H-1, C-13 AND N-15 NMR-STUDY OF THE SOLUTION STRUCTURE OF META-BRIDGED BIS(BENZO-15-CROWN-5 ETHER)S

The H-1 and C-13 NMR spectra of the three isomers of the bis(benzo-15- crown-5 ether) studied and their complexes with K+ and Na+ cations wer e recorded and assigned by H,H-COSY and HMQC 2D-NMR experiments. The t hree isomers with respect to the amide C(=O)-NH bonds could be assigne d by N-15 NMR spectroscopy and heteronuclear coupling constants. Relev ant conclusions about the isomerism were deduced from the (1)J(15-N,1- H) coupling constants of the various NH amide groups and the (1)J(13-C ,1-H) coupling constants of (CH)-H-3 and (CH2)-H-alpha-C(=O)- groups, respectively. From 2D-ROESY NMR experiments, further stereochemical in formation about the preferred conformation of the free and complexed b is(benzo-15-crown-5 ether) was obtained. The conformational study is a ccompanied and corroborated by molecular dynamics and quantum chemical calculations. In the case of the K+ complex, a ''sandwich''-like comp lex could be determined, This structure clearly proved to be already p reorganized in the non-complexed host molecule. For the corresponding Na+ complex, the ''usual'' 2:1 host-guest complex was detected.