States And Migration Of Excess Electrons In Novel Liquid Media Revealed By Ab Initio Molecular Dynamics Simulation

The solvation of excess electrons(EE) in various media including molecular solvents, molecular clusters, and even solid surfaces or solid defects, to form transient species due to EE addition is an interesting topic for basic science. EE properties have been extensively investigated by various methods, and many important applications to charge transfer, radical reactions, and polarons follow from their special physical characterization and reactivity. Despite these studies the details of EE in different solvents and the involved solvent-mediated mechanism are still insufficiently understood. There is still a long way to go for the exploring the potential novel applications of solvated electron. Keep in mind the concept of "green chemistry", as greatly promoted all over the world; we have studied the solvation of excess electron in two environmental benefit novel media, room-temperature ionic liquid and supercritical carbon dioxide using ab initio molecular dynamics simulation technique. The special electronic and structural properties of both these solvents lead to their different performances in mediating electron transfer processes thus attract intense interests from both experimental and theoretical scientists. We carried out a series of significative work and obtained some valuable results on these issues. The primary innovations are related as follows.1) Excess electron solvation in an imidazolium-based room-temperature ionic liquid we present the first approach to the excess electron solvation in a novel medium, room-temperature ionic liquid, using ab initio molecular dynamics simulation technique in this work. Results indicate that an excess electron can be solvated in the [dmim]+Cl- IL as long-lived delocalized states and two short-lifetime localized states:one a single-cation-residence parasitical type and the other a double-cation-based solvated type state. The presence of a low-lyingπ*-LUMO as the site of excess electron residence in the cation moiety disables the C-H unit as an H-bond donor, while the aromaticity requirement of the rings and the effect of the counterion Cl-s make the resulting ion-pairs a weak stabilizer for an excess electron. Although no large solvent reorganization in IL was found at the picosecond scale, the IL fluctuations sufficiently modify the relative energy levels of the excess electron states to permit facile state-to-state conversion and adiabatic migration. The binding energy of the excess electron is only～0.2 eV, further indicating that it is in a quasi-free state, with large drift mobility, suggesting that ILs are unreactive, and promising mediators for transport of excess electrons, in agreement with the experimental findings. The present study provides insight into the novel electron solvation character in a new class of promising media for physical and chemical processes, which are fundamental for understanding of electron migration mechanisms in IL-based applications.2) States and eigration of excess electron in a pyridinium-based room-temperature ionic liquid the structural and electronic properties of the EE in 1-methylpyridinium chloride IL were explored using ab initio molecular dynamics simulations and quantum chemistry calculations to give an overall understanding of the solvation and transport behavior of an EE in such an IL. Results indicate that the cationπ*-type orbitals are the residence of EE and the relative electronic states can be characterized by alternative appearance of localized and delocalized states during evolution. The types of electronic states of EE in IL are determined by the number of cations which contribute to the LUMO of IL. In a localized state, one or two cations contribute to the LUMO of the bulk ionic liquid, while the IL LUMO is composed of theπ*-type orbitals over nearly all the cations in the cell in the delocalized state. The arrangement and fluctuation-induced changes of the orbital components in the empty band lead to alternation of different states, thus exhibiting electron migration in the system. These findings could be attributed to the special features of electronic structures and geometries of the IL, which can be also used to explain similarities and differences of the performance between the pyridinium-based and imidazolium-based ILs in mediating electron migration.3) Dielectrons in a pyridinium-based room temperature ionic liquid we approach series of problems that about dielectrons in 1-methyl pyridinium chloride room temperature ionic liquid such as, the solvation mechanism and electronic evolution features of dielectrons as well as the differences between the corresponding electronic states of solvated single electron and dieletrons, by performing Car-Parrinello molecular dynamics simulations of two excess electrons in the ionic liquid 1-methylpyridinium chloride. Both singlet and triplet multiplicity are considered. The results show that dielectrons mainly resides in cationπ*-type orbitals as single electron dose in the IL, whereas solvated dielectrons are much more extensively distributed since the addition of a second excess electrons to the cation is much more difficult than that of a single EE. The dielectrons tends to distribute on more cation to avoid accumulation of negative charge on a certain cation too much, which is also inhabited by the aromatic requirement of the cation rings. Singlet dielectrons tend to diffuse to larger areas when compared with the triplet ones. Since singlet dielectrons are in diaparallel configuration, the migration of one electron always brings the other to follow in the same direction due to the coupling interaction between them. While for the triplet dielectrons, parallel configuration causes repulsion between them, which prevents the diffusing of each other. Although there is no observation of transformation of the multiplicity of the system between singlet and triplet, due to the limit of different multiplicity setting for independent simulation of ionic liquid systems with either singlet or triplet dielectrons, the same fluctuating ranges of the total potential energy of the different simulated systems suggests the possibility of the transfer of the multiplicity between singlet and triplet.4) Solvation dynamics of excess electron in supercritical CO2 the solvation dynamics including states and migration behavior of an excess electron (EE) in supercritical carbon dioxide (scCO2) were explored using ab initio molecular dynamics (AIMD) simulations combined with density functional theory (DFT) calculations. Results suggest that an EE can be trapped in homogeneous scCO2 with frequently alternative appearance of the localized and delocalized states, distinctly different from the cases in water, ionic liquids, and solids. In particular, two interesting phenomena, spontaneous sustained breathing events and core-transformation diffusion migration, were observed to characterize the existence and evolution dynamics of an EE in scCO2. The delocalized EE can gather to one core molecule, forming a core-seeding CO2- solvated by the remaining CO2 solvent molecules, a localized state. After a short period for surviving, the localized EE expands out (breathing out), becoming a delocalized state. Then, the EE re-gathers to the same core, forming the localized state again. The localization-delocalization oscillation period is about 50 fs. The repeated events are continued, which can last for 1-3 ps. After a certain oscillation period, the EE can gather to another core molecule, forming a new-core-localized state, leading to electron transfer via a core-transformation diffusion migration mechanism, followed by the similar sustained breathing events. Further analyses indicate that bending vibration of the core CO2 is responsible to the oscillation phenomenon with an assistance of the stretching vibration and relative vibrational amplitudes of two candidate cores regulated by the surrounding CO2 molecules determines the electron transfer. These observations could apply to a kind of EE solvation systems with a seeding behavior where the attaching-scattering of an electron from the seeding molecule can be inversely mixed stretching/bending vibrations-regulated and thermal-fluctuation-promoted. These findings have potentially important ramifications for understanding electron localization and migration in the breathing-functional group-containing condensed media as well as biological long-range charge transfer.In summary, ab initio molecular dynamics simulations have been performed to study the solvation of excess electrons in two environmental benefit novel media, room-temperature ionic liquid and supercritical carbon dioxide. The novel findings of the dynamic properties of solvated electrons in these media are important for getting better understanding the electron transfer mechanisms involved in various physical and chemical processes.