Significant bromine (Br) isotope composition variations are found in natural salts and brines, which are often even larger than those of co-existing chlorine isotope compositions. The cause of such large Br isotope variations remains elusive. In this study, equilibrium Br isotope fractionations among Br-bearing gaseous molecules, aqueous species and crystalline minerals are provided via the density functional theory (DFT) based first-principles calculations. These fractionation factors form the base for future Br isotope geochemistry studies. Specifically, the first-principles molecular dynamics (FPMD) method is used to evaluate the tricky solvation effects, i.e., dozens of snapshots of FPMD trajectories are selected and re-optimized to their lowest energy atomic coordinates, then the reduced partition function ratios (i.e., β factors) of aqueous Br-bearing species are obtained by computing the average values of those snapshots. In this way, the configurational effects of aqueous Br-bearing species are included. For crystalline compounds, the static first-principles periodic DFT methods are used. We find that the β factors generally increase as the oxidation states of Br increase, indicating that heavy Br isotopes tend to be enriched in substances with higher Br oxidation states. The aqueous Br-bearing species in brines have detectable but small Br isotope fractionations with each other. However, the fractionations between gaseous molecules and NaBr_(aq) are significantly larger, e.g., Δ⁸¹Br_CH3Br(gas)-_NaBr(aq) is 0.95 ± 0.041‰ at 20 °C. As for minerals, the fractionation between Br-halite_(p) (pure NaBr crystal) and NaBr_(aq) is close to zero, but for Br-halite_(1/124) (Na₁₂₅Cl₁₂₄Br₁) and NaBr_(aq), the fractionation is 0.206 ± 0.041‰. Similar results are found for the fractionations between Br-sylvite, Br-bischofite and Br-bearing solutions, indicating that Br isotope compositions are closely related to Br/Cl ratios in minerals.

By using the calculated isotope fractionation factors, we have modeled the Br isotope composition variations during salt precipitation processes. At 20 °C, the total variations of Br isotopes of salts and brines are found to be −0.45‰ to +0.21‰ and −0.39‰ to 0‰, respectively. They are much smaller than the observed Br isotope variations in natural samples. There must be other ways to fractionate Br isotopes to the extent that observed in nature. Based on the predicted equilibrium Br isotope fractionations among substances with different Br oxidation states, the large Br isotope variations in nature can be explained by the change of Br oxidation state. The sudden change of Br isotope compositions hence can be used as a new indicator for the change of redox conditions.

在天然盐和盐水中发现了溴（Br）同位素组成的显着变化，它们通常甚至比共存的氯同位素组成更大。如此大的Br同位素变化的原因仍然难以捉摸。在这项研究中，通过基于密度泛函理论（DFT）的第一性原理计算，提供了含溴气体分子，水性物质和晶体矿物之间的平衡溴同位素分馏。这些分馏因子构成了未来Br同位素地球化学研究的基础。具体而言，第一原理分子动力学（FPMD）方法用于评估棘手的溶剂化效果，即，选择了几十个FPMD轨迹快照并重新优化到其最低能量原子坐标，然后降低了分配函数比（即，通过计算这些快照的平均值，可以得出含Br的水物种的β因子。以这种方式，包括了含水的含Br物质的构型效应。对于结晶化合物，使用静态第一性原理定期DFT方法。我们发现，β因子通常随着Br的氧化态的增加而增加，这表明重质Br同位素往往富含具有更高Br氧化态的物质。盐水中的含溴水物种彼此之间具有可检测的但较小的Br同位素分馏。但是，气态分子与NaBr之间的分馏 使用静态第一性原理定期DFT方法。我们发现，β因子通常随着Br的氧化态的增加而增加，这表明重质Br同位素往往富含具有更高Br氧化态的物质。盐水中的含溴水物种彼此之间具有可检测到的但较小的Br同位素分馏。但是，气态分子与NaBr之间的分馏 使用静态第一性原理定期DFT方法。我们发现，β因子通常随着Br的氧化态的增加而增加，这表明重质Br同位素倾向于富含具有更高Br氧化态的物质。盐水中的含溴水物种彼此之间具有可检测到的但较小的Br同位素分馏。但是，气态分子与NaBr之间的分馏_{（水溶液）}是显著更大，例如，Δ ⁸¹溴_{CH 3 Br（上气）} -_{的NaBr（水溶液）}为0.95±0.041‰在20℃。对于矿物，Br-卤石_（p）（纯NaBr晶体）和NaBr _（aq）之间的分馏接近于零，但是对于Br-_{卤石（1/124）}（Na ₁₂₅ Cl ₁₂₄ Br ₁）和NaBr _{（aq） ）}，分馏为0.206±0.041‰。在溴-钾铝榴石，溴-亚硫酸锂和含溴溶液之间的分馏中发现了相似的结果，表明溴同位素组成与矿物中的溴/氯比密切相关。

通过使用计算出的同位素分馏因子，我们对盐析出过程中的Br同位素组成变化进行了建模。在20°C，盐和盐水的Br同位素总变化分别为-0.45‰至+ 0.21‰和-0.39‰至0‰。它们比在自然样品中观察到的Br同位素变化小得多。必须有其他方法可以将Br同位素分级到自然界所能观察到的程度。根据具有不同氧化态的物质之间平衡的同位素同位素分馏预测值，可以用氧化态的变化来解释自然界中较大的同位素变化。因此，Br同位素组成的突然变化可以用作氧化还原条件变化的新指标。