Hydrocarbons carbon-13 chemical shifts

Further doubt about the validity of the original Grant-Cheney model was expressed by Seidman and Maciel (185), whose INDO calculations of proximity effects in hydrocarbons revealed that there is no simple correlation between carbon chemical shifts and calculated electron-density increases caused by steric C-H bond polarization they report the conformational relation of interacting bonds and groups to be at least equally important, if not more so (185). 

Although all change of the Y carbon chemical shift from that of the parent hydrocarbon in compounds I and II is caused by charge, the overall change in the a position s shift is composed... 

Fig. 5.12. Charge density change (AQ) of the a-carbons relative to the corresponding carbon atoms of the parent hydrocarbons versus chemical shifts [%].

H and 13C NMR Data. A ketone or aldehyde carbonyl group bound to a CF2H group shields its proton slightly (0.1 ppm), and even more surprisingly it also has a shielding effect upon its carbon chemical shift of about 8 ppm (Scheme 4.42). By comparison, a hydrocarbon... 

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

This section contains the carbon-13 NMR chemical shifts of a selection of aromatic hydrocarbons. The chemical shifts of benzenes substituted by other than hydrocarbon groups are included in the section. 

It is needless to say how useful have been NMR spectra in chemistry. A simple proof of this is tables and tables of chemical shifts for various types of atomic nuclei, particularly for proton chemical shifts and chemical shifts. Clearly, chemical shifts are atomic properties, but if one would add all carbon chemical shifts in a hydrocarbon, one would obtain a numerical quantity that can be viewed as a molecular entity. Let us elaborate on this. Paraphrasing what one finds in Wikipedia and the Merriam-Webster Dictionary, one can write the following explanations for entity ... 

Likewise, a primary —CH2F fluorine adjacent to silicon is shielded by more than 50 ppm compared to the respective hydrocarbon (Scheme 3.29), with the value of -277ppm observed for fluoromethyltrimethyl-silane being the largest chemical shift known for a single carbon-bound fluorine. 

H and13C NMR Data. Some typical proton and carbon NMR data for trifluoromethyl ethers, sulfides, and esters are given in Scheme 5.11. Continuing the trend observed going from CH2F to CF2H to CF3 carbons, the 13C chemical shift of a trifluoromethyl ether is actually more shielded (by about 5 ppm) than that of a trifluoromethyl hydrocarbon. Scheme 5.12 summarizes the relative impact of an ether substituent upon the chemical shifts of various fluorinated carbons. 

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,... 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e... 

The results suggest that chinoid type structures are the predominant resonance contributors for 88. The IGLO/DZ//3-21G calculated 13C NMR chemical shifts of benzylic monocations 88 correlate reasonably well with the experimentally obtained data. The 13C NMR chemical shifts of the carbocation centers (CH2 carbon) are calculated 10.6-12.5 ppm too deshielded. Similar results were obtained for benzylic dications 89. NMR chemical shifts of arenium ions derived from various classes of polycyclic aromatic hydrocarbons have been calculated using GIAO-DFT methods.103... 

A striking feature of trimethyltin substituents is the linear correlation between the chemical shift of the a-carbon atom in RSn(CH3)3 and that of the corresponding atom in the parent hydrocarbon RH (136) ... 

C-13 nmr spectra were taken of all the alkali metal compounds of I and II from Li to Cs. As expected, changing the counter-ion had very little effect on the chemical shifts of the carbons in the neopentyl group in either I or II, or on the extra methyl group in II, compared with the parent hydrocarbon ( 3). 

In Section lU-B we showed that the chemical shift of methyl carbons in polypropylene can be calculated considering the relative position of the adjacent substituents, expressed in configurational terms (m or r, Oor 1). An alternative interpretation, with a better physical basis, takes into account only conformational aspects. Experimental evidence from hydrocarbon spectra has established that the presence of a gauche conformation between carbon atoms separated by three bonds causes an upfield shift(7 effect) (206). For flexible molecules the magnitude of this shift depends on the fraction of conformers having gauche interactions with the observed carbons (207). 

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