Cyclodextrin inclusion of four phenylurea herbicides: determination of complex stoichiometries and stability constants using solution 1H NMR spectroscopy

An inclusion complex formation between alpha- and beta-cyclodextrin and four phenylurea analogues, namely metobromuron, monolinuron, monuron and fenuron, is reported. Complex formation was established using solution H-1 NMR spectroscopy. Complex stoichiometries were determined by the method of continuous variation using the chemically induced shifts of both the host and guest protons. An analysis of the spectroscopic data revealed the stoichiometry as 1:1 in all cases while a further analysis of the same data yielded values for the association constant K ranging from 208 to 2749M(-1). From the observed chemical shifts it was deduced that in all cases, only the guest aromatic ring enters the host cavities, the substituted urea moiety protruding from the secondary rim in the case of alpha-cyclodextrin, but from the primary rim in the case of beta-cyclodextrin.