Infrared photodissociation spectroscopy of D2-tagged CH3CO2-(H2O)0-2 anions

Infrared photodissociation spectroscopy of D-2-tagged anions is used to obtain the vibrational spectra of microsolvated acetate, (n = 0-2), in the CH/OH stretching (similar to 4000-2500 cm(-1)) and fingerprint (similar to 1800-800 cm(-1)) spectral regions. These results are analysed by comparison to anharmonic IR spectra from MP2 calculations as well as Born-Oppenheimer molecular dynamics (BOMD) simulations. In agreement with prior work, we find that the first water molecule adds to the acetate anion by donating two hydrogen bonds, yielding a symmetrical structure involving a six-membered hydrogen-bonded ring. Two nearly degenerate binding motifs that differ in energy by less than 1 kJ/mol are identified for n = 2 anion, where the lowest-energy geometry has two ion-water hydrogen bonds as well as a water-water hydrogen bond. The molecular dynamics simulations confirm that this lower-energy structure is preferred over a slightly higher-lying configuration possessing three ion-water hydrogen bonds and no water-water interactions. Analysis of the molecular motion contributing to peaks in the BOMD spectra via a generalised normal mode approach provides assignment of all observed transitions to the lower-energy structure, and enables distinction of the vibrational signatures associated with ion-water and water-water intermolecular motions.