Isotope Effects, Comments

Wolfsberg, M. Comments on selected topics in isotope theoretical chemistry. In Kohen, A. and Limbach, H. H., Eds. Isotope Effects in Chemistry and Biology, CRC Press/Taylor Francis, Boca Raton, FL (2006), Chapter 3, p. 89. 

Van Hook, W. A. and Wolfsberg, M., Comments on H/D isotope effects on polarizabilities. Correlation with virial coefficient, molar volume and electronic second moment isotope effects. Z Naturforsch. 49A, 563 (1994)... 

Lipscomb has commented that glutamic acid-245 might act either as a general base or a nucleophile. The available mechanistic information has been reviewed by Kaiser and Kaiser (1972), who postulated that the carboxylate anion of glutamic acid-245 acts as a nucleophile forming an anhydride intermediate [equation (29)]. The divergent D2O solvent isotope effects, =... 

The intramolecular kinetic isotope effect determined in reaction of BTNO with p-MeO-C6H4CH(D)0H gave a h/ d ratio of 5.6 in MeCN , consistent with a rate-determining H-abstraction step. Additional determinations gave a h/ d of 7 with PhCH(D)OH, and 12 for the intermolecular competition of fluorene vs. 9,9-dideuteriofluorene. The latter value supports the contribution of tunnelling already commented on for reaction of PINO with various C—H donors ( h/ d values in the 11-27 range) . ... 

Kaiser, E. W and T. J. Wallington, Comment on Inverse Kinetic Isotope Effect in the Reaction of Atomic Chlorine with C2H4 and C2D4, J. Phys. Chem. A, 102, 6054-6055 (1998). 

It now remains to comment on the change in trQ2 for cyclopropane updn perdeuteration, for which the data are shown in Table XI. The observed isotope effect is much smaller in this case than for propane or butane. This reduced isotope effect can be readily explained in terms of the... 

Figure 5 shows the EPR spectra and intensity for the x=0.06 sample. It is seen that in contrast to the x <0.06 samples at x=0.06 only a single EPR line is observed and the temperature dependence of the signal intensity recovers an usual Curie-Weiss behavior. On the other hand there is still a substantial isotope effect on the EPR line. To understand the change of the EPR spectra at x=0.06, one should first comment on the observability of the phase separation in our EPR experiments. The main difference of the EPR signals from the hole-rich and hole-poor regions is the spin relaxation rate of the Cu... 

There are two levels of assumption in the internal reference method. The more basic is that there is not complete assurance that the rate coefficients of the reference reaction are the same in the differently iso topically labelled molecules. This comment is particularly pertinent in the case of metastable ions, where it has been shown that secondary isotope effects can be very large (> 10) (see Sect. 7.5.4). 

The comment has been made that, in general, isotope effects on hydrogen rearrangements of molecular ions of alcohols [338, 340] and ketones are lower than might be expected, given that isotope effects... 

Some physical insight into how this cancellation arises is considered next. In particular, we comment on why the dramatic and large exit channel mass-dependent isotope effect observed [18] under "unscrambled" experimental conditions is not relevant when the experimental conditions are, instead, the usual ones... 

An experiment that may be simpler in its reactions, more sensitive than the detection limit of O isotope exchange need to explain the discrepancy, and as free as possible from OH vibrational excitation, would be helpful to explore the issue further. On the other hand the isotope effect is very small and the system sufficiently complicated that the theoretical calculations themselves may be the source of the discrepancy. In either case we infer that the reaction itself is mass anomalous rather than mass-independent in the strict sense of the term, both experimentally and in terms of current theoretical insight. In concluding this section we comment in physical terms on the small isotope effects, about —5 to —10 per mil, observed for and in this system at low pressures. 

The isotope effects of particular interest in this chapter are those that arise from substitution of protium, deuterium, or tritium at one or two hydrogenic sites as described in Fig. 11.2. With this many isotopic labels, a few comments about nota-... 

Many isotope effects due to substitution of deuterium for hydrogen at positions g- or more remote from the reaction center have been observed. (18) There is only space here to refer the reader to more detaile3"summaries or to the original literature and to comment that each of the "big three" mechanisms of intramolecular interaction between reaction and isotopic centers has been shown in one case or another to cause isotope effects 1) Hy-perconjugative electron release from a C-H bond causes normal... 

HYNES - You emphasized that the pH depends strongly on the Zundel hydrogen bond polarizability. Since that might depend on proton (presumably quantum) motion in the bond there might be a big (solvent) isotope effect. Can you comment on this and also indicate what is known experimentally for the H O/D O pH ratio ... 

Regardless of the comments above, we need to speak about nuclear spins, since they are everywhere (at least in the form of protons) and because they also can be connected with quantum effects in bio-systems. Recent experiments (Buchachenko and Kouznetsov 2008 Buchachenko et al. 2005) demonstrate that intramitochondrial nucleotide phosphorylation is a nuclear spin controlled process because the magnetic magnesium isotope Mg(ll) increases the rate of mitochondrial ATP synthesis in comparison with the spinless nonmagnetic Mg(ll), Mg(lI) ions. Such nuclear spin isotope effect is usually interpreted in terms of radical pair theory for separated spins in solvent cage (Buchachenko 1977), but an alternative explanation based on... 

To answer these questions, comment the results obtained after a complete Kinetic Isotope Effect (KIE) study on the hydrolysis reactions of methyl D-xylopyra-nosides 1 and methyl D-thioxylopyranosides 2 (both anomers in each case). 

A.J.T. Jull, DJ. Donahue and P.E. Damon, Factors affecting the apparent radiocarbon age of textiles a comment on Effects of fires and biofractionation of carbon isotopes on results of radiocarbon dating of old textiles the Shroud of Turin , by D.A. Kouznetsov et al., J. Archaeol. Sci. 23, 157 160 (1996) D.A. Kouznetsov, A.A. Ivanov and P.R. Veletsky, Effects of fires and biofractionation of carbon isotopes on results of radiocarbon dating of old textiles the Shroud of Turin, J. Archaeol Sci. 23, 109 121 (1996). 

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