Solid-state C-13 NMR study of beta-cyclodextrin substituted aromatic ketone complexes: Evidence for two kinds of complexation of the guest molecules

Solid-state C-13 cross polarization magic angle spinning nuclear magnetic resonance measurements on beta-cyclodextrin (beta-CD)/substituted aromatic ketone inclusion complexes indicate that the interaction between the beta-CD (host) and the substituted ketone (guest) depends markedly on the substituent. The carbon chemical shifts of both the host and the guest molecules depend on the substituent. In the beta-CD/alpha-aminoacetophenone complex, each carbon of the guest molecule exhibits two C-13 NMR peaks. One of these peaks is shifted with respect to that of the free molecule. This shifted peak gradually disappears when the temperature increases, whereas the unshifted peak is unchanged. The same phenomenon occurs when the inclusion complex is washed. This result is related to the existence of two kinds of complexed molecule (inside the beta-CD cavity and outside but non-covalently bonded to beta-CD). In beta-CD/alpha-amino-4-bromoacetophenone, only one kind of complexed molecule exists. The disposition of alpha-amino-4-bromoacetophenone inside the beta-CD cavity is different from that of alpha-aminoacetophenone.