Chemical shift values deuterium isotope effects

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. 

Deuterium isotope effects on 1SN chemical shifts Deuterium isotope effects on 15N chemical shifts AN(D), similarly as for AC(D), can be employed in determination of mole fraction of the proton transferred form Schiff bases.44 A similar correlation between the mole fraction of NFI-form and AN(D) values was found (Figure 2). For the Schiff bases in which proton transfer takes place, the AN(D) values varied from —2 to + 5 ppm and depend on solvent and temperature. The AN(D) values of... 

Deuterium isotope effects on 15N chemical shift in CDC13 solution as well as in solid state were measured for a series of symmetrical and unsymmetrical di-Schiff bases being derivatives of fra s-l,2-diaminocy-clohexane and various aromatic ort/io-hydroxy-aldehydes [22],57 The AN (D) value determined in solid state for symmetrical di-Schiff base which was a derivative of salicylaldehyde was —1.8 ppm, which was typical of... 

The position of the proton transfer equilibrium for the Schiff bases being derivatives of rac-2-aminobutane [24] or rac-a-methylbenzylamine [25] and their adducts with dirhodium complex has been estimated in CDCI3 solution on the basis of measurements of deuterium isotope effects on 15N chemical shift.12 It was shown that adduct formation significantly influenced the position of the equilibrium which was manifested by AN(D) values. 

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... 

FIGURE 10. Correlation between the absolute value of the deuterium isotope effects on chemical shifts and the dihedral angle. (A) Axial and equatorial 4-tert-butylcyclohexane-di (B) Adamantane-2-di. (C) Dicyanocobyrinic acid heptamethyl ester. (D) ewdo-Fenchol-2-exo-di. Reproduced by permission of John Wiley Sons from Ref. 161... 

For 1 1 mixtures of cations 16 and [Di]16 a negative primary deuterium isotope effect on the hydrogen chemical shift (A5( H, H) = -0.30) is detected. Negative (A ( H, H) values indicate a single minimum potential [10] and are found for symmetrical bridged carbocations [11]. 

For the monodeuteriated cycloalkanes 37-41 the isotope effects shown in the scheme have been measured Remarkably, in 39 and 41 some sign changes for long-range deuterium effects were revealed. The A values correlate with the chemical shifts. 

Chemical shifts are shifted to higher field when deuterium replaces hydrogen either geminally or vicinally the stereochemical dependence of the vicinal-shift isotope effect has been investigated by means of cis- and trans-[2- H]fluorocyclohexane with spectra obtained under conditions of slow ring-inversion at — 85°C. The observed F isotope shifts (/p.p.m.) for (34) and (35) are shown. Values for (36) and (37) and the norbornyl systems (38) and (39) are also shown these were determined in order to ascertain the stereochemical dependence of the isotopic shift. The smallest... 

The isotopic perturbation of resonance (IPR) method for ti -H2 characterization depends on the observation of differing chemical shifts and temperature-dependent isotope fractionation in partially deuterated polyhydrides. IPR has been shown to correctly indicate classical configurations, despite low Ti values, for many rhenium polyhydrides.However, IPR sometimes proves useless due to insignificantly small chemical shifts and broad signals. Alternatively, deuteration may exert an influence on equilibria, such as that shown in Eq. (1). Deuteration increases the abundance of (2). However, this effect is offset by greater intramolecular fractionation of deuterium into the ri site of (2) resulting in little IPR shift. 

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