Raman scattering of water in vicinity of polar complexes: Computational insight into baseline subtraction

Water is a greatly convenient solvent in Raman spectroscopy. However, non-additive effects sometimes make its signal difficult to subtract. To understand these effects, spectra for clusters of model ions, including transition metal complexes and water molecules, were simulated and analyzed. A combined molecular mechanics/quantum mechanics approach was taken to reveal how relative Raman scattering intensities depend on the distance from the solute and the excitation wavelength. The computations indicate a big effect of solute charge; for example, the sodium cation affects Raman scattering by water to a lesser extent than the chlorine anion. The modeling was able to qualitatively reproduce the experimental observation that a solution of a simple salt may work as a baseline better than pure water in many Raman experiments. For absorbing species, an additional scattering boost occurs due to the resonance effect. Simulations thus provide useful insight into solute-solvent interactions and their effects on measured spectra.