Effects of Halogenations and Conformational Isomers on Positron Binding in Halogenated Hydrocarbons

We studied isomeric conformer effects on the positron affinity (PA) of halogenated hydrocarbons using density functional theory combined with the electron-positron correlation-polarization potential (CPP) model. PA values are computed for 75 halogenated hydrocarbons, including fluorine (F), chlorine (Cl), and bromine (Br) derivatives of methane, ethylene, and ethane molecules. The positive PA values can be described by a linear combination of the dipole moment and polarizability of parent molecules. For Cl-substituted methane derivatives, PA increased with the number of Cl substitutions. Such a trend is consistent with the increase in the polarizability of the Cl-substituted methane derivatives. For Cl-substituted ethylene derivatives, PA differences among C₂H₂Cl₂ isomers (PA(cis-C₂H₂Cl₂) > PA(1,1-C₂H₂Cl₂) > PA(trans-C₂H₂Cl₂)) correlated well with the dipole moments of the respective parent isomers. The isosurfaces of positronic density in the cis isomer revealed that the positron is localized near the halogen atoms, whereas those in the trans isomer are more diffusive due to the spatial separation of the 2 Cl atoms. While a similar overall feature of positron density is observed in Br-substituted C₂H₂Br₂ species, positron densities of C₂H₂Br₂ are more contracted than those of C₂H₂Cl₂, reflecting that PA(Br) > PA(Cl) due to polarizability differences. These tendencies are also found in halogenated ethane species.