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

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 epsilon = -0.12 parts per thousand +/- 0.07 parts per thousand (benzene), epsilon(C) = 0.49 parts per thousand +/- 0.23 parts per thousand (triethylamine), and epsilon(H) = 1.79 parts per thousand +/- 0.54 parts per thousand (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.

Automated summary

This study investigated the isotope fractionation of benzene, trimethylamine, and chloroform, revealing different directions and magnitudes of fractionation. Computational analysis showed that the isotope effect arises from contributions of various natures and extents, with influences from intramolecular vibrations weakening and complexation with surrounding water molecules. The study also found that dispersion played a dominant role in benzene's stabilization patterns, while electrostatics dominated for chloroform and trimethylamine.