title: Ion association in aqueous solutions probed through vibrational energy transfers among cation, anion, and water molecules.
abstract: KSCN and NH4SCN aqueous solutions were investigated with intermolecular vibrational energy transfer methods. In a KSCN/H2O (1/10 molar ratio) solution, 90% of the initial excitation of the CN stretch (~2066 cm(-1)) of the SCN(-) anion is transferred to the HOH bending mode (~1636 cm(-1)) of water molecules with an energy transfer time constant 3.1 ps. In a NH4SCN/H2O (1/10 molar ratio) solution, only 49% of the CN excitation flows to the water HOH bending mode with a time constant 6.3 ps. Most of the remaining CN excitation goes to the NH bending mode (~1460 cm(-1)) of the NH(+) cation with a time constant of 7.0 ps. The results indicate that about 50% of the energy transfer channel from the CN stretch to the HOH bending observed in the KSCN solution is overpowered by the NH4(+) cations in the NH4SCN/H2O solution. Ion concentration dependent measurements support this argument. According to the dipole/dipole approximation, the CN/OH energy transfer occurs most efficiently between SCN(-) anions and the water molecules closest to them. The experimental results therefore suggest that more than 50% of the water molecules closest to the SCN(-) anions are replaced by the NH4(+) cations in the NH4SCN/H2O (1/10 molar ratio) solution. The percentage is much larger than the NH4(+)/water ratio of 10%, indicating that the ion association between NH4(+) and SCN(-) is caused by the chemical nature of the solution rather than the statistical "forced contact" because of the high ion concentration.
authors.affiliation: Department of Chemistry, Rice University, Houston, Texas 77005, USA.
authors.forename: Jiebo
authors.initials: J
authors.lastname: Li
authors.forename: Hongtao
authors.initials: H
authors.lastname: Bian
authors.forename: Hailong
authors.initials: H
authors.lastname: Chen
authors.forename: Xiewen
authors.initials: X
authors.lastname: Wen
authors.forename: Bryan T
authors.initials: BT
authors.lastname: Hoang
authors.forename: Junrong
authors.initials: J
authors.lastname: Zheng
mesh_terms: D000838:Anions / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D003486:Cyanides / Q000737:chemistry; D017666:Deuterium Oxide / Q000737:chemistry; D004735:Energy Transfer; D007700:Kinetics; D013055:Spectrophotometry, Infrared; D013861:Thiocyanates / Q000737:chemistry*; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D000838:Anions; D002412:Cations; D003486:Cyanides; D013861:Thiocyanates; D014867:Water; C026976:thiocyanic acid; D017666:Deuterium Oxide; C009941:potassium thiocyanate