Investigating the Heaviest Halogen: Lessons Learned from Modeling the Electronic Structure of Astatine’s Small Molecules

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2023-01, Vol.127 (1), p.46-56
• Main Authors: Casetti, Vincent T., MacLean, James, Ayoub, Adam D., Fredericks, Rain J., Adamski, Jacob A., Rusakov, Alexander A.
• Format: Article
• Language: English
• Subjects: A: Structure, Spectroscopy, and Reactivity of Molecules and Clusters
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.2c06039

We present a systematic study of electron-correlation and relativistic effects in diatomic molecular species of the heaviest halogen astatine (At) within relativistic single- and multireference coupled-cluster approaches and relativistic density functional theory. We establish revised reference ab initio data for the ground states of At2, HAt, AtAu, and AtO+ using a highly accurate relativistic effective core potential model and in-house basis sets developed for accurate modeling of molecules with large spin–orbit effects. Spin-dependent relativistic effects on chemical bonding in the ground state are comparable to the binding energy or even exceed it in At2. Electron-correlation effects near the equilibrium internuclear separation are mostly dynamical and can be adequately captured using single-reference CCSD­(T). However, bond elongation in At2 and, especially, AtO+ results in rapid manifestation of its multireference character. While useful for evaluating the spin–orbit effects on the ground-state bonding and properties, the two-component density functional theory lacks predictive power, especially in combination with popular empirically adjusted exchange-correlation functionals. This drawback supports the necessity to develop new functionals for reliable quantum-chemical models of heavy-element compounds with strong relativistic effects.