Computational Isotope Geochemistry

Rustad, James R.

doi:10.1002/9781118845226.ch5

Cited by 2 publications

(3 citation statements)

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“…For the gaseous monomers, polarization functions on heavy atoms are absolutely necessary, triple-ζ valence splitting is better, and polarization functions on H help match experimental harmonic frequencies better, as indicated by Figure 7. These results largely match those reported by Rustad 69 on a wide range of gaseous molecules, in which additional polarization functions on all atoms improved O isotope fractionation predictions, and triple-ζ Dunning basis sets performed better than double ζ. Diffuse functions on heavy atoms are probably necessary to correctly account for charge transfer in aqueous systems, especially since dissolved inorganic C is mostly anionic at circumneutral pH, but the necessity of placing diffuse functions on H is less clear. 11 The 6-311++G(2d,p) basis set paired with the X3LYP functional reproduced 18/16 α eq (CO 2 (g)−H 2 O(g)) most closely; this combination receives our recommendation for calculations involving aqueous CO 2 and related species.…”

Section: Discussionsupporting

confidence: 87%

“…No systematic method for selecting the best, cheapest method has been developed, but Rustad et al point out that the best fast methods rely on cancellations of errors to achieve near-experimental fractionation factor values. It is not clear that a method with good error cancellation in one scenario will maintain that in another scenario, e.g., when extending from equilibrium to kinetic fractionation factors, which involve atoms far from equilibrium positions, or when extending from O isotopes to C isotopes . This work seeks to suggest a manner to select a good computational method using experimental data other than known fractionation factors.…”

Section: Discussionmentioning

confidence: 99%

“…It is not clear that a method with good error cancellation in one scenario will maintain that in another scenario, e.g., when extending from equilibrium to kinetic fractionation factors, which involve atoms far from equilibrium positions, or when extending from O isotopes to C isotopes. 69 This work seeks to suggest a manner to select a good computational method using experimental data other than known fractionation factors. Application of this method requires the availability of experimental harmonic frequencies, which are not always available, especially for large molecules or intermolecular clusters.…”

Section: Journal Of Chemical Information and Modelingmentioning

confidence: 99%

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Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)

Boettger

Kubicki

2019

J. Chem. Inf. Model.

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Quantum mechanical calculations can be useful in predicting equilibrium isotopic fractionations of geochemical reactions. However, these computational chemistry methods vary widely in their effectiveness in the prediction of various physical observables. Most studies employing the approach known as density functional theory (DFT) to model these observable quantities focus on predictive accuracy for energetics and geometries. In this study, several density functionals are evaluated against experimental bond lengths, harmonic vibrational frequencies, frequency shifts upon isotopic substitution, and 18 O/ 16 O isotopic fractionation between CO 2 (g) and H 2 O(g). Successful prediction of harmonic vibrational frequencies strongly correlates with successful prediction of isotopic fractionation, despite the possible introduction of errors by the harmonic approximation. Harmonic experimental frequencies, not anharmonic ones, must be used when comparing spectra and when predicting isotope fractionation. The B3LYP and X3LYP functionals perform more accurately in the evaluation of both harmonic vibrational frequencies and isotopic fractionation factors using the 6-311+G(d,p) and 6-311++G(2d,p) basis sets, achieving fractionation factor errors of 0.2−0.6‰ at 25 °C out of a total fractionation of 51‰. Error cancellation between vibrational frequencies and the harmonic approximation is crucial to their success. The above combination of exchange−correlation functionals and basis sets also well predicts the vibrational properties of interacting CO 2 and H 2 O molecules, suggesting that they may be applicable to more complex geochemical reactions involving C and O isotopic fractionations.

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Section: Discussionsupporting

confidence: 87%

Section: Discussionmentioning

confidence: 99%

Section: Journal Of Chemical Information and Modelingmentioning

confidence: 99%

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Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)

Boettger

Kubicki

2019

J. Chem. Inf. Model.

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Theoretical isotopic fractionation between structural boron in carbonates and aqueous boric acid and borate ion

Balan

Noireaux

Mavromatis

et al. 2018

Geochimica et Cosmochimica Acta

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International audienceThe 11B/10B ratio in calcite and aragonite is an important proxy of oceanic water pH. However, the physico-chemical mechanisms underpinning this approach are still poorly known. In the present study, we theoretically determine the equilibrium isotopic fractionation properties of structural boron species in calcium carbonates, BO33-, BO2(OH)2- and B(OH)4- anions substituted for carbonate groups, as well as those of B(OH)4- and B(OH)3 species in vacuum. Significant variability of equilibrium isotopic fractionation properties is observed among these structural species which is related to their contrasted coordination state, B-O bond lengths and atomic-scale environment. The isotopic composition of structural boron does not only depend on its coordination number but also on its medium range environment, i.e. farther than its first coordination shell. The isotopic fractionation between aqueous species and their counterparts in vacuum are assessed using previous investigations based on similar quantum-mechanical modeling approaches. At 300K, the equilibrium isotope composition of structural trigonal species is 7 to 15 ‰ lighter than that of aqueous boric acid molecules, whereas substituted tetrahedral borate ions are heavier than their aqueous counterparts by 10 to 13 ‰. Although significant uncertainties are known to affect the theoretical prediction of fractionation factors between solids and solutions, the usually assumed lack of isotopic fractionation during borate incorporation in carbonates is challenged by these theoretical results. The present theoretical equilibrium fractionation factors between structural boron and aqueous species differ from those inferred from experiments which may indicate that isotopic equilibrium, unlike chemical equilibrium, was not reached in most experiments. Further research into the isotopic fractionation processes at the interface between calcium carbonates and aqueous solution as well as long duration experiments aimed at investigating the kinetics of equilibration of boron environment and isotopic composition are therefore required to refine our understanding of boron coprecipitation in carbonates and thus the theory behind the use of boron isotopes as an ocean pH proxy

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Computational Isotope Geochemistry

Cited by 2 publications

References 50 publications

Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)

Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)

Theoretical isotopic fractionation between structural boron in carbonates and aqueous boric acid and borate ion

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Computational Isotope Geochemistry

Cited by 2 publications

References 50 publications

Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO2(g) and H2O(g)

Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO2(g) and H2O(g)

Theoretical isotopic fractionation between structural boron in carbonates and aqueous boric acid and borate ion

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Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)

Evaluating Computational Chemistry Methods for Isotopic Fractionation between CO₂(g) and H₂O(g)