Deuterium field effect

If only the field effect were operating, 1 would be more stable than 2, since deuterium is electron-donating with respect to hydrogen (p. 18), assuming that the field effect of deuterium could be felt two bonds away. 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

Electrophilic substitution, aromatic, 31, 130-167, 381 1,2-ti. 1,4-addition, 195 as addition/elimination, 133 complexing with substituent, 160 deuterium exchange, 131,158 electronic effects in, 148, 158, 159 energetics of, 132, 136 field effect in, 152 hyperconjugation in, 153 inductive effect in, 22,152,153,156, 160... 

The static reactor method used for absolute rate determinations, and almost always for ortho -para deuterium studies, is generally that based on the micro-Pirani gauge analysis chamber as adapted by Ashmead et al. (3). The time necessary for a single determination of the extrinsic field effect by this method is unfortunately likely to be measured in hours or days rather than in seconds as for the flow reactor. To date the only application of this method to the extrinsic field effect appears to be that of Eley et al. (4). Van Cauwelaert and Hall (5) have described a recirculating adaptation of the static reactor that would seem to be useful for studying the field effect. 

The propene ion is one system in which determination of isotope effects is hampered by hydrogen randomisation. Nevertheless, deuterium isotope effects upon ion abundances following El have been obtained by making allowances for the extent of hydrogen randomisation [see Sect. 7.5.1(f)]. For hydrogen atom elimination, the isotope effect/H//D has been put at 1.7—2.0 for source reactions [372] and 2.3—3.0 and 3.6 (first and second field-free regions, respectively) for metastable ions [510]. For hydrogen atom elimination from propenoic acid ions, the isotope effect /H//D has been put at 4.3 for metastable ions [587]. 

To supplement the data on prolyl isomerization, I will draw on the literature describing rotation about the C-N bond in secondary amides. Early studies in this field were described by Stewart and Siddall in an excellent 1970 review. As we will see, these reactions are related to prolyl isomerization and support the mechanism to be proposed for prolyl isomerization. The mechanism is based on results from a variety of experimental approaches. In all cases, experiments employing kinetic-based probes will be used to obtain an accurate picture of the activated complex in the rate-limiting transition state. The experiments that will be described include thermodynamics, in which activation parameters (i.e., AG, AHt, and ASt) will be described solvent effects, in which the influence of organic solvents and deuterium oxide will be reviewed acid-base catalysis substituent effects and secondary deuterium isotope effects. 

The acidity of the hydrogen atoms at C-2 and C-4, as measured by hydrogen-deuterium exchange data, could not be correlated with Huckel MO, Pariser-Parr-Pople (PPP), or variable / ,y-PPP localization energies calculations. The deficiency of /S-electron methods was probably because of the field effects of the heteroatoms and their lone pairs of electrons. ... 

The greater acidity of hydrogen in H than I has been interpreted by certain workers to imply predominance of inductive or field effects While it is clear that in H lone pair-C—F interaction is not possible, it is also apparent that the interaction of the lone pair with an anti acceptor F2C—CF2 bond is optimized and can account for the relative rate of deuterium exchange. 

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