title: Hydration of guanidinium: second shell formation at small cluster size.
abstract: The structures of hydrated guanidinium, Gdm(+)(H2O)n, where n = 1-5, were investigated with infrared photodissociation spectroscopy and with theory. The spectral bands in the free O-H (∼3600-3800 cm(-1)) and free N-H (∼3500-3600 cm(-1)) regions indicate that, for n between 1 and 3, water molecules bind between the NH2 groups in the plane of the ion forming one hydrogen bond with each amino group. This hydration structure differs from Gdm(+) in solution, where molecular dynamics simulations suggest that water molecules form linear H-bonds with the amino groups, likely a result of additional water-water interactions in solution that compete with the water-guanidinium interactions. At n = 4, changes in the free O-H and bonded O-H (∼3000-3500 cm(-1)) regions indicate water-water H-bonding and thus the onset of a second hydration shell. An inner shell coordination number of n = 3 is remarkably small for a monovalent cation. For Gdm(+)(H2O)5, the additional water molecule forms hydrogen bonds to other water molecules and not to the ion. These results indicate that Gdm(+) is weakly hydrated, and interactions with water molecules occur in the plane of the ion. This study offers the first experimental assignment of structures for small hydrates of Gdm(+), which provide insights into the unusual physicochemical properties of this ion.
authors.affiliation: Department of Chemistry, University of California , Berkeley, California 94720-1460, United States.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cooper
authors.forename: Sven
authors.initials: S
authors.lastname: Heiles
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: DiTucci
authors.forename: Evan R
authors.initials: ER
authors.lastname: Williams
mesh_terms: D019791:Guanidine / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007536:Isomerism; D008956:Models, Chemical; D012996:Solutions; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*
chemical_list: D012996:Solutions; D014867:Water; D019791:Guanidine