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 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 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 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 and SCN , which can facilitate the intermolecular vibrational energy redistribution of the SCN .