This research seeks to develop a better understanding of hydrogen bonding and solvation of simple ions in liquid electrolytes. The electrolytes considered range from dilute salt solutions in water and dipolar solvents, to ionic liquids, and concentrated mixtures of ions and dipolar components such as are present in deep eutectic solvents. The focus is on fundamentals, and much of the initial work will be on simple, well-known systems. The work will then progress to the new, more complex electrolyte systems of growing importance in many energy-related technologies, most notably in the areas of Li-ion and related batteries, supercapacitors, and electroplating. This work is most closely aligned with the two CSGB themes Charge Transport and Reactivity and Chemistry in Aqueous Environments, particularly complex multicomponent solutions and concentrated electrolytes. Three interrelated projects are proposed, which combine infrared and NMR spectroscopy, and computer simulation.
The first and primary project entails the use of far-infrared (FIR) spectroscopy to study solvation of simple ions such as alkali metal salts in liquid electrolytes. FIR absorption bands of such ions result from rattling motions of the ions within the cage formed by surrounding solvent molecules. The spectra of these ion-solvent vibrations, particularly when augmented with computer simulations, as proposed here, provide a direct window on solvation structure and dynamics of these ions in different environments. Although some work of this sort was initiated decades ago, improvements in instrumentation and computational power have greatly enhanced the quality of both the spectra obtained and our ability to interpret them. Almost no work along these lines has so far been reported in the more complex liquid electrolytes of interest here. The proposed projects therefore explore what new insights can be gained from this type of spectroscopy coupled with modern computational methods.
The second project involves characterization and use of phosphine oxides as mid-IR solvation probes. The resonance between P=O and P+-O- forms in phosphine oxides renders the PO bond highly sensitive to solvent polarity, particularly a solvent’s hydrogen bond donating ability. For this reason, the 31P chemical shift of triethylphosphine oxide (TEPO) was previously used to establish the well-known acceptor number scale of solvent electrophilicity. Like the 31P chemical shift, the frequency of the P=O stretch of TEPO is highly sensitive to its environment, and multiple bands corresponding to differently hydrogen-bonded solvates can be observed in protic solvents. Quantum chemical calculations and molecular dynamics simulations will be used to characterize the P=O stretch of trimethylphosphine oxide (TMPO) and create a spectroscopic map based on calibration against experimental data in a number of conventional solvents. IR spectroscopy of TMPO and related molecules will then be used to measure polarity and hydrogen bonding in the liquid electrolytes mentioned above. Classical molecular dynamics simulations, together with the spectroscopic map, will be used to interpret the observed spectra in terms of specific molecular interactions and dynamics.
Finally, in the third project, computer simulations undertaken for the FIR work will be extended to include calculation of the electric field gradient autocorrelation function (EFG ACF), which summarizes the dynamics relevant to quadrupole relaxation of atomic ions. These functions will be used to predict quadrupole relaxation times of ions both in simple solvents, where experimental times are already available, as well as in electrolyte solvents, which have yet to be studied in this manner. New NMR measurements of relaxation rates will be performed as needed to test these predictions. Comparison of the EFG-ACFs to suitable representations of the dynamics underlying the FIR spectra will be made in order to better appreciate the different perspectives on ion solvation dynamics provided by FIR and NMR spectroscopies.
