Correlation function protonated carbons

Figure 5.24 shows that this approach fails not only quantitatively but also qualitatively. Neither is the strong increase of the collective times relative to the self-motion in the peak region of Spair(Q) explained (this is the quantitative failure) nor is the low Q plateau of tpair(Q) predicted (this is the quaUtative shortcoming). We note that for systems hke polymers an intrinsic problem arises when comparing the experimentally accessible timescales for self- and collective motions the pair correlation function involves correlations between all the nuclei in the deuterated sample and the self-correlation function relates only to the self-motion of the protons. As the self-motion of carbons is experimentally inaccessible (their incoherent cross section is 0), the self counterpart of the collective motion can never be measured. For PIB we observe that the self-correlation function from the protonated sample decays much faster than the pair... 

The carbon nuclei are to be identified with I, the proton nuclei with S, and the carbon-proton internuclear distance with r. The spectral density is the Fourier transform of a correlation function which is usually based on a probabilistic description of the motion modulating the dipole-dipole interactions. The spin-spin relaxation time, T2, is usually written directly as a function of spectral densities ( ). 

A very useful source on various NMR techniques and chemical shifts of functionalized carbon atoms that contains tabulated data on monosaccharides is Reference [23]. Specifically correlating functional groups with chemical shifts of protons and the associated carbon is an important tool for elucidating the stmcture of the monosaccharide moiety of many complex oligosaccharides. Since monosaccharides contain one or more chiral centers, absolute configuration must be known in order to predict the conformational shape of the molecules. The outcome of many stereoselective functionalizations of free and partially functionalized monosaccharides depends on conformation. 

Figure 66 shows correlation functions for several protonated carbons of BPTI obtained from a molecular dynamics simulation.329 In these, as in most cases, the motional averaging over the time scale of the simulation is largely... 

NMR linewidths in gels and near-solid polymers. One limitation common to all the motional models described so far is that, at least for proton-decoupled carbon spectra, they predict exponential T2 decay and hence a simple Lorentzian lineshape, even when the correlation function is complex. Furthermore, the predicted linewidths become almost infinite as the assumed underlying isotropic rotation slows to zero. In fact, the linewidths of polymers well below the glass transition temperature reach a maximum, determined... 

Theoretical calculations of the addition of HCl across the C=C and C=C bonds of 12 model compounds revealed similar or slightly lower enthalpies of protonation and lower activation energies for alkenes compared to alkynes. Linear correlation has been found for both protonation and activation energies as a function of carbon Is energies, which in turn were obtained experimentally from high-resolution carbon Is photoelectron spectra measured in the gas phase. ... 

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. 

Figure 3 Creation of the longitudinal order by cross-correlation as a function of the mixing time fm which follows the inversion of a carbon-13 doublet (due to a./-coupling with a bonded proton). The read-pulse transforms the longitudinal polarization into an in-phase doublet and the longitudinal order into an antiphase doublet. The superposition of these two doublets leads to the observation of an asymmetric doublet.

The discussion above has been more or less empirical and descriptive. However, considerable effort has been made to interpret 3-SCS on a more physical basis. Electric-field effects (71-75) were invoked to explain signal shifts of 3-carbon atoms induced by protonation of amines (157,158) (cf. Section II-B-3). This approach was later extended to other functionalities by Schneider and coworkers, who assumed that the SEF component (E2) rather than inductive properties of the substituents should be responsible for 3-SCS (113). They found fairly linear correlations of 3-SCS(X ) and 3-SCS(X ) in cyclohexyl derivatives (76) and attributed the difference between these for a given X to a widening of the C -Cp-Cv bond angle by 2.2° in the axial conformer (114,159). The decrease of 3-SCS in the order primary Cp —> secondary Cp — tertiary Cp — quaternary Cp was explained by electron-charge polarization in the Cp-C" bond(s) induced by the LEF component of the C -X dipole, which is already of significance at this distance, though ( 2) still dominates (160). Such an electron flow toward the 3 carbon is expected to be much more pronounced in C-C than in C-H bonds because of the polarizability difference (aCH = 0.79 acc = 1.12) (150,151,160). 

Olah has determined the pnmr and cnmr spectra of a number of protonated carbonyl compounds in fluorosulphuric acid-antimony pentafluoride-sulphur dioxide solution (Olah and White, 1968 Olah et al. 1972). The carbon-13 data are given in Tables 9 and 10. The proton data are not included in Table 9, since the anisotropy of the oxygen function obscures any interpretation. A correlation of some... 

Results of ( 1/2)0/( 1/2) determinations as a function of temperature for the protonated aromatic carbons are shown in Fig. 103. The higher content of antiplasticiser (19 wt %) is required to see the occurrence of the onset of mobility of the aromatic carbons at a higher temperature than in pure matrix. Such a behaviour is quite similar to that observed for HPE units, in agreement with the conclusion, reached in pure epoxy networks, of a likely correlation between the motions of the aliphatic units and the ring flips. 

---