Using 1 H and 13 C NMR spectroscopy, the interaction of tetrapropoxy-p-tert-butylcalix[4]arene (1) with H 3 O + ions produced by hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) and traces of water was studied in nitrobenzene-d 5 . It was shown that 1 readily forms an equimolecular complex with H 3 O + . The equilibrium constant K of its formation is 2.6 at 296 K. Exchange between bound and free 1 is fast even under mild excess of HDCC, the correlation time τ τ τ τ ex being about 0.13 ms. NMR

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... s that H 3 O + is bound to the aryl-oxygen atoms and this binding forces the calixarene cup to adopt a more open and symmetrical conformation. This conclusion is in a full accord with high precision quantum DFT calculations which find one structure of the complex corresponding to a global energy minimum, in which the H 3 O + ion is bound to three of the oxygen atoms by strong hydrogen bonds and to the remaining oxygen by two weaker hydrogen bonds. The calixarene part is forced into a C 4 symmetrical opened form. When stored for weeks, the complex gradually transforms into other forms, most probably its hydrates, according to spectral evidence and DFT calculations. Using 1 H and 13 C NMR spectroscopy, the interaction of tetrapropoxy-p-tert-butylcalix[4]arene (1) with H 3 O + ions produced by hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) and traces of water was studied in nitrobenzene -d 5 . It was shown that 1 readily forms an equimolecular complex with H 3 O + . The equilibrium constant K of its formation is 2.6 at 296 K. Exchange between bound and free 1 is fast even under mild excess of HDCC, the correlation time τ ex being about 0.13 ms. NMR shows that H 3 O + is bound to the aryl-oxygen atoms and this binding forces the calixarene cup to adopt a more open and symmetrical conformation. This conclusion is in a full accord with high precision quantum DFT calculations which find one structure of the complex corresponding to a global energy minimum, in which the H 3 O + ion is bound to three of the oxygen atoms by strong hydrogen bonds and to the remaining oxygen by two weaker hydrogen bonds. The calixarene part is forced into a C 4 symmetrical opened form. When stored for weeks, the complex gradually transforms into other forms, most probably its hydrates, according to spectral evidence and DFT calculations.