Isotopes effects

If isotopic effects on the differential Franck-Condon factors are neglected, it is indeed found that the hydride is four times more strongly predissociated 

The isotope effect on differential Franck-Condon factors has been investigated by Child (1974) (see Fig. 7.29), and it can be predicted that the absolute maximum value of T varies roughly as /x1/6. This dependence for the magnitude of T is weaker and in the opposite sense to the n 2 dependence for gyroscopic predissociations. The oscillation frequency of r(i ) versus E is also sensitive to the reduced mass. Since the phase difference, (f (EVyj) [Eq. (7.6.10)] increases approximately in proportion to the area, 

Anomalous isotope effects occur at accidental or indirect predissodations, which are discussed in Section 7.13. The accidentally predissociated v, J-level is perturbed by a v, 7-level that is directly predissociated by a third (unbound) state. The accidentally predissociated level, having acquired an admixture of the perturber s wavefunction, borrows part of the characteristics of the perturber, 

Another type of anomalous isotope effect, which is observed in the Nj C2X+ state, is discussed in the following section. 

Most elements are composed of more than one naturally occurring isotope, i.e., having nuclei of the same atomic number but different mass numbers due to different numbers of neutrons. [64] The mass number of an isotope is given as a superscript preceding the element symbol, e.g., and (D) or and C (Chap. 3). 

Obviously, mass spectrometry is ideally suited for distinguishing between isotopic species, and isotopic labeling is used for mechanistic as well as analytical applications (Chap. 3.2.9). However, the effect of isotopic substitution is not only an effect on ionic mass, but isotopic substitution can have several simultaneous effects, and this complication sometimes produces results which are at first sight curious. [65] 

Any effect exerted by the introduction of isotopes are termed isotope effects. Isotope effects can be intermolecular, e.g., upon D loss from CD4 versus H loss from CH4, or intramolecular, e.g., upon H loss versus D loss from CH2D2.  

Up to now in our discussion of radiotracers, we have assumed that all isotopes of a given element, stable or radioactive, would behave alike chemically and physically. We will now examine this point more critically to see how different isotopes behave and how this difference in behavior (the isotope effect) can be detected and used to 

Some examples of physical isotope effects follow  

Distillation For a given temperature, the velocity of a light isotope will be greater than that of a heavy isotope, so that the lighter isotope will have a greater vapor pressure. 

Chemical isotope effects are divided into two classes—those affecting the position of the equilibrium in a chemical reaction and those affecting the rate of a chemical reaction. Equilibrium isotope effects have their origin in the fact that the extent to which any chemical reaction goes is governed by the number of possible ways it can proceed (the phase space available). The more equally probable reaction paths available, the more likely the reaction will go. To illustrate this point, consider the exchange reaction 

Kinetic isotope effects are very important in the study of chemical reaction mechanisms. The substitution of a labeled atom for an unlabeled one in a molecule will cause a change in reaction rate for Z 10, and this change can be used to deduce the reaction mechanism. The change in reaction rate due to changes in the masses of the reacting species is due to differences in vibrational frequency along the reaction coordinate in the transition state or activated complex. 

The kinetic isotope effect, a change of rate that occurs upon isotopic substitution, is a widely used tool for elucidating reaction mechanism.48 The most common isotopic substitution is D for H, although isotope effects for heavier atoms have been measured. Our discussion will be in terms of hydrogen isotope effects the same principles apply to other atoms. 

To a good approximation, substitution of one isotope for another does not alter the potential energy surface. The electronic structure, and thus all binding forces, remain the same. All differences are attributable solely to the change in mass, which manifests itself primarily in the frequencies of vibrational modes. For a hypothetical model of a small mass m attached to a much larger mass by a spring of force constant k, the classical vibrational frequency is given by 49 

48 For general treatments of the isotope effect, see (a) K. B. Wiberg, Physical Organic Chemistry, Wiley, New York, 1964, p. 273 and p. 351 (b) L. Melander, Isotope Effects on Reaction Rates, Ronald Press, New York, 1960 (c) F. H. Westheimer, Chem. Rev., 61, 265 (1961) (d) J. Bigeleisen and M. Wolfs-berg, Adoan. Chem. Phys., 1, 15 (1958), (e) C. J. Collins and N. S. Bowman, Eds., Isotope Effects in Chemical Reactions, ACS Monograph 167, Van Nostrand Reinhold, New York, 1970. 

48 If the two masses joined by the spring are comparable, m in Equation 2.67 must be replaced by the 

The quantum mechanical treatment of the same model leads to energy levels 

One of the subtlest cases of social isomerism was encountered with the isotopic substitution of one of the ends of an encapsulated molecule. Equilibrium isotope effects are somewhat of a rarity in molecular recognition but are not insignificant [29, 30]. Deuterium substitution of C-H bonds in molecules tends to make behave as 

The above studies still left open the possibility of two steps that could be rate determining alkene coordination or insertion in the rhodium hydride bond. To this end, the rate-determining step in the hydroformylation of 1-octene, catalyzed by the rhodium-xantphos catalyst system, was determined using a combination of experimentally determined H/ H and kinetic isotope effects and a theoretical 

Phosphine platinum complexes give active hydroformylation catalysts and both terminal and internal alkenes can be hydroformylated by selectively employing platinum-diphosphine complexes, often activated by an excess of tin chloride as the cocatalyst [25,26]. The combination of platinum chloride andtin(II) chloride leads to the formation of the trichlorostannate anion, which presumably acts as a weak coordinating anion, as tin-free catalyst systems have also been reported [27]. The group of Vogt found that the preformation of the catalyst also proved to be effective with only one equivalent of the tin source [28]. 

These systems have mainly been applied to asymmetric hydroformylation [29], although their strength in normal alkene hydroformylation rests in their high selectivity for linear aldehyde. 

Hess and Tully [8] have looked at the effect of deuterium substitution on reactions (15) and (16). 

In the experiments on reaction (10), described in detail in Section 2.3.2, Tully etal. [7] investigated the reaction of OH with deuterated and partially deuterated ethanes 

Ratio of rate coefficients for the isotopic reactions of OH + CH3OH and OH + CDjOH as a function of temperature. 

It was found that the temperature data for reactions (10) and (18) could be represented by modified Arrhenius equations with identical pre-exponential factors. In its thermodynamic formulation TST defines a bi-molecular rate coefficient as 

The neglect of pre-exponential factors in isotope analyses of activation energies is only valid if the entropy changes on forming the TS or the temperature dependence of the entropy change is identical for the two 

A study of the tautomerism of dehydroacetic acid using (solution and 

CP/MAS), gated H-decoupling techniques, deuterium-induced isotope effects chemical shifts, and molecular modeling has shown that this 

A review with 116 references was given. Isotope effects on chemical shifts, nA C(D), nA H(D), 1A N(D) and 1A C( 0), and solvent isotope effects in proteins are reviewed and references are provided to related cases. 

If the reaction A -i- BC AB + C proceeds by spectator stripping, the internal excitation energy of the product AB ion is given by 

As applied to reaction (87), the equation indicates that there should be an isotope effect of a factor of about 2 in the critical energies for H and D atom transfer (M, = 1 and 2, respectively, and Mq). Thus, the calculation shows that ArH and ArD should be unstable at about 164 eV (LAB) and 84 eV (LAB), respectively, and should have cross-sections tending towards zero. This difference in the stability of the isotopic product ions was first considered to be responsible for the rapid increase in the isotopic ratio ArH /ArD at higher incident energies [87]. 

In order to confirm the existence of the critical energy and hence the instability of the products above this energy, Henglein and co-workers [27, 91, 92] extrapolated the cross-section versus energy curve to zero cross-section for the reactions Ar + Hj and Ar + D2 (and also for the corresponding reactions of Nj and CO ). As expected the cross-sections 

These effects have been investigated in detail, the former by Light and Chan [124] and the latter by George and Suplinskas [123]. In Light and Chan s treatment, the reaction radius rA nc of Herman et al. [103] is 

A schematic diagram of the surface is shown in Fig. 15. From the centre-of-mass of the HD molecule, a cone of acceptance for the H isotope 

Replacement of hydrogen by deuterium or tritium causes all of these bands to shift toward the infrared, as predicted by Eq. 10.11. Hydrogen 

Glass Absorbing Species Wavelength (cm ) Extinction Coeff. (L mob cmr ) 

As illustrated above, nonadiabatic dynamics exhibits vividly how electrons move in and between molecules. Complex natural orbitals, in particular SONO in the present case, clearly illustrate how the electronic wavefunction evolves in time. In addition to the time scale, the driving mechanism for the electron migration has also been illustrated. By clarifying such complex electron behavior, not available using stationary-state quantum chemistry, our understanding of realistic chemical reactions is greatly enhanced. As a result, it has clearly been shown that nonadiabatic electron wavepacket theory is invaluable in the analysis of non-rigid and mobile electronic states of molecular systems. 

Note The importance of KER measurements results from the fact that the potential energy surface between transition state and products of a reaction can be reconstmcted [55]. Thus, KER and AE data are conplementary in determining the energy of the transition state. 

The observed KER consists of two components, one from Eex and one from This splitting becomes obvious from the fact that even if there is no Eq, a small KER is always observed, thus demonstrating partitioning of Eex between ,a, and Etrans (KER)  

Diatomic molecular ion dissociations represent the only case with clear energy partitioning, as all excess energy of the decomposition has to be converted to translational energy of the products (Egx = For polyatomic ions the parti- 

Consequently, Etrms becomes rather small for substantial values of s, e.g., 0.3 eV/(0.44 x 30) = 0.023 eV. Therefore, any observed KER in excess of Etrans must originate from Eor being the only alternative source [61]. The analogous partitioning of Eor is described by  

Note In practice, most of the observed KERs (except at 50 meV), can be attributed to Etrans frotn [61], and as a rule of thumb, E 0.33 E  

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Rabinovitch B S and Setser D W 1964 Unimolecular decomposition and some isotope effects of simple alkanes and alkyl radicals Adv. Photochem. 3 1-82... 

Limbach H H 1991 Dynamic NMR spectroscopy in the presence of kinetic hydrogen/deuterium isotope effects NMR Basic Principles and Progress vol 23, ed P Diehl, E Fluck, H Gunther, R Kosfeld and J Seelig (Berlin ... 

Study of secondary D-isotope effects have indicated a highly symmetrical T.S. 

It is clear, then, that the measurement of primary kinetic isotope effects will not give a wholly unambiguous clue to mechanism in the absence of other evidence. Nevertheless, the absence of a kinetic isotope effect is most easily understood in terms of the /S 2 mechanism... 

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... 

One way in which the step of the reaction in which the proton is lost might be slowed down, and perhaps made kinetically important (with i), would be to carry out nitration at high acidities. Nitration of pentadeuteronitrobenzene in 97-4% sulphuric acid failed to reveal such an effect. In fact, nitrations under a variety of conditions fail to show a kinetic isotope effect. 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

Melander, L. (a) (i960). Isotope Effects on Reaction Rates. New York Ronald Press. (6) (1950). Ark. Kemi 2, 211. 

The occurrence of a hydrogen isotope effect in an electrophilic substitution will certainly render nugatory any attempt to relate the reactivity of the electrophile with the effects of substituents. Such a situation occurs in mercuration in which the large isotope effect = 6) has been attributed to the weakness of the carbon-mercury bond relative to the carbon-hydrogen bond. The following scheme has been formulated for the reaction, and the occurrence of the isotope effect indicates that the magnitudes of A j and are comparable ... 

However, the existence of the Wheland intermediate is not demanded by the evidence, for if the attack of the electrophile and the loss of the proton were synchronous an isotope effect would also be expected. The... 

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). 

A primary isotope effect /ch/ d of 6.4 (extrapolated for 35 C) is observed for the metalation and the methylation of 171b when the C-5 position is deuterated. This value is in excellent agreement with the primary isotope effect of 6.6 reported for the metalation of thiophene (392) and it confirms that the rate-determining step is the abstraction by the base of the acidic proton. 

An isotopic effect (H or D) has been demonstrated when starting from 2-methyl-4-phenylthiazole or from 2-methyl-4-phenyl-5-D-thiazole (224) in the dimerisation reaction. 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Kinetic isotope effects are an important factor in the biology of deuterium. Isotopic fractionation of hydrogen and deuterium in plants occurs in photosynthesis. The lighter isotope is preferentially incorporated from water into carbohydrates and tipids formed by photosynthesis. Hydrogen isotopic fractionation has thus become a valuable tool in the elucidation of plant biosynthetic pathways (42,43). 

Isotope Effects on Superconductivity. Substitution of hydrogen by deuterium affects the superconducting transition temperature of palladium hydride [26929-60-2] PdH2 (54,55), palladium silver hydride, Pd Ag H D ( 6), and vanadium—2itconium—hydride, N(57). 

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). 

C. J. Collins and N. S. Bowman, eds.. Isotope Effects in Chemical Reactions, American ChemicalSociety Monograph 167, Van Nostrand Reinhold Co., New York, 1971. 

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

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