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 been studied by EPR spectroscopy. The spectra have first-order, fast-motion line shapes with anisotropic linewidths. For both compounds the spectra 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 conformations is obtained from the intensities of their EPR signals and is accounted for in terms of the balance between guest-to-guest and guest-to-host hydrogen bonding. The transverse relaxation rate constants for the different hyperfine components have been obtained by computer simulation of the spectra. The relaxation originates from the librational motion of the beta methylene group and from the hindered rotation 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