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Abstract
The solid and liquid-crystalline phases of two long chain lead(II) carboxylates have been studied by 13C and 1H NMR spectroscopy. High resolution 13C NMR spectra of the solid phase of lead(II) decanoate and octadecanoate, reveal splittings of the peaks attributed to the carboxylate and adjacent methylene groups. This may result from two different environments for the carboxylate chains coordinated to the same Pb(II) ion. On going from the solid to liquid-crystalline or liquid phases, this splitting is lost, and small changes in chemical shift of the bands due to the methyl and methylene groups are observed. These are attributed to the onset of conformational disordering, and analysis of the data gives a gauche population in good agreement with that from Raman spectra and theoretical calculations. Longitudinal 13C relaxation times are reported. From these, and from spectral data, the -CH3 group is seen to have different dynamics from the rest of the chain, and to retain high mobility even in the solid phase. Further information on the structural changes comes from proton NMR spectroscopy. The transverse 1H relaxation of lead(II) decanoate shows two components, which are suggested to arise from a crystalline, and a relatively amorphous region of the compound. Study of the evolution of these as a function of temperature shows that the amorphous fraction is relatively low up to 335 K. but then increase rapidly, until at 365 K, close to the transition to the L, phase, it is 99 per cent- In parallel to these observations, changes in band shape were also observed and suggest a progressive fusion of the chains up to the temperature of this transition. The global 1H spin-lattice relaxation was also studied as a function of temperature. and two components were observed between room temperature and approximately the tint phase transition. These are associated with populations of hydrogen nuclei with different mobilities.
