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The liquid structure of five different amide-based deep eutectic solvents (DESs) as a function of the chemical structure of the hydrogen bond acceptor (HBA) was investigated by linear Fourier transform infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopies. Linear FTIR spectroscopy shows that the amide band of the DESs is not significantly affected by the chemical structure and symmetry of the HBA cation. However, its excitonic nature does not allow us to draw further conclusions. Analysis of the 13C amide line shapes derived from the 2DIR spectra reveals that the different DESs do not show appreciable differences in the level of disorganization. The vibrational dynamics, derived from the photon echo experiments on the 13C amide, shows that there is a fast component with a time scale of ∼1 ps irrespective of the HBA. The ultrafast dynamics is assigned to hydrogen bond making and breaking between amides. In addition, a slow dynamical component is observed in the time evolution of the photon echo signal. This contribution appears to be correlated with the asymmetry and polarity of the moieties of the HBA. The overall dynamics is rationalized in terms of a microscopic heterogeneous structure of the DESs, where the heterogeneities create domains that slow the hydrogen bond making and breaking. Molecular dynamics simulations provide additional support for our modeling of the data. In addition, the presence of nanoscopic heterogeneities is consistent with the observation of an endortherm at 23 °C in the differential scanning calorimetry thermogram, which evidenced a phase transition at 23 °C, even though the tested DESs have a melting temperature below -40 °C. [ABSTRACT FROM AUTHOR]

Thevibrational population relaxation and hydration shell dynamicsof the symmetric tricyanomethanide (TCM) anion are investigated ina sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle as a functionof the water pool radius. Two-dimensional infrared (IR) spectroscopyin combination with linear absorption and ultrafast IR pump–probespectroscopy is utilized in this study. Spectroscopic measurementsshow that the anion has two bands in the 2160–2175 cm–1region, each with its own spectroscopic signatures. Analysis ofthe vibrational dynamics shows that the two vibrational bands areconsistent with the anion located either at the interface or in thewater pool. The sensitivity of the TCM anion to the environment allowsus to unequivocally monitor the vibrational and hydration dynamicsof the anion in those two different environments. A TCM anion locatedat the interface does not show any significant variation of the vibrationaldynamics with the water pool size. On the contrary, the TCM anioninside the water pool exhibits a large and nonlinear variation ofthe vibrational lifetime and the frequency–frequency correlationtime with the pool radius. Moreover, for the solvated anion in waterpools of 49 Å in radius (W0= 30),the vibrational lifetime reaches the values observed for the anionin bulk water while the frequency–frequency correlation timeshows a characteristic time higher than that observed in the bulk.In addition, for the first time a model is developed and used to explainthe observed nonlinear variation of the spectroscopic observableswith the pool size. This model attributes the changes in the vibrationaldynamics of the TCM anion in the water pool to the slow and radius-dependentwater dynamics present in the confined environment of a reverse micelle. [ABSTRACT FROM AUTHOR]

The degenerate transition corresponding to asymmetric stretches of the D3h tricyanomethanide anion, C(CN)3–, in aqueous solution was investigated by linear FTIR spectroscopy, femtosecond pump–probe spectroscopy, and 2D IR spectroscopy. Time resolved vibrational spectroscopy shows that water induces vibrational energy transfer between the degenerate asymmetric stretch modes of tricyanomethanide. The frequency–frequency correlation function and the vibrational energy transfer show two significantly different ultrafast time scales. The system is modeled with molecular dynamics simulations and ab initio calculations. A new model for theoretically describing the vibrational dynamics of a degenerate transition is presented. Microscopic models, where water interacts axially and radially with the ion, are suggested for the transition dipole reorientation mechanism. [ABSTRACT FROM AUTHOR]

The structure of poly( N -isopropylacrylamide) (PNIPAM) in solution is still an unresolved topic. Here, the PNIPAM structure in water was investigated using a bottom-up approach, involving the monomer, dimer, and trimer, and a combination of infrared (IR) spectroscopies as well as molecular dynamics simulations. The experiments show that the monomer and oligomers exhibit a broad and asymmetric amide I band with two underlying transitions, while PNIPAM presents the same major transitions and a minor one. Analysis of the 2D IR spectra and theoretical modeling of the amide I band indicates that the two transitions of the monomer do not have the same molecular origin as the oligomers and the polymer. In the monomer, the two bands originate from the ultrafast rotation of its ethyl group, which leads to different solvation structures for the various rotational conformers. In the case of the oligomers, the asymmetry and splitting of the amide I band is caused by the vibrational coupling among adjacent amide side chains. Moreover, it is deduced from the simulations that the oligomers have three distinct backbone conformations for neighboring amides. In particular, two of the backbone conformations have a closed and compact structure, while in the third, the backbone is open and elongated. The bottom-up approach allowed us to infer that such backbone conformations exist in PNIPAM as well. Consequently, the two major amide I transitions of the polymer are also assigned to split amide I transitions resulting from the vibrationally coupled nearest-neighboring amides. In contrast, the additional minor transition observed in PNIPAM is assigned to unsolvated amide units of the polymer. The proposed molecular model successfully describes that PNIPAM amide I band changes with temperature in terms of its molecular structure. This new model strongly suggests that PNIPAM does not have a completely random backbone structure, but has distinct backbone conformers between neighboring amides.

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C-H group and an electron-rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C-H group and an amide carbonyl oxygen atom in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6-20 kJ mol -1 . Ab-initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen-bond making and breaking., (© 2020 Wiley-VCH GmbH.)