Ion association in aqueous solutions probed through vibrational energy transfers among cation, anion, and water molecules.

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.