Reverse isotope effect

The replacement of light isotope with heavy isotope in activated state also lowers the energy. Let the lowering in energy in activation state be represented as AZ o H has been observed that AE is less than AE0 and, therefore, ratio of k/kj > 1. When the bond involving the isotope element in activated complex is completely broken AE and k/k will be maximum. However, when the bonds in activated complex is stronger than in the initial molecule, i.e. AE > A/i o, the value of ktk will be less than unity. This is called reverse isotope effect. 

Extrapolation of the observed separation factors indicates that the reverse isotope effect persists to about 155°C in TiMo. Although the observed isotope effect is determined for the 7-phase hydrides, it is likely that absorption data at low solute contents will exhibit the same trend. 

This is not energetically reasonable since the enthalpy change for such a process is probably > 65 kcal.mole". Lapidus et subsequently studied deuterium and isotope effects in the oxalic acid decomposition. The deuterium effect (kiilko) was found to vary from 1.3 (400 °K) to 0.87 (435 °K), a reverse isotope effect. A similar reversal was found in the study, viz. 

Reverse Isotopic Effect in Plasma-Chemical Kinetics... 

Distributions (3-t67) for the mixture of two isotopes are illustrated in Fig. 3-14. The population of lower vibrational levels is larger for the heavier isotope (1, usually small additive), which corresponds to the Treanor effect and relation (3-165). The situation is opposite at higher levels of excitation, where the vibrational population of a relatively light isotope exceeds that of a heavier one. This phenomenon is known as the reverse isotopic effect (Macheret et al., t980a,b). 

The reverse isotopic effect takes place if the activation energy of the plasma-chemical process appears in the specific interval E] < E < E2 (see Fig. 3-14). The light isotope is excited mnch more and reacts mnch faster in this case than the heavier one. The coefficient of selectivity for the reverse isotopic effect can be calcrrlated from (3-167) and (3-168) as (Macheret et al., 1980) ... 

An analogous reverse isotope effect has been known for some time (100) with respect to the dissociation of FsBO(CH3)2 complex. The ratio of dissociation constants, [B Falg [F3B 0(CH3)2]i/[B F3]g [F3B 0(CH3)2]i, is 0.968 0.002 at 4° (101), in reasonably good agreement with the theoretical value of 0.96.3 calculated (101) from spectroscopic data. It has been pointed out (101) that the reverse isotope effect is due to the greater strength of B-F bonds in free BFs... 

Regardless of the possibility of providing a quantitative theoretical rationalization of the measured primary isotope effect, the data are significant because of the analogy between the measured equilibrium process and the pseudo equilibria between reactants and activated complexes in SN-1 heterolyses. Stabilization of a carbonium ion or activated complex which has h h carboniiun ion character must invariably involve delocalization of positive chaige and attendant increase in bond order around the carbon atom undergoing reaction. If delocalization in the transition state is sufficient, then a reversed isotope effect is to be anticipated. The one reported case of a primary isotope effect on SN-1 heterolysis involves the solvolysis of 2-methyl-2-chloropropane-2-C in 60% dioxane- 40% water at 25° (102). The rate ratio, = 1.027 0.015, corresponds to = 1.01 and... 

The effect of ring deuterium contrasts strongly with the effect of p-a-deuterium a significant increase in ionization constant obtains, i.e., there is a reverse isotope effect, and, as far as can be judged... 

Experimental results of the reverse isotope effect observed upon replacement of Hj by D2 provide an interesting illustration of the difference in methane kinetics compared to that of the hydrocarbon chain growth reaction (see Scheme 16.2). 

Infrared Spectrophotometry. The isotope effect on the vibrational spectmm of D2O makes infrared spectrophotometry the method of choice for deuterium analysis. It is as rapid as mass spectrometry, does not suffer from memory effects, and requites less expensive laboratory equipment. Measurement at either the O—H fundamental vibration at 2.94 p.m (O—H) or 3.82 p.m (O—D) can be used. This method is equally appticable to low concentrations of D2O in H2O, or the reverse (86,87). Absorption in the near infrared can also be used (88,89) and this procedure is particularly useful (see Infrared and raman spectroscopy Spectroscopy). The D/H ratio in the nonexchangeable positions in organic compounds can be determined by a combination of exchange and spectrophotometric methods (90). 

Friedel-Crafts acylation sometimes shows a modest kinetic isotope effect. This observation suggests that the proton removal is not much faster than the formation of the (j-complex and that the formation of the n-complex may be reversible under some conditions. 

A direct irreversible proton transfer in limiting stage of 1-ethoxybut- l-en-3-yne hydration is confirmed by the value of kinetic isotopic effect k ilk = 2.9. For fast reversible proton transitions this value is less than 1. 

The small isotope effects found most likely arise from the reversibility of step 1 by a partitioning effect The rate at which ArHY reverts to ArH should be essentially the same as that at which ArDY" " (or ArTY ) reverts to ArD (or ArT), since the Ar H bond is not cleaving. However, ArHY" should go to ArY faster than either ArDY or ArTY", since the Ar—H bond is broken in this step. If 2 > 1. this does not matter since a large majority of... 

For each catalyst, the mechanism for one direction is the exact reverse of the other, by the principle of microscopic reversibility. As expected from mechanisms in which the C—H bond is broken in the rate-determining step, substrates of the type RCD2COR show deuterium isotope effects (of 5) in both the basic- and the acid -catalyzed processes. 

Here the effects of any one fractionating step can be expressed in a change in isotopic composition in a wider range of body tissue components, including the product as well as the precursor of a (reversible) reaction. The details depend on the explicit model, for example how rates depend on metabolite concentrations. Therefore, where a metabolic pathway is, or becomes, reversible, the effect on isofractionation on measured body components can be more widespread. 

The basicities of some phosphinamides (84) have been measured and the acid-catalysed hydrolysis studied. Unsubstituted and A -alkyl derivatives follow an A2 mechanism of reversible protonation followed by ratedetermining water attack. However, the rates for the A -aryl derivatives follow Hq (but with a slope of 0.5), and an A mechanism was suggested as most consistent with this fact and the solvent isotope effect. The anomalous dependence on Ho, together with the large negative value of A5, while not necessarily excluding an ionization mechanism, leaves the question in some doubt. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

Aromatic compounds react with mercuric salts to give arylmercury compounds.69 Mercuric acetate or mercuric trifluoroacetate are the usual reagents.70 The reaction shows substituent effects that are characteristic of electrophilic aromatic substitution.71 Mercuration is one of the few electrophilic aromatic substitutions in which proton loss from the a complex is rate determining. Mercuration of benzene shows an isotope effect kB/kD = 6,72 which indicates that the [Pg.1026]

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