Photoelectron Spectroscopy and Circular Dichroism of an Open-Shell Organometallic Camphor Complex

TL;DR

This study uses photoelectron spectroscopy (PES) and photoelectron circular dichroism (PECD) to probe the chiral open-shell organometallic system Eu(HFC)$_{3}$ and its ligand HFC, the heaviest organometallic molecule studied by PECD to date. By combining VUV PES/PECD experiments with density functional theory and correlated methods (PBE0-D3/cc-pVTZ; OVGF; IP-ADC(3[4+]); ΔSCF for the Eu complex), the authors assign valence orbital energies and interpret chiral asymmetries in terms of molecular structure and tautomerism. They report PECD magnitudes up to around $8 ext{\%}$ for HFC and $7 ext{\%}$ for Eu-HFC$_{3}$, indicating that PECD remains a practical probe for large, complex chiral systems, while also underscoring the theoretical challenges in modeling such open-shell, heavy-molecule continua. The work reveals that keto-enol tautomerism in HFC can strongly influence the electronic structure and thus the PECD signal, and suggests that coordination to Eu(III) stabilizes an enol-like ligand arrangement; these insights pave the way for future PECD studies in other lanthanide–camphorate systems and for advancing the theory needed to describe PECD in large, relativistic, multiplet-bearing molecules.

Abstract

We present an investigation of one-photon valence-shell photoelectron spectroscopy and photoelectron circular dichroism (PECD) for the chiral molecule (1R,4R)-3-(heptafluorobutyryl)-(+)-camphor (HFC) and its europium complex Eu(III) tris[3-(heptafluorobutyryl)-(1R,4R)-camphorate] (Eu-HFC$_{3}$), the latter of which constitutes the heaviest organometallic molecule for which PECD has yet been measured. We discuss the role of keto-enol tautomerism in HFC, both as a free molecule and complexed in Eu-HFC$_{3}$. PECD is a uniquely sensitive probe of molecular chirality and structure such as absolute configuration, conformation, isomerisation, and substitution, and as such is in principle well suited to unambiguously resolving tautomers; however modeling remains challenging. For small organic molecules, theory is generally capable of accounting for experimentally measured PECD asymmetries, but significantly poorer agreement is typically achieved for the case of large open-shell systems. Here, we report PECD asymmetries ranging up to $\sim8\%$ for HFC and $\sim7\%$ for Eu-HFC$_{3}$, of similar magnitude to those reported previously for smaller isolated chiral molecules, indicating that PECD remains a practical experimental technique for the study of large, complicated chiral systems.

Figures (5)

• Figure 1: Commercially provided molecular structures of (1R,4R)-3-(heptafluorobutyryl)-(+)-camphor (HFC, left) and Eu(III) tris[3-(heptafluorobutyryl)-(1R,4R)-camphorate] (Eu-HFC$_{3}$, right).
• Figure 2: Representative photoion mass time-of-flight spectra (left) and corresponding photoelectron spectra collected in coincidence with the parent ion peaks (right) for (a,b) HFC and (c,d) Eu-HFC$_{3}$. The top row HFC spectra (a,b) were recorded with 13 eV photon energy; the bottom row Eu(HFC)$_3$ (c,d) measurements used 12 eV photons. Expanded views of the parent ion peaks are presented in (a) and (c). Photoelectron circular dichroism traces, showing $b^{\{+1\}}_{1}$ values derived from the photoelectron velocity map images, are presented as blue traces in (b) and (d).
• Figure 3: Photoelectron circular dichroism measured as a function of photoelectron kinetic energy for the two highest occupied molecular orbitals of (a,b) HFC and (c,d) Eu-HFC$_{3}$. Statistical error bars are included, but are generally smaller than the markers.
• Figure 4: Molecular structures of the calculated lowest-energy conformers of the keto (a) and enol (d) tautomers of (1R,4R)-3-(heptafluorobutyryl)-(+)-camphor, along with visualizations of the two highest occupied molecular orbitals for each tautomer (b: keto HOMO; c: keto HOMO-1; e: enol HOMO; f: enol HOMO-1). For the molecular orbital visualizations, green and red represent the phases of the wavefunction with an isovalue of 0.1. All molecular orbitals shown are representative of the neutral species.
• Figure 5: Optimized molecular structure of Eu-HFC$_{3}$ (a) with molecular orbitals visualized for the two highest occupied molecules orbitals (b: HOMO; c: HOMO-1). For the molecular orbital visualizations, green and red represent the phases of the wavefunction with an isovalue of 0.05. All molecular orbitals shown are representative of the neutral species. The differing perspective in (a) was chosen to highlight the helical ligand packing arrangement, while (b) and (c) utilize a different perspective to better illustrate the MOs.