Ethylene, isotopic substitution effects

These results are valid and apply for all addition reactions involving olefinic double bonds. Addition reactions are characterized by an increase in the number of normal modes of vibration. In this case the ethylene molecule has 12 normal modes of vibration while thiirane has 15. One of these, the CS stretching mode, coincides with the reaction coordinate and does not contribute to the isotope effect. Out of the net gain of two, the CCS bending mode is not sensitive to isotopic substitution and does not generate an isotope effect, but the twist of the CH2 group which... 

Table 10.1)/ which is the inverse isotope shift assuming the MW as the isotope mass within the framework of the BCS theory. On the other hand, the D-salt of Cu[N(CN)2]Br has a lower than the H-salt (normal isotope shift).These isotope substitution results have been confirmed not only by resistance measurements but also by the magnetization and/or if penetration measurements. It is noteworthy that the D-salt of Cu[N(CN)2]Br has an exceptionally small volume fraction of magnetization due to superconducting (one order of magnitude lower than that of the H-salt). The C substitution of the terminal ethylene groups has a similar but smaller isotope effect to that of the D substitution. However, C or isotope substitution of other parts of the ET molecule has almost no or a normal isotope effect on 7),. 

The behavior described above has been verified by experiment and calculation on numerous substituted dienes and dienophiles. For example Fig. 10.13 shows results for 2°-D isotope effects on Diels-Alder reactions of 2-methyl-butadiene with cyano-ethylene and 1,1-dicyano-ethylene. The calculated and experimental isotope effects are in quantitative agreement with each other and with the results on (butadiene + ethylene). In each case the excellent agreement between calculated and observed isotope effects validates the concerted mechanism and establishes the structure of the transition state as that shown at the bottom center of Fig. 10.11 and the left side of Fig. 10.12a. 

Studies of the secondary isotope effect also confirmed the two-step mechanism, involving the formation of a dipolar or diradical intermediate in the rate determining step. The observed isotope effects in the reaction of 1 -(p-methoxyphenyl)- ethylene 70 and its deuterated analogues 1 -(p-methoxyphcnyl)-ethylene-rfi 70-rfi and l-(p-methoxyphenyl)-ethylene-d3 70-d with C60, when taken in conjunction, exclude the formation of transition state TSm (concerted mechanism, see Fig. 30) because in that case substitution at either Ca or Cp would have given an inverse isotope effect. 

The rate coefficient of the acid catalyzed hydrolysis of ethylene oxide is strongly increased by methyl substitution (Table 10). On the other hand, kH is decreased by substitution of halogen or hydroxy in the 3-position of 1,2-epoxypropane. The kinetic solvent isotope effect is kH/kD = 0.45 for the hydrolysis of ethylene oxide and feH/ftD = 0.53 for the hydrolysis of epichlorohydrin [107]. 

Steric isotope effects are less clear cut, possibly because many small effects considered to be steric are in fact a mix of steric and stereoelectronic. Early work on the racemisation of optically active biphenyls gave a value of kne/ De of 0.85 for a 2,2 -dimethylbiphenyl also containing a 6,6 ethylene bridge (4,5-dimethyl 9,10 dihydrophenanthrene), and 1-deuteriocyclohexane prefers the deuterium-axial conformation by 25 J mol V but the preference decreases next to a heteroatom. Effects of deuterium substitution of carbon-bound protons in glucose on the anomeric equilibrium in water cannot be simply rationalised by a single effect the equilibrium isotope effect (defined as [P]H[a]D/[p]D[a]H) being 1.043 for HI, 1.027 for H2, 1.027 for H3, 1.001 for H4, 1.036 for H5 and 0.998 for H6,6. ... 

The role of the metal ion in ester hydrolysis catalysed by CPA has been examined with both Zn +- and Co +-substituted enzymes. When the terminal carboxyl of the substrate is electrostatically linked to argenine-145 and the aromatic side-chain lies in a hydrophobic pocket, the only residues close enough to the substrate to enter catalysis are glutamate-270, tyrosine-248, the metal ion, and its associated water. Low-temperature studies aid the elucidation of the mechanism. Between - 25 and - 45 °C in ethylene glycol-water mixtures two kinetically discrete processes are detected, the slower of which corresponds to the catalytic rate constant. The faster reaction is interpreted as deacylation of a mixed anhydride acyl-enzyme intermediate formed by nucleophilic attack by glutamate-270 on the substrate (Scheme 6). Differences in the acidity dependences of the catalytic rate constant with the metal ions Zn + (p STa 6.1) and Co +-(pATa 4.9) suggest that ionization of the metal-bound water molecule occurs and is involved in the decay of the anhydride. The catalytic rate constant shows an isotope effect in DgO. 

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