1.5- carbon shift

The above interpretation of the factors indicated by current theory to be important in determining proton and carbon-13 chemical shifts does not offer much encouragement for the optimistic statement that [carbon-13 chemical shifts] should provide a more reliable index of charge than the hydrogen shifts (Fraenkel and Farnum, 1968, p. 251). It is true that the other atom terms are a more serious perturbation on proton than on carbon-13 chemical shifts, and are difficult to evaluate. However it is also apparent that carbon-13 shifts are determined by a number of terms and 0 whose charge dependence may well be in... 

Figure 10. Correlation of proton chemical shifts of methyl groups in carbonium ions with carbon-13 shifts of the adjacent trigonal carbon atoms. See Table 3 for data and references, o Aryldimethylcarbonium ions cycloalkcnyl cations a phenylmethyl-carbonium ions.

Studied by Olah (Olah and Liang, 1972 and Olah et al., 1972a) were chosen—the methyl substituted cycloalkenyl cations, represented hy solid dots, and the substituted aryldimethylcarbonium ions, represented by open circles. The correlation to a line of slope 0 0146 is excellent for all but two of the points. If the Spiesecke-Schneider value of 160 p.p.m. is used for the linear charge dependence of the carbon-13 shifts, then the calculated value for the methyl shifts is... 

Substitution of heteroatoms on carbonium ion centres often has a profound and unexpected effect on the carbon-13 chemical shift which is hardly designed to bolster our confidence in the simplicity of the charge density-chemical shift correlation. A few examples will illustrate the point (in the following structures carbon-13 shifts are given in regular type, proton shifts in italics). 

Figure 23. Correlation of observed and calculated changes in carbon-13 shifts for a number of heterocyclic systems upon protonation. For data and references see Table 12. ° Data from Adam et al. (1969) (Column A) Data from Pugmire and Grant (1968) ignoring changes in A (Column B). x Data from Pugmire and Grant (1968) corrected for changes in A (Column C).

Comparison of carbon-13 shifts of methyl phosphines and methyl substituted methanes... 

Proton NMR shifts have been interpreted in terms of a ring current, and some coupling constants also indicate aromatic properties. Boron-11 shifts alone can apparently not be used as criteria of aromaticity, in the same way that carbon-13 shifts are not meaningful. 

Carbon-13 magnetic resonance spectra of the naturally occurring cytidines have been described in several papers.79-82 The electronic structure of the compounds is reflected in the carbon-13 shifts. For instance, the observed chemical shifts for these and other pyrimidine and purine nucleosides were correlated, at least qualitatively, with the calculated charge densities (see Section VIII) and with the known reactivity of these molecules. It is difficult to draw conclusions from the carbon-13 spectra about the tautomerism of cytosine. 

In the case of a heteronuclear AX system, e.g. a CH bond with A = 1H and X = 13C, the transverse magnetization dephases to the doublet components with v0 + J(H and v0 — 7CI, because of CH coupling (Fig. 2.38(a)). A 13C- H pair thus responds to the echo sequence shown in (Fig. 2.37 (a-c), provided the 180" pulse covers the range of carbon-13 shifts At time 2z after the initial pulse, a spin-echo builds up along the negative y axis. Subsequent Fourier transformation computes a negative signal (Fig. 2.38(a)). 

Fig. 2.54 presents a two-dimensional carbon-proton shift correlation of D-lactose after mutarotational equilibration (40% a-, 60% / -D-lactose in deuterium oxide), demonstrating the good resolution of overlapping proton resonances between 3.6 and 4 ppm by means of the larger frequency dispersion of carbon-13 shifts in the second dimension. The assignment known for one nucleus - carbon-13 in this case - can be used to analyze the crowded resonances of the other nucleus. This is the significance of the two-dimensional CH shift correlation, in addition to the identification of CH bonds. For practical evaluation, the contour plot shown in Fig. 2.54(b) proves to be more useful than the stacked representation (Fig. 2.54(a)). In the case of D-lactose, selective proton decoupling between 3.6 and 4 ppm would not afford results of similiar quality. 

For 13C shift/structure correlations and for tabulations of ppm values one generally accepted reference should be used. Carbon disulfide, which appears in the low field region of, 3C spectra, was widely used in the early literature [73a, b]. Later, tetramethylsilane (TMS), known from proton NMR, became the generally accepted carbon-13 shift reference, particularly because of some parallels in the behavior of H and 13C shifts. 

Carbon-13 shifts of some methylcycloalkanes are given in Table 4.5. Methyl substitution increments have been derived for methylated cyclopentanes (Table 4.6 [210]) and cyclohexanes (Table 4.7 [87]) in order to predict carbon shift values. While y effects in methylated cyclopentanes are small (Table 4.6), shieldings of carbon atoms in y position... 

An increment system has been derived for methyldecalins [230] and methylated perhy-drophenanthrenes [231] by regressional analysis, because these compounds are steroid models and the increments, in fact, have proved to be of help in carbon-13 shift assignments of steroids (Section 5.2). Eq. (4.2) permits prediction of <5C using the increments A, of structural elements in position l to the carbon atom k to be considered (Table 4.9). These increments clearly indicate the configurational influence of the y substituent, decreasing for example from eclipsed via gauche to trans interactions (I , > Vq > Vt and... 

Table 4.9. Increments A, of Structural Elements in Position l Relative to the Carbon Atom k in order to Predict its Carbon-13 Shift Sc(k) according to eq. (4.2).

Similarly to alkanes, an increment system has been proposed in order to predict carbon-13 shifts of olefinic carbons from the reference value of ethene (122.1 ppm) [237] and the increments A, for a, f , y, and 6 alkylation, as well as multiple substitution corrections S (Table 4.11). Accuracy is about 1 ppm. 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

The large carbon-13 shifts of the central sp carbons in allenes (200-210 ppm) relative to the terminal ones (70-90 ppm, Table 4.15) are attributed to an increased paramagnetic shielding due to two localized n bonds originating from the central carbon. The / effect of methyl substitution to the central carbon is considerably smaller than that found for alkenes and alkynes. 

Fig. 4.4. Carbon-13 shift increments of chlorocyclohexane for rapid (a) and frozen (b) cyclohexane ring inversion (X = Cl).

Two empirical increment systems (Table 4.22) derived from experimental data as collected in Table 4.23 permit prediction of alkanol carbon-13 shifts. One is related to the shift value B of the hydrocarbon R —H and involves, as usual, addition of the increments Z, = <5,(r oh) — r)(- R > according to eq. (4.8 a) [268]. The other employs a linear equation (4.8 b), correlating the shifts of an alkanol R — OH and the corresponding methyl-alkane R —CH3 by a constant bk and a slope ak, which is 0.7-0.8 for a and about 1 for ji and y positions [269]. Specific parameter sets characterize primary, secondary, and tertiary alcohols (Table 4.22). The magnitudes of Z) increments in eq. (4.8 a) decrease successively from primary to tertiary alcohols (Table 4.22), obviously as a result of reduced populations of conformers with yliauche interactions in the conformational equilibrium when the degree of alkylation increases. 

Table 4.22. Increment Sets for Prediction of Alkanol Carbon-13 Shifts According to the Equations (4.8 a) [268] and (4.8 b) [269].

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