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

Abstract

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-C20 H42 is observed at 11.7 T for solutions in C6 D6 or C6 D5 CD3 . A (13) C T1 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-C16 or longer chain alkanes in CDCl3 . 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 CDCl3 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-C20 H42 , n-C21 H44 , and n-C22 H46 . However, none of the solvents used could even partially resolve the C-10 and C-11 signals of n-C21 H44 or n-C22 H46 at 11.7 T, which may result from a different distribution of conformers for n-C21 H44 or n-C22 H46 than for n-C20 H42 and shorter n-alkanes.