Methylene spin-lattice relaxation

Table XIII. Calculated Methylene Spin-Lattice Relaxation Times and Nuclear Over-hauser Enhancement Factors for the C-l Carbon...

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

Also displayed in Table II are spin-lattice relaxation data for liquidlike (CH2) groups that were observable in DPMAS experiments. Both the dependence on temperature and the particular Ti values suggested rapid segmental motions within long runs of methylene groups, quite similar to the dynamic behavior reported for soft-segment CH2 s in synthetic polyesters (19). 

Now it will be necessary to elucidate the location of the butyl isopropenyl ketone unit in the polymer chain. The spin-lattice relaxation time, Tj., of the protons in the polymer and oligomer was measured. The Ti of methylene protons adjacent to the carbonyl group was nearly the same level as the T of methyl protons in the terminal butyl group or terminal methine proton (Table ) but much longer than the T of the protons in interior sequences of polymer (13). These indicate that the butyl carbonyl group in the polymer or oligomer locates at or near to the forefront or the end of the chain. 

Differing 7j values for CH3, CH2, and CH carbon nuclei within a molecule can arise not only by methyl rotation or anisotropic molecular motion, but also from the segmental mobility of partial structures, even when the dipolar mechanism predominates. Thus the spin-lattice relaxation times of methylene carbon atoms in long alkane chains pass through a minimum at the middle of the chain. In the presence of heavy nonassociating... 

A theory which explains spin lattice relaxation of methylene chain groups of phospholipid bilayers has been developed (Gent and Prestogard, 1977). Synthetic micellar structures formed from p-t octylphenol [25] and p-t-... 

Figure 13A. C-13 relaxation times for the methyl (A). methylene (9), and methine (O) carbons of PP as a function of temp erature spin-lattice relaxation times at 1.41...

Figure 13.1 13C spin-lattice relaxation times (nTl) for methylene carbons in bulk polyisoprene at two different field strengths (62.5 (O) and 25.15 ( ) MHz) as a function of inverse temperature. The lines are the best fits of the data to the theory of Dejean de la Batie and co-workers [24] (long dashes) and for the modified relaxation function described in reference [27] (dotted lines)... 

The decay time constants for the polymeric radicals clearly vary from solvent to solvent, ranging over an order of magnitude from about 2 ps in tetrahydrofuran (THF) to 200 ns in methylene chloride. A similar solvent dependence of NMR Ti values has been reported by Spyros et al. They studied poly(naphthyl methacrylate) using the inversion recovery technique and found that the spin-lattice relaxation time of... 

Spin-Lattice Relaxation. In order to determine whether each resonance line comprises a single component, we first measured the spin-lattice relaxation time Tic by the pulse sequence developed by Torchia [53] or by the standard saturation-recovery pulse sequence. The Tic values thus obtained were 2560,263 and 1.7 s for resonance line I and 0.37 s for line II. As reported by several investigators, the line at 33 ppm is associated with three different Tic values [ 17,54,55]. This means that this line is contributed to by three components with different molecular mobilities. However, since each component was represented by a single Lorentzian line shape at 33 ppm, they are all assignable to methylene groups in the orthorhombic crystalline form or in the trans-trans conformation. The component with a Tic of s can be assigned to methylene groups with a some-... 

The results are shown in Table 16. The spin-lattice relaxations of the o-methyl and methoxy protons were found to arise mainly from the dipolar interactions between the protons in the methyl group itself. In the relaxation of backbone methylene protons the contributions from the protons in the neighbour-... 

Figure 2. The 13C spin-lattice relaxation times at 15 MHz for isotactic poly (propylene) methylene ( ), methine (O), and methyl (A) carbons.

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