Deuterium isotopic perturbation method

Saunders, Telkowski and Vogel developed another powerful method for distinguishing nonclassical from classical cations using the deuterium isotopic perturbation method. ... 

The isotopic perturbation method was first developed using H-nmr spectroscopy and deuterium as the perturbing isotope. The isomerization of 3-deuterio-2,3-dimethyl-2-butyl cation (Saunders et al., 1971), 2,3,3-trimethyl-2-butyl cation and cyclopentyl cation (Saunders and Vogel, 1971a) and methyl interchange in 2,3-dimethyl-2-butyl cation [10] (Saunders and Vogel, 1971b) were studied. 

By the study of 12 isotopomers of protoadamantane, the stereochemical dependence of isotope effects was also observed. A Karplus-type relationship similar to that for spin-spin coupling constants was proposed Recently, the first quantitative stereochemical dependence between isotope effects and dihedral angle was reported for a series of deuteriated norbornanes, as shown in Figure Observations of the influence of substitution with deuterium on conformational equilibria led to a new method in physical organic chemistry called isotopic perturbation of equilibrium. Details can be found in a recent review The effect was first observed for the chair-tO"Chair interconversion of deuteriated 1,3-dimethylcyclohexane 43 and later in 4-ethyl-l-methylcyclohexane " as well as in 1,1,4,4-tetramethylcyclohexane. In contrast, the isotope effect on conformational equilibrium in related systems turned out to be too small to be observable... 

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. 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

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