Cooperative interaction of n ‐butylammonium ion with 1,3‐alternate tetrapropoxycalix [4]arene: NMR and theoretical study

The interaction of 1,3‐alternate tetrapropoxycalix[4]arene (1) with n ‐butylammonium ion (2) in CD 2 Cl 2 was examined using 1 H, 13 C and 14 N NMR spectroscopy and DFT (density functional theory) calculations. NMR shows that 1 forms with 2 an equimolecular hydrogen‐bonded complex with the equilibrium constant 5.91 × 10 3 l/mol at 296 K. The structure of the complex can be shown to be asymmetric at 203 K, with 2 interacting by hydrogen bonds with the two ethereal oxygen atoms of one half of 1 and with the π system of the other half, but is rapidly averaged to an apparent C 4h symmetry by chemical exchange at higher temperatures. Using two related but independent techniques based on transverse and rotating‐frame proton relaxation, it is shown that only an intermolecular exchange of 2 between the bound and free states takes place, in contrast to previously studied interaction of 1 with H 3 O + . Its correlation time is 0.169 ms. It is shown by DFT calculations that such swift exchange is not possible without a cooperative interaction of both 2 and 1 with several molecules of water present. Similarities and contrasts between the exchange processes of 2 and H 3 O + bound to 1 are discussed, in particular with respect to the apparent quantum tunneling of the latter inside the molecule of the complex. Copyright © 2008 John Wiley & Sons, Ltd.