Conformation of alkali metal ion-calix[4]arene complexes investigated by IR spectroscopy in the gas phase.

We measure the IR spectra of calix[4]arene (C4A) complexes with K+, Rb+, and Cs+ ions in the 3200-3700 cm-1 region by IR-UV double-resonance spectroscopy performed under cold (∼10 K) gas-phase conditions. All the complexes show two bands that can be assigned to the stretching vibrations of hydrogen-bonded OH groups in the C4A part. Quantum chemical calculations predict several isomers having different IR spectra, but the IR spectrum of the "cone" conformer reproduces the IR-UV spectrum very well, indicating that all the complexes adopt the cone conformation including the metal ions in the cone. The frequency of the OH stretching vibrations decreases with increasing the ion size from K+ (3357 and 3513 cm-1) to Rb+ (3323 and 3463 cm-1) and Cs+ (3279 and 3379 cm-1), but it is substantially higher than that of hydrogen-bonded OH groups in bare C4A (3158 cm-1). These results suggest that C4A encapsulates the metal ions by distorting the cone cavity, and that the distortion of the cone conformation is reduced more and the hydrogen bond between the OH groups becomes stronger with increasing the ion size from K+ to Cs+. The Cs+ complex has the smallest distortion of the C4A cavity among the alkali metal ion complexes. This can be one origin for the predominant encapsulation of Cs+ ions by C4A over smaller alkali metal ions in solution.