Structures of Thermal, Mass-Selected Water Clusters Probed with Hydrophobic Ion Tags and Infrared Photodissociation Spectroscopy

The structures of tetramethylammonium (TMA(+)) and phenyltrimethylammonium (PTMA(+)) with 1-8 water molecules attached were investigated using infrared photodissociation spectroscopy (IRPD), blackbody infrared radiative dissociation, and theory to elucidate the effects of ion-water interactions and water-water hydrogen bonding in clusters containing these hydrophobic ions. Several pieces of evidence suggest that these ions have only a small effect on water structure. The binding energy of a water molecule to TMA(+) is 44 kJ/mol, a value that is significantly lower than that reported for other cations and close to the condensation energy of bulk water. The OH asymmetric stretch of the water molecule in TMA(+) center dot (H2O)(1) at 3718 cm(-1) is less red-shifted from that of an isolated water molecule (3756 cm-1) than for those of other cations, and water-water hydrogen bonding is extensive for all clusters with two or more water molecules. These results indicate that charge transfer from water to these hydrophobic ions is much less than that for other cations. There are two predominant structures for these ions with three water molecules: one in which water molecules form a chain and one with a cyclic structure analogous to that of neutral water trimer. With four water molecules attached, water forms a cyclic structure similar to that of neutral water tetramer. The spectra of the larger clusters follow trends reported for neutral water molecule clusters, which indicates that these hydrophobic ions have only a minimal effect on the structure of water in clusters with four or more water molecules. These results suggest that these hydrophobic ions can be used as "tags" that could make possible acquisition of IRPD spectra of even larger clusters, such as clathrates, for which precise mass selection of neutral clusters is not possible by other methods.