Chemical shifts hydrogen bonding

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). 

Recently, solid state nmr spectroscopy has been used to probe hydrogen-bonded materials and first reports showed that there was a direct relationship between / o o anisotropic chemical shift as bonds become... 

Recently, solid state nmr spectroscopy has been used to probe hydrogen-bonded materials and first reports showed that there was a direct relationship between R0...0 and cr, the anisotropic chemical shift as bonds become shorter the chemical shift moves downfield. Twenty-four compounds were examined and some, such as potassium hydrogen malonate, had a o-value below 20 ppm (Berglund and Vaughan 1980). 

It has been shown [73-75] that the plot of the chemical shifts vs. bond lengths is useful for obtaining the positions of the hydrogen atoms. Due to the intrinsic difficult of the X-ray technique in detecting the position of the hydrogen atoms, an... 

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... 

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... 

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. 

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