Magnesium isotope effect

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

Magnesium bis(hexamethyldisilazide), Mg(HMDS)2, catalyses the enolization of ketones.287 On addition to propiophenone in toluene at ambient temperature, a ca 3 1 E Z mixture of enolates (103, R=SiMe3) is formed. These enolates, and an initial ketone complex, have been characterized by NMR, X-ray, IR, and UV-visible spectroscopy and computational studies. Kinetics of tautomerization have been measured, with proton transfer confirmed as rate determining ( hAd = 18.9 at 295 K). The significant temperature dependence of the primary isotope effect is indicative of tunnelling. 

The V(OH)2/Mg(OH)2 gel is believed to be a solid solution with a lattice similar to Cdl2, and the yield of hydrazine reaches a maximum with the magnesium to vanadium ratio in the range 1 5-10 (133). Shilov finds (1) no 14N isotope effect corresponding to the Schrauzer mechanism, (2) that there is no evidence for vanadium IV), (3) that the reduction of allyl alcohol is independent of, and competitive with, dinitrogen reduction, and (4) a different dependence on P 2 from the square dependence claimed. In short, no evidence for diazene, but much more for a direct reduction to hydrazine (136). 

Equations (16) and (17) were derived for the isotopic effects associated with condensation in the idealized limit of no re-evaporation of the condensed material. Such a situation can arise for highly supersaturated systems and then Equation (16) gives the isotopic fractionation of the gas and Equation (17) that of the condensate as a function of the amount condensed. The same result obtains if we consider evaporation when recondensation is negligible, and then Equation (16) applies to the evaporation residue and Equation (17) to the evaporated gas. The magnitude of fractionations given by Equations (16) and (17) are illustrated in Figure 4 in the specific case of magnesium isotopes. The key point is that for systems not in... 

Because magnesium has only three stable isotopes, double-spiking techniques cannot be employed to study magnesium isotopic fractionation by MC-TIMS. However, this is possible using MC-ICP-MS, which yielded clear evidence for isotope fractionation effects for magnesium in Nature [218-220]. However, no such measurements have been conducted so far in humans, although significant isotope fractionation effects similar to those observed for calcium can clearly be expected. 

For the mechanism of the metal-mediated Grignard-type reactions in water, Li previously proposed a carbanion/allylmetal/radical triad, in which the specific mechanism of the reaction is dependent on the metal being used (Fig. 4.6). Recently, mechanistic studies of the aUylation detected secondary deuterium kinetic isotope effects in the metal-mediated aUylation ofbenzaldehyde in aqueous media.The inverse SDKIE observed for the indium and tin cases are consistent with the polar addition mechanism. For magnesium and antimony, normal SDKIE were observed. These were interpreted as single electron transfer processes on metal surface in the magnesium case, or between the allyhnetal and the carbonyl compound in the antimony case. 

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... 

Another isotopic anomaly, discovered in Allende inclusions, concerns magnesium, for which an intrinsically low abundance in these samples makes its isotope ratios sensitive to small effects. Certain of the inclusions show a correlation between 26Mg and 27 Al, indicating an origin of excess 26Mg from radioactive decay of 26 A1 (mean life 1 Myr), the existence of which had previously been postulated as a heat source for meteorite parent bodies (Fig. 3.32). Other short-lived activites that seem to have been alive in the early Solar System are 10Be (mean life 2.2 Myr) from a correlation of 10B with 9Be, and 41Ca (mean life 0.15 Myr) from a correlation of... 

Sverjensky DA, Shock EL, Helgeson HC (1997) Prediction of flie thermodynamic properties of aqueous metal complexes to 1000°C and 5 kb. Geochim Cosmochim Acta 61 1359-1412 Tarutani T, Clayton RN, Mayeda TK (1969) The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim Cosmochim Acta 33 987-996... 

Magnesium is mass fractionated by 2 to 3 %/amu in favor of the heavy isotopes (Wasserburg et al. 1977). This is about 2 times the fractionation of O. When canceling out this mass fractionation by normalizing to the terrestrial Mg/ Mg ratio, a dehcit of Mg of about 3 %o is found in both inclusions. This is generally not believed to be a deficit of radiogenic Mg (from Al) but to some unidentified nucleosynthetic effect on one of the 2 other isotopes of Mg Mg, Mg. 

The interplay of factors leading to the formation of carbonate rocks is extremely complicated, but several avenues of investigation have yielded fruitful results. These approaches include studies of surface energies, isotopic composition and radiochemistry (including the use of carbon-14 to study the rates of carbonate deposition), solubilities under ambient conditions, and the effect of magnesium-calcium ratios in the solutions from which carbonate minerals are precipitated. 

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