Alkanes carbon-13 chemical shifts

Comparison of C NMR Absorptions of Alkenes Table 11-3 with the Corresponding Alkane Carbon Chemical Shifts (in ppm)... 

Grant and Paul Chemical Shifts. The technique of obtaining branch content information from NMR for polymers utilizes an empirical relationship given by Grant and Paul [29,79,80]. The Grant and Paul empirical relationship [29,79,80] can be used to calculate the values of the chemical shifts for carbon atoms in the vicinity of a branch point in a hydrocarbon polymer. The empirical relationship was obtained from NMR studies on alkanes. The chemical shift of any carbon atom in a 13C-NMR can be decomposed as a sum of contributions from its nearest five neighboring carbon atoms. The value of the chemical shift for any carbon atom C, is given as,... 

Chemical Shift Sum (CSS) was proposed as single molecular descriptor derived from spectra as the sum of the chemical shifts of the carbon atoms from C-NMR spectroscopy [Randic, 1980a]. It was shown that at least for alkanes, the chemical shift sum varies regularly with some physico-chemical properties of alkanes. QSAR models for CSS, based on artificial neural networks, were proposed by using the first four path counts [Ivanciuc, Rabine et al., 1997]. 

The most distinctive feature of the H-NMR spectra of ethers is the chemical shift of hydrogens on the carbons bonded to the ether oxygen. Signals for this type of hydrogen fall in the range 8 3.3 to 4.0, which corresponds to a downfield shift of approximately 2.4 units compared with their normal position in alkanes. The chemical shifts of H—C—O— hydrogens in ethers are similar to those seen for comparable H—C—OH hydrogens of alcohols. 

The carbon NMR absorptions of the alkenes also are highly revealing. Relative to alkanes, the corresponding alkenyl carbons (with similar substituents) absorb at about 1(X) ppm lower field (see Table 10-6). Two examples are shown in Table 11-3, in which the carbon chemical shifts of an alkene are compared with those of its saturated counterpart. Recall that, in broad-band decoupled NMR spectroscopy, all magnetically unique carbons absorb as sharp single lines (Section 10-9). It is therefore very easy to determine the presence of sp carbons by this method. 

For program SIMULA, the values of A, Zj, and corr are stored in tables. There are currently 39 basic carbon chemical shifts and 2479 values of for 150 substituents. Theoretically there are many more different values of Z for these substituents (e.g., the program now contains 467 out of a possible 600 for the alkane substituent, but only 39 of the 300 for the cyclopropane substituent), leading to some limitations, but largely in the case of the less common substituents. The tables of VODIK contain 22 basic proton shifts and the values of Zj for 85 substituents. 

Table 7.50 Estimation of Chemical Shifts of Alkane Carbons 7.102...

If an H atom in an alkane R-// is replaced by a substituent X, the C chemical shift 8c in the a-position increases proportionally to the electronegativity of X (-/ effect). In the (1-position, Sc generally also increases, whereas it decreases at the C atom y to the substituent (y-effect, see Section 2.3.4). More remote carbon atoms remain almost uninfluenced (dSc 0). 

Replacement of a hydrogen atom within an organic molecule, for example an alkane, by a substituent X changes the electronic environments of directly bonded and of more remote carbon nuclei. Thereby l3C NMR signals are shifted either upheld or downfield the difference between the chemical shifts 8 of a certain carbon atom in the substituted and the unsubstituted parent compound is called the substituent effect. For this term the abbreviation SCS (substituent-induced chemical shift) has generally been adopted in the literature and will also be used here. The SCS is given by the equation... 

J. Prud homme I think here the same situation arises as with hydrogenated 1,4-polydimethylbutadiene. It would appear that when two methylene units separate two chiral carbons, each methylene unit shows little sensitivity to the meso and racemic configurations of the two adjacent chiral carbons. Carman et al. have reported spectra measured on alkanes which show that when two tertiary chiral carbons are separated by two methylene units, the difference in the chemical shifts of the methylene units in the meso and racemic configurations is close to 0.3 ppm. It was not possible to observe this kind of resolution in the spectra of the present polymers. Only a broadening effect occurred. 

From Mark s RIS model for ethylene-propylene copolymers (J. Chem. Phys. 1972, 57, 2541) it is determined that P(t) = 0.380, P g+) = 0.014, and Pig") = 0.606 in 2,4-dimethylhexane (2,4-DMH). Using this RIS model, furthermore, for all the branched alkanes considered whose isopropyl groups are separated by at least one methylene carbon from the next substituted carbon and the RIS model developed by Asakura et at. (Makromol. Chem, 1976, 177, 1493) for head-to-head polypropylene to treat 2,3-dimethyl pentane, AS s are calculated for a large number of branched alkanes. The agreement between the observed and the calculated nonequivalent 13C NMR chemical shifts is quite good, including the prediction that separation of the isopropyl group from the next substituted carbon by four or more methylene carbons removes the nonequivalence. 

Additional parameters Sw are required in order to calculate the chemical shift values of branched chain alkanes (standard deviation of predicted chemical shifts + 0,3 ppm, B = — 2.3 ppm) (Table 4.3). The values symbolized by Greek letters indicate the change in chemical shift due to substitution of hydrogen by a methyl group at the a to s carbon atoms. The remaining 8 correction parameters Sk, account for the effect of branching. 

Estimates of the kinetics of methyl loss from energy-selected CztHg" species have been made by calculation.23 The hydride transfer from alkanes to carbenium ions in the gas phase is calculated to involve a species with a symmetric potential well, which is different from the situation in superacid or zeolite media.24 A correlation between the charge on a carbon and the in-plane tensor component of its 13 C chemical shift has been observed for a number of simple cationic and anionic species.25 High-level calculations... 

---