Near Resonant Raman Optical Activity Study of the Complex Cation Tris(ethylenediamine)cobalt(III) in Water: Experiment and DFT Calculation.

Chiral metal complexes are important in catalysis and medicinal chemistry, where understanding their structural and optical properties is crucial. This study presents a combined experimental and theoretical Raman optical activity (ROA) investigation of the chiral tris(ethylenediamine)cobalt(III) complex, Δ- and Λ-[Co(en)3]3+, in aqueous solution. ROA spectra were recorded using near-resonance excitation at 532 nm. Under these conditions, a significant ECD-Raman contribution from the solution was observed and quantitatively corrected, enabling extraction of the intrinsic ROA spectra. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to simulate ROA, Raman, UV-visible absorption, and electronic circular dichroism (ECD) spectra. Four representative conformers were analyzed, and their stability and spectral properties were evaluated using both the polarizable continuum model (PCM) and the solvation model based on density (SMD). While both models reproduced key experimental trends, SMD provided a more accurate description of vibrational features, especially NH2 scissoring and CH2 rocking modes. This work shows that near-resonant ROA spectra of a cationic transition-metal complex in water can be accurately reproduced using first-principles methods without empirical adjustments, highlighting the importance of conformational averaging and solvation effects.