Hydrated electrons as nodes in porous clathrate hydrates

Abstract

Understanding the nature of hydrated electrons (e-aq) in various aqueous systems has always been a nontrivial and challenging task. Here, using periodic density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, we unveil e-aq electronic structures in one of water's most exciting crystalline structures, namely porous clathrate hydrates (CHs). Specifically, we propose e-aq acts as a node (e-aq@node) in CHs. Different from e-aq in water and ice, e-aq@node resides in a H2O-defect in the presence of four unsaturated hydrogen bonds (H-bonds) and forms a rigid tetrahedral structure. Due to the unique H-bond networks of CHs, e-aq@node is shared by four stacking H2O cavities. Intriguingly, these cavities can accommodate guest molecules and adjust the electronic states of e-aq@node, leading to an obvious blue shift of its optical absorption spectrum compared to liquid water and ordinary ice. Our findings have a general interest and extend knowledge of e-aq into new porous aqueous systems.

Two-photon chemistry of tetrahydrofuran in clathrate hydrates

“…The atomic radicals quickly decay when our samples are annealed above ∼120 K, consistent with other reports. 24,26,29,38 Only at these elevated temperatures does the thermally activated diffusion of H˙/D˙ between cages 59,60,66 enable reaction (R3) with in-tact THF molecules to form the cyclic THF-2-yl radicals which we only detect after annealing (Fig. S12 and Table S7, ESI†).…”

“…This is strong evidence in favor of the acyl radical because its measured linewidth will be broadened chiefly by unresolved HFC to intramolecular protons. Conversely, since the preferred trapping site for an electron in clathrate hydrates is likely to involve tetrahedral coordination by four water molecules at a nodal defect in the host lattice, 60 an ESR singlet corresponding to e t − should instead experience this significant linewidth contraction in D 2 O hydrates. We conclude that the most likely identity of this signal is an acyl radical:Even if we do not observe e t − by steady-state ESR, it may be relevant at timescales not probed in this work; we will discuss the likelihood of this in Section 2.3.…”

“…Based on our estimate of 1d THF /17d H 2 O in Section 2.1, some ionization of water is likely to occur in our system under UV irradiation, analogous to reaction (R2). The ionization threshold of water has an early onset in the condensed phase, 51,52,[67][68][69] beginning at energies as low as B8 eV in amorphous ice at 77 K. 70,71 Existing H-bond defects in the crystal lattice can serve as trapping sites 60 which may prolong recombination with protons. Likewise, the combined energy of two pump photons (9.36 eV) happens to fall just below the adiabatic ionization threshold of gas-phase THF (9.42 eV).…”