Anomalous or Mass-Independent Isotope Effects

Weston, Ralph E.

doi:10.1021/cr9800154

Cited by 105 publications

(127 citation statements)

References 86 publications

Supporting

Mentioning

127

Contrasting

Order By: Relevance

“…[1][2][3][4][5] Our interest in the subject was prompted by studies of ozone whose formation and isotopic effects have been of much recent interest. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] In general, the formation of a molecule AB is described by A + B AB ‫ء‬ , ͑1͒…”

Section: Introductionmentioning

confidence: 99%

On collisional energy transfer in recombination and dissociation reactions: A Wiener–Hopf problem and the effect of a near elastic peak

Zhu

Marcus

2008

The Journal of Chemical Physics

View full text Add to dashboard Cite

The effect of the large impact parameter near-elastic peak of collisional energy transfer for unimolecular dissociation/bimolecular recombination reactions is studied. To this end, the conventional single exponential model, a biexponential model that fits the literature classical trajectory data better, a model with a singularity at zero energy transfer, and the most realistic model, a model with a near-singularity, are fitted to the trajectory data in the literature. The typical effect of the energy transfer on the recombination rate constant is maximal at low pressures and this region is the one studied here. The distribution function for the limiting dissociation rate constant k 0 at low pressures is shown to obey a Wiener-Hopf integral equation and is solved analytically for the first two models and perturbatively for the other two. For the single exponential model, this method yields the trial solution of Troe. The results are applied to the dissociation of O 3 in the presence of argon, for which classical mechanical trajectory data are available. The k 0 's for various models are calculated and compared, the value for the near-singularity model being about ten times larger than that for the first two models. This trend reflects the contribution to the cross section from collisions with larger impact parameter. In the present study of the near-singularity model, it is found that k 0 is not sensitive to reasonable values for the lower bound. Energy transfer values ͗⌬E͘'s are also calculated and compared and can be similarly understood. However, unlike the k 0 values, they are sensitive to the lower bound, and so any comparison of a classical trajectory analysis for ͗⌬E͘'s with the kinetic experimental data needs particular care.

show abstract

Section: Introductionmentioning

confidence: 99%

On collisional energy transfer in recombination and dissociation reactions: A Wiener–Hopf problem and the effect of a near elastic peak

Zhu

Marcus

2008

The Journal of Chemical Physics

View full text Add to dashboard Cite

show abstract

“…56 -58. Ozone formed by photolysis 56 -58 [60][61][62][63][64] and provides insight into data on many different experiments, including those where there is extensive isotopic exchange, [55][56][57][58][59] those where there is none, 65-68 the different pressure effects for each of these conditions, 68 -70 and experiments using oxygen heavily enriched in 17 O and 18 O. [71][72][73][74][75] We apply the concepts here, particularly the symmetry-based effect described for ozone in Refs.…”

Section: Introductionmentioning

confidence: 99%

Mass-independent isotope effect in the earliest processed solids in the solar system: A possible chemical mechanism

Marcus

2004

The Journal of Chemical Physics

135

View full text Add to dashboard Cite

A major constraint is described for a possible chemical origin for the ''mass-independent'' oxygen isotope phenomenon in calcium-aluminum rich inclusions ͑CAIs͒ in meteorites at high temperatures ͑ϳ1500-2000 K͒. A symmetry-based dynamical effect is postulated for O atom-monoxide recombination on the surface of growing CAIs. It is the surface analog of the volume-based effect occurring in a similar phenomenon for ozone in the gas phase ͓Y. Q. Gao, W. C. Chen, and R. A. Marcus, J. Chem. Phys. 117, 1536 ͑2002͒, and references cited therein͔: In the growth of CAI grains an equilibrium is postulated between adsorbed species XO (ads)ϩO (ads) XO 2 * (ads), where XO 2 * (ads) is a vibrationally excited adsorbed dioxide molecule and X can be Si, Al, Ti, or other metals and can be C for minerals less refractory than the CAIs. The surface of a growing grain has an entropic effect of many order of magnitude on the position of this monoxide-dioxide equilibrium relative to its volume-based position by acting as a concentrator. The volume-based effect for ozone in the earlier study is not applicable to gas phase precursors of CAIs, due to the rarity of three-body recombination collisions at very low pressures and because of the high H 2 and H concentration in solar gas, which reduces gaseous O and gaseous dioxides and prevents the latter from acting as storage reservoirs for the two heavier oxygen isotopes. A surface effect yields XO 2 * (ads) that is mass-independently rich in 17 O and 18 O, and yields XO (ads)ϩO (ads) that is mass-independently poor in the two heavier oxygen isotopes. When the XO 2 * (ads) is deactivated by vibrational energy loss to the grain, it has only one subsequent fate, evaporation, and so undergoes no further isotopic fractionation. After evaporation the XO 2 again has only one fate, which is to react rapidly with H and ultimately form O. Some O (ads) used to form the minerals are necessarily in excess of the XO (ads), because of the stoichiometry of the mineral, and modify the fractionation pattern. This effect is incorporated into the mechanistic and mathematical scheme. A merit of this chemical mechanism for the oxygen isotope anomaly is that only one oxygen reservoir is required in the solar nebula. It also does not require a sequestering of intermediate products which could undergo isotopic exchange, hence undoing the original isotopic fractionations. The gas phase source of adsorbed O atoms in this environment is either O or H 2 O. As inferred from data on the evaporation of Mg 2 SiO 4 taken as an example, the source of O (ads) is primarily H 2 O rather than O and is accompanied by the evolution of H 2 . Nonisotopic kinetic experiments can determine more sharply the mechanism of condensed phase growth of these minerals. Laboratory tests are proposed to test the existence of a surface effect on the growing CAI surfaces at these high temperatures.

show abstract

“…A satisfactory theoretical explanation of mass-independent isotope effects, including the prediction of their magnitude, has proved remarkably elusive ever since this phenomenon was first reported (19), with significant progress having been achieved only recently (20,21). Complementary experimental investigations (9,22) hitherto have generally (and necessarily) involved gaseous reactants in electronically excited states, which introduces additional complexity and uncertainties to the system. The thermal decomposition of ozone (23,24) is an exception, involving only groundstate molecular and atomic species.…”

mentioning

confidence: 99%

“…Since the initial discovery of mass-independent isotopic fractionation (19), the role of molecular symmetry in the formation of such anomalies has been the subject of much debate (8,9), culminating in the quantum-level model of Gao and Marcus (21) for ozone formation. We note here that there are qualitative, geometrical similarities between the respective thermal decompositions of ozone and carbonates.…”

mentioning

confidence: 99%

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Miller

Franchi

Thiemens

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

Nearly all chemical processes fractionate 17 O and 18 O in a massdependent way relative to 16 O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen threeisotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (⌬ 17 O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 ؎ 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by ؊0.241 ؎ 0.042‰. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the massindependent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 ؎ 0.0007). These findings of a 17 O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation. Measurements have been conducted of the oxygen tripleisotope composition of calcium oxide, magnesium oxide, and mixed calcium-magnesium oxides formed by thermal decomposition of naturally occurring terrestrial carbonates under high vacuum conditions. The rationale was originally to devise a rapid and simplified procedure, compared with existing methods (1, 2), for high-precision measurements of oxygen isotopic anomalies in meteoritic carbonates. Carbonates were pyrolyzed by using infrared laser heating, with continuous pumping to ensure the removal of CO 2 as produced. Because calcium oxide readily hydrates and also reacts with CO 2 , it was important that samples remained isolated from atmosphere after pyrolysis. For this purpose, 3-5 mg of carbonate (consisting of one or two individual grains, except in the case of fine-grained international reference materials) was thermally decomposed in the same vacuum chamber as used for subsequent fluorination of the residual solid oxide. A 25-W infrared laser, with attenuated beam, provided the heat source. For carbonate decomposition, the temperature was increased gradually from ambient, in a carefully controlled manner, ensuring that the pressure as monitored in the reaction cell did not exceed Ϸ10 Ϫ4 mbar at any stage, until evolution of gas ceased and the residual solid was incandescent. It is difficult to estimate the final temperature, but it is likely to have exceeded 1,000°C. Released volatiles were continually removed during pyrolysis, to minimize the extent of back-reaction between the respective decomposition products. Oxygen triple-isotope analysis ...

show abstract

Anomalous or Mass-Independent Isotope Effects

Cited by 105 publications

References 86 publications

On collisional energy transfer in recombination and dissociation reactions: A Wiener–Hopf problem and the effect of a near elastic peak

On collisional energy transfer in recombination and dissociation reactions: A Wiener–Hopf problem and the effect of a near elastic peak

Mass-independent isotope effect in the earliest processed solids in the solar system: A possible chemical mechanism

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Contact Info

Product

Resources

About