Bulletin of the Korean Chemical Society

Korean Chemical Society (대한화학회)
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NMR Relaxation Study of Segmental Motions in Polymer-n-Alkanes

  • Chung Jeong Yong (Department of Chemistry, College of Natural Sciences Seoul National University) ;
  • Lee Jo Woong (Department of Chemistry, College of Natural Sciences Seoul National University) ;
  • Park Hyungsuk (Department of Chemistry, College of Natural Sciences Seoul National University) ;
  • Chang Taihyun (Department of Chemistry, Pohang Institute of Science and Technology)
  • Published : 1992.06.20
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Abstract

$^{13}C$ spin-lattice relaxation times were measured for n-alkanes of moderate chain length, ranging from n-octane to n-dodecane, under the condition of proton broad-band decoupling within the temperature range of 248-318 K in order to gain some insight into basic features of segmental motions occurring in long chain ploymeric molecules. The NOE data showed that except for methyl carbon-13 dipole-dipole interactions between $^{13}C$ and directly bonded $^1H$ provide the major relaxation pathway, and we have analyzed the observed $T_1data$ on the basis of the internal rotational diffusion theory by Wallach and the conformational jump theory by London and Avitabile. The results show that the internal rotational diffusion constants about C-C bonds in the alkane backbone are all within the range of $10^9\;-10^10\;sec^{-1}$ in magnitude while the mean lifetimes for rotational isomers are all of the order of $10^{-11}\;-10^{-10}$ sec. Analysis by the L-A theory predicts that activation energies for conformational interconversion between gauche and trans form gradually increase as we move from the chain end toward the central C-C bond and they are within the range of 2-4 kcal/mol for all the compounds investigated.

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Cited by

  1. Additional insights from very‐high‐resolution 13C NMR spectra of long‐chain n‐alkanes vol.51, pp.10, 1992, https://doi.org/10.1002/mrc.3988