Vibrational spectra, force field and normal modes of 4,13-diaza-18-crown-6

Abstract

Raman and IR spectra of uncomplexed 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (4,13-diaza18-crown-6) have been measured in the solid state. Normal coordinate calculations have been carried out on the C 2h conformation of this macrocyclic compound. The force field of the diazacrown ether has been determined by adjusting several important several force constants associated with the imino group, other force constants being constrained, as transferred, to those of 18-crown-6. The vibrational assignments of 4,13-diaza-18-crown-6 are made on the basis of the previous assignments of a KSCN complex of 18-crown-6 with a similar ring conformation and of the results of the normal coordinate calculations. The normal modes of low-wavenumber vibrations are depicted. The lowest two vibrations represent out-of-plane deformations of folding the macrocyclic ring double. These modes are favourable to complexation of the macrocycle with guest. It is likely that these low-wavenumber vibrations are possible paths of the inclusion of the guest by the macrocyclic ring.