Microhydration of nitromethane anions from both a solute and solvent perspective

We present experimental infrared spectroscopic data on mass-selected, hydrated nitromethane anion clusters with up to four water ligands. The vibrational bands in the OH stretching region encode the solvent structure, while the CH stretching bands contain information on the influence of the hydration shell on the solute ion. We interpret our findings using density functional theory calculations. The first water molecule binds symmetrically to the two oxygen atoms of the nitro group but couples to low-frequency vibrational modes that impart a very complicated structure on the OH stretching region. Competition between water-ion and water-water interaction makes the dihydrate very floppy and precludes unambiguous structural assignment. The tri- and tetrahydrate spectra can be interpreted on the basis of H-bonded ring structures. The excess electron is polarized by hydration, which can clearly be seen by shifting CH stretching frequencies.