Studying protonated ion hydration by infrared spectroscopy of size-selected NH4+(H2O)n clusters in a free jet expansion

This paper investigates heterogeneous water nucleation on ammonium ions in a supersonic jet using vibrational predissociation spectroscopy. For NH4+(H2O)(n), it is revealed that nonhydrogen bonded NH stretching of NH4+ is resonant at 3300-3400 cm(-1), which is fairly independent of the cluster size of n = 2-4. The frequencies, however, shift strongly from similar to 2900 cm(-1) of n 2 to similar to 3100 cm(-1) of n = 4 for hydrogen bonded NH stretches. Hydration at the second shell starting with n = 5 has little effect on the ion core vibrations. Similar to the NH oscillations, nonhydrogen bonded and hydrogen bonded OH stretches are found for the solvent molecules at 3600-3800 and 3300-3600 cm(-1), respectively. Simultaneous observations of these four distinct stretches allow us to identify a number of structural isomers (both cyclic and noncyclic) at n = 4-6. Of particular interest is that the clusters form an energetically favored four-membered ring at n = 5 that is evidenced by its characteristic bonded OH stretching absorptions at similar to 3550 cm(-1). This ring shaped structure remains at n greater than or equal to 5, where a sharp feature that can be ascribed to the nonhydrogen bonded OH stretching of three-coordinated H2O emerges at similar to 3690 cm(-1). All the results, both frequencies and assignments, are in close agreement with recent ab initio calculations. The association of the present observations with heterogeneous water nucleation in the gas phase and the infrared spectroscopy of neutral water clusters and crystalline ice is discussed. (C) 1998 Elsevier Science B.V.