Ion association in aqueous solutions probed through vibrational energy transfers among cation, anion, and water molecules.
Ion association in aqueous solutions probed through vibrational energy transfers among cation, anion, and water molecules.
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DOI：
10.1021/jp3053373
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发表时间：
2013-04
期刊：
影响因子：
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通讯作者：
Jiebo Li;Hongtao Bian;Hailong Chen;Xiewen Wen;B. T. Hoang;Junrong Zheng
中科院分区：
文献类型：
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作者：
Jiebo Li;Hongtao Bian;Hailong Chen;Xiewen Wen;B. T. Hoang;Junrong Zheng
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.
