13C NMR line broadening

The values of AG (Af-Meax -> transition state) of 9.1 0.3 kcal mol-1 and AG (N-Mecq - transition state) of 11.0 0.3 kcal mol-1 for 1,2,2,6-piperidine determined by a 13C-NMR line-broadening study108 (detailed discussion in Section II,B,4) are higher than the corresponding values for iV-methylpiperidine. These differences in barrier heights result from the interaction between the three vicinal methyl groups. 

In a study of rates of degenerate 1,2-shifts in tertiary carbocations, Saunders and Kates854 used higher-field (67.9 MHz) 13C NMR line broadening in the fast-exchange limit. The 2-butyl cation showed no broadening at — 140°C. Assuming the hypothetical frozen out chemical shift difference between C(2) and C(3) to be 227 ppm, an upper limit for AG was calculated to be 2.4 kcal mol 1. 

The 13C NMR spectra were measured regularly over 6 months (190 days) and the polymer structure became almost stable after this period. After 6 months, the relative intensities in the unsaturated carbon region were independent of CP time. The alkyl signal positions became constant after 11 days (Figure 38(b)). However, their line width gradually broadened to double the width after 6 months. 

Figure 2. 50.33 MHz 13C NMR spectrum of lime cutin, obtained with cross polarization (contact time 1.5 ms, repetition rate 1.0 s), magic-angle spinning (5.0 kHz), and dipolar decoupling (762/211 = 48 kHz). This spectrum was the result of 6000 accumulations and was processed with a digital line broadening of 20 Hz. Chemical-shift assignments are summarized in Table I. Reproduced from Ref. 7 of the American Chemical Society.

Figure 4. 31.94 MHz 13C NMR data for intact lime cutin (bottom) and the solid residue of a depolymerization treatment (top). Both spectra were obtained with a 1H-13C contact time of 1.0 ms, repetition rate of 1.0 s, spinning rate of 3.0 kHz, a H decoupling field of 60 kHz, and a line broadening of 20 Hz. (For the chosen contact time, peak intensities within each spectrum reflect the approximate numbers of each carbon type.) Only the intact cutin spectrum retained signal intensity near 30 ppm when decoupling was delayed before acquisition (13,14).

Dipole-moment measurements in benzene solution on 1,4-dimethylpiper-azine and on a series of 1 -alkyl-4-tm-butylpiperazines120 show clearly that the N-alkyl groups exist predominantly in the expected position and indicate a value of 1.8 kcal mol-1 for the N-methyl group. This must now be considered a minimum value, and kinetic protonation of 1,4-dimethylpiperazine with 13C-NMR analysis of the products326 gives 2.96 + 0.05 kcal mol". An attempt312 to apply the line-broadening method failed.313... 

Standard (gel-phase) NMR spectra of polymers usually show significant line broadening, mainly because of chemical shift anisotropy and dipolar coupling [98], Only nuclei with strong chemical shift dispersion, e.g. 13C [99-106], 15N [107], 19F [108-112], and 31P [113] give sufficiently resolved gel-phase NMR spectra. The resolution of... 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

Very slow molecular motion (tc > 10 9 s/rad at B0 x 2.1 Tesla) leads to an increase in T, while T2 decreases (Fig. 3.20). The signals then broaden (line width at half-maximum intensity zlv1/2 1/7V). Therefore, the more sluggish macromolecules usually give poorly resolved 13C NMR spectra having a bandlike shape. 

The observed fivefold symmetry in the 1H and 13C NMR spectra even at very low temperature (— 150°C) with no line broadening leaves only two alternatives for the structure of the dication the nonclassical fivefold symmetrical, static structure 437 or... 

Thus, at room temperature, the molecular motions responsible for the mechanical losses of poly(cyclopentyl methacrylate) and poly(cycloheptyl methacrylate) are very rapid processes, having too-high a frequency to broaden the line widths of MAS CP DD 13C NMR spectra, in agreement with the absence of observable motional broadening in the spectra of these two polymers (Fig. 8a and c). 

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