Additional insights from very-high-resolution ~(13)C NMR spectra of long-chain n-alkanes

摘要

New information has been obtained from very-high-resolution ~(13)C NMR studies of a series of long-chain n-alkanes. These compounds are fundamentally important in the petroleum industry and are essential to the life of some plants, flowers, and insects. At least partial resolution of the ten different ~(13)C NMR signals of n-C_(20)H_(42) is observed at 11.7 T for solutions in C_6D_6 or C_6D_5CD_3. A ~(13)C T_1 inversion-recovery experiment provides much more detailed information than in previous studies of long-chain n-alkanes, demonstrates a steady increase in the relaxation times of the ten different carbons proceeding from the middle to the end of the chain because of segmental motion, and thus confirms the assignments for the interior carbons. In contrast, there is significant overlap for the signals for C-7 and the more interior carbons in a solution of n-C_(16) or longer chain alkanes in CDCl_3. Not only are the chemical shifts sensitive to the solvent used, but also the relative chemical shifts change. Signals for the interior carbons of the odd-number alkanes in CDCl_3 are better resolved than in the spectra of their even-number counterparts. Some mixed aromatic solvent systems give increased dispersion of the cluster of C-6 through C-10 signals of n-C_(20)H_(42), n-C_(21)H_(44), and n-C_(22)H_(46). However, none of the solvents used could even partially resolve the C-10 and C-11 signals of n-C_(21)H_(44) or n-C_(22)H_(46) at 11.7 T, which may result from a different distribution of conformers for n-C_(21)H_(44) or n-C_(22)H_(46) than for n-C_(20)H_(42) and shorter n-alkanes.