Zur Elektronenstruktur metallorganischer Komplexe der ƒ -Elemente, 81. Berechnung der Normalschwingungen von La(η5-C5H5)3(NCCH3)2 auf der Basis der Dichtefunktionaltheorie sowie vibronische Kopplungen und Auffindung weiterer rein elektronischer Absorptions- und Lumineszenzüberg¨ange bei [La(η5-C5H5)3(NCCH3)2:Nd3+] / Electronic Structures of Organometallic Complexes of f Elements, 81. Calculation of the Normal Modes of La(η5-C5H5)3(NCCH3)2 on the Basis of Density Functional Theory as well as Vibronic Couplings and Identification of Further Purely Electronic Absorption and Luminescence Transitions of [La(η5-C5H5)3(NCCH3)2:Nd3+]

Abstract

The planned model calculations of normal modes of pseudo-trigonal-bipyramidal LaCp3(NCCH3)2 (Cp = η5-C5H5-) (1) adopting density functional theory (DFT), and using the molecular structure as suggested by X-ray investigations, did not converge. Alternatively, DFT calculations assuming molecular C3h symmetry were performed. Unfortunately, these calculations did not reproduce the experimentally derived frequencies of the skeletal modes very well, including a wrong energetic sequence of the four previously unambiguously assigned (close lying) Raman-active ν(La-Cp) skeletal modes. The same presumably also holds for DFT calculations made for LaCp3 · NCCH3 (2) and base-free LaCp3 (3), assuming molecular C3 and C3h symmetry, respectively. In order to check whether the calculated incorrect sequence is produced by the five-membered Cp rings, a DFT calculation also has been performed for the hypothetical model complex [La(η6-C6H6)3]3+ (4) of D3 symmetry. A closer examination of the vibronic sidebands of the hypersensitive absorption transition 4I9/2 → 4G5/2 of [LaCp3(NCCH3)2:Nd3+] (5) showed that first of all, totally symmetric intraligand and not skeletal vibrations are coupling as it was the case for [LaCp3(NCCH3)2:Ln3+] (Ln = Pr, Sm). Applying this result to the vibronic sidebands of some purely electronic crystal field (CF) transitions, which are hampered by strong binary combination vibrations, the energies of these CF levels could be determined. The CF state 1Γ8 of the ground multiplet 4I9/2 of [LaCp3(NCCH3)2:Nd3+], which previously could not be detected by absorption measurements, could be derived from the luminescence transition 4F3/2 → 4I9/2. Considering these additional assignments, the goodness of the fit increased from 32.3 to 29.9 cm-1 for 61 assignments