in icy surrounding with internal or H-bonding network permeating state electron distributions. The dielectron presents significant antiferromagnetic or ferromagnetic spin couplings through a novel electron-permeating H-bonding network superexchange mechanism.

This work opens up a possibility to explore novel icy crystal

magnetic materials based on the clathrate hydrates.

INTRODUCTION Solvated electrons, a unique kind of electron carriers, have been widely studied since they were first observed in liquid ammonia 1,2 due to their close associations with quite more important phenomena,1-16 and especially the discovery of electrides clathrates

23-26

18-22

and electron

has further stimulated researchers’ interest because it opens a possibility for

potential applications of weakly bounded electrons.

Furthermore, the existence of solvated

dielectrons are also well-documented experimentally and theoretically in various media,27-36 and the spin interactions among correlated excess electrons are known to govern more fundamental electronics properties of the systems such as magnetic properties, spintronics,

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and nonlinear optical properties.18,22,27 Thus, solvated dielectrons are expected to show more intriguing characteristics and potential applications compared with solvated single electrons, and have attracted a wide range of attention and aroused a strong interest in research.27-36 For example, a thermally stable electride can exhibit different solvated dielectron structures at different temperatures, and thus exhibit the metallic and hopping conductivities,18 while structural fluctuations in liquids could derive rich ferromagnetic coupling dynamics of solvated dielectrons.27

Clearly, the spin coupling is a measure of solvated dielectron

distinguishing from two independent solvated single electrons, and may exhibit some novel coupling modes and properties as in organic diradicals.37-40

It is thus an unexplored topic to

find novel structures and to reveal unique spintronics in various solvated dielectron systems. Clathrate hydrates have recently received great attention both from theoretical and experimental aspects

41-46

because of their great potential for the gas storage and prospective

applications in icy materials.46 In clathrate hydrates, water molecules form cages by means of H-bonding networks to enfold small guest molecules, and the cages usually have special polyhedral structures and may vary subject to the size and shape of the guest molecules. Interestingly, many more small molecules can be confined in the voids of the cages not only for storage

41-46

but also for exchange and conversion.45,47,48 In particular, more magnetic

molecules (e.g. radicals, high-spin species) can be encapsulated in the water cages, exhibiting rich magnetic and spintronics properties in their icy structures and making them promising building blocks for icy materials.46 Besides, electron clathrate hydrates were experimentally observed and theoretically studied using water cluster anions.23-26 Such electron clathrate hydrates have rigid structures with a single excess electron inside a cage consisting of more than 20 H2O molecules, in contrast to traditional hydrated electrons in water, aqueous solutions or ice which usually have a (H2O)4-6 cage.2 Dielectron clathrate hydrates were observed experimentally in the X-irradiated (CH3)4N+OH− clathrate hydrate although relevant structures and properties were not reported,33 and some radical or electron-added molecule clathrate hydrates were also observed experimentally.49-51 Structurally, clathrate hydrates are usually referred to their cage-stacked structures where more polyhedral cages are fused together with shared faces and the center distances between adjacent two cages are generally ca. 6-10 Å.

Thus, it is expectable that there exist

2


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interactions among the guest species especially the charged or high-spin ones in clathrate hydrates which are clearly the basis for further applications.46 Unfortunately, there hardly are the studies on the structures and spin interactions between the trapped excess electrons or radicals although some studies on electron clathrate hydrates were reported and the existence of dielectron clathrate hydrates was experimentally documented.23-26,33 In view of the experimentally observed inclusion phenomena of various guest species including electrons and the guest-guest interactions, we here investigate four kinds of dielectron clathrate hydrates ((H2O)n2−, n=36, 44, 48, and 52) with different double-cavity structures extracted from clathrate hydrates observed experimentally using the density functional theory method, clarifying the structures and spin coupling properties and mechanisms in the representative dielectron hydrates.

Besides, quantum chemical

calculations are the most frequently used techniques at now days and have been extensively utilized to explore the structures and properties of various molecules and assemblies, providing much more exact and useful information. COMPUTATIONAL DETAILS Structural Consideration.

By surveying the structures of various clathrate hydrates, we

use three representative water cavities (512, 4668 and 51262)

12,23

to construct double-cavity

clusters and investigate the structures, stability, magnetic couplings and mechanisms of the corresponding dielectron hydrates.

Each (H2O)24 cavity structure (4668 or 51262) has two

isomers which support the H-bonding network permeating state (A-type) and internal state (B-type) for excess electrons and are denoted by 4668A and 4668B or 51262A and 51262B, respectively (Fig. 1).

However, as for the (H2O)20 512 cavity, only the A-type one is found

(denoted by 512A).12

They are further fused together to construct double-cavity structures as

dielectron binding motifs.

Three combination types are considered in construction: A-type

plus A-type (AA-type), A-type plus B-type (AB-type) and B-type plus B-type (BB-type), and each type of combinations includes three modes: i) sharing one H-bonding ring as bridge (one-layer water bridge); ii) two cavities are directly H-bonded together (two-layers water bridge); and iii) an additional H-bonding ring is used to H-bond two cavities (three-layers water bridge).

All constructed double-cavity dielectron hydrates are denoted as e2@512AA, 3



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e2@4668AB(n), e2@4668BB(n), and e2@51262BB, where n=1-3, the numbers of the intervening water layers between two cavities (Fig. 1 and Fig. S2 in the Supporting Information (SI)). Computational Methods.

Geometry optimizations for all considered e2@CHs in their

closed-shell (CS) singlet, broken-symmetry (BS) open-shell singlet and triplet (T) states were conducted at the B3LYP/6-31+G(d) level without symmetry constraint with vibrational frequency analyses to get the optimized local minimum structures.

The total energies, radii

of gyration and electron center separations of two EEs and radii of water cavities were further determined

through

single

point

calculations

using

the

B3LYP/6-311++G(d,p)//B3LYP/6-31+G(d) method to understand the spatial distributions and states of e@CHs or e2@CHs.

Besides, vertical detachment energies (VDE) were calculated

for their ground states at the LC-ωPBE/6-311++G(d,p) Level (ω = 0.165 bohr-1 was used according to ref. 63 for similar sizes of the water clusters, the SI) to check the stability of hydrated electrons or hydrated dielectrons, including VDE(1), VDE(2) and VDE(t) which denote the first, second and total VDEs of EEs in e2@CHs, respectively, and VDE(t) = VDE(1) + VDE(2).

To mimic the realistic icy-hydrate surrounding of clathrate hydrates, the

surrounding effects were also considered for all e@CHs and e2@CHs by using ice as medium (dielectronic constant ε = 3.2) and the polarizable continuum model (PCM) in determining the spin coupling constants (J) and energies of e@CHs and e2@CHs through single-point energy calculations based on their gas phase geometries. The broken-symmetry approach was used to determine the BS state in the DFT framework and ground state and to calculate J through the Noodleman scheme:64-65 J = (EBS ET)/(T - BS), where EBS and ET refer to the energies of the BS and T states, while BS and T denote the average spin square values of the two states, respectively. Electron distribution proportions combined with the corresponding spin density maps at different isovalues, electronic localization function (ELF) and localized orbital locator (LOL) were also calculated to further illustrate the spin coupling mechanism in e2@CHs.

In

addition, the effects of diffuse functions were examined on the spatial distributions, VDEs and J values of e2@CHs using two types of diffuse functions which include extra atom-centered and ghost atom based diffuse functions.

The former is realized by augmenting n sets of 4


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diffuse sp functions on each hydrogen atom to a standard basis group 6-311+G(d,p), denoted as 6-311+n+G(d,p), while the latter is realized by using the ghost H-atoms that are placed at the centers of or intervening H-bonding bridge between two cavities. All of the calculations were performed using the Gaussian 09 suite of program,60 and relevant spatial maps were visualized using the VMD

61

and Multiwfn

62

softwares.

All

relevant detailed results are given in the SI. RESULTS AND DISCUSSION A) Structures and Stability of Dielectron Clathrate Hydrates Structures and EE States.

All the optimized ground state structures of dielectron

hydrates are depicted in Fig. 1 and other low-lying excited state structures and total energies of all anionic hydrates are given in the SI (Fig. S2 and Table S1 in the SI).

According to spin

density distributions (Fig. 1), these anionic clusters can be divided into three categories: 1) double internal states, e2@4668BB(n) (n=1-3) and e2@51262BB; 2) mixed states including an internal state and a network permeating state, e2@4668AB(n) (n=1-3); 3) double surface states (e2@512AA). Clearly, the differences in electronic states mainly originate from the structural differences of their component cavities. The open-shell singlet or triplet ground states of these dielectron hydrates are attributed to unique interactions between the trapped two electrons.

Taking the BB-type structure as an

example (Fig. 2), linear combination of two cavity orbitals (φ1, φ2) leads to two molecular orbitals (ψ+ and ψ−) and excess electron filling yields a closed-shell singlet state or a triplet state (Fig. S4 in the SI).

Due to the weak overlap between two single cavity orbitals (φ1, φ2)

which leads to a very small energy gap between ψ+ and ψ−, almost all dielectron hydrates have a triplet lower than the closed-shell singlet state with very small triplet-singlet energy gaps but e2@4668AB(1) which has a closed-shell singlet lower than the triplet state.

Further

configuration interaction gives rise to an open-shell singlet ground state (Fig. 1, Fig. S3 in the SI). Examination of the distribution percentages of the trapped excess electrons (Fig. 1) indicates that the AA-type cavities capture around 86 % and the BB-type ones approximately accommodate 98-99 %, while the AB-type cavities nearly bind ca. 95 % of two excess 5



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electrons. Clearly, these two excess electrons are either inhabiting on the interior or surface of a cavity or permeating on the cavity-shell H-bond network.

Their radii of gyration (Table S3

in the SI) also confirm the states of two excess electrons: those in the cavity states are ca. 3 Å, being very close to those of hydrated electrons in water (ca. 2.5 Å)

52

and aqueous glucose

solution (ca. 2.6 Å),7 while those in the network permeating states are ca. 6.5 Å. Electron Center Distances.

The separations between two excess electrons (de…e) were

also measured to clarify the states of excess electrons in the hydrates. For the n=1 double cavity structures, the structure is compact and two excess electrons have the shortest distances compared with the n=2,3 cases (Table S5 in the SI).

de…e in e2@4668BB(1) and e2@51262BB

are longer than that in e2@4668AB(1), while de…e in e2@512AA is considerably longer (12.58 Å) than those of them due to its different bridging H-bond network and cavity structure. Increase of the number of the intervening water layers connecting two cavity moieties enlarges de…e (Table S5 in the SI) to be 9-10 Å for n=2 and further 12-13 Å for n=3 in both the e2@4668AB(n) and e2@4668BB(n) series.

These de…e are comparable to those (5-11 Å with a

main distribution at 7-9 Å for the open-shell singlet state and 7-12 Å with an average of ca. 9.3 Å for the triplet state) of dynamics bipolarons in the methylpyridinium chloride ionic liquid,27 and are also close to those (5-7 Å) found in liquid ammonia, water, and water clusters from experiment and molecular dynamics simulations.34,36 Clearly, these spatial separations of two excess electrons provide a strong support for dielectron hydrate structures. Stability.

We further examined the stability of such dielectron hydrates by comparing

relative energies among their closed-shell singlet, triplet and possible open-shell singlet states and calculating VDEs of dielectrons (Table S5 in the SI).

All the calculated VDEs are

positive, indicating that dielectron clathrate hydrates are quite stable and both excess electrons can be tightly bound inside cavities or permeating the cavity shells.

For the e2@4668BB(n)

series, all total VDEs (e.g. VDE(t)/3.18-3.34 eV) are larger than those (VDE(t)/2.07-2.13 eV) for the e2@4668AB(n) series.

e2@51262BB has the largest VDEs (VDE(t)/4.26 eV ), whereas

e2@512AA has the smallest VDEs (VDE(t)/1.70 eV).

These VDE results are also very close

to those of the solvated dielectron in ionic liquids (VDE(t)/3.25-3.75 eV),27 indicating similar stability in two media.

Actually, de…e observed in dielectron hydrates are appropriate to

maximize both the stability and spin coupling interactions of the captured two electrons for 6


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promising magnetic and other spintronics properties.

In addition, the calculations of the

Gibbs free energies of formation of them also confirm their thermodynamic stability (Table S8 in the SI). B) Magnetic Properties and Spin Coupling Mechanism Magnetic Properties.

To clarify how the two excess electrons interact with each other in

such clathrate hydrates, we further calculated the spin coupling constants (J) at the B3LYP/6-311++G(d,p) level of theory (Table S5 in the SI).

Surprisingly, all dielectron

hydrates except e2@512AA have an open-shell singlet ground state and are thus antiferromagnetic (J0), as evidenced by their spin density distributions (Fig. 1).

The J for three sets

of the open-shell singlet ground state dielectron hydrates are quite mild with the values ranging from -4 to -430 cm-1 and that for e2@512AA is ca. 32 cm-1, mainly due to unique coupling modes and large spatial separation between two excess electrons.

Actually, these J

in dielectron hydrates are quite close to those (J = -37 ~ -117 cm-1) in many organic magnets with large J constants

53-57

and considerably larger than the experimental ones in the O2…O2

double-cavity hydrate (J=-1.1 cm-1 for O2@(H2O)20...O2@(H2O)20)46 and in the directly linked Cu(II) diporphyrins (J=0.55-3.64 cm-1)56 and in many other organic or inorganic magnets (|J|

Dielectron Clathrate Hydrates with Unique Superexchange Spin

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