²⁰⁷Pb N.M.R. of lead(II) soaps in solid, liquid-crystalline and liquid phases

Abstract

²⁰⁷Pb N.M.R. data are reported for a number of even chain length lead(II) carboxylates (soaps) at various temperatures. At room temperature, the solid lead(II) decanoate and tetradecanoate show similar spectra, with a single metal ion site, and modest shielding anisotropy. As the temperature is increased, the soaps (hexanoate to octadecanoate) all form a highly ordered smectic phase, which gives a very broad ²⁰⁷Pb signal of linewidth comparable to that of the solid phase. At higher temperatures, the hexanoate to dodecanoate soaps form a lamellar L _α (smectic A) phase, whilst the longer chain length carboxylates melt directly to the liquid phase. Both the lamellar L _α and liquid phase give fairly sharp, isotropic signals, whose chemical shifts and linewidths are strongly temperature dependent. Possible explanations for this effect include paramagnetic contributions to the shielding tensor from low-lying electronic states of Pb(II), and contributions to the observed signal from different coordination species produced in the lead(II) carboxylate system. Although there are discrete changes in chemical shift at the phase transition, the magnitudes observed in all the phases are similar, suggesting that there are no dramatic changes in the metal coordination environment.