Kinetic isotope effects temperature dependence

Kinetic complexity can produce apparently temperature-independent isotope effects. For example, a rise in temperature produces a smaller intrinsic isotope effect, in agreement with the conventional expectations of Chart 3, for an isotope-sensitive step that is partially rate limiting. If at the same time the rise in temperature makes other steps relatively more rapid so that the isotope-sensitive step then becomes more nearly rate-limiting, then the intrinsic isotope effect will be more fully expressed (Chart 4). If these effects roughly balance, then the isotope effect may appear to be independent of temperature while in fact fully in accord with semiclassical expectations. Seymour and Klinman have discussed in detail the problem of kinetic complexity in isotope-effect temperature dependences. 

A recent paper by Leffek and Matheson (1971) nicely complements this work, as it describes the results of a careful investigation of the temperature dependence of the kinetic isotope effect in the reaction studied by Kaplan and Thornton (1967). It is found that AAH = 134 + 30 cal mol and dd/S = 0-15 + 0-09 cal mol deg , demonstrating that the isotope effect is primarily due to an enthalpy difference, and providing support for the steric interpretation suggested by Kaplan and Thornton (1967). 

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

Based on C-H versus C-D zero point vibrational differences, the authors estimated maximum classical kinetic isotope effects of 17, 53, and 260 for h/ d at -30, -100, and -150°C, respectively. In contrast, ratios of 80,1400, and 13,000 were measured experimentally at those temperatures. Based on the temperature dependence of the atom transfers, the difference in activation energies for H- versus D-abstraction was found to be significantly greater than the theoretical difference of 1.3kcal/mol. These results clearly reflected the smaller tunneling probability of the heavier deuterium atom. 

Interestingly, in a comparison of the CD3 and CHj carbenes, an unusual temperature dependence of the kinetic isotope effect was observed. In contrast to typical reactions, the ratio of rates of H versus D shift, k /ko, actually increased as temperature was raised. In fact, k was measured to be larger than k at 248 K. It was suggested that these results required a normal temperature dependence of the isotope effect for the classical component of the reaction, but an unusual diminished isotope effect for the QMT reaction. 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

San Filippo and coworkers163 have determined the temperature dependence of the primary hydrogen-deuterium kinetic isotope effects for the hydrogen transfer reactions between several organic radicals and tributyltin hydride (deuteride) see equation 116. 

The photolyses of diazirines 9a and 9b were similarly studied in Ar matrices at 10-34.5 K 59 Eq. 10. Benzylchlorocarbene (10a) and its ct,a-d2 analogue (10b) were observed by UV or IR (10b) spectroscopy, and their decay to styrenes 11 and 12 could be monitored. Tunneling in these 1,2-H(D) shifts was indicated by (a) much higher rates of carbene decay at 10 K than could be anticipated from extrapolation of the 298 K LFP kinetic data, (b) a kinetic isotope effect (KIE) for the 1,2-H(D) shifts estimated at 2000, and (c) little temperature dependence of the rate at low temperature.59 Accepting that QMT is important in the very low temperature H shifts of carbenes 10 and 18, the obvious question becomes is QMT important at higher or even ambient temperatures ... 

Not only primary but also secondary hydrogen isotope effects can be indicative of tunneling. The most frequently employed criteria of tunneling are the temperature dependence of kinetic isotope effects and the isotopic ratio of the pre-exponential factors in Arrhenius plots, but the pre-exponential criterion has been shown to be invalid for small secondary isotope effects. 

Fig. 1 Lewis and Funderburk found that the H/D primary kinetic isotope effects (25 °C in aqueous t-butyl alcohol) for proton abstraction from 2-nitropropane by pyridine derivatives all exceed the maximum isotope effect that could have been derived from the isotopic difference in reactant-state zero-point energies alone (a value around 7). The magnitude of the isotope effect increases with the degree of steric hindrance to reaction presented by the pyridine derivative, the identical results for 2,6-lutidine and 2,4,6-collidine ruling out any role for electronic effects of the substituents. The temperature dependence shown for 2,4,6-collidine is exceedingly anomalous the pre-exponential factor Ahis expected to be near unity but is instead about 1/7, while the value of AH — AH = 3030 cal/mol would have generated an isotope effect at 25 °C of 165 if the pre-exponential factor had indeed been unity.

Fig. 2 Schematic representation of the so-called semiclassical treatment of kinetic isotope effects for hydrogen transfer. All vibrational motions of the reactant state are quantized and all vibrational motions of the transition state except for the reaction coordinate are quantized the reaction coordinate is taken as classical. In the simplest version, only the zero-point levels are considered as occupied and the isotope effect and temperature dependence shown at the bottom are expected. Because the quantization of all stable degrees of freedom is taken into account (thus the zero-point energies and the isotope effects) but the reaction-coordinate degree of freedom for the transition state is considered as classical (thus omitting tunneling), the model is ealled semielassieal.

Kinetic complexity definition, 43 Klinman s approach, 46 Kinetic isotope effects, 28 for 2,4,6-collidine, 31 a-secondary, 35 and coupled motion, 35, 40 in enzyme-catalyzed reactions, 35 as indicators of quantum tunneling, 70 in multistep enzymatic reactions, 44-45 normal temperature dependence, 37 Northrop notation, 45 Northrop s method of calculation, 55 rule of geometric mean, 36 secondary effects and transition state, 37 semiclassical treatment for hydrogen transfer,... 

It is difficult to correlate the kyi/k values with the operation of a hydride, proton or H atom transfer. The temperature dependence of the kinetic isotope effect, which is now easier to measure accurately, is more diagnostic of mechanism but has been applied mainly to organic systems. ... 

The deuterium kinetic isotope effect for intramolecular CH insertion of the nitrene (87), generated by photolysis of the corresponding azide, is 14.7 0.3 at 20 °C and is consistent with the H-abstraction-recombination mechanism from the triplet state. The temperature dependence of the kinetic isotope effect suggests that quanmm mechanical tunnelling is important in this process. 

Observation of the temperature dependence of ku/kj and Td/Tt and kn/ko kinetic isotope effects via Arrhenius plot is very useful in detecting and characterizing proton tunneling ... 

TEMPERATURE CONTROL TEMPERATURE DEPENDENCE ARRHENIUS LAW VAN T HOFF RELATIONSHIP TRANSITION-STATE THEORY TEMPERATURE DEPENDENCE OF KINETIC ISOTOPE EFFECTS... 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

In order to determine whether QMT may contribute to the overall reaction of diarylcarbenes with hydrogen atom donors in solution at ambient temperature, kinetic isotope effects for the benzylic hydrogen atom abstractions of the triplet states of several diarylcarbenes with toluene-toluene-i g in fluid solution were determined over the temperature ranges of —75 to 135 °C. The results are very much dependent on the structure of the carbene (Table 9.11)." The differential... 

From the temperature dependence of the substantial kinetic isotope effect (KIE) observed in the oxidation of diols to hydroxycarbonyl compounds by 2,2/-bipyridinium chlorochromate (BPCC), it is proposed that hydride transfer occurs in a chromate ester intermediate, involving a six-electron Hiickel-type transition state.9 A similar conclusion is drawn for the oxidation of substituted benzyl alcohols by quinolinium chlorochromate.10... 

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