Shifts addition

Table 19. Examples of Chemical Shift Additivity Method for Fluorinated Benzenes [56]...

The shielded chemical shift is in turn due to the strong magnetic anisotropy of the sp -hybridized carbons. All the above dications are definitively classical, nonbridged cations, as also shown by the chemical shift additivity criterion of Schleyer and coworkers. 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

Magnetic field inhomogeneities and chemical shifts additionally dephase each doublet component in Fig. 2.38. These effects are also refocussed by a 180" pulse, provided it rotates the components about the x -axis, as is the case in experiments (a) and (c) of Fig. 2.38. 

Fifolt [ 130] reported this chemical shift additivity method for fluorobenzenes in two deuterated solvents d6 acetone and d6 dimethyl sulfoxide (DMSO) Close correlations between experimental and calculated fluorine chemical shifts were seen for 50 compounds Data presented in Table 18 result from measurements in deuterochloroform as (he solvent [56] Fluorine chemical shifts calculated by this additivity method can be used to predict approximate values for any substituted benzene with one or more fluorines and any combination of the substituents, to differentiate structural isomers of multisubstituted fluorobenzenes [fluoromtrotoluenes (6, 7, and 8) in example 1, Table 19], and to assign chemical shifts of multiple fluorines in the same compound [2,5 difluoroamline (9) in example 2, Table 19] Calculated chemical shifts can be in error by more than 5 ppm (upfield) in some highly fluonnated systems, especially when one fluonne is ortho to two other fluorines Still, the calculated values can be informative even in these cases [2,3,4,6-tetrafluorobromobenzene (10) in example 3, Table 19]... 

C2 is even smaller as both C and C carbons show identical C chemical shifts ( C 39.3) at -73 °C. Olah and Schleyer s chemical shift additivity criterion showed a classical dicationic nature and the difference of the summation of the chemical shifts of all carbons in the cationic species is greater than in the related hydrocarbon by at least 893 ppm, which is about twice the value for a typical classical monocarbocation. ... 

We chose this reaction since it has been proposed as a model for the rearrangement of 2-methyleneglutarate to 3-methylitaconate, catalyzed by the coenzyme-B,2-dependent enzyme, 2-methyleneglutarate mutase [16, 26, 57]. More specifically, equation 2 represents the second step in the addition-elimination pathway (reaction c. Scheme 4) for a 1,2-shift. Additionally, this reaction has been widely studied experimentally [58] and has been described as the most precisely calibrated radical reaction [59]. 

Figure 5.7 Frequency dependences of the storage (Q) and loss (-h) moduli for poly(dimethylsilox-ane) (PDMS) samples whose reactions were quenched at the times indicated (see Fig. 5-6). The data are time-temperature-shifted to the reference temperature T gf of 34°C, and they are shifted additionally by an amount A on the logarithmic axis to keep the curves from overlapping. The vertical shift factors bf are given by p T sf)T d/ p T)T), where p is the density. (From Winter and Chambon 1986, with permission from the Journal of Rheology.)...

Fig. 2. Optimized histograms for the two-dimensional ferromagnetic Ising model. After the feedback of the local diffusivity a peak evolves near the critical energy of the transition Ec —1.41 N. The feedback thereby shifts additional resources towards the bottleneck of the simulation which were identified by a suppressed local diffusivity...

In summary, 2-norbornyl cations, generated by solvolysis of exo-2-norbornyl halides of sulfonates, behave as symmetrical (achiral) intermediates. Other modes of generation (deamination, hydrogen shifts, addition of HX to norbomene) lead to partial asymmetry, excluding the bridged norbomyl cation as the only intermediate. Although classical 2-norbornyl cations may intervene in some of these reactions, no unequivocal identification has been achieved and alternative interpretations are possible. 

Figures 14.1(a,b) show typical CP/MAS spectra of two types of PET yarns, an amorphous yarn wound at relatively low speed and a 36% crystalline yarn wound at relatively high speed, respectively [2]. The ethylene and carbonyl carbon peaks of the amorphous yarn are shifted about 1 ppm downfield with respect to the semicrystalline yarn, as opposed to the aromatic carbons which are shifted slightly upheld. Besides differences in chemical shift, the spectrum of the 36% crystalline yarn shows narrower lines with a better S/N ratio than the spectrum of the amorphous yarn. The broader lines in Fig. 14.1(a) are attributed to a broader orientation distribution of polymer molecules, which results in a larger distribution of isotropic chemical shifts. Additional differences between both spectra are observed in the lineshape the ethylene and carbonyl carbon peaks in Fig. 14.1(a) have a symmetric lineshape, whereas, these lines in Fig. 14.1(b) are asymmetric. The asymmetric lineshape is resolvable into two partially overlapping resonances a relatively broad low-field component and a relatively narrow high-field...

E. A., and Chen, C. L. (1992) Biphenyl-type lignin model compounds Synthesis and car-bon-13 NMR substiment chemical shift additivity rule. Holzforschung 46(2), 127-134. 

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