Isotope effects for reactions

SECOND-ORDER RATE COEFFICIENTS (Ar2) AND KINETIC ISOTOPE EFFECTS FOR REACTION... 

RATE COEFFICIENTS, SOLVENT AND SUBSTRATE ISOTOPE EFFECTS FOR REACTION OF... 

SOLVENT ISOTOPE EFFECTS FOR REACTION OF ARYLBORONIC ACIDS WITH ACID IN... 

RATE COEFFICIENTS AND SOLVENT ISOTOPE EFFECT FOR REACTION OF 4-ClC6H4SiMe3 WITH AQUEOUS TRIFLUOROACETIC ACID AT 25 "C471... 

The effect of steric hindrance on the rates and kinetic isotope effects for reactions of l-nitro-l-(4-nitrophenyl)alkanes and their deuterated analogues with two bicyclic guanidines of comparable basicity (l,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD, and its 7-methyl derivative, MTBD) in THF has been studied. The results disagree with the notion that deuterium kinetic isotope effects are enhanced by steric hindrance, since for the reactions of MTBD with various carbon acids the KIEs decrease with steric hindrance in the carbon acid but the converse is true for reactions of TBD. 

Aldol reactions have continued to attract attention.28-39 hi order to determine the mechanism of addition of lithium pinacolone enolate [CH2=C(OLi)C(Me)3] to benzaldehyde the carbonyl-carbon KIE (xlk/nk = 1.019) and the substituent effects (p = 1.16 0.31) have been compared with those for other lithium reagents.28,29 The small positive KIE, which is larger than the equilibrium IE (nK/nK = 1.006) determined by ab initio MO calculations (HF/6—31 + G ), is in contrast with nk/l4k = 1.000 for MeLi addition which proceeds by the rate-determining ET mechanism, characterized by a much smaller p value. Since probe experiments showed no evidence of single electron transfer, it has been concluded that the significant isotope effect for reaction of lithium pinacolone enolate is indicative of rate-determining polar attack (PL) rather than radical coupling (RC) (Scheme 2). 

Tunneling of atomic nuclei in the course of an elementary act of chemical reaction was first considered theoretically in Refs. [5-8] soon after quantum mechanics had been created. It has been shown that nuclear tunneling may lead to unusually large isotope effects for reactions in which light atoms (hydrogen, deuterium, tritium) are transferred and to a decrease in the effective activation energy of chemical processes as the temperature decreases. 

If the calculated equilibrium lsO-isotope effects for reaction (2) are used... 

KIE calculated from this equation corresponds to the overall effect including primary and secondary isotope effects. For reaction with a C-H bond breakage in a methyl or methylene group the primary isotope effect (KIEr) is observed for... 

TABLE 6. Absolute rate constants and deuterium kinetic isotope effects for reaction of 1,1-diaryl-silenes (XC6H4)2Si=CH2 (19a-e) with acetic acid (AcOH) in acetonitrile solution at 23 °C (in units... 

TABLE 1. Isotope effects for reactions of alkyl halides with nucleophiles... 

Finally the temperature dependence of the primary isotope effects was determined. Here the traditional expectations of Chart 3 were fully met the results translate into AH/AD = 1.1 0.1, aD — aH = 0.8 kcal/mol. Thus the amount of tunneling present, adequate to produce the observed exaltation of secondary isotope effects, violations of the Swain-Schaad relationship, and violations of the Rule of the Geometric Mean in the neighborhood of room temperature, does not lead to anomalies in either the ratio of isotopic pre-exponential factors nor the isotopic activation energy difference over the temperature range studied (approximately 0-40 °C). As will be seen later, the temperature dependence of isotope effects for reactions that include tunneling is in general a complex, unresolved issue. 

Actually this equilibrium constant was measured for an analogous reaction involving a similar carbon acid (9-phenylfluorene). The value of KT /KD for (83) was calculated from Kr /KH with the Swain—Schaad relation. Using this analysis for the isotope exchange of triphenylmethane, a value of aT = 0.66 0.04 was obtained which is similar to the value deduced from other experiments for a closely related reaction [22]. However, the assumptions involved in the analysis may not be entirely valid and the derived values of a1 which measure the amount of internal return may not always be reliable. For the reaction of triphenylmethane the discrepancy between the measured value obs/ obs = 1-77 0.05 and the value 2.6 0.2 which is calculated for this isotope effect from the measured value of kobs/ obs = 1.34 0.03 using the Swain—Schaad relation is well outside experimental error. For the malononitriles discussed earlier in this section the experimental isotope effects for reaction in aqueous solution (Table 3) are well correlated by the Swain—Schaad relation which probably means that in these cases internal return is not important. 

A number of very interesting conclusions are arrived at in this study.86 (1) Reaction of hydroxide ion as a base on carbocation 77 to form epoxide (k 2[HO-]) is energetically more favorable than its reaction with 77 as a nucleophile to form diols. (2) The reaction pathway for formation of ketone 128 is completely separate from the stepwise mechanism for diol formation. (3) The observed kinetic deuterium isotope effect for reaction of 76-f>-d at pH 11 is 2.15 the kinetic deuterium isotope effect on the 1,2-hydrogen migration is estimated to be 4. Hydrogen migration must be occurring at the transition state for ketone formation. 

Table 3 Kinetic isotope effects for reactions of pNPP with members of the PTPase family the Yersinia PTPase (YopH), PTP1, the dual-specific VHR, and the low-molecular-weight PTPase Stpl...

The effect of steric hindrance on the rates and kinetic isotope effects for reactions of l-nitro-l-(4-nitrophenyl)alkanes and their deuterated analogues with two bicyclic guanidines of comparable basicity (l,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD, and its... 

The hexenes form besides hydrogen the largest product group in the radiolysis of the paraffins. They can be formed by Reaction 1 and by disproportionation of hexyl radicals. The decrease of their G-value on deuteration is 1.5, the corresponding decrease of G( molecular hydrogen) is 1.6. This is additional proof for an isotope effect for Reaction 1. This reaction explains also the observation that the isomeric distribution of... 

Typical deuterium isotope effects for reactions in which C—H bond breaking is rate-determining lie in the range 3-8. If the C—H bond breaks after the ratedetermining step, the overall reaction rate is affected only slightly and k /kj) = 1-2. Thus, measuring the deuterium isotope effect can tell us if a C—H bond breaks in the rate-determining step. 

SO that the figures in the last colunm of Table 23 should represent the maximum H/D isotope effects for reactions involving these bonds. 

Isotope effect for reactions of methane derivatives RCH3 -h X... 

Table 1 Isotope effects for reactions of dianion of pNPP (pNPP ) and pNPPT . t-BuOH stands for terf-butyl alcohol Species...

Eyring (56) is of the opinion that transition state theory in its present form cannot—nor was it intended to— confidently handle rate differences of the magnitude encountered in secondary isotope effects. Still, if these are to be treated formally at all, it must be done in the framework of kinetic isotope effect theory—which is all we have. Fortunately, to use the words of Bigeleisen and Wolfsberg (49a) the effect of isotopes on the various quantities can often be predicted with more reliability than the quantities themselves, so that one might try to predict quantitative isotope effects for reactions so complex that quantitative predictions of fc would not be attempted. ... 

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