Isotope Effects on the Vaporization of Organic Compounds from an Aqueous Solution–Insight from Experiment and Computations

Abstract

An isotope fractionation analysis of organic groundwater pollutants can assess the remediation at contaminated sites yet needs to consider physical processes as potentially confounding factors. This study explores the predictability of water–air partitioning isotope effects from experiments and computational predictions for benzene and trimethylamine (both H-bond acceptors) as well as chloroform (H-bond donor). A small, but significant, isotope fractionation of different direction and magnitude was measured with ε = −0.12‰ ± 0.07‰ (benzene), ε C = 0.49‰ ± 0.23‰ (triethylamine), and ε H = 1.79‰ ± 0.54‰ (chloroform) demonstrating that effects do not correlate with expected hydrogen-bond functionalities. Computations revealed that the overall isotope effect arises from contributions of different nature and extent: a weakening of intramolecular vibrations in the condensed phase plus additional vibrational modes from a complexation with surrounding water molecules. Subtle changes in benzene contrast with a stronger coupling between intra- and intermolecular modes in the chloroform–water system and a very local vibrational response with few atoms involved in a specific mode of triethylamine. An energy decomposition analysis revealed that each system was affected differently by electrostatics and dispersion, where dispersion was dominant for benzene and electrostatics dominated for chloroform and triethylamine. Interestingly, overall stabilization patterns in all studied systems originated from contributions of dispersion rather than other energy terms.

Insights into Generalization of the Rate-Limiting Steps of the Dehalogenation by LinB and DhaA: A Computational Approach

“…rotational and translational) were omitted since it was shown that they do not introduce significant differences in isotope effects in computations of similar processes. [38][39][40][41][42] BIEs were calculated for each possible pair of one structure of ligand bound within the selected enzyme and one structure of ligand in water (10x10). The results were shown as means of all the individual values obtained for the specific system (enzyme-ligand) together with standard deviation of samples.…”

Non-Covalent Isotope Effects

“…Phase transfer equilibria and associated isotope effects , are vast areas that can only briefly be touched on here. They play an important role in a plethora of different disciplines, including geochemistry, food authentication, and of course isotopic separation techniques.…”