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

Perturbation of Fermi Resonance on Hydrogen-Bonded > CO: 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