SAMOP 2021 – scientific programme

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MO: Fachverband Molekülphysik

MO 5: Miscellaneous

MO 5.6: Talk

Thursday, September 23, 2021, 12:00–12:15, H2

Combined IR/UV spectroscopic investigations on neutral isolated ion pairs in a molecular beam — •Pol Boden, Markus Gerhards, and Christoph Riehn — TU Kaiserslautern, Fachbereich Chemie and Research Center Optimas, Erwin-Schrödinger-Str. 52, 67663 Kaiserslautern

Investigations of neutral isolated ion pairs in the gas phase within the context of Stark effects constitute a still very new field. For these studies, molecular beam experiments combined with IR/UV-laser spectroscopic techniques are an ideal tool, yielding spectra of the free ion pairs which are not affected by any environmental effects, facilitating comparison with quantum chemical predictions. Within this context, the alkali salts (Li-Cs) of para-aminobenzoic acid (PABA) were studied in different electronic states (S0 and D0). PABA is a so-called push-pull system, showing rather unique electronic features, with the carboxylate group acting as electron density acceptor, while the amino group donates electron density to the aromatic ring. Hence, for the series of ion pairs, the influence of the reigning electric fields on the electronic behaviour of the distinct compounds can be investigated. More specifically, the often small but considerable IR- and UV-frequency shifts provoked by exchanging the alkali cation (e.g. Li to Na) were followed in detail. A clear correlation between the measured spectral shifts and the distance separating the coordinating alkali cation from the carboxylate anion, depending on the size of the alkali ion (increasing from Li to Cs), is observed and confirmed by quantum chemical calculations (DFT/B3LYP-D3(BJ)/def2-TZVP).