Chemical shift alkanes

The aliphatic additivity constants presented in many of the section heading discussions are usually very useful when utilized with these parent alkane chemical shifts in the calculation of theoretical chemical shifts for straight aliphatic compounds. 

Empirical equations have been developed for the 13C chemical shifts of alkanesulphonic acids, and the substituent effects of S03- on alkane chemical shifts, relative to H, have been calculated58 to be a-C, 39.7 /2-C, 3.7 and y-C, — 0.2 to — 2.1 ppm. These effects are quite small in relation to the electron-withdrawing effect the N02 group deshields an a-carbon nucleus by 64 ppm59. 

Over the past decade it has been established that for various substituents the i C chemical shift increment is a constitutive property. This applies to many systems e.g. benzenes, alkanes and alkenes. The availability of over 200 allenes, randomly substituted with groups of different nature, enabled us to prove that in the case of allenes the chemical shift increment is a constitutive property too, thus establishing a convenient method for estimating i ( C) values for allenes. 

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). 

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,... 

The high shift values for NO+ (474 ppm for 170 and —3 ppm for nitrogen) relative to lower values for X—N=0 compounds is good evidence for differences in chemical shift anisotropy as pertain in 13C NMR for the alkynes relative to alkanes and alkenes64. The higher symmetry of acetylene leads to a high field shift. 

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... 

One of the earliest empirical rules for predicting 13C chemical shifts was that of Grant and Paul (169) for alkanes ... 

The ability of the Lindeman-Adams (LA) rule to predict, 3C chemical shifts of even highly branched alkanes satisfactorily was proven impressively by calculations of the shieldings of the dodecanes 292 to 294 (393). 

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. 

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