Additional insights from very‐high‐resolution ¹³C 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.

Interpretation of NMR Relaxation in Bitumen and Organic Shale Using Polymer–Heptane Mixes

“…In fact, as pointed out previously, it is remarkable that eq even holds for the light alkanes, since R R is presumed to increase with increasing chain length. However, recent MD simulations of the light alkanes confirm that R R remains roughly constant with increasing chain length, implying that NMR is indeed a local probe of the molecular dynamics (see also 13 C T 1 inversion–recovery spectra of n -C 15 H 32 and n -C 20 H 42 ). Nevertheless, due to differences in molecular structure, it is plausible that R R 3 is up to a factor of ∼4 larger for these three samples (i.e., R R is up to a factor of ∼1.6 larger), resulting in the observed left shift along the x axis of Figure relative to the other samples and the fit.…”

Stretching and folding of 2-nanometer hydrocarbon rods

“…22,92,93 Of course, the usually moderate temperature in solvents means that entropy aspects complicate the issue and a statistical analysis of various folded structures is necessary. 94 There is some indirect experimental NMR evidence in deuterated non-protic solvents 95 that n ¼ 21, 22 may show a different conformational distribution than shorter chains, but n ¼ 17 also exhibits some anomaly and its origin remains unclear. Calculations predict the folding in such solvents to be signicantly more delayed.…”