Bromine isotope effects: Predictions and measurements

Journal Pre-proof Bromine Isotope Effects: Predictions and Measurements

Faina Gelman, Agnieszka Dybala-Defratyka PII:

S0045-6535(19)32987-X

DOI:

https://doi.org/10.1016/j.chemosphere.2019.125746

Reference:

CHEM 125746

To appear in:

Chemosphere

Received Date:

10 September 2019

Accepted Date:

23 December 2019

Please cite this article as: Faina Gelman, Agnieszka Dybala-Defratyka, Bromine Isotope Effects: Predictions and Measurements, Chemosphere (2019), https://doi.org/10.1016/j.chemosphere. 2019.125746

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Journal Pre-proof

Bromine Isotope Effects: Predictions and Measurements Faina Gelmana and Agnieszka Dybala-Defratykab

a Geological b Institute

Survey of Israel, Jerusalem, Israel

of Applied Radiation Chemistry, Faculty of Chemistry, Lodz University of Technology, Lodz, Poland

1

Journal Pre-proof Abstract Brominated organic compounds (BOCs), abundant in Nature, originate from its own sources or anthropogenic activity. Many of these compounds are harmful and constitute a serious threat, therefore it is important to study and understand their behavior and fate. In situ, BOCs undergo various chemical and biochemical reactions through distinctive mechanistic pathways. However, breaking C-Br specific bond is a crucial step in the transformation of brominated organic compounds. Understanding the mechanisms of debromination can be substantially enhanced by studying Br isotope effects. In this Mini-review we provide overlook of existing experimental techniques for Br isotope analysis, discuss Br kinetic isotope effects measured for selected chemical and biochemical reactions in the light of underlying reaction mechanisms, and review the outcome from computational study of performed to provide more insightful interpretation of observed findings.

Keywords: isotope effect, bromine isotopes, reaction mechanism, bromine isotopic ratio measurements

1. Introduction One of the useful research tools for exploring the mechanism of organic reactions is kinetic isotope effect approach. Although the field of utilizing chlorine kinetic isotope effects (KIEs) for the elucidation of mechanisms of chemical and biochemical reactions seems to be already established with respect to analytical techniques as well as the library of known pathways, the application of bromine isotope effects for mechanistic investigations is still on the initial stages. Works on the methodology of bromine stable isotope determination had started as early as the 20s of XX century, but only last two decades have brought significant 2

Journal Pre-proof development allowing for precise, quick and yet requiring small amount of sample measurements of bromine isotopic ratios (Du et al. 2013). This state of the matters led to an increased number of studied transformations involving brominated organic species ranging from physical processes, through chemical changes and ending at microbiological degradation. One of the reasons for the increased number of studies on bromine isotope effect in organic reactions is the fact that brominated organic compounds (BOCs) are abundant in the environment (Neilson, 2003; Putshev 2003). They originate from natural sources, e.g. the marine habitats (Gribble, 1999; 2000) or anthropogenic activity focused on the production of flame retardants, pesticides, fungicides in the wood preservative process, and germicides (Ezechias et al. 2014). Many of these chemical compounds are harmful and constitute a serious environmental threat, therefore it is important to study and understand their behavior and fate. In situ, BOCs undergo various chemical and biochemical transformations through distinctive mechanistic pathways. However, cleavage of the carbon-bromine specific bond seems to be a significant step in the transformation of brominated organic compounds leading to the formation of dehalogenated product. Understanding the mechanisms of chemical and biochemical debromination can be substantially enhanced by studies of Br isotope effects. However, very often due to the complexity of transformations which brominated compounds undergo in the natural environment and different environmental conditions, one experimental technique is not sufficient and additional approaches to study the fate of brominated species are sought. One of them is computational chemistry which with its predictive power has already settled its prominent position in exploring mechanisms of various chemical and biochemical processes along with determination of isotope effects using stable isotopes of elements like C, N - the most frequently, S, H, and halogens - Cl (Kohen and Limbach, 2006; Harris and Anderson, 2017) and Br more recently.

3

Journal Pre-proof In the present review we would like to discuss reports on Br isotope effects studied so far in chemical and biological systems as well as in physical processes. We also provide overlook of existing experimental techniques for bromine isotope analysis along with their feasibilities and major drawbacks. Modern computational chemistry tools for prediction of Br-KIEs in chemical and physical processes are also reviewed. 2. Measurements The first attempts to analyze bromine isotopes were made by Aston (1920). During the next three decades, more studies on mass spectrometric determination of bromine isotopic abundances have been reported in the literature (Blewett, 1936; Williams and Yuster, 1946; Cameron et al. 1956). Through this period gradual improvement of analytical precision from up to ±4‰ was achieved, however the reported absolute values for

79Br/81Br

were

inconsistent. In 1964 Catanzaro (1964) measured

79Br/81Br

values in commercial and natural

bromides by thermal emission mass spectrometer using standards of known isotopic composition for mass bias correction. No significant isotopic variations within the method uncertainty (±2‰) among analyzed commercial and natural bromides were observed (Cantanzaro et al. 1964). Later, Xiao et al. (1993) reported determination of the bromine isotopic ratio based on the positive thermal ionization mass spectrometry (TIMS) of the Cs2Br+ ion. The analyzed bromides were converted first to HBr by cation ion exchange and then neutralized with Cs2CO3 solution to form CsBr which was deposited on the filament. The intensity of Cs2Br+ emitted from CsBr was enhanced by the addition of graphite to the filament. High mass of Cs2Br+ ion and low and reproducible mass spectrometric discrimination effect resulted in

4

Journal Pre-proof precision of 0.11‰. Significant variance of bromine isotopic composition has been observed in the commercial bromides of different origin. Isotope analysis of bromine in organic compounds was for the first time reported in 1978 by Willey and Taylor (1978). Bromine isotope composition in methyl bromide was performed by dual inlet – Isotope Ratio Mass Spectrometry (DI-IRMS). Isotopic composition of the sample was determined as a relative difference between the measured ratios for the standard and sample gases. The relative standard deviation of measured ratios for methyl bromide of 7 parts in 106 was reported. The analytical approach proposed by Willey and Taylor was later adapted for analysis of inorganic bromides by Eggenkamp and Coleman (2000). The whole method consisting of chemical transformation of inorganic bromides to methyl bromide, off-line gas chromatographic separation of methyl bromide and DI- IRMS analysis resulted in the total precision better than 0.18‰ (1sd) (Eggenkamp and Coleman, 2000). Chemical conversion of inorganic bromides to methyl bromide comprised of bromide precipitation as AgBr following by its reaction with methyl iodide. For natural samples containing large amounts of chlorides along with bromides, separation of Br- from Cl- was carried out by chemical oxidation with potassium dichromate. Isotope ratio of inorganic bromides was determined relative to isotope composition of Standard Mean Ocean Bromide (SMOB). Further improvement and optimization of bromine isotope analysis in methyl bromide by continuous-flow IRMS (CF-IRMS) was performed by Shouakar-Stash et al. (2005). Although this analytical technique included chemical transformation of Br- to methyl bromide as well, CF-IRMS enabled to analyze much smaller samples (containing as small as 1μmol of Br-) with external precision better than ±0.06‰ (1sd). Additionally, total analysis time for one sample was shortened from 75 min (in the case of DI-IRMS) to 16 min. More possibilities for analysis of organic bromides appeared with introduction of hyphenated Gas Chromatography (GC) / Multi-Collector- Inductively Coupled Plasma- Mass 5

Journal Pre-proof Spectrometer (MC-ICPMS) technique. For the first time, Sylva et al. demonstrated compound-specific bromine isotope analysis in a mixture of brominated organic compounds in 2007 (Sylva et al. 2007). Using GC/MC-ICPMS system for Br isotope ratio analysis in BOC does not require any preliminary chemical conversion of the sample. The analysis includes an on-line separation of the mixture of organic compounds by gas chromatography, following by consequent introduction of individual organic compounds into MC-ICPMS for bromine isotope analysis. Analytical precision around 0.3‰ (1SD) was attained with 0.3 nmol of Br injected into the GC, whereas for samples larger than 0.3 nmol Br, the precision was within a factor of 3 of the shot-noise limit. δ81Br values for the analyzed compounds were determined relative to dibromobenzene (DBB) used as an arbitrary reference standard (δ81Br=0‰ for DBB was supposed). Later, it was suggested to improve analytical precision of GC-MC-ICPMS analysis by applying strontium as an external spike for instrumental mass bias correction (Gelman and Halicz, 2010). This allowed attaining precisions of 0.1‰ (2SD) for the signals resulted from less than 1nmol of Br injected. Holmstrand et al. adapted GC/MC-ICPMS method for analysis of high-boiling point polybrominated aromatic compounds by construction of heated transfer line from GC to MC-ICPMS (2010). Brominated diphenyl ethers (BDEs) in Bromkal 70-5DE, a technical flame-retardant mixture, were used as test substances, using monobromobenzene (MBB) with a known δ81Br composition as an internal standard. Although MBB was analyzed with a precision of 0.4‰ (1SD), the precision for BDEs was in the range 1.4–1.8‰ (1SD). It was hypothesized that the lower precision for the BDEs than for MBB may reflect partial condensation of the analytes in ICP torch assembly. Bromine isotope composition (δ81Br) analysis in methyl bromide by GC/MC-ICPMS was reported by Horst et al. (2011). Sample amounts of >40 ng were measured with a precision of 0.1% (1σ). Using this analytical method δ81Br of atmospheric CH3Br was measured for the first time (Horst et al., 2013).

6

Journal Pre-proof A possibility of hyphenation of ion chromatography (IC) to MC-ICPMS ion-specific 81Br/79Br

ratio analysis was reported by Zakon et al. (2013). In this instrumental setup,

separation of bromine containing anions was performed on-line by IC, followed by introduction of ions into MC-ICPMS for Br isotope ratio analysis. Applying low-resolution mode in MC-ICPMS, precision of ±0.1‰ (1SD) for signals containing down to 0.6 nmol of Br attained. δ81Br values for inorganic bromine samples were reported relative to SMOB using sample-standard bracketing approach for instrumental drift correction. Recently, Wei et al. demonstrated that precision of bromide isotope analysis by MC-ICPMS can be further improved by optimization of optics parameters and using of high-resolution mode (Wei et al. 2015). Novel approach for measuring bromine isotope composition in brominated organic compounds by their on-line conversion into gaseous hydrogen bromide was proposed by Hitzfeld et al. (2011). In this method, quantitative conversion of brominated organic compounds into HBr was achieved by using high-temperature reactor and hydrogen make-up gas. Determination of bromine isotope ratio was suggested to be performed on tracing HBr+ ions with m/z = 80 (H79Br) and m/z = 81 (H80Br). However, the proposed method has not found broad application for bromine isotope analysis, probably, due to the complicity of the proposed analytical system. Recently, Zakon et al. demonstrated a possibility of using GC/qMS for δ81Br analysis in brominated organic compounds (2016). Precisions in the range 0.2–0.3‰ were attained for sample amounts in the range of tens to thousands pmol. Good correlation between the results obtained by GC/qMS and GC/MC-ICPMS for laboratory standard materials was achieved. However, a proper validation of the method is required prior to application since its performance depends on several parameters such as type and amount of the analyte, calculation scheme applied and instrumental parameters. 7

Journal Pre-proof 3. Predictions A detailed description of the transformations of brominated species at the molecular level may provide meaningful insight into the existing knowledge on strategies employed by more complex, environmental systems and leading to the elimination of substituents responsible for the toxicity of organic compounds. Furthermore, such analysis may constitute a ground for an interpretation tool of trends observed during isotopic analysis of environmental samples. In particular when the results coming from isotopic ratio measurements are supported by theoretical predictions using the most advanced methods of computational chemistry. Thus, for chlorine kinetic isotope effects, computational chemistry tools have been shown to provide many important details on mechanisms of reactions catalyzed by dehalogenating enzymes (Lewandowicz et al. 2001; 2002; Soriano et al. 2005; Papajak et al. 2006; Szatkowski et al. 2011; Szatkowski and Dybala-Defratyka, 2013; Manna and Dybala-Defratyka, 2013; Siwek et al. 2013; Schurner et al. 2015). An overview focused on the techniques in Cl KIEs theoretical determination was presented recently by Szatkowski et al. (2017). In principle, very similar approach might be used to model the carbon-bromine cleavage reaction and to predict bromine kinetic isotope effects. Bromine as an element may, however, cause some issues related to its less common presence in various databases, parameters sets, etc. that are frequently used in calculations. Therefore, there might be a need for additional treatment but despite this fact, the rest should be the same. In order to predict any isotope effect on a reaction under study, reliable geometries of the involved species (reactants and products for equilibrium IEs and reactants and transition state for KIEs), and a reliable approximation for the second derivatives of the potential energy with respect to the nuclear coordinates of the system (force constants matrix or Hessian) calculations are needed. Since changes in bonding between the reactants and transition states results in electronic structure change a use of quantum mechanical methods is necessary to 8

Journal Pre-proof describe them. Systems of rather small size (up to 100 atoms) can be easily satisfied with respect to the reasonably accurate level of theory for both geometry optimization and Hessian calculations. The situation becomes, however, a bit complicated when reactions in the condensed phase are of interest, like the processes taking place in enzymatic active sites or in solutions. Accounting for the presence of such environment will result in significant system size increase and will naturally lead to using lower (less accurate) level of theory to describe changes in the electronic structure. Modern computational chemistry is fortunately equipped with several tools allowing to treat such models as accurately as possible even if a compromise between the computational cost and date accuracy is needed. If there is no need to include specific interactions between a solute and a solvent (environment in general) molecules, simplified treatment of the environment is feasible using so-called implicit solvation (condensed phase in general) models or mixed-solvation where only small (the most important) part of the overall system is treated explicitly and the rest is approximated by the charged field mimicking certain conditions (polar, non-polar, etc.). Most commonly, in particular for heavy atom isotope effects estimates certain 'special' effects are neglected. These would include coupling of other than reaction coordinate modes, rotations and translations, anharmonicity (resulted from internal rotations in very flexible molecules, like hydrocarbons and their derivatives, including halogenated ones), quantum effects such as tunneling, recrossing. Of course their contribution is crucial only in some cases, and they do not have to be taken into account every time when isotope effects are to be predicted. Computational chemists apart from some obvious situations, like, for instance hydrogen transfer reactions, usually start from as simple model of a studied process as possible. If it allows for reproducing all experimentally observed KIEs then pretty often researchers remain at the original choice.

9

Journal Pre-proof Experimental measurements due to technical reasons or kinetic complexity of a reaction do not always provide unambiguous information of a given chemical reaction. In order to interpret such data, it is mandatory to understand the relationship between the isotope effect(s) on steps that occur during the conversion of the initial reactant to the final product and the overall isotope effect measured by the method. Isotope effects coming from theoretical calculations, allow for mechanistic elucidation and give detailed information on transition state structure. As each step of a multistep reaction can be treated independently and predicted isotope effects carry position-specific isotopic information, they allow for recognizing the contribution of every single atom of a molecule to the overall calculated effect and prevent erroneous interpretations. Computational chemistry tools can, therefore, provide information on molecular positions and reaction steps which are often experimentally non-accessible. Combination of measurements and calculations for interpretation of Br IEs for a few chemical systems has been recently demonstrated by our research groups (Szatkowski et al. 2013; Zaczek et al. 2017; Manna et al. 2018; Vasquez et al. 2018). We will address the details of these studies in the section devoted to chemical transformations of selected brominated compounds.

4. Classical chemical reactions Experimental determination of bromine isotope effect along with theoretical calculations were performed for a number of classical chemical reactions following different mechanistic pathways. 4.1 Solvolysis of n-butyl and t-butyl bromides in organic solvents

10

Journal Pre-proof One of the first studies aiming at determination of Br KIE is the report by Willey and Taylor (1980) focusing on solvolysis of n-butyl and tert-butyl bromides in organic solvents (mainly methanol). Apart from providing temperature dependence of leaving group Br KIE that study also discusses mechanistic aspects and differentiation between the SN1-E1 and E2 pathways. Within the range of -10 ÷ 40 C Br KIEs on the solvolysis of t-butyl bromide in lithium nitrate/lutidine/methanol and for the reaction of n-butyl bromide with thiophenoxide anion in lithium methoxide/methanol solvent were measured. In general, at each temperature the determined Br KIE for n-butyl bromide was lower than the one obtained for tert-butyl halide (1.0016  1.0019 vs. 1.0030  1.0033, respectively). That finding was treated as an indication that SN2 reactions exhibit less pronounced leaving group KIEs than SN1 reactions. The study was accompanied by discussion on terms contributing to the overall KIE, namely, the tunneling contribution, the reaction coordinate mode contribution (so-called temperature independent factor, TIF), and the vibrational energy contribution (temperature dependent factor, TDF). The performed analysis was quite limited due to the lack of access to certain information, like, for instance vibrational frequencies of transition state by that time but enabled to explain differences in magnitude of Br KIE and ascribe them to different factors. In the SN1 reaction TIF constituted around 70% of the overall effect, whereas in SN2 only 31%. 4.2 Dehydrobromination of 2-bromoethylbenzene Chemical transformation of 2-bromoethylbenzene (2-BEB) promoted by the hydroxyl ion (Scheme 1) was modeled and the outcome from the theoretical approach was compared to the measured magnitudes of isotope effects. (Manna et al. 2018) Specifically the effect of solvent (water, ethanol, acetonitrile) on Br KIEs was explored. No interpretable influence of the solvent on the transition state structure and the predicted KIEs was found. The only product of this reaction is styrene, hence the elimination pathway seemed to dominate the 11

Journal Pre-proof transformation of 2-BEB by the hydroxyl anion (Table 1). A thorough computational analysis that took into account possible conformers of 2-BEB and the transition states of the reaction led to the conclusion that styrene is formed via a concerted E2 mechanism. It was also demonstrated that the C-Br bond is the easiest to break among the tested bonds consisting of other halogens (F, Cl) and the reaction mode responsible for its cleavage along with the hydrogen abstraction from 2-BEB (TIF) by OH- constitutes 68% of the overall Br KIE predicted for this reaction.

H H

H H Br

CH

+ OH-

CH2

+ Br- + H2O

Scheme 1. Dehydrobromination of 2-bromoethylbenzene by OH-.

4.3 Finkelstein reaction Another example of debromination reaction occurring in the SN2 fashion is the Finkelstein reaction which was used to benchmark available electronic structure methods to study its energetics as well as C and Br KIEs (Żaczek et al. 2017). Isotope effects were measured and predicted specifically for the reaction between 2-BEB and the iodide ion in acetone and acetonitrile (Scheme 2). In part, the study was motivated by the need to establish a framework for prediction of Br KIEs for a reaction taking place in the condensed phase. For the purpose of assessing the performance of computational methods, 25 various density functional and ab initio chemistries along with two implicit solvation models, PCM (Miertus et al. 1981) and SMD (Marenich et al. 2009) were used. Different sets of methods that provided the best agreement of both activation parameters and kinetic isotope effects betweeen the experiment and theory were found for the reaction modeled using PCM and 12

Journal Pre-proof SMD methods. In general PCM turned out to be a better choice and along with its use density functionals belonging to the B97 family of density functional (Chai and Head-Gordon, 2008a; 2008b) appeared to agree the most with the experimental data (Table 1). It was demonstrated that neither anharmonic effects nor tunneling should affect Br KIE to any interpretable extent.

H

H

H H

H

Br

+

I

-

H

H

H I

+ Br-

Scheme 2. SN2 reaction between 2-BEB and the iodide ion. 4.4 Grignard reagent

In 2013 Br KIEs measured on Grignard reagent (GR) formation was reported for the first time in the study by Szatkowski et al. (2013). Experimental determination of Br KIEs was accompanied by their theoretical predictions. Additionally within that study the potential of Br isotope analysis was tested with respect to capability of providing reliable bromine isotope effects as it was one of the first measurements on chemical reaction by that time. Following the earlier, general suggestion that GR can be formed either via outer sphere electron transfer within which a tight radical anion pair is formed (so-called radical pathway) and no bond involving carbon and halogen are either cleaved or formed; or inner sphere electron transfer based on a loose radical pair (called non-radical pathway) resulting in the CBr bond cleavage and the Mg-Br bond formation. Thus, one can expect that such simplified picture of the GR formation mechanism will provide neither carbon nor bromine KIE for the former pathway, whereas for the latter scenario will (Vogler et al. 1978) Taking also into account different reactivity of alkyl and aryl bromides two groups of organic halides were considered.

For

experimental

determination

1-bromohexane,

1-bromodecane

and 13

Journal Pre-proof bromobenzene, 1-bromonapthalene were used, respectively. In order to explore two operating mechanistic scenarios different models of reacting species were constructed. For radical pathway models with the magnesium surface omitted were built. They consisted of only bromoethane as a model for alkyl bromides and bromobenzene as a representative for aryl halide. To account for the direct intervention of magnesium atoms in the C-Br bond cleavage, models mimicking the metal surface were included. Measured magnitudes of Br KIEs showed clear distinction between the values obtained for alkyl bromides (1.0005  1.0007) and aryl bromides (1.0026  1.0027). Calculations of KIEs, on the other hand, led to no effect for 1e reduction of ethyl bromide and 1.0038 for bromobenzene. Moreoever, predictions showed that in the case of alkyl bromide radical pathway occurs via concerted fashion whereas in the case of aryl halide it is rather a stepwise process. In contrast, non-radical pathway involving an intimate contact between the magnesium surface and an organic compound for both halides resulted in quite significant Br KIEs ranging from 1.0013 to 1.0034. Findings from both approaches led to the conclusion that in the case of aliphatic halides the GR formation proceeds via the outer-sphere mechanism, whereas in the case of aromatic substrates it should be rather the inner-sphere pathway. 5. Environment-relevant chemical and biological processes Brominated organic compounds (BOCs) are known to have both natural and anthropogenic sources. Environmental fate of some brominated organics is difficult to understand and monitor, since they can undergo different transformation processes and their distribution in aquatic systems is affected not only by chemical/biochemical transformations, but also by some physical processes such as mass-transfer, dilution, and sorption. In analogy to the known isotope fractionation in organochlorides, it can be expected that environmental transformations of organobromides will be accompanied by significant carbon and bromine isotope fractionation. Pretty often isotopic fractionation studies are focused on the destructive 14

Journal Pre-proof chemical/biochemical transformations of the contaminants since they are expected to be accompanied by significant carbon and bromine isotope effects due to the C-Br bond breakage in the rate-limiting step. 5.1 Transformations of brominated aliphatic compounds Ethylene dibromide (EDB) One of the abundant groundwater brominated contaminants, 1,2-dibromoethane (also known as ethylene dibromide, EDB) can be transformed in the environment via different mechanistic

pathways

such

as

nucleophilic

substitution,

dehydrobromination,

dibromoelimination, or radical oxidation depending on reaction conditions. Some of these mechanisms have been recently studied (Kuntze et al., 2016; Schurner et al., 2015). Chemical degradation of EDB can be realized via alkaline hydrolysis, dibromoelimination (e.g. by Zn(0) and reduced corrinoids) as well as oxidative degradation (e.g. via Fenton-like reaction) (Scheme 3). Nucleophilic substitution under alkaline conditions resulted in the apparent C and Br KIE of 1.0301 and 1.0010, respectively. Experimental values were in a good agreement with the values predicted via electronic structure calculations using the mixed model of aqueous solvation (EDB along with attacking OH- and six water molecules placed in the reaction field mimicking the solvent) - C KIE: 1.0413, Br KIE: 1.0012. Dibromoelimination either by Zn(0) or corrinoids led to lower carbon isotope fractionation C KIE (1.0223-1.035) but higher Br KIE (1.0042-1.0079) (Jin et al., 2018). Oxidative debromination (Fenton-like oxidation) resulted in significant carbon isotope fractionation without any detectable bromine isotope effect indicating that C-Br bond cleavage is not a rate limiting step of the reaction (Table 1). Biodegradation of ethylene dibromide by Ancylobacter aquaticus occurring under aerobic conditions, assumed to follow the SN2 nucleophilic substitution pathway accompanied by both carbon and bromine isotope effects (Vogler et al. 1978). These isotope effects were

15

Journal Pre-proof lower than those determined for the chemical

hydrolysis. Higher bromine isotope

fractionation along with significant carbon isotope effect were observed during anaerobic biotransformation of ethylene dibromide with crude extract of Sulfurospirillium multivorans, following dibromoelimination pathway (Vogler et al. 1978). However, the observed isotope effects were still lower than the effects determined during EDB transformation by the reduced corrinoid co-factors resulting in dibromoelimination as well (Table 1).

H A.

C.

H

Br H Br

Br H

Br

H B.

H

H

H

Br

OH H

H

Br H

H

H

H

Br H

H

CO2

Scheme 3. Transformation pathways of BCE: A) Hydrolytic debromination, B) Dibromoelimination; C) Oxidative degradation

Alkaline hydrolysis was also computationally compared to dehydrobromination of EDB (Schurner et al., 2015). The E2 mechanism resulted in slightly lower Br KIE and significantly lower C KIE. Treating those pathways theoretically allowed to discuss differences in located transition states that can be accounted for differences in the predicted isotope effects. Those differences included the carbon-bromine bond elongated to a lesser extent at the E2 transition state, smaller negative charge developed on the bromide leaving group. Interesting approach toward studying mechanism of chemical as well as biotic 16

Journal Pre-proof transformation of EDB has been proposed by Jin et al. (2018). It is based on the reaction mechanisms and the experimental data discussed and generated in the study by Kuntze et al. (2016). Based on validated model the approach was tested for complex reaction systems involving sequential and parallel reactions. For the former scenario a multistep SN2 pathway was used and for the latter the EDB degradation was modeled via two concurrent reactions; SN2 and dehydrobromination. Within the presented model it was possible to predict not only the isotopic behavior of the parent compound but also the one of the intermediate and the end product. Validated model provided different isotopic fractionation of the parent compound and intermediate. Modeling of the parallel reactions also allowed to provide the contribution of each of them to the overall reaction taking place in the aqueous environment. Methyl bromide Methyl bromide is emitted to the atmosphere by both natural and anthropogenic sources and is known as an ozone depleting substance. Stable isotope analysis may be used as a tool for identification and quantification of its emission. In the recent study by Horst et al. (2019) three-dimensional isotope measurements (δ13C, δ2H and δ81Br) were applied for the first time for characterizing major abiotic degradation processes of methyl bromide in oceans. Isotopic enrichment factors (ε) for the isotopes of all three elements were measured for hydrolysis and halide exchange processes following nucleophilic substitution (SN2) pathway. The enrichment factors determined in the study for hydrolysis and halide exchange processes were indistinguishable within the analytical uncertainty (Table 1). Based on that, it was concluded that the two processes cannot be individually characterized and quantified with isotopic methods and both reactions may be included as a combined abiotic degradation process in future isotope-based budget estimates (Horst et al., 2019). Dibromoethene (DBE) and tribromoethene (TBE)

17

Journal Pre-proof Both carbon and bromine isotope effect were observed during reductive debromination of TBE and cis- and trans- DBE by Sulfurospirillum multivorans and Desulfitobacterium hafniense PCE-S strain (Zakon et al. 2013). Although significant difference in carbon isotope fractionation was observed for TBE and DBEs transformation by S. multivorance and D. hafniense PCE-S, bromine isotope fractionation was very similar (Table 1). Tribromoneopentyl alcohol (TBNPA) Yet

another

brominated

compound,

used

as

flame

retardant

additive

-

tribromoneopentyl alcohol (TBNPA) was investigated with respect to its transformation pathway described and quantified via carbon and bromine compound-specific isotope analysis (Kozell et al., 2015). Significant variations in carbon isotope fractionation were observed for SN2 hydrolytic, reductive debromination and oxidative degradation pathways (Scheme 4). The magnitudes of Br KIE obtained within that study ranged from unity for oxidative degradation to 1.0012 for hydrolytic debromination and to 1.0057  0.0003 for reductive debromination pathways (Table 1). Undetectable bromine isotope fractionation along with significant carbon isotope effect were observed in microcosm biodegradation of the brominated flame retardant tribromoneopentylalcohol (TBNPA) by an aerobic bacterial consortium enriched from the polluted groundwater underlying an industrial site in the northern Negev (Israel) (Kuntze et al. 2016). Considering the lack of Br isotope fractionation during the degradation and necessity of oxygen for biodegradation occurrence, authors hypothesized that the C-H bond cleavage is a rate-limiting step of the reaction.

Br

Br C A.

Br

Br OH

C

O

Br

18

Journal Pre-proof Br

Br C B.

Br

Br OH

C Br

OH

Br H CH Br C

C.

CH3

Br

CO2

OH

Scheme 4. Transformation of TBNPA: A) Hydrolytic debromination; B) Reductive debromination; C) Oxidative degradation

5.2 Transformations of brominated phenols Several studies regarding the application of isotope analysis for tracing the transformations of brominated phenols have been reported during the last years. Experiments on degradation of 4-bromophenol (4-BP), 2,4-dibromophenol (2,4-DBP), and 2,4,6tribromophenol (2,4,6-TBP) under anaerobic conditions by a microbial consortium enriched from a contaminated site of the northern Negev (Israel), resulted in stepwise reductive debromination with formation of phenol. The studied biotransformations yielded Br apparent KIE (AKIE) of 1.00078±0.0008, 1.00092±0.00038, and 1.0006±0.00018 for the debromination of 4-BP, 2,4-DBP, and 2,4,6-TBP, respectively (Bernstein et al. 2013) (Scheme 5). Recently, isotope effects during aerobic transformation of 4-BP by Ochrobactrum sp. HI1 microbial strain was reported (Woods et al. 2018). Degradation followed the ring hydroxylation pathway and resulted in a small carbon isotope effect (C AKIE = 1.0066±0.0005) and negligible bromine isotope fractionation (Br KIE = 1.00007±0.0001), suggesting no direct involvement of Br in the rate-limiting step of the reaction. Degradation of 2,4,6-TBP by Achromobacter piechaudii strain TBPZ under aerobic 19

Journal Pre-proof conditions resulted in small carbon and bromine isotope effects (Bernstein et al., 2019). The degradation process utilized by the TBPZ strain was accompanied by stoichiometric release of halogen atoms with no detection of lower halogenated phenols. Although any metabolite has not been detected in this study, it was suggested that the process follow ring hydroxylation pathway (Bernstein et al., 2019). Based on the obtained results, it was suggested that the observed isotope effects do not reflect the ring hydroxylation step but represents rate-limiting steps preceding the catalysis of hydroxylation. The results also indicated that application of dual-element isotope effects for distinguishing between oxidation and reduction of TBP in the environment is questionable. In the work by Zakon et al. (2013) bromine isotopic fractionation during UV-photolysis of brominated phenols were measured for the first time. Specifically light-induced degradation of 2-, 3- and 4-bromophenols (2-BP, 3-BP, and 4-BP, respectively) was studied in ethanol and aqueous solutions. It was found that in ethanol photodegradation of bromophenols is accompanied by inverse bromine isotope effects whereas in aqueous solution the reaction with 2-BP resulted in no effect, with 3-BP and 4-BP again inverse Br KIEs were obtained (Table 1). Instead it was proposed that the observed Br IE should be composed of conventional Br KIE and less conventional mass independent isotope effect (Br MIE) arising from different nuclear magnetic moments of bromine isotopes that allow them to undergo spin conversion however, at different rates and this way leading to Br MIE. Taking all of these into account mechanistic scenarios were proposed for each of the studied bromophenols and reaction conditions. OH

OH Br

Br

Br 2,4,6-TBP

OH Br

OH Br

Br 2,4-DBP

2-BP

Scheme 5. Microbial transformations of brominated phenols under anaerobic conditions 20

Journal Pre-proof 5.3 Source apportionment of brominated organic compounds Bromine isotope composition may serve as a potential diagnostic tool for source apportionment of brominated organic compounds in the environment. Holmstrand et al. (2010) analyzed polybrominated diphenyl ethers (PBDEs) by using standard bracketing with monobromobenzene. The measured 81Br values for BDE-47, BDE-99, and BE-100 in a technical mixture Bromokal were indistinguishable within the analytical uncertainty (-0.12 ± 1.43; -0.70 ± 1.67; -0.65 ± 1.81; transfer line 2). In addition, similar δ81Br values were measured for BDE-47 and methoxy-BDE-47 extracted from whale blubber. Carrizo et al. (2011) reported δ81Br values in the range from -4.3 to -0.4‰ for six industrially synthesized BOCs and +0.2 ± 1.6‰ for the naturally produced dibromophenol. Although δ81Br value for the natural sample was statistically different from the values obtained for four out of six industrially produced chemicals, the source apportionment of BOCs based on δ81Br values seemed questionable. Later Chen et al. (2007) suggested to apply two-dimensional δ13C–δ81Br isotope analysis

for

source

identification

of

polybrominated

flame

retardants.

Tetrabromodiphenylether (BDE-47) and decabromodiphenyl ether (BDE-209) from different suppliers were analyzed for the carbon and bromine isotope composition. All the analyzed samples showed negative δ13C values in the range from -25.81 to -28.26‰. Significantly different δ13C values were observed for three BDE-209 samples, while two analyzed BDE209 samples showed similar δ13C values. Bromine isotope analysis of the congeners revealed that they have relatively narrow range of δ81Br values (from -0.34 to 0.5‰), when negative δ81Br values (from -0.34 to -0.26‰) were detected for three BDE-47 samples and positive δ81Br values (+0.43 and +0.50‰) were observed for two BDE-209 samples. The study

21

Journal Pre-proof indicates that the caution should be taken for using isotopic composition for source identification of polybrominated flame retardants. Table 1. Measured and Predicted Bromine Kinetic Isotope Effects for Reported Transformation Pathways of Studied Brominated Contaminants. Compound EDB

Methyl bromide 2-BEB

2-BEB

n-BuBr t-BuBr TBNPA

Reaction alkaline hydrolysis

Mechanism S N2

Br AKIEexp/theory 1.0010 /1.0012

dehydrobromination

E2

n/d / 1.0008

dibromoelimination by Zn (0)

reduction

Fenton-like (via OH) dibromoelimination by corrinoids

oxidation reduction

Hydrolysis Halide exchange dehydrobromination

S N2

Concerted: 1.0042  0.0006 Stepwise: 1.0021  0.0003 No effect Concerted: 1.0079  0.0008 Stepwise: 1.0039  0.0004 1.00116±0.00042 1.0022±0.00023 Ethanol:1.0010  0.0003 / 1.0011 Acetonitrile: 1.0006  0.0002/ 1.0011 Acetone: 1.0021  0.0004 / 1.0017 Acetonitrile: 1.0013  0.0004/ 1.0016 1.00169 0.00003 1.00310  0.00004 1.0012  0.0003

Finkelstein reaction

solvolysis solvolysis alkaline hydrolysis nZVI in anoxic conditions H2O2 in the presence of nCuO catalyst

anti-E2

S N2

S N2 S N1 Intramolecular nucleophilic substitution Reductive debromination Oxidative C-H cleavage

4-BP 2,4- BP

Kuntze et al. 2016

Horst et al. 2019 Manna et al. 2018

Żaczek et al. 2017 Willey and Taylor, 1980

Kozell et al. 2015

No effect 1.00076±0.00008

Anaerobic degradation by microbial consortium

Reductive debromination

2,4,6-TBP

2-BP

1.0057  0.0003

Reference Manna et al. 2018; Kuntze et al. 2016 Manna et al. 2018; Kuntze et al. 2016

1.00092±0.00038

Bernstein et al. 2013

1.00060±0.00018

photolysis

Radical mechanism/ionic

Ethanol: 0.9979  0.0003/water: no

Woods et al. 2018 22

Journal Pre-proof mechanism Radical mechanism/ionic mechanism Radical mechanism

3-BP

4-BP

Alkyl bromide Aryl bromide

GR formation

Outer-sphere ET

GR formation

Inner-sphere ET

effect Ethanol: 0.9967 0.0004/water: 0.9949  0.0002 Ethanol: 0.9968  0.0005/water: 0.9978  0.0003 1.0005  1.0007 / no effect 1.0025 1.00027 / 1.0030

Szatkowski et al. 2013

6. Vaporization from Pure Organic Phase Evaporation is one of the important pathways of volatile organic compounds (VOCs) attenuation driven by the vapor pressure that these compound possess. To quantify this process one can study the isotopic composition of the compound phases. Depending on the isotopic composition of a compound phase (liquid vs vapor) such analysis can lead to either normal (liquid phase enriched in the heavier isotopologue) or inverse (vapor phase enriched in the heavier isotopologue) vapor pressure isotope effects (VPIEs). According to isotope effect theory, VPIEs are mainly governed by intermolecular forces of a different kind. However, till the date, evaluation of isotope fractionation during evaporation process is still a developing experimental field, for example, the little experimental data available mainly refers to pure organic phase evaporation while more realistic environment like aqueous solutions is unexplored but more important. Different experimental setups frequently result in an inconsistent and/or misleading interpretation of the results. Hence, it is necessary to seek independent tools that allow not only for interpreting experimental results, but also offering additional information that could not be achieved through the experiments. In order to meet this goal some of the available computational approaches to predict VPIEs were tested and compared to the experimentally determined values. For this purpose, dibromomethane (DBM) and bromobenzene (BB) as representatives of brominated VOCs,

23

Journal Pre-proof clear examples of aliphatic and aromatic compound, respectively were selected. With the use of carbon and bromine isotopic analysis, possible differences in the direction of measured and predicted isotope effects were explored. To this end, the path integral formalism of quantum chemistry, (Feynman and Hibs, 1965) as well as ONIOM (Dapprich et al. 1999) scheme and QM cluster calculations, were used (Vasquez et al., 2018). Within all tested possibilities only for very limited settings a kind of agreement between measured and predicted values was obtained and the best match is presented in Table 2. Table 2. Average (bulk) C and Br VPIEs for evaporation of bromobenzene and dibromomethane measured and predicted using various computational approaches at 300 K. compound BB DBM

PIMD C Br 1.0037 1.0000 0.9972 1.0001

theory QM cluster C Br 1.0004 1.0001 0.9995 1.0001

experiment ONIOM C Br 1.0008 1.0002 0.9996 1.0001

C 1.0004 0.9994

Br 1.0009 1.0008

7. Summary and Future Outlook Although the first attempts to measure stable isotope composition of bromine were made almost 100 years ago, significant progress in investigating bromine isotope effects in different chemical and the environmental process has been made only during the last decade. The reason for this significant progress has been, likely, initiated by the establishment of a sensitive and precise Br isotope analysis. Currently, GC/MC-ICPMS analysis seems to be one of the most prominent techniques for the determination of bromine isotope composition of organobromides. Although good analytical performance has been demonstrated for Br isotope analysis by GC/MC-ICPMS, there is still a need for the method validation for different types of BOCs. The development of international standard materials with defined bromine isotope composition is of high importance in order to make possible comparison of data obtained by different laboratories. Employing Br isotope analysis significantly extended the knowledge on 24

Journal Pre-proof transformations of BOCs in the environment relevant processes. Although the magnitude of bromine isotope effect is usually relatively small, still Br isotope analysis, especially as a supplementary to carbon isotope analysis, may serve as a valuable tool for characterizing reaction mechanisms. Considering the data reported in the literature until now, source identification of BOCs based on δ81Br values is challenging and a larger database is necessary to reach the conclusion. Application of bromine isotope analysis for studying the fate of atmospheric bromo-organic compounds seems promising for estimating their transport in the atmosphere. Bromine isotope analysis may be potentially helpful for studying environmental transformations of brominated flame retardants. However, for high-precision analysis of highboiling point polybrominated compounds, further development of GC/MC-ICPMS method should be performed. In general, the knowledge on the bromine isotope fractionation in chemical and environmental processes is still little relative to organochlorides and further research is necessary for more detailed understanding behavior of brominated vs. chlorinated compounds. This can be especially important for any biotransformations of these compounds as their fate in the active site of dehalogenating enzymes can differ. Bromine atoms are larger than chlorine ones and this may induce different patterns of interactions of brominated and chlorinated compounds with surrounding residues and result in distinctive binding for each of them. Differences in binding may have their consequences for subsequent catalytic steps and lead sometimes to different apparent halogen KIE which obviously does not have to arise from differences in the fashion of breaking C-Cl versus C-Br bond. These issues would be ideal to be tackled by computational methods, however, to date no test comprising both chlorinated and brominated substrate to the same degree for the same enzyme has been performed. Furthermore, in order to run such tests, one would have to first make sure that appropriate method and/or level of theory is available, in terms of its accuracy and reasonable

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Journal Pre-proof computational cost, to treat highly halogenated compounds in the environment such as enzymes. Acknowledgments This work was supported by the National Science Center in Poland (Sonata BIS grant UMO2014/14/E/ST4/00041), the United States - Israel Binational Science Foundation, BSF grant number 2014374, and in part by PLGrid Infrastructure (Poland). References Aston, F.W., 1920. LXXII. The mass-spectra of chemical elements. (Part 2). The London, Edinburgh, and Dublin Phil. Mag. and J. Sci. 40, 628. Bernstein, A., et al. 2013. Kinetic bromine isotope effect: example from the microbial debromination of brominated phenols. Anal. Bioanal. Chem. 405, 2923-2929 Bernstein, A., Golan, R., Gelman, F., Kuder, T., 2019. Microbial oxidation of tri-halogenated phenols-Multi-element isotope fractionation. Int. Biodeter. Biodeg. 145, 104811 Blewett, J. P., 1936. Mass spectrograph analysis of bromine. Phys. Rev. 49, 900 Cameron, A. E., Herr, W., Herzog, W., Lunden, A., 1956. Isotopen-Anreicherung beim Brom durch elektrolytische Überführung in geschmolzenem Bleibromid. Z. Naturforsch. A 11, 203. Carrizo, D., Unger, M., Holmstrand, H., Andersson, P., Gustfsson, O., Sylva, S. P., Reddy, C. M., 2011. Compound-specific bromine isotope compositions of one natural and six industrially synthesised organobromine substances. Environ. Chem. 8, 127-132. Catanzaro, E.J., Murphy, T.J., Garner, E.L., Shields, W.R., 1964. Absolute Isotopic Abundance Ratio and the Atomic Weight of Bromine. J. Res. Natl. Bur. Stand. 68A, 593-599. Chai, J.-D., Head-Gordon, M., 2008. Systematic optimization of long-range corrected hybrid density functional. J. Chem. Phys. 128, 084106. Chai, J.-D., Head-Gordon, M., 2008. Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections. Phys. Chem. Chem. Phys. 10, 6615-6620. Chen L., Shouakar-Stash, O., Ma, T., Wang, C., Liu, L., 2012. Significance of stable carbon and bromine isotopes in the source identification of PBDEs. Chemosphere 186,160-166. 26

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Graphical Abstract

Highlights   

Br isotope effects are crucial in describing brominated organics fate in the environment Modern analytical methods allow for precise measurement of bromine isotopic ratios Combined the experimental and theoretical approaches to determine Br IEs are discussed

31

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Declaration of interests x The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: