Specific Host-Guest Interactions in the Crown Ether Complexes with K + and NH 4 + Revealed from the Vibrational Relaxation Dynamics of the Counteranion.

The specific host-guest interactions in the corresponding complexes of K ⁺ and NH ₄ ⁺ with typical crown ethers were investigated by using FTIR and ultrafast IR spectroscopies. The counteranions, i.e., SCN ^- , were employed as a local vibrational probe to report the structural dynamics of the complexation. It was found that the vibrational relaxation dynamics of the SCN ^- was strongly affected by the cations confined in the cavities of the crown ethers. The time constant of the vibrational population decay of SCN ^- in the complex of NH ₄ ⁺ with the 18-crown-6 was determined to be 6 ± 2 ps, which is ∼30 times faster than that in the complex of K ⁺ with the crown ethers. Control experiments showed that the vibrational population decay of SCN ^- depended on the size of the cavities of the crown ethers. A theoretical calculation further indicated that the nitrogen atom of SCN ^- showed preferential coordination to the K ⁺ ions hosted by the crown ethers, while the NH ₄ ⁺ can form hydrogen bonds with the oxygen atoms in the studied crown ethers. The geometric constraints formed in the complex of crown ethers can cause a specific interaction between the NH ₄ ⁺ and SCN ^- , which can facilitate the intermolecular vibrational energy redistribution of the SCN ^- .