¹H NMR Relaxation Time Studies of Molecular Motions in a 1,3-Alternate-Shaped Calix[4]arene Ensemble

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The systematic studies through ¹H NMR longitudinal relaxation times (T₁) of an ionophoric 1,3-alternate-shaped calix[4]arene ester 1 were reported. The T₁ values were measured under proton-noise-decoupling conditions by the inversion-recovery technique. The terminal groups in an ethoxycarbonylmethoxybenzene unit in 1 move vigorously whereas the motion of the central ArCH₂Ar moiety is rather suppressed. K⁺ ion plays an important role as a template ion specifically to freeze the fluctuational motion of these terminal groups. The ethoxycarbonylmethoxybenzene unit features a ‘seesaw motion’ around the ArCH₂Ar moiety as central support. It is clear that the molecular motion of the calixarene skeleton is efficiently frozen through complexation with metal cations.