STRUCTURE AND DYNAMICS OF HYDROGEN-BONDING GUESTS IN UREA INCLUSION-COMPOUNDS

The radical obtained by gamma-irradiation of the 2-nonadecanone/urea ( 2-NDOU) and nonadecanoic acid/urea (NDAU) inclusion compounds have bee n studied by EPR spectroscopy. The spectra have first-order, fast-moti on line shapes with anisotropic linewidths. For both compounds the spe ctra show the presence of two similar species. They originate from the two possible arrangements of the molecules inside the host channels, i.e. head-to-head or head-to-tail. The relative abundance of the two c onformations is obtained from the intensities of their EPR signals and is accounted for in terms of the balance between guest-to-guest and g uest-to-host hydrogen bonding. The transverse relaxation rate constant s for the different hyperfine components have been obtained by compute r simulation of the spectra. The relaxation originates from the librat ional motion of the beta methylene group and from the hindered rotatio n of the radical inside the host channel. Evidence of pretransitional effects is shown by the spin relaxation rates above the order-disorder transition in NDAU.