Multiple frequency (field) carbon-13 spin-lattice relaxation studies. Probe to cooperativity of complex molecular motions

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

“…While one can reasonably infer that the most deshielded signal for 15 in Fig. is from C‐6 in light of the other spectra, confirmation comes from a 13 C T 1 inversion‐recovery experiment demonstrating that the most deshielded signal in the cluster relaxes more slowly than the other two signals because of segmental motion along the chain (Fig. ).…”

“…The C‐10 signal nulls slightly earlier than the C‐9 signal, and the null point for C‐5 is about 50% longer than for C‐10 (about 1.55 s vs. about 1.02 s). Such information has not been available for long‐chain n ‐alkanes, as previous 13 C T 1 studies have yielded only four or five T 1 values for neat 13 , 14 , 15 , 16 , 18 , and 20 ; for 16 in CDCl 3 ; for 18 and 20 in CCl 4 ; for 18 and 20 in 1‐chloronaphthalene; and for 16 and 18 in 4.1 : 0.8 : 5.1 v/v/v alkane : indan : CCl 4 . One of these four or five T 1 values is a composite for all the overlapping signals for the interior carbons.…”

“…n ‐Alkanes shorter than n ‐C 17 H 36 are liquid at room temperature and can be used as solvents. n ‐Alkanes ranging from n ‐C 6 H 14 to n ‐C 24 H 50 have also served as prototypes for studying segmental motion in long chains, but the amount of information obtained has been limited by being able to determine only a composite value for the spin–lattice relaxation time for all the overlapping signals from the interior carbons of the longer‐chain alkanes . In addition, 13 C NMR and labeled compounds have been used to study the biosynthesis of two long‐chain n ‐alkanes .…”

“…However, far more common have been reports of just four or five 13 C signals resolved in the spectra of long‐chain n ‐alkanes: four 13 C signals for n ‐nonadecane ( n ‐C 19 H 40 , 19 ) in CCl 4 ; four 13 C signals for 20 neat or in CCl 4 ; five 13 C signals for 11 neat, in CDCl 3 , or in 4.1/0.8/5.1 v/v/v 11 /indan/CCl 4 ; five 13 C signals for 12 neat, in CHCl 2 CHCl 2 , in C 6 D 6 , or in 4.1/0.8/5.1 v/v/v 12 /indan/CCl 4 ; five 13 C signals for 13 neat; five 13 C signals for n ‐tetradecane ( n ‐C 14 H 30 , 14 ) neat or in CDCl 3 ; five 13 C signals for n ‐pentadecane ( n ‐C 15 H 32 , 15 ) neat 13 or in CDCl 3 ; five 13 C signals for 16 neat, in CDCl 3 , or in 4.1/0.8/5.1 v/v/v 16 /indan/CCl 4 ; five 13 C signals for 17 in CDCl 3 ; five 13 C signals for 18 neat, in CCl 4 , in 4.1/0.8/5.1 v/v/v 18 /indan/CCl 4 , or in 1‐chloronaphthalene; five 13 C signals for 19 in CDCl 3 ; five 13 C signals for 20 neat, in CDCl 3 , in CCl 4 , or in 1‐chloronaphthalene; and five 13 C signals for n ‐docosane ( n ‐C 22 H 46 , 22 ) in CDCl 3 . Thus, one could reasonably infer from the 1986 report of at least partial resolution of the ten different 13 C signals of 20 in 10/90 v/v C 6 D 6 /1‐chloronaphthalene at just 9.4 T (100.56 MHz 13 C) that this mixed solvent, rather than either of the individual solvent components, and a field strength higher than those used to study 20 in 1‐chloronaphthalene (20.09 MHz 13 C) or 18 in C 6 D 6 (67.8 MHz 13 C) are required to achieve such resolution.…”

Nuclear relaxation and solute reorientation in a locally ordered medium

“…(2/3) where ß is the H-C-H bond angle. The ratio JA/J°t akes on an especially simple form in the extreme narrowing limit (D||,Z)± » ) JC/JA = (1 + 9 cos (2/3) + 2X)(10 + 2X)'1 (6) where X = D^/D±. For chloromethane, ß = 112°.…”