Hydrogen-bonding effects chemical shift

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). 

It has become clear in the past decade that strong hydrogen bonding has associated with it several characteristic properties. In particular, as hydrogen-bond strength changes, maxima or minima are observed in nmr chemical shifts, the isotope effect on the chemical shift A[5( H) — 8( H)] defined on p. 271, ir Vah/vad band ratios, and in the isotope-fractionation factor, p. 

This picture of the chemical shift and isotopic effects also explain the somewhat puzzling fact that the downfield limits of <5(OHO) at ca. 20 ppm do not represent the strongest of hydrogen bonds. This situation indicates case 3 type bonds. 

Ando, S., Ando, I., Shoji, A., andOzak, T., Intermolecular hydrogen-bonding effect on C NMR chemical shifts of glycine residue carbonyl cations of peptides in the solid state, J. Am. Chem. Soc. 110, 3380-3386(1988). 

For 13C NMR studies, the intermolecular effects which can affect chemical shifts have been documented (25). These are of two types hydrogen-bonding effects and conformational or y effects. 

Very favourable hydrogen bonding may occur in substituted 8-hydroxyquinoline N-oxides (substituted 32) judging from the 50H value (in the 5,7-dinitro-8-quinolinol N-oxide a value of 20.38 ppm is found) as well as the deuterium isotope effects on the C chemical shifts. Complicated substituent effects are found, because substiments such as bromine may interact with both the OH and the N—O group. No tautomerism was observed... 

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