NMR shift data

Differences in reactivity of the double bond among the four isomers are controlled by substitution pattern and geometry. Inductive effects imply that the carbons labeled B in Table 3 should have less electron density than the A carbons. nmr shift data, a measure of electron density, confirm this. 

These correlations are of generally poorer precision than those for reactivity and F-nmr shift data, and in the case of the ortho data set they required rejection of data for the substituents CHjCO, NO2, and CO2R (which give large deviations if included). Ihe latter deviations may be associated with ortho chelation effects. In any case, the Kj and X patterns, as noted above, appear essentially as expected. 

Contrary to conventional reactivity arguments, which imply that substitution at carbonyls by electronegative atoms reduces electron density at the carbonyl carbon and hence promotes addition to carbonyls, a systematic study of 13C NMR shift data for ester carbonyls shows that electron density is actually greater at such carbons (reactivity enhancement is actually due to destabilisation of the ground states of the esters by the electron-withdrawing substituents).132,133 Our observations are in line with those of Neovonen et al. Electron-withdrawing nitrogen in... 

Appendix Table 3 H NMR shift data and UV-vis data for selected porphyrins and porphyrinoids ... 

STC411). In the monosubstituted compounds (Me, Et, /Pr, and OMe on C-2), the substituents are axially oriented contrary to their normal equatorial orientation on C-3 and C-4. The disubstituted compounds (l,X-dialkyl—X = 3, 4, 5, 6) exist as cis/trans isomers. In the more stable isomeric form the 2-substituent is axial and the other is equatorial and in the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derviative where the 13C NMR shift data point to a predominance of the di-axial form. 

The NMR shift data refer to the chemical shifts measured by NMR for the amide proton in die coil conformation (37). 

The effect of Pt coordination on the acid-base properties of the purine ligands / -ethylguanine, N -methylhypoxanthine, inosine, Af -methyladenine, and trimethyladenine has been quantitatively evaluated The acidity constants of the complexes were calculated by curve-fitting procedures, using the pH (pD) dependence of the H NMR shift data. It was found that Pt coordinated to the N -site acidifies the N-7 unit to the same extent as N -coordinated Pt acidifies the N-1 site. 

Although no structural data are available, a tenfold increase in O2 affinity was observed between an ester-strapped porphyrin, offering no hydrogen-bonding possibilities, and its conformationally very similar amide analogue. O2 amide hydrogen bonding was demonstrated by means of NMR shift data (Zn and Fe—CO... 

Table 4. H-NMR shift data for alkali metal-crownophane complexes Chem. shift difference (A o = (complex) — 5(free))...

Fig. 3). Acid hydrolysis of 41 liberates approximately 1 mol of proline, [( )] +460° (224 nm, 0.1 HCl), which establishes the S conformation at C-12. The stereochemistry at position 9 is based on the 95,125 configuration of deoxybre-vianamide E, an apparent bioprecursor. The relative configuration of the spiro atom (position 2) to that of position 9 is based on NMR-shift data. The proposed structure and absolute configuration of (125)-tetrahydroaustamide was subsequently confirmed by X-ray crystallography (77). d/-Austamide was synthesized by Hutchison and Kishi (78). 

Theoretical studies of six-membered ring molecules with two or more heteroatoms containing at least one selenium or tellurium have been relatively limited since CHEC-II(1996). One such study deals with determining the optimum geometries of 1,2-dichalcogenins 1-3. These compounds are predicted to be nonplanar with puckered structures <2000JA5065>. These results are consistent with Se NMR shift data. 

The differences in behavior of the Eu ion in the ground and excited states may also account for the disagreement in the extent of outer-sphere complexation by chloroacetates as obtained from La-NMR data (Choppin 1980), solvent extraction (Choppin 1980) and luminescence measurements (Barthelemy and Choppin 1989a). Although NMR shift data and solvent extraction studies gave estimates of 60% and 25% inner-sphere character in the EulClAc) " and Eu(a2Ac) complexes, respectively, luminescence lifetimes data provided estimates of 100% and 50% inner-sphere character for the two complexes. 

A simplified reaction field theory was used by Abboud and Taft [Ab 79] for the description of the solvent dependence of NMR shift data. Their concept proved to be useful for the generalization of solvent properties in polar solvents only. 

Thus, NMR shift data can be used to detect and to estimate the presence and the strength of HBs (Scheme 1). 

Although NMR is a powerful tool for studying solvation, it suffers the same limitation outlined above for ESR spectroscopy, namely, that systems are almost always in fast exchange. In our experience, the only way to reach unambiguous interpretations of NMR shift data is to link the results with infrared studies, when possible. (Shifts are by far the most widely studied parameters coupling constants are generally relatively insensitive to solvent changes.)... 

Luteijn, J.M., A. Van Veldhuizen, and A. De Groot The assignment of NMR shift data in clerodanes and related structures. Organic Magnetic Resonance 19, 95 (1982). 

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