Composite ab initio vibrational spectroscopy and thermochemistry of low-valency lanthanide compounds: europium dihalides EuX2 (X = F, Cl, Br, I)

Abstract

A unique diversity in chemical properties of lanthanide compounds making them very useful in various important applications stems not least from the existence of less common Ln oxidation states, in particular +2. However, the molecular properties of low-valency lanthanide compounds are not yet well studied even for the most stable divalent Ln-containing species, such as europium dihalides. In this paper, highly accurate molecular structures, vibrational spectra, and atomisation energies of the europium dihalides EuX₂ (X = F, Cl, Br, I) are obtained at the complete basis set CCSD(T) level. All of the EuX₂ species are calculated to be non-linear (C_2v) exhibiting a regular increase in X–Eu–X bond angle on passing through the halogen series (F → Cl → Br → I), from 117° in EuF₂ to 141° in EuI₂, which is accompanied by a rapid decrease in the barriers to linearity, h = E(D_∞h) − E(C_2v), from 2180 cm⁻¹ to 166 cm⁻¹, respectively. The Eu–X bonds in EuX₂ appear to be longer but energetically stronger than those in the respective monohalides EuX, whose properties are studied in this work at the same level of theory as EuX₂. The performance of the Ln 4f-in-core pseudopotential (PP) approximation for EuX₂ is assessed by comparing the Eu PP-based coupled-cluster (CC) calculations with all-electron CC benchmarks. Incorporating the Eu 4f electrons into the PP is shown to cause large errors: up to 10 deg. in X–Eu–X bond angles, 80% in barriers to linearity, and 0.05 Å in the Eu–X bond lengths, which proves the necessity of an explicit treatment of 4f electrons if high accuracy is the goal. The present results set a benchmark in the field of low-valency lanthanide chemistry, in particular for the calibration of DFT functionals.