Structures and Isomeric Transitions of NH₄⁺(H₂O)₃_-₆: From Single to Double Rings

Abstract

The NH4+(H2O)3-6 cluster ions synthesized by a free jet expansion contain a variety of structural isomers. This investigation identifies some of these isomers by employing vibrational predissociation spectroscopy (VPS) in conjunction with ab initio calculations. The NH4+(H2O)n ions are produced by corona discharge of NH3/H2O seeded in a H2 beam. They are mass-selected and then vibrationally cooled in an octopole ion trap for infrared spectroscopic measurements. In the VPS, four distinct stretching vibrations (hydrogen-bonded and non-hydrogen-bonded NH and OH) are closely examined. The characteristic absorptions of these stretches, together with systematic temperature dependence measurements of their band intensities, allow us to identify both cyclic and noncyclic isomers in the supersonic jet. Such identification is corroborated by ab initio calculations performed at the B3LYP and MP2 levels using the 6-31+G* basis set. The satisfactory agreement in both vibrational frequencies and absorption intensities between observations and calculations demonstrates that small-sized NH4+(H2O)n cluster ions can be structurally determined in the gas phase.