Nuclear magnetic relaxation studies for investigating the hydration of 15-crown-5 and 18-crown-6 ethers in aqueous and aqueous salt solutions
✍ Scribed by K.J. Patil; T.M. Kirschgen; M. Holz; M.D. Zeidler
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 782 KB
- Volume
- 81
- Category
- Article
- ISSN
- 0167-7322
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✦ Synopsis
The results of experimental studies of 'H and 2H nuclear magnetic relaxation rates in the solutions of 15-crown-5 and 18-crown-6 ethers in D20 and CDC13 (concentration range 0 to 5 m) at 298 K are reported. It is observed that the extreme narrowing condition is not fulfilled for 'H relaxation data above 1 m concentration in case of 18-crown-6 in D20 solutions. Some measurements for 'H and 2H spin-lattice relaxation rates for a fixed concentration of ethers in the presence of varying amounts of alkali bromides (LiBr, NaBr and KBr) were also carried out. It has been noted that the relaxation behaviour of crown ethers is not influenced appreciably by the presence of ions in solution except in the case of LiBr in the dilute concentration region. However, the solvent (D20) behaviour is found to be strongly and in a specific way influenced by the nature of the electrolytes. The 2H relaxation data in the case of 1 m solutions of 18-C-6 in D20 with variable concentrations of KBr indicate the formation of 1:l and I:2 host-guest type complexes. Attempts are being made to analyse the results in terms of hydrophobic interactions of the crown molecules in D20, the occupation of cavities by bridging water molecules (Dsd conformation of the crown), the replacement of these bridging water molecules by K' ions, corresponding ion-pair formation and structural interactions. It is concluded that preorganisation of the appropriate conformation of crown molecules and hydrogen bonding interaction play an important role in the formation of hostguest type complexes of crown ethers with alkali bromides.