An NMR Study on Complexation of Ethylammonium Ion by Alkyl p-tert-Butylcalix[6]aryl Ester Derivatives

The complexation of ethylammonium ion by alkyl p-tert-butylcalix[6]aryl ester derivatives was studied via measurements of proton and carbon spin-lattice relaxation times <TEX>$(T_1)$</TEX> and chemical shift changes in solution state <TEX>$(CDCl_3)$</TEX>. The results indicate that the endo-type complexes are formed and that the overall tumbling rates of these complexes are more rapid than those of the corresponding free hosts. The association constants for these complexes in <TEX>$THF-d_8$</TEX> were determined by <TEX>$^1H$</TEX> NMR titration at several different temperatures to estimate the relevant thermodynamic parameters. The logK's for ethylammonium complexes of methyl, ethyl, and propyl esters at 313 K, for example, were found to be 1.56, 3.41, and 3.08, respectively. The complexes formed may be thought of as being kinetically stable in view of their <TEX>$^1H$</TEX> NMR behavior in 2 : 1 host/guest solution.