Halogenation-induced enhancement of positron binding in small hydrocarbons

Abstract We investigated relationships between positron affinities and molecular properties of halogenated hydrocarbons using the electron–positron correlation–polarization potential calculation with the generalized gradient approximation. It is well established experimentally that, for the simple hydrocarbons and their typical derivatives, the electrostatic properties, such as the permanent dipole moment and dipole polarizability, strongly influence the positron affinity. In addition, the number of π bonds, reflecting the π -electron contributions to the molecular polarizability or heteroatomic bond polarity, are effective factors in increasing the positron affinity. In this work, for the family of the halogenated hydrocarbons, the dataset is expanded by incorporating ten fluorinated, chlorinated, and brominated derivatives of acetylene, for which experimental positron affinities are unavailable, to our previously computed set of 75 halogenated hydrocarbons with including ethane and ethylene derivatives. The calculated positron affinities for 53 molecules, excluding fluorinated species due to their near-zero positron affinities, revealed no significant difference between halogenated acetylene-based and ethylene/ethane-based species, while the identity of the substituted halogen atoms and the degree of halogenation have substantial effects on the observed differences. The decomposition analysis of the electron–positron contact density into the electronic orbital contributions showed that, for acetylene-based systems, the higher occupied π orbitals make prominent contributions commonly, whereas for halogenated systems, such (near-) highest occupied states are dominated by halogen-derived lone pair electrons. The bound positrons for halogenated unsaturated hydrocarbons extend from the vicinity of the carbon–carbon bond axis toward the substituted halogen atoms, manifesting significance of local polarizabilities of halogen atoms.