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 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.