Deuterium isotope effects hydrogen shifts

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... 

Next reviews were dedicated to problems of hydrogen-bonded systems. Hydrogen/deuterium isotope effects on NMR parameters in liquids and solids have been reviewed by Limbach et al.11 Review covers period to 2004 and illustrates the correlation of intermolecular hydrogen-bonded systems geometry and H/D isotope shifts and coupling constants, particularly measured in the solid state and in liquids at low temperature. Several reviews concern the isotope effects on intramolecular hydrogen-bonded systems.12-17 Since that time several new papers dedicated to hydrogen-bonded systems were published, mostly on intramolecular systems.18-24... 

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... 

Primary and secondary kinetic isotope effects are of general importance in the study of neighboring group participation. Isotopic substitution a to the incipient carbo-cation produces a secondary isotope effect whereas 0 and y substituents may give rise to both primary and secondary effects. For example, if the rate determining step of a solvolytic reaction involves a hydrogen shift or elimination, primary deuterium isotope effects are clearly implicated. 

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... 

This work also determined the activation parameters for the overall loss of 1,3-dimethylcyclopropene as log k = 13.36 — 39 300/2.3/ T and that for the formation of 1-ethylcyclopropene as log A = 12.29 — 39 800/2.3/ 7. The activation parameters for loss of the latter material were found to be very similar to that of the former cyclopropene. Still later work using 2-deuterium-labeled material not only demonstrated that the ethyl group migrated in preference to the methyl group in the vinylidene by a factor of roughly 3, but also found a deuterium isotope effect of 1.32 at 509 K. This isotope effect could result from a pathway like that depicted in Scheme 4.2, but the authors suggested that it arises from the 1,3-hydrogen shift to form the vinylidene. 

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]. 

Upheld shifts of up to 1.5 ppm may be experienced by carbon atoms when hydrogen atoms attached to them are replaced by deuterium atoms. Deuterium isotope effects are dependent on the degree of deuteration. 

The primary isotope effects used in the study of tautomeric equilibria are primarily those of deuterium or tritium. As these are proportional [51], they can be used interchangeably as long as one remembers that they should be scaled. Primary deuterium isotope effects in hydrogen-bonded nontautomeric systems are found to correlate with the OH chemical shifts in a nonhnear manner. This is illustrated in Figure 6.25. 

Isotope effects on chemical shifts can be used to reveal tautomeric equilibria in symmetric as well as asymmetric tautomeric systems. The isotope effects may also provide information about hydrogen-bond potentials. A number of different isotope effects, short-range as well as log-range, may be used. Deuterium isotope effects are by far the most useful, both from an effect point of view and also because deuterium is easy to introduce in most situations. Heavy isotopes are not used extensively. and are excellent as observing nuclei for secondary isotope... 

As resonances are broad only large deuterium isotope effects on chemical shifts can be observed. This means, for example, OH groups involved in strong hydrogen bonding or involved in tautomeric equilibria. An example is shown in Figure 3.18 in which A O(OD) isotope effects are plotted versus AC(OD) isotope effects for a series of o-hydroxy acyl compounds [18]. The tautomeric compounds (open symbols) are easily pointed out. 

P, and Limbach, H.H. (1996) Hydro-gen/deuterium isotope effects on the NMR chemical shifts and geometries of intermolecular low-barrier hydrogen bonded complexes. /. Am. Chem. Soc., 118, 4094-4101. 

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