Perturbation of Fermi Resonance on Hydrogen-Bonded > C═O: 2D IR Studies of Small Ester Probes.

We utilized linear and 2D infrared spectroscopy to analyze the carbonyl stretching modes of small esters in different solvents. Particularly noteworthy were the distinct carbonyl spectral line shapes in aqueous solutions, prompting our investigation of the underlying factors responsible for these differences. Through our experimental and theoretical calculations, we identified the presence of the hydrogen-bond-induced Fermi resonance as the primary contributor to the varied line shapes of small esters in aqueous solutions. Furthermore, our findings revealed that the skeletal deformation mode plays a crucial role in the Fermi resonance for all small esters. Specifically, the first overtone band of the skeletal deformation mode intensifies when hydrogen bonds form with the carbonyl group of esters, whereas such coupling is rare in aprotic organic solvents. These spectral insights carry significant implications for the utilization of esters as infrared probes in both biological and chemical systems.

Theoretical Spectroscopy Aided Validation of the Hydration Structure of Trimethylamine N-Oxide (TMAO).

The molecular-level understanding of the hydration structure of external solutes is extremely challenging. In the context of molecular simulation, particularly sampling proper solvation structure by classical force fields remains always a matter of concern. In the present work, we use theoretical vibrational spectroscopy to understand the microscopic solvation structure of TMAO in water in the cases of five different classical force fields of TMAO. We find that the Netz (J. Phys. Chem. B 2013, 117, 8310-8321) force field agrees better with the experimental results. We also observe that the O-H stretching frequency gets red-shifted compared to the bulk water response, suggesting that the TMAO-water forms stronger hydrogen bonds than water-water. We further investigate the O-H stretching frequency in different solvation shells and the hydrophobic and hydrophilic regions of TMAO. We find that, in the hydrophilic region, O-H stretching frequencies show a strong orientational correlation; however, that is absent in the hydrophobic region. These are further supplemented by hydrogen-bond analysis and local structure index data.

Theoretical spectroscopy of isotopically dilute water and hydrophobicity.

Hydrophobic solutes significantly alter the water hydrogen bond network. The local alteration of solvation structures gets reflected in the vibrational spectroscopic signal. Although it is possible to detect this microscopic feature by modern infrared spectroscopy, bulk phase spectra often come with a formidable challenge of establishing the connection of experimental spectra to molecular structures. Theoretical spectroscopy can serve as a more powerful tool where spectroscopic data cannot provide the microscopic picture. In the present work, we build a theoretical spectroscopic map based on a hybrid quantum-classical molecular simulation approach using a methane-water system. The single oscillator O-H stretch frequency is well correlated with a collective variable solvation energy. We construct the spectroscopic maps for fundamental transition frequencies and also the transition dipoles. A bimodal frequency distribution with a blue-shifted population of transition frequency illustrates the presence of gas like water molecules in the hydration shell of methane. This observation is further complemented by a shell-wise decomposition of the O-H stretch frequencies. We observe a significant increase in the ordering of the first solvation water molecules, except those which are directly facing the methane molecule. This is manifested in the redshift of the observed transition frequencies. Temperature dependent simulations depict that the water molecules facing the methane molecule behave similarly to the high temperature water, and a few of the first shell water molecules behave more like cold water.

Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG)2; M = Ni2+, Cu2+] Complexes: A Comparative Theoretical Study.

Dimethylglyoxime (DMG) usually forms thermodynamically stable chelating complexes with selective divalent transition-metal ions. Electronic and spectral properties of metal-DMG complexes are highly dependent on the nature of metal ions. Using range-separated hybrid functional augmented with dispersion corrections within density functional theory (DFT) and time-dependent DFT, we present a detailed and comprehensive study on structural, electronic, and spectral (both IR and UV-vis) properties of M(DMG)2 [M = Ni2+, Cu2+] complexes. Ni(DMG)2 results are thoroughly compared with Cu(DMG)2 and also against available experimental data. Stronger H-bonding leads to greater stability of Ni(DMG)2 with respect to isolated ions (M2+ and DMG-) compared to Cu(DMG)2. In contrast, a relatively larger reaction enthalpy for Cu(DMG)2 formation from chemically relevant species is found than that of Ni(DMG)2 because of the greater binding enthalpy of [Ni(H2O)6]2+ than that of [Cu(H2O)6]2+. In dimers, Ni(DMG)2 is found to be 6 kcal mol-1 more stable than Cu(DMG)2 due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm-1) is predicted between two spin-1/2 Cu2+ ions present in the Cu(DMG)2 dimer. Additionally, the potential energy curves calculated along the O-H bond coordinate for both complexes suggest asymmetry and symmetry in the H-bonding interactions between the H-bond donor and acceptor O centers in the solid-state and in solution, respectively, well corroborating with early experimental findings. Interestingly, a lower proton transfer barrier is obtained for the Ni(DMG)2 compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu(DMG)2 results in blue-shifted O-H stretching modes compared to that in Ni(DMG)2. On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu(DMG)2. Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M(DMG)2 complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination.