Spin lattice methyl rotation

Temperature-dependent lineshape changes were observed in an early study of the fluo-renyllithium(TMEDA) complex. A detailed study by lineshape analysis, which was also applied to the TMEDA complex of 2,3-benzofluorenyllithium(TMEDA) (Figure 29f, yielded barriers AG (298) of 44.4 and 41.9 kJmoD for the 180° ring flip in these systems, respectively . A second dynamic process, which was detected via the temperature dependence of, the spin-lattice relaxation time in the rotating frame, is characterized by barriers of 35.1 and 37.6 kJmoD, respectively, and may be ascribed to the ring inversion process. For the fluorenyl complex, a barrier AG (298) of 15.9 kJmoD for the methyl rotation in the TMEDA hgand was determined from temperature-dependent NMR spectra of the deuteriated system. 

The barrier to methyl rotation in 4-methylpyridine has been measured by means of proton spin lattice relaxation time (74MI20405), and found to be very low, about 0.06 kJ mol-1. This is in line with ab initio calculations using a minimal STO-3G basis set (76JST(32)67), and MINDO/3 MO calculations (79JST(57)209), the second of which also show that the equilibrium position for the methyl group is with one hydrogen atom in the plane of the pyridine ring. 

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... 

Another way of using JH NMR to study the dynamics of phenyl protons in BPA-PC consists in selective deuteration of the methyl groups (BPA-d6-PC) [32]. Thus, the temperature dependence of the JH spin-lattice relaxation time, Ti, and spin-lattice relaxation time in the rotating frame, T p, has been determined, and is shown in Fig. 38. 

The interpretation of carbon T p data is complicated by the fact that spin-spin (cross-relaxation) processes as well as rotating frame spin-lattice processes may contribute to the relaxation (40). Only the latter process provides direct information on molecular motion. For the CH and CH2 carbons of PP, the Tip s do not change greatly over the temperature interval -110°C to ambient and, as opposed to the T behavior, the CH2 carbon has a shorter T p than the CH carbon. These results suggest that spin-spin processes dominate the Tip (46). However, below ca. -115°C, the Tip s for both carbons shorten and tend toward equality. McBrierty et al. (45) report a proton Ti minimum (which reflects methyl group reorientation at KHz frequencies) at -180°C. No clear minimum is observed in the data, perhaps due to an interplay of spin-spin and spin-lattice processes. Nonetheless, it is apparent that the methyl protons are responsible for the spin-lattice portion of the Tip relaxation for CH and CH2 carbons. 

Scheme 65). The conformational state and barrier to rotation of methyl groups have attracted much theoretical and experimental interest. The barriers to rotation of methyl in various aza aromatics have been determined in the solid state from H-spin-lattice relaxation times (85JOC2972). Such barriers for 84a-i are given (in kJ mol-1) in Scheme 66. 

Chang et al. reported the miscibility of poly(vinylphenol) (PVPh) with poly(methyl methacrylate) (I MMA) Figure 1 shows the C CP/MAS spectra of pure PVPh, PMMA, PVPh-co-PMMA, PEG, and PVPh-co-PMMA/ poly(ethylene oxide) (PEO) blends of various compositions with peak assignments. VPh contents of PVPh-co-PMMA is 51 mol% and Mn of PEO is 20,000. The spin lattice relaxation time in the rotating frame (Tip ) was measured to examine the homogeneity of PVPh-co-PMMA/PEO blends on the molecular scale. 

The miscibility of poly(methyl acrylate) (PMAA, Mw= 150,000)/PVAc (Mw= 167,000) blends at various mixing ratios was investigated by both Ti and Tip measurements. C CP/MAS NMR spectra of PMAA, PVAc and the PMAA/PVAc blends are shown in Fig. 2. Figure 3 shows the plots of the spin-lattice relaxation times in the laboratory (Ti , A) and in the rotating (Ti, B) frames against the molar ratio of PMAA (xpmaa)-The ll relaxation times from the CI 12 (O) and OCH (A) carbons for PMAA and PVAc, respectively, can be observed because these two carbons are observed separately even in the blends (Fig. 2), so that it is possible to obtain each relaxation time for PMAA or PVAc in the blends independently. 

One such analysis in the review period involves the characterization of the rotation of the methyl groups in pyridoxine (vitamin B6).49 The temperature dependencies of the 1H spin-lattice relaxation time T, and Tld (the relaxation time constant characterizing the relaxation of dipolar order, a population distribution over the Zeeman spin levels, which corresponds to a density operator component T20, he. I z - Ii. I2, to equilibrium) at three different applied field strengths and for a variety of temperatures were determined, yielding the curves in Fig. 30. The only motion that could affect... 

Von Schiitz and Wolf [29] have likewise investigated a second typical stochastic reorientation motion using NMR in molecular crystals the hindered rotation of methyl (CH3 -) groups in ten different methyl-substituted naphthalene crystals. The reorientation motions are, at not-too-low temperatures, the dominant source of nuclear spin-lathce relaxation in these highly purified molecular crystals. Only at very low temperatures do thermally-activated reorientation processes cease to play a role. The spin-lattice relaxation is then determined essentially by paramagnetic impurities. 

The self-diffusion coefficients of toluene in polystyrene gels are approximately the same as in solutions of the same volume fraction lymer, according to pulsed field gradient NMR experiments (2fl). Toluene in a 10% cross-linked polystyrene swollen to 0.55 volume fraction polymer has a self-diffusion coefficient about 0.08 times that of bulk liquid toluene. Rates of rotational diffusion (molecular Brownian motion) determined from NMR spin-lattice relaxation times of toluene in 2% cross-linked ((polystytyl)methyl)tri-/t-butylphosphonium ion phase transfer catalysts arc reduced by factors of 3 to 20 compai with bulk liquid toluene (21). Rates of rotational diffusion of a soluble nitroxide in polystyrene gels, determined from ESR linewidths, decrease as the degree of swelling of the polymer decreases (321. 

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