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.