The Hydrated Electron as a Pseudo-Atom in Cavity-Bound Water Clusters

Photo-excited charge transfer from adamantane to electronic bound states in water

Aqueous nanodiamonds illuminated by UV light produce free solvated electrons, which may drive high-energy reduction reactions in water. However, the influence of water conformations on the excited-state electron-transfer mechanism are still under debate. In this work, we offer a theoretical study of charge-transfer states in adamantane-water structures obtained by linear-response time-dependent density-functional theory. Small water clusters with broken hydrogen bonds are found to efficiently bind the electron from adamantane. A distinction is made with respect to the nature of the water clusters: some bind the electron in a water cavity, others along a strong permanent total dipole. These two types of bound states are more strongly binding, the higher their electron affinity and their positive electrostatic potential, the latter being dominated by the energy of the lowest unoccupied molecular orbital of the isolated water clusters. Structural sampling in a thermal equilibrium at room temperature via molecular dynamics snapshots confirms under which conditions the underlying waters clusters can occur and verifies that broken hydrogen bonds in the water network close to adamantane can create traps for the solvated electron.

Switching from an electride-like molecule to the molecular electride K-F6C6H6 driven by an oriented external electric field

The exploration of innovative molecular switches has resulted in large developments in the field of molecular electronics. Focusing on a single molecular switch with different forms exhibiting different electride features, potassium-atom-doped all-cis 1,2,3,4,5,6-hexafluorocyclohexane K-F6C6H6 was studied theoretically. It was found that an oriented external electric field can drive excess electron transfer from the region outside of the K atom to that outside of F6C6H6. Subsequently, the electride-like molecule K-F6C6H6 (1) switches into the molecular electride K-F6C6H6e- (3) through another electride-like molecule K-F6C6H6 (2). The static first hyperpolarizabilities (β0) are increased over 12- and 5-fold when moving from 1 to 2 and 3, respectively. The rise of each β0 value constitutes an order of magnitude improvement. Between them, the different β0 values suggest that K-F6C6H6 is a good candidate for use as a multiple-response nonlinear optics switch. The order of the β0 values of 1-4 for M-F6C6H6 (M = Li and Na) coincide with that of K-F6C6H6, also exhibiting a switch effect.

Thermal Equilibration Controls H-Bonding and the Vertical Detachment Energy of Water Cluster Anions

One of the outstanding puzzles in the photoelectron spectroscopy of water anion clusters, which serve as precursors to the hydrated electron, is that the excess electron has multiple vertical detachment energies (VDEs), with different groups seeing different distributions of VDEs. We have studied the photoelectron spectroscopy of water cluster anions using simulation techniques designed to mimic the different ways that water cluster anions are produced experimentally. Our simulations take advantage of density functional theory-based Born-Oppenheimer molecular dynamics with an optimally tuned range-separated hybrid functional that is shown to give outstanding accuracy for calculating electron binding energies for this system. We find that our simulations are able to accurately reproduce the experimentally observed VDEs for cluster anions of different sizes, with different VDE distributions observed depending on how the water cluster anions are prepared. For cluster anion sizes up to 20 water molecules, we see that the excess electron always resides on the surface of the cluster and that the different discrete VDEs result from the discrete number of hydrogen bonds made to the electron by water molecules on the surface. Clusters that are less thermally equilibrated have surface waters that tend to make single H-bonds to the electron, resulting in lower VDEs, while clusters that are more thermally equilibrated have surface waters that prefer to make two H-bonds to the electron, resulting in higher VDEs.

Electron Detachment and Subsequent Structural Changes of Water Clusters

A cost-effective equation of motion coupled cluster method, EOMIP-CCSD(2), is used to investigate vertical and adiabatic ionization potential as well as ionization-induced structural changes of water clusters and compared with CCSD(T), CASPT2, and MP2 methods. The moderate N(5) scaling and low storage requirement yields EOMIP-CCSD(2) calculation feasible even for reasonably large molecules and clusters with accuracy comparable to CCSD(T) method at much cheaper computational cost. Our calculations shed light on the authenticity of EOMIP-CCSD(2) results and establish a reliable method to study of ionization energy of molecular clusters. We have further investigated the performance of several classes of DFT functionals for ionization energies of water clusters to benchmark the results and to get a reliable functionals for the same.

On the electrostatic nature of electrides

The nature of electron localization in electrides is explored by examining their electrostatic features. Ab initio investigations of three experimentally synthesized and two theoretically modeled organic electrides are performed in order to unveil the characteristics of the trapped electron and to understand the reason for their low thermal stability. A single molecular unit of the electride extracted from the crystal structure shows an unusually deep minimum in its electrostatic potential, located far away from its van der Waals surface. A comparison of electrostatic features of the usual electron localization such as lone pairs has been drawn against those of the trapped electron in the crystal voids of electrides. Further characterization of the MESP minimum brings out the isotropic behavior of the trapped electrons as compared to the lone-pair minimum which is strongly directional. The analysis of single molecular behavior of an electride has been extended to the set of molecules in the unit cell of the crystal lattice. The present study also suggests the criteria for ligands to achieve thermally stable organic electrides.

Exploring water binding motifs to an excess electron via X2(-)(H2O) [X = O, F]

X(2)(-)(H(2)O) [X = O, F] is utilized to explore water binding motifs to an excess electron via ab initio calculations at the MP4(SDQ)/aug-cc-pVDZ + diffs(2s2p,2s2p) level of theory. X(2)(-)(H(2)O) can be regarded as a water molecule that binds to an excess electron, the distribution of which is gauged by X(2). By varying the interatomic distance of X(2), r(X1-X2), the distribution of the excess electron is altered, and the water binding motifs to the excess electron is then examined. Depending on r(X1-X2), both binding motifs of C(s) and C(2v) forms are found with a critical distance of ∼1.37 Å and ∼1.71 Å for O(2)(-)(H(2)O) and F(2)(-)(H(2)O), respectively. The energetic and geometrical features of O(2)(-)(H(2)O) and F(2)(-)(H(2)O) are compared. In addition, various electronic properties of X(2)(-)(H(2)O) are examined. For both O(2)(-)(H(2)O) and F(2)(-)(H(2)O), the C(s) binding motif appears to prevail at a compact distribution of the excess electron. However, when the electron is diffuse, characterized by the radius of gyration in the direction of the X(2) bond axis with a threshold of ∼0.84 Å, the C(2v) binding motif is formed.