Bromine and Carbon Isotope Effects during Photolysis of Brominated Phenols

In the present study, carbon and bromine isotope effects during UV-photodegradation of bromophenols in aqueous and ethanolic solutions were determined. An anomalous relatively high inverse bromine isotope fractionation (ε_{reactive position} up to +5.1‰) along with normal carbon isotope effect (ε_{reactive position} of −12.6‰ to −23.4‰) observed in our study may be attributed to coexistence of both mass-dependent and mass-independent isotope fractionation of C–Br bond cleavage. Isotope effects of a similar scale were observed for all the studied reactions in ethanol, and for 4-bromophenol in aqueous solution. This may point out related radical mechanism for these processes. The lack of any carbon and bromine isotope effects during photodegradation of 2-bromophenol in aqueous solution possibly indicates that C–Br bond cleavage is not a rate-limiting step in the reaction. The bromine isotope fractionation, without any detectable carbon isotope effect, that was observed for 3-bromophenol photolysis in aqueous solution probably originates from mass-independent fractionation

A Benchmark Study of Kinetic Isotope Effects and Barrier Heights for the Finkelstein Reaction

Herein, we present a combined (experimental and computational) study of the Finkelstein reaction in condensed phase, where bromine is substituted by iodine in 2-bromoethylbenzene, in the presence of either acetone or acetonitrile as a solvent. Performance of various density functional theory and ab initio methods were tested for reaction barrier heights as well as for bromine and carbon kinetic isotope effects (KIEs). Two different implicit solvation models were examined (PCM and SMD). Theoretically predicted KIEs were compared with experimental values, while reaction barrier heights were assessed using the CCSD­(T)-level and experimental energies as reference. In general, although the tested parameters (energies and KIEs) do not exhibit any substantial difference upon a change of the solvent, the different behavior of the theoretical methods was observed depending on the solvent. With respect to isotope effects, both PCM and SMD seem to perform very similarly, though results obtained with PCM are slightly closer to the experimental values. For predicting reaction barriers, utilization of either PCM or SMD solvation models yielded different results. Functionals from the ωB97 family: ωB97, ωB97X, and ωB97X-D provide the most accurate results for the studied system

Can Path Integral Molecular Dynamics Make a Good Approximation for Vapor Pressure Isotope Effects Prediction for Organic Solvents? A Comparison to ONIOM QM/MM and QM Cluster Calculation

Isotopic fractionation of volatile organic compounds (VOCs), which are under strict measures of control because of their potential harm to the environment and humans, has an important ecological aspect, as the isotopic composition of compounds may depend on the conditions in which such compounds are distributed in Nature. Therefore, detailed knowledge on isotopic fractionation, not only experimental but also based on theoretical models, is crucial to follow conditions and pathways within which these contaminants are spread throughout the ecosystems. In this work, we present carbon and, for the first time, bromine vapor pressure isotope effect (VPIE) on the evaporation process from pure-phase systemsdibromomethane and bromobenzene, the representatives of aliphatic and aromatic brominated VOCs. We combine isotope effects measurements with their theoretical prediction using three computational techniques, namely path integral molecular dynamics, QM cluster, and hybrid ONIOM models. While evaporation of both compounds resulted in normal bromine VPIEs, the difference in the direction of carbon isotopic fractionation is observed for the aliphatic and aromatic compounds, where VPIEs are inverse and normal, respectively. Even though theoretical models tested here turned out to be insufficient for quantitative agreement with the experimental values, cluster electronic structure calculations, as well as two-layer ONIOM computations, provided better reproduction of experimental trends

δ¹³C and δ³⁷Cl Isotope Fractionation To Characterize Aerobic vs Anaerobic Degradation of Trichloroethylene

Trichloroethylene (TCE) is a carcinogenic organic chemical impacting water resources worldwide. Its breakdown by reductive vs oxidative degradation involves different types of chemical bonds. Hence, if distinct isotope effects are reflected in dual element (carbon and chlorine) isotope values, such trends could help distinguishing both processes in the environment. This work explored dual element isotope trends associated with TCE oxidation by two pure bacterial cultures: Pseudomonas putida F1 and Methylosinus trichosporium OB3b, where the latter expresses either soluble methane-monooxygenase (sMMO) or particulate methane-monooxygenase (pMMO). Carbon and chlorine isotope enrichment factors of TCE (ε¹³C = −11.5, −2.4, and −4.2‰; ε³⁷Cl = 0.3, −1.3, and −2.4‰, respectively) differed strongly between the strains. The dual element isotope trend for strain F1 (ε¹³C/ε³⁷Cl = −38) reflected, as expected, primary carbon and negligible chlorine isotope effects, whereas unexpectedly large chlorine isotope effects became apparent in the trend obtained with strain OB3b (ε¹³C/ε³⁷Cl = +1.7 for sMMO and pMMO). Therefore, although dual element isotope analysis partly reflects predicted differences in oxidative vs reductive (ε¹³C/ε³⁷Cl = 3.4–5.7) degradation, the unexpected OB3b fractionation data may challenge field interpretation

Dual Carbon–Bromine Stable Isotope Analysis Allows Distinguishing Transformation Pathways of Ethylene Dibromide

The present study investigated dual carbon–bromine isotope fractionation of the common groundwater contaminant ethylene dibromide (EDB) during chemical and biological transformations, including aerobic and anaerobic biodegradation, alkaline hydrolysis, Fenton-like degradation, debromination by Zn(0) and reduced corrinoids. Significantly different correlation of carbon and bromine isotope fractionation (Λ_C/Br) was observed not only for the processes following different transformation pathways, but also for abiotic and biotic processes with, the presumed, same formal chemical degradation mechanism. The studied processes resulted in a wide range of Λ_C/Br values: Λ_C/Br = 30.1 was observed for hydrolysis of EDB in alkaline solution; Λ_C/Br between 4.2 and 5.3 were determined for dibromoelimination pathway with reduced corrinoids and Zn(0) particles; EDB biodegradation by <i>Ancylobacter aquaticus</i> and <i>Sulfurospirillum multivorans</i> resulted in Λ_C/Br = 10.7 and 2.4, respectively; Fenton-like degradation resulted in carbon isotope fractionation only, leading to Λ_C/Br ∞. Calculated carbon apparent kinetic isotope effects (¹³C-AKIE) fell with 1.005 to 1.035 within expected ranges according to the theoretical KIE, however, biotic transformations resulted in weaker carbon isotope effects than respective abiotic transformations. Relatively large bromine isotope effects with ⁸¹Br-AKIE of 1.0012–1.002 and 1.0021–1.004 were observed for nucleophilic substitution and dibromoelimination, respectively, and reveal so far underestimated strong bromine isotope effects