Hydrogen chemical shifts

Proton Chemical Shifts. Hydrogen chemical shifts are affected by a number of factors, but ring current and heteroatom effects are particularly noticeable. 

The variations of /uc- h d 7ih-ih acetone and dimethyl sul-phoxide in various solvents show an almost linear relationship with the carbonyl-and chemical shifts/ " Hydrogen bonding with the solvent, or the presence of large solvent dipoles, may increase the relative importance of polar resonance structures of the carbonyl or sulphoxide group (33), and the electron withdrawl experienced by the methyl group will result in increased values of C—H couplings. 

Gorenstein A priori it is not possible to say that a trianionic phosphate is a much better reference standard than a simple monoanionic diester since we know so little about intrinsic temperature dependencies to chemical shifts. Hydrogen-bonding interactions to solvent and electrostatic effects to counter-ions may be quite different between the two types of phosphates and thus 1 am not sure that one "standard" is necessarily better than another. Uhat is important is the relative difference in temperature dependency to P-31 shifts between any reasonable simple model and the nucleic acids. Ue will though investigate your suggestion. Note that our choice of lock standards (P2O or C5F5) also exerts an influence on these measurements (see discussion in ref. 8). 

Chemical shift Hydrogen number Radical Chemical shift Hydrogen number Radical... 

Tjandra N and Bax A 1997 Solution NMR measurement of amide proton chemical shift anisotropy in N-15-enriched proteins. Correlation with hydrogen bond length J. Am. Chem. Soc. 119 8076-82... 

The third alternative is a more robust, sensitive and specialized fonn of the first, in that only hydrogen nuclei indirectly spin-spin coupled to in a specific molecular configuration are imaged. In achieving selectivity, the technique exploits the much wider chemical shift dispersion of compared to H. The metliod involves cyclic transfer from selected H nuclei to indirectly spin-spin coupled C nuclei and back according to the sequence... 

No nitration of thiazole occurs with the classical nitration reagents, even in forcing conditions (341-343). In a study concerning the correlation between the ability of thiazole derivatives to be nitrated and the HNMR chemical shifts of their hydrogen atoms, Dou (239) suggested that only those thiazoles that present chemical shifts lower than 476 Hz can be nitrated. From the lowest field signal of thiazole appearing at 497 Hz one can infer that its nitration is quite unlikely. Thiazole sulfonation occurs... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

The chemical shift of the carbonyl carbon (8 209) is not included because it has no attached hydrogens... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

The same group of hydrogens in a meso dyad [XIII] produce a quartet of peaks two different chemical shifts, each split into two by the two hydrogens in the methylene. 

Proton chemical shift data from nuclear magnetic resonance has historically not been very informative because the methylene groups in the hydrocarbon chain are not easily differentiated. However, this can be turned to advantage if a polar group is present on the side chain causing the shift of adjacent hydrogens downfteld. High resolution C-nmr has been able to determine position and stereochemistry of double bonds in the fatty acid chain (62). Broad band nmr has also been shown useful for determination of soHd fat content. 

Each hydrogen atom in a unique chemical environment is shielded differently from the external field and has a slightly different resonance frequency. The chemical shift in ppm, 5, is defined as... 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

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