Theoretical estimates of equilibrium chlorine-isotope fractionations

Abstract

Equilibrium chlorine-isotope (^(37)Cl/^(35)Cl) fractionations have been determined by using published vibrational spectra and force-field modeling to calculate reduced partition function ratios for Cl-isotope exchange. Ab initio force fields calculated at the HF/6-31G(d) level are used to estimate unknown vibrational frequencies of ^(37)Cl-bearing molecules, whereas crystalline phases are modeled by published lattice-dynamics models. Calculated fractionations are principally controlled by the oxidation state of Cl and its bond partners. Molecular mass (or the absence of C-H bonds) also appears to play a role in determining relative fractionations among simple Cl-bearing organic species. Molecules and complexes with oxidized Cl (i.e., Cl^0, Cl^+, etc.) will concentrate ^(37)Cl relative to chlorides (substances with Cl^−). At 298 K, ClO_2 (containing Cl^(4+)) and [ClO_4]^− (containing Cl^(7+)) will concentrate ^(37)Cl relative to chlorides by as much as 27‰ and 73‰, respectively, in rough agreement with earlier calculations. Among chlorides, ^(37)Cl will be concentrated in substances where Cl is bonded to +2 cations (i.e., FeCl_2, MnCl_2, micas, and amphiboles) relative to substances where Cl is bonded to +1 cations (such as NaCl) by ∼2 to 3‰ at 298 K; organic molecules with C-Cl bonds will be even richer in ^(37)Cl (∼5 to 9‰ at 298 K). Precipitation experiments, in combination with our results, provide an estimate for Cl-isotope partitioning in brines and suggest that silicates (to the extent that their Cl atoms are associated with nearest-neighbor +2 cations analogous with FeCl_2 and MnCl_2) will have higher ^(37)Cl/^(35)Cl ratios than coexisting brine (by ∼2 to 3‰ at room temperature). Calculated fractionations between HCl and Cl_2, and between brines and such alteration minerals, are in qualitative agreement with both experimental results and systematics observed in natural samples. Our results suggest that Cl-bearing organic molecules will have markedly higher ^(37)Cl/^(35)Cl ratios (by 5.8‰ to 8.5‰ at 295 K) than coexisting aqueous solutions at equilibrium. Predicted fractionations are consistent with the presence of an isotopically heavy reservoir of HCl that is in exchange equilibrium with Cl^−_(aq) in large marine aerosols.

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