Protonated Tetramethyl p - tert -Butylcalix[4]arene Tetraketone: NMR Evidence and Probable Structures

Both NMR spectra in nitrobenzene-d ₅ and high-precision quantum mechanical DFT calculations proved that tetramethyl p-tert-butylcalix[4]arene tetraketone binds hydroxonium cation H₃O⁺ quite strongly to form an equimolar complex. Three different structures of the resulting complex species were indicated by the NMR spectra and the DFT calculations.