Shift for / -protons

A relatively small training set of 744 NMR chemical shifts for protons from 1 20 molecular structures was collected from the literature. This set was designed to cover as many situations of protons in organic structures as possible. Only data from spectra obtained in CDCl, were considered. The collection was restricted to CH protons and to compounds containing the elements C, H, N, 0, S, F, Cl, Br. or I. 

Table 7.43 Estimation of Chemical Shift for Protons of —CHj— and Methine...

TABLE 7.44 Estimation of Chemical Shift for Proton Attached to a Double Bond... 

In practical terms, it is invariably a nitrogen atom that is protonated in salt formation. This always leads to a downfield shift for protons on carbons both alpha and beta to the nitrogen concerned. In alkyl amines, the expected shifts would be about 0.7 and 0.3 ppm respectively. Remember that some heterocyclic compounds (e.g., pyridine) contain nitrogen atoms that are basic enough to protonate and comparable downfield shifts can be expected (Spectrum 5.9). 

Approximate chemical shifts for protons in various functional groups relative to TMS. Chemical shift is highly dependent on hydrogen bonding and... 

The first attempt to correlate proton chemical shifts of methyl groups in carbonium ions with charge on the adjacent carbon atom seems to have been made by Mac Lean and Mack or (1961). They examined the pnmr spectra of a number of cyclohexadienyl cations (benzenium ions) [6] where R was CH3, and found that chemical shifts for protons directly attached to positive carbons were linearly... 

TABLE 10 Chemical Shifts for Protonated a, j3-Unsaturated Aldehydes and Ketones" ... 

The chemical shift 5, defined by Equation (22), was measured at 40.0 and 15.6 Mc./sec. and was found to be —3 2 relative to water for both SA and SG. The derivatives of the resonance absorptions were recorded in the measurements. If the total anisotropy of the chemical shift of protons in the solid is somewhat less than the line width, the cross-over point of the derivative will correspond to the average value of S as for liquids, and will be directly comparable with the shifts for protons in the liquid state. Comparison of the shift value with those of H3O+ (aqueous) (1 1), S = -1-11, OH (aqueous) (121), S = -1-10 dilute solutions of alcoholic-type protons... 

Tables 5.2 and 5.3 give characteristic shifts for nuclei in some representative organic compounds. Table 5.4 gives characteristic chemical shifts for protons in common alkyl derivatives. Table 5.5 gives characteristic chemical shifts for the olefinic protons in common substituted alkenes. To a first approximation, the shifts induced by substituents attached an alkene are additive. So, for example, an olefinic proton which is trans to a -CN group and has a geminal alkyl group will have a chemical shift of approximately 6.25 ppm [5.25 + 0.55(tra .s-CN) + 0.45(gew-alkyl)].

Note that 5h is the chemical shift for protons in the residual protonated solvent (6c is that for the deuterated solvent). The intensities of the peaks in the multiplets will not be the same as for the same multiplets in H spectra as the coupling is to D (= "H), a spin I nucleus... 

The values of chemical shifts for protons attached to different functional groups are reasonably constant, so that we can predict the chemical shift of protons in various molecular environments (especially protons attached to an aromatic ring or those of a methylene group) by using some empirical rules (Boxes 4.5 and 4.6). 

Let us first try to discuss the contact contribution to the shift for protons and deuterons. As we shall see, there are conditions under which the pseudocontact shifts are small and negligible, or can be determined separately and subtracted from the hyperfine shifts. Furthermore, in the case of a monodentate ligand which exchanges rapidly from bound to free, the average pseudocontact shift in the absence of geometric constraints is zero. 

Jeffreys, J.A.D., A rapid method for estimating NMR shifts for protons attached to carbon, J. Chem. Educ., 56, 806, 1979. 

The ability of methoxy adducts 42-44 to add a second molecule of methanol across the C=N bond appears to depend on the C=N carbon electrophilicity. The latter can be estimated by chemical shifts for proton and carbon-13 resonances of the HC=N fragment in the 1H- and 13C-NMR spectra of dihydropyrazines 42-44. 

Stereochemical differences often result in different chemical shifts for protons on the same carbon atom. For example, the two protons on Q of allyl bromide (3-bromopropene) are not equivalent. Ha is cis to the —CH2Br group, and Hb is trans. Ha absorbs at 8 5.3 Hb absorbs at 8 5.1. There are four different (by NMR) types of protons in allyl bromide, as shown in the structure at right in the margin. 

Compound Symbol Retention time (sec) Molecular weight Calc. Found Chemical shifts for protons ... 

Symbol Retention Molecular weight Chonical shifts for protons time (sec) Calc. Found... 

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