Methyl protons chemical shifts

As shown in Table II, spin-spin coupling constants between methyl protons and the thallium nucleus 2j(t1-CH2) of the thiacrown-ether complexes of dimethylthallium(III) ion in CD CN are somewhat larger than that of the free dimethylthallium(III) ion in the same solvent. These values have a tendency to decrease, accompanied by a downfield shift of the methyl proton chemical shift, as the number of sulfur atom in 18-crown-6 increases. Similar results were obtained in J(T1-CH2-) and S(T1-CH2-) values of diethylthallium(lll) complexes. The magnitude of 2j(t1-CH3) values has been found to be an indication of increasing interaction between solvent molecules and the thallium atom in the solvent dependence of 2j(t1-CH3) values of dimethylthallium ion. O)... 

H NMR data has been reported for the ethylzinc complex, Zn(TPP—NMe)Et, formed from the reaction of free-base N-methyl porphyrin H(TPP—NMe) with ZnEti. The ethyl proton chemical shifts are observed upheld, evidence that the ethyl group is coordinated to zinc near the center of the porphyrin. The complex is stable under N2 in the dark, but decomposed by a radical mechanism in visible light.The complex reacted with hindered phenols (HOAr) when irradiated with visible light to give ethane and the aryloxo complexes Zn(TPP—NMe)OAr. The reaction of Zn(TPP—NMe)Et, a secondary amine (HNEt2) and CO2 gave zinc carbamate complexes, for example Zn(TPP—NMclOiCNEti."" ... 

Wells and coworkers53 have prepared a series of deuterated tetramethyltin compounds, which they used to study the long-range deuterium isotope effects on the proton chemical shifts of tetramethyltin. The various deuterated tetramethyltin compounds, with one to four trideuteromethyl groups on the tin atom, were prepared by a series of methyl group exchanges beginning with tri-trideuteromethyltin chloride and undeuterated tetramethyltin (Scheme 12). 

With time, the CD3 group is distributed throughout the system resulting in the formation of (CH3) (CD3)4- Pb (n = 3, 2, 1) and (CH3) (CD3)3- PbCl (n = 2, 1), as shown in Scheme 32. These species were used to determine the effect that a deuteron positioned four bonds away, 13, would have on the proton chemical shifts of a methyl group. These long-range four-bond deuterium isotope effects, 4AH( ) ), are summarized in Table 14. 

However, if side-chain carbon assignments are wanted, C(CC)(CO)NH experiments [33] that start directly with carbon magnetization and transfer it further to the amide proton for detection are available. If protonated substituents, for example methyl groups, have been introduced into the otherwise perdeuterated protein, the usual HC(C)(CO)NH-TOCSY pulse sequence can be used to obtain the proton chemical shifts. These protons can provide a small number of NOEs that, together with residual dipolar couplings and the secondary structure identification from chemical shifts, make the determination of the global fold of large proteins possible. 

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... 

Figure 8. Parameters used to calculate the charge dependence of the proton chemical shift for methyl groups on carbonium ion centres.

Figure 9. Charge dependence of proton chemical shifts of methyl groups on ions. See Table 2 for data and references, o Raw data corrected for ring current coirected for ring current and. ...

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.

The question of additivity of SCS is difficult to probe, because ortho interactions, notoriously difficult to account for, are bound to arise in even disubstituted pyridine rings, apart from those with the 3,5-disposition. To test this, proton chemical shifts have been measured for six series of substituted methyl pyridinecarboxylates (71), (72), (73), (74), (75) and (76) <750MR(7)4l), and SCS ortho and para can be accounted for by assuming additive substituent, ester, and nitrogen effects. However, for protons meta to the substituent, particularly for the case where both the proton and the substituent are adjacent to N, the additivity breaks down, and there is evidence of substituent-nitrogen interactions. 

For example, the proton chemical shifts of the methyl halides (Table 9-4) show decreasing shielding, hence progressively low-field chemical shifts with increasing halogen electronegativity (F > Cl > Br > I) ... 

Methyl group proton chemical shifts in structurally related inside yohimbanes are shown in [503] and [504]. (306)... 

It is concluded that one isomer has the methyl group below the basal plane of the boron framework (endo-) with the CF3 group nearly in the plane of the base in an axial (exo-) position and in the other isomer the conformation is the opposite, but no assignment was made. Marynick and Onak 180> have suggested, on the basis of their ring current model for the correlation of chemical shifts in pyramidal boron compounds, that the proton chemical shifts reported for the methyl groups of these two isomers 174> favor assignment of the endo-methyl conformation to isomer A. 

The proton chemical shift of H20 is 4.73 ppm downfield from that of the methyl group of 2,2-dimethyl-2-silapentane 5-sulfonate (DSS) at 29°C. [Modified from Ho and Russu (1981)]. The particular resonance can be observed in both H20 and D20, unless specifically stated otherwise. 

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