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 molecule...

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Published in:	The journal of physical chemistry. B Vol. 117; no. 16; pp. 4274 - 4283
Main Authors:	Li, Jiebo, Bian, Hongtao, Chen, Hailong, Wen, Xiewen, Hoang, Bryan T, Zheng, Junrong
Format:	Journal Article
Language:	English
Published:	Washington, DC American Chemical Society 25-04-2013
Subjects:	Anions Anions - chemistry Atomic and molecular collision processes and interactions Atomic and molecular physics Bending Cations Cations - chemistry Cyanides - chemistry Deuterium Oxide - chemistry Energy Transfer Exact sciences and technology Excitation Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions Intermolecular and atom-molecule potentials and forces Ion association Ion concentration Kinetics Physics Spectrophotometry, Infrared Thiocyanates - chemistry Time constant Vibration Water - chemistry Intermolecular interaction Chemical solution Aqueous solutions Dipole approximation Thiocyanates Vibrationally excited state Energy transfer
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Summary:	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.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	1520-6106 1520-5207
DOI:	10.1021/jp3053373