Deuterium isotope effects, chemical shifts

CP/MAS), gated H-decoupling techniques, deuterium-induced isotope effects chemical shifts, and molecular modeling has shown that this... 

Figure 1.9. NMR spectra of a mixture of ethanol and hexadeuterioethanol [27 75 v/v, 25 °C, 20 MHz], (a) H broadband decoupled (b) without decoupling. The deuterium isotope effect Sch - d on chemical shifts is 1.1 and 0.85 ppm for methyl and methylene carbon nuclei, respectively...

FIGURE 2.11. 19F NMR spectrum of l,6-difhiorohexane-l,l-d2, demonstrating the deuterium isotope effect on the fluorine chemical shift... 

Scheme 2.16 provides further insight regarding both a- and 3-deuterium isotope effects on fluorine chemical shifts with a series of deuterated chloro, fluoroethylenes.16... 

FIGURE 2.12. CH2F/CD2F region of the 13C NMR spectrum of 1,6-difluorohexane-1,1 -d2 demonstrating deuterium isotope effect on 13C chemical shifts... 

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. 

T. Deuterium isotope effects on 13 C chemical shifts In the studies of proton transfer equilibrium of Schiff bases, the most informative are deuterium isotope effects measured for carbons bonded with proton donor groups (C-2 or C-7 for gossypol derivatives). For imines in which... 

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. 

Wells and coworkers53 have prepared a series of deuterated tetramethyltin compounds, which they used to study the long-range deuterium isotope effects on the proton chemical shifts of tetramethyltin. The various deuterated tetramethyltin compounds, with one to four trideuteromethyl groups on the tin atom, were prepared by a series of methyl group exchanges beginning with tri-trideuteromethyltin chloride and undeuterated tetramethyltin (Scheme 12). 

Wei and collaborators180 studied the long-range deuterium isotope effects on the proton chemical shifts of tetramethyllead and chlorotrimethylplumbane. The perdeuterated analogues of these organolead compounds were generated in situ by reaction of equimolar amounts of (CD3)3SnCl and (CH3)4Pb in methanol-d, in equation 118. 

With time, the CD3 group is distributed throughout the system resulting in the formation of (CH3) (CD3)4- Pb (n = 3, 2, 1) and (CH3) (CD3)3- PbCl (n = 2, 1), as shown in Scheme 32. These species were used to determine the effect that a deuteron positioned four bonds away, 13, would have on the proton chemical shifts of a methyl group. These long-range four-bond deuterium isotope effects, 4AH( ) ), are summarized in Table 14. 

The 13 C resonances in derivatives of aniline reveal that the carbon bearing a partially deuterated NH2 group appears as a multiplet because of the deuterium isotope effect on the 13C chemical shift, when the hydrogen exchange is low. This effect is larger for... 

Shifts due to either a- or /1-deuterium substitution are also quite significant, usually leading to well-resolved fluorine signals for the deuterated and undeuterated species, which can be useful in characterization of deuterium-labeled fluorinated compounds. An example of the a-effect is shown in Figure 2.20, which depicts the fluorine NMR spectrum of F-CH2CH2CH2CH2CH2CD2-F, a molecule that has magnetically equivalent fluorines, except for the presence of the deuteriums. The observed isotope effect on the chemical shift is a 1.31 ppm upfield shift (or 0.65 ppm upheld per deuterium). This effect is an a-deuterium isotope effect, in this case a two-bond effect on fluorine. 

Other examples of stereoelectronic effects on three-bond transmission of deuterium isotope effects on fluorine chemical shift are provided in the 2-deuteriofluorocyclohexane system, where an anh-deuterium gives rise to an incremental shift of 0.35 ppm, whereas gauche deuterium has only about half the effect (Scheme 2.33).31... 

Figure 2.21 depicts the CH2F/CD2F region of the13 C NMR spectrum of l,6-difluorohexane-d2, and it provides a measure of the deuterium isotope effect upon the carbon chemical shift of the CH2F carbon... 

T. Dziembowska, Z. Rozwadowski, Application of the deuterium isotope effect on nmr chemical shift to study proton transfer equilibrium. Curr. Org. Chem. 5, 289-313 (2001)... 

Reuben J (1986) Intramolecular hydrogen bonding as reflected in the deuterium isotope effects on carbon-13 chemical shifts. Correlation with hydrogen bond energies. J Am Chem Soc 108 1735-1738... 

B. Coxon, Deuterium isotope effects in carbohydrates revisited. Cryoprobe studies of the anomeriza-tion and NH to ND deuterium isotope induced 13C NMR chemical shifts of acetamidodeoxy and aminodeoxy sugars, Carbohydr. Res., 340 (2005) 1714-1721. 

Deuterium isotope effects on chemical shifts of phenols of which the OH proton has been exchanged by deuterium can be measured in two different ways. If the OH(D) proton is exchanging slowly (see Section II.B) two different resonances are observed, one due to the protio and one due to the deuterio species (see Figure 1). The relative intensities will depend on the H D ratio, perhaps not in a quantitative way due to fractionation (see Section II.O). If exchange is fast on the NMR time scale only one resonance for the X-nuclei (e.g. C) is observed, the position of which depends on the H D ratio. In order to determine the isotope effects properly, a series of experiments must be conducted varying the H D ratios of the exchanging species, typically 1 5, 1 2, 1 1, 2 1 and pure solvent . The exchanging species is typically H2O D2O but could equally well be deuteriated alcohols, ROD. 

---