Combined Experimental and Theoretical Study of the Transient IR Spectroscopy of 7‑Hydroxyquinoline in the First Electronically Excited Singlet State

The photophysics of 7-hydroxyquinoline (7HQ) in protic media results from an interplay of acid–base chemistry, prompted by the effects of photoacidity of the hydroxyl group and photobasicity of the nitrogen atom in the quinoline aromatic system. With ultrafast IR spectroscopic measurements, we follow the proton transfer dynamics of 7HQ in its four possible charged forms in methanol solution. Using deuterated methanol as solvent, we determine deuteron transfer rates from the neutral to the zwitterionic form to be 330 ps, those from the cationic form to the zwitterionic form to be 170 ps, and those from the anionic form to the zwitterionic form to be 600 ps. We compare the observed IR-active fingerprint marker patterns in the electronic ground state and the first electronically excited ¹L_b-state with those calculated using density functional theory and time-dependent density functional theory, respectively, and find good correspondence between experimental and calculated transitions. The calculations provide insight into the nature of electronic excitation of these four different charged forms of 7HQ, suggesting the key role of electronic charge distribution changes upon electronic excitation of 7HQ and hydrogen bond changes at the donor hydroxyl and acceptor nitrogen moieties

Ultrafast Proton Transport between a Hydroxy Acid and a Nitrogen Base along Solvent Bridges Governed by the Hydroxide/Methoxide Transfer Mechanism

Aqueous proton transport plays a key role in acid–base neutralization and energy transport through biological membranes and hydrogen fuel cells. Extensive experimental and theoretical studies have resulted in a highly detailed elucidation of one of the underlying microscopic mechanisms for aqueous excess proton transport, known as the von Grotthuss mechanism, involving different hydrated proton configurations with associated high fluxional structural dynamics. Hydroxide transport, with approximately 2-fold-lower bulk diffusion rates compared to those of excess protons, has received much less attention. We present femtosecond UV/IR pump–probe experiments and ab initio molecular dynamics simulations of different proton transport pathways of bifunctional photoacid 7-hydroxyquinoline (7HQ) in water/methanol mixtures. For 7HQ solvent-dependent photoacidity, free-energy–reactivity correlation behavior and quantum mechanics/molecular mechanics (QM/MM) trajectories point to a dominant OH–/CH3O– transport pathway for all water/methanol mixing ratios investigated. Our joint ultrafast infrared spectroscopic and ab initio molecular dynamics study provides conclusive evidence for the hydrolysis/methanolysis acid–base neutralization pathway, as formulated by Manfred Eigen half a century ago. Our findings on the distinctly different acid–base reactivities for aromatic hydroxyl and aromatic nitrogen functionalities suggest the usefulness of further exploration of these free-energy–reactivity correlations as a function of solvent polarity. Ultimately the determination of solvent-dependent acidities will contribute to a better understanding of proton-transport mechanisms at weakly polar surfaces and near polar or ionic regions in transmembrane proton pump proteins or hydrogen fuel cell materials

Soft X‑ray Spectroscopy of the Amine Group: Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid–Base Pairs

We use N K-edge absorption spectroscopy to explore the electronic structure of the amine group, one of the most prototypical chemical functionalities playing a key role in acid–base chemistry, electron donor–acceptor interactions, and nucleophilic substitution reactions. In this study, we focus on aliphatic amines and make use of the nitrogen 1s core electron excitations to elucidate the roles of N–H σ* and N–C σ* contributions in the unoccupied orbitals. We have measured N K-edge absorption spectra of the ethylamine bases Et_<i>x</i>NH_3–<i>x</i> (<i>x</i> = 0...3; Et– = C₂H₅−) and the conjugate positively charged ethylammonium cation acids Et_<i>y</i>NH_4–<i>y</i>⁺ (<i>y</i> = 0...4; Et– = C₂H₅−) dissolved in the protic solvents ethanol and water. Upon consecutive exchange of N–H for ethyl-groups, we observe a spectral shift, a systematic decrease of the N K-edge pre-edge peak, and a major contribution in the post-edge region for the ethylamine series. Instead, for the ethylammonium ions, the consecutive exchange of N–H for ethyl groups leads to an apparent reduction of pre-edge and post-edge intensities relative to the main-edge band, without significant frequency shifts. Building on findings from our previously reported study on aqueous ammonia and ammonium ions, we can rationalize these observations by comparing calculated N K-edge absorption spectra of free and hydrogen-bonded clusters. Hydrogen bonding interactions lead only to minor spectral effects in the ethylamine series, but have a large impact in the ethylammonium ion series. Visualization of the unoccupied molecular orbitals shows the consecutive change in molecular orbital character from N–H σ* to N–C σ* in these alkylamine/alkylammonium ion series. This can act as a benchmark for future studies on chemically more involved amine compounds

Cr L‑Edge X‑ray Absorption Spectroscopy of Cr^III(acac)₃ in Solution with Measured and Calculated Absolute Absorption Cross Sections

X-ray absorption spectroscopy at the L-edge of 3d transition metals is widely used for probing the valence electronic structure at the metal site via 2p–3d transitions. Assessing the information contained in L-edge absorption spectra requires systematic comparison of experiment and theory. We here investigate the Cr L-edge absorption spectrum of high-spin chromium acetylacetonate CrIII(acac)3 in solution. Using a transmission flatjet enables determining absolute absorption cross sections and spectra free from X-ray-induced sample damage. We address the challenges of measuring Cr L absorption edges spectrally close to the O K absorption edge of the solvent. We critically assess how experimental absorption cross sections can be used to extract information on the electronic structure of the studied system by comparing our results of this CrIII (3d3) complex to our previous work on L-edge absorption cross sections of MnIII(acac)3 (3d4) and MnII(acac)2 (3d5). Considering our experimental uncertainties, the most insightful experimental observable for this d3(CrIII)–d4(MnIII)–d5(MnII) series is the L-edge branching ratio, and we discuss it in comparison to semiempirical multiplet theory and ab initio restricted active space calculations. We further discuss and analyze trends in integrated absorption cross sections and correlate the spectral shapes with the local electronic structure at the metal sites