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Ultrafast two-dimensional infrared (2DIR) spectroscopy is a relatively new methodology, which has now been widely used to study the molecular structure and dynamics of molecular processes occurring in solution. Typically, in 2DIR spectroscopy the dynamics of a system is inferred from the evolution of 2DIR spectral features over waiting times. One of the most important metrics derived from the 2DIR is the frequency–frequency correlation function (FFCF), which can be extracted using different methods, including center and nodal line slope. However, these methods struggle to correctly describe the dynamics in 2DIR spectra with multiple and overlapping transitions. Here, a new approach, utilizing pseudo-Zernike moments, is introduced to retrieve the FFCF dynamics of each spectral component from complex 2DIR spectra. The results show that this new method not only produces equivalent results to more established methodologies in simple spectra but also successfully extracts the FFCF dynamics of individual component from very congested and unresolved 2DIR spectra. In addition, this new methodology can be used to locate the individual frequency components from those complex spectra. Overall, a new methodology for analyzing the 2D spectra is presented here, which allows us to retrieve previously unattainable spectral features from the 2DIR spectra. [ABSTRACT FROM AUTHOR]

The microscopic molecular structure and dynamics of a new deep eutectic solvent (DES) composed of an ionic liquid (1-hexyl-3-methylimidazolium chloride) and an amide (trifluoroacetamide) at various molar ratios were investigated using linear and non-linear infrared spectroscopy with a vibrational probe. The use of the ionic liquid allows us to investigate the changes that the system undergoes with the addition of the amide or, equivalently, the changes from an ionic liquid to a DES. Our studies revealed that the vibrational probe in the DES senses a very similar local environment irrespective of the cation chemical structure. In addition, the amide also appears to perceive the same molecular environment. The concentration dependence studies also showed that the amide changes from being isolated from other amides in the ionic liquid environment to an environment where the amide–amide interactions are favored. In the case of the vibrational probe, the addition of the amide produced significant changes in the slow dynamics associated with the making and breaking of the ionic cages but did not affect the rattling-in-cage motions perceived by it. Furthermore, the concentration dependence of slow dynamics showed two regimes which are linked to the changes in the overall structure of the solution. These observations are interpreted in the context of a nanoscopic heterogeneous environment in the DES which, according to the observed dynamical regimes, appears at very large concentrations of the amide (molar ratio of greater than 1:1) since for lower amide molar ratios, the amide appears to be not segregated from the ionic liquid. This proposed molecular picture is supported by small angle x-ray scattering experiments. [ABSTRACT FROM AUTHOR]

Carbonate-based lithium-ion electrolytes are of great importance due to their close relationship with the resulting battery efficiency and safety. Modifying the organic electrolyte has been paramount for achieving more efficient and safer lithium-ion batteries. However, the molecular picture of the electrolyte is still under scrutiny. Lately, ultrafast infrared spectroscopic studies have investigated the solvation structure and dynamics of the lithium ion (Li+) in both linear and cyclic carbonates. However, theoretical studies describing the molecular arrangements and transformation occurring in such time scales are scarce. In this study, ab initio molecular dynamics simulations were used to obtain the molecular structure and dynamics of the Li+ solvation shell in cyclic and linear carbonates. The theoretical results showed that molecular arrangement of the carbonates directly coordinating Li+ is not significantly altered by the carbonate chemical nature. However, the cyclic and linear carbonates showed significant different pictures of the overall solvation shell due to the intercalation phenomenon observed for cyclic carbonates, which significantly alters the motions of coordinated solvent. In addition, the intercalation appears to affect the propensity of ion pair formation and/or solvent exchange. Finally, the dynamics of the geometrical changes of the carbonates solvating Li+ is found to occur with characteristic times of tenths of picoseconds, while ion pair and solvent chemical exchange appear to happen in time scales which are at least an order of magnitude larger. Our study provides a comprehensive picture of the structure and dynamics of the molecular components in different carbonate-based lithium-ion electrolytes occurring in picosecond time scales. [ABSTRACT FROM AUTHOR]