RESUMEN
Ultrafast infrared spectroscopy has become a very important tool for studying the structure and ultrafast dynamics in solution. In particular, it has been recently applied to investigate the molecular interactions and motions of lithium salts in organic carbonates. However, there has been a discrepancy in the molecular interpretation of the spectral features and dynamics derived from these spectroscopies. Hence, the mechanism behind spectral features appearing in the carbonyl stretching region was further investigated using linear and nonlinear spectroscopic tools and the co-solvent dilution strategy. Lithium perchlorate in a binary mixture of dimethyl carbonate (DMC) and tetrahydrofuran was used as part of the dilution strategy to identify the changes of the spectral features with the number of carbonates in the first solvation shell since both solvents have similar interaction energetics with the lithium ion. Experiments showed that more than one carbonate is always participating in the lithium ion solvation structures, even at the low concentration of DMC. Moreover, temperature-dependent study revealed that the exchange of the solvent molecules coordinating the lithium ion is not thermally accessible at room temperature. Furthermore, time-resolved IR experiments confirmed the presence of vibrationally coupled carbonyl stretches among coordinated DMC molecules and demonstrated that this process is significantly altered by limiting the number of carbonate molecules in the lithium ion solvation shell. Overall, the presented experimental findings strongly support the vibrational energy transfer as the mechanism behind the off-diagonal features appearing on the 2DIR spectra of solutions of lithium salt in organic carbonates.
RESUMEN
While cheminformatics skills necessary for dealing with an ever-increasing amount of chemical information are considered important for students pursuing STEM careers in the age of big data, many schools do not offer a cheminformatics course or alternative training opportunities. This paper presents the Cheminformatics Online Chemistry Course (OLCC), which is organized and run by the Committee on Computers in Chemical Education (CCCE) of the American Chemical Society (ACS)'s Division of Chemical Education (CHED). The Cheminformatics OLCC is a highly collaborative teaching project involving instructors at multiple schools who teamed up with external chemical information experts recruited across sectors, including government and industry. From 2015 to 2019, three Cheminformatics OLCCs were offered. In each program, the instructors at participating schools would meet face-to-face with the students of a class, while external content experts engaged through online discussions across campuses with both the instructors and students. All the material created in the course has been made available at the open education repositories of LibreTexts and CCCE Web sites for other institutions to adapt to their future needs.
RESUMEN
Lithium-ion batteries have become ubiquitous to modern life because of their use in the energy storage needs of our daily lives. In past several decades, much effort has been put into studying the molecular structure of electrolytes composed of organic carbonates. However, other solvents with similar properties but better thermal stabilities, such as tertiary amides, have not received the same level of scrutiny. In this work, solutions of lithium salts in ureas, tertiary amides with the structure RR'N-CO-NRâ³Râ´, with different sizes and connectivity are studied. Ureas present an interesting case study because unlike organic carbonates, the amide bond is planar and has restricted conformational change. In addition, ureas cannot bind the lithium ion through their nitrogen atoms. By using steady-state and time-resolved infrared spectroscopies and ab-initio computational methods, detailed descriptions of the changes to the lithium-ion solvation structure as a result of the urea structure were derived for three ureas bearing a strong resemblance to commonly used organic carbonates. These results show that the solvation shell of ureas has a tetrahedral structure similar to that of other organic solvents. Although the structure of the amide bonds in these ureas is similar to that of carbonate molecules, the atomic connectivity differs. In addition, the dynamics of the cation solvation shell formed by ureas shows a picosecond motion, which is attributed to deformation of the tetrahedral structure. Our investigations also indicate that the deformation dynamics is controlled directly by the size of the urea because of the rigidity of the amide bond in these molecules. Overall, this work shows that ureas share similarity with their organic carbonate analogues, but the rigid urea structure provides an easier framework for interpreting the vibrational observations in terms of the solvent molecular structure.
RESUMEN
Solutions of lithium hexafluorophosphate (LiPF6) in linear organic carbonates play a significant role in the portable energy storage industry. However, many questions remain about the solution structure at the molecular-level. An atomic characterization of these solutions is important for determining their structure-property relations, which will allow for the rational design of new and improved lithium ion based energy storage technologies. In this study, a combination of infrared spectroscopies and density functional theory calculations was used to investigate the speciation of the lithium ion (free ion, solvent separated ion pair, contact ion pairs, and aggregates) in dimethyl carbonate solutions having concentrations ranging from 1 M to 3 M. The experimental data shows that at typical battery electrolyte concentrations the lithium ion exists predominantly as free ions and solvent separated ion pairs, but charged contact ion pairs are also present in small concentrations. In contrast, at high concentrations the lithium ion is present in aggregates, but a noticeable fraction remains present as free ions.
