Effects on Chemical Shifts

Because fluorine is relatively sensitive to its environment and has such a large range of chemical shifts, considerable changes in chemical shift can be observed when a nearby atom is replaced by an isotope. For example, replacement of 12C by 13C for the atom to which the fluorine 

FIGURE 2.20. 19F NMR spectrum of 1,6-difluoro hexane-1, demonstrating the deuterium isotope effect on the fluorine chemical shift 

For the cz s-l-chloro-2-fluoroethylene, an a-deuterium isotope effect (one D) of 0.6 ppm is observed, along with a trans-P-deuterium isotope effect of 0.4 ppm. Looking at the Zra .v-l-chloro-2-fluoroethylene system, the a-deuterium isotope effect is 0.5 ppm and the cis-fi-isotope 

From these results, it appears that anri-deuterium substitution transmits its isotope effect better than gauc/ze-deuterium substitution, the same trend as is observed in transmission of coupling constants. 

FIGURE 2.21. CH2F/CD2F region of the 13C NMR spectrum of 1,6-difluorohexane-l,l-d2 demonstrating deuterium isotope effect on 13C chemical shifts 

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Give a clear indicaUon of solvent, concentration, and temperature. These parameters have a much greater effect on chemical shifts and coupling constants for fluorine than for protons. 

MI1 P. E. Hansen, Isotope Effects on Chemical Shifts as a Tool in Struc-... 

Amino groups have a shielding effect on chemical shift similar to that of an OH group (Scheme 4.28). 

The measurement of deuterium isotope effects on chemical shifts is a useful tool for studies of tautomeric equilibrium (for details see the reviews 42 44). The deuterium isotope effect AX(D) is defined as the difference between chemical shifts in deuterated and non-deuterated sample [26]. 

The deuterium isotope effects on chemical shift consists of intrinsic isotope effect (direct perturbation of the shielding of X atom) and equilibrium isotope effect (perturbation of the equilibrium caused by the isotopic substitution). The values of deuterium isotope effects are to some extent independent of chemical shifts and allow determination of the mole fraction of the forms in equilibrium. 

By using PHIP-NMR studies, various intermediates such as the previously elusive dihydrides of neutral and cationic hydrogenation catalysts, as well as hydrogenation product/catalyst complexes, have already been detected during the hydrogenation of styrene derivatives using cationic Rh catalysts. Information about the substituent effect on chemical shifts and kinetic constants has been obtained via time-resolved PASADENA NMR spectroscopy (DYPAS). 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

The chemical shift <5 of a given nucleus in a molecule contains stereochemical information which, however, might often be uninterpretable because the corresponding chemical shift of another stereoisomer is not available for comparison. Thus, the evaluation of an isolated chemical shift is possible only if the contribution of the stereochemical influence can be identified unequivocally among other influences. This is, for example, the case when substituent effects on chemical shifts (SCS substituent chemical shift), instead of chemical shifts themselves, are determined, since some of these values are known to possess a well-defined stereochemical dependence. An SCS is defined as the chemical shift of a given nucleus in a substituted molecule (R—X) relative to the unsubstituted parent compound (R H) ... 

Six-membered rings containing one nitrogen atom resemble cyclohexanes in their geometry, and the stereochemical dependence of substituent effects is often similar. However, there is a complication involving nitrogen inversion of neutral amines which results in different orientations of the lone pair with respect to the ring621 for such effects on chemical shifts see Section 4.1.1.1. and on /H C and 2/HH values see Section 4.1.1.2. 

Observation of a great number of NMR spectra reveals general factors which are responsible for predictable and cumulative effects on chemical shifts. 

A DFT-based third order perturbation theory approach includes the FC term by FPT. Based on the perturbed nonrelativistic Kohn-Sham orbitals spin polarized by the FC operator, a sum over states treatment (SOS-DFPT) calculates the spin orbit corrections (35-37). This approach, in contrast to that of Nakatsuji et al., includes both electron correlation and local origins in the calculations of spin orbit effects on chemical shifts. In contrast to these approaches that employed the finite perturbation method the SO corrections to NMR properties can be calculated analytically from... 

Scheme 2.4 Substituent effects on chemical shifts of bridgehead bicyclic fluorides...

Substitutional and Configurational Effects on Chemical Shift in Pyranoid Carbohydrate Derivatives, R. U. Lemieux and J. D. Stevens, Can. J. Chem., 43 (1965) 2059-2070. 

EFFECT ON CHEMICAL SHIFTS BY TWO OR THREE DIRECTLY APPENDIX B ATTACHED FUNCTIONAL GROUPS... 

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