Experimentelle Bestimmung des aktiven Zentrums im heteronuklearen Redox‐System [AlVO_x]^+./CO/N₂O (x=3, 4) durch Gasphasen‐Infrarotspektroskopie

Das aktive Zentrum der Sauerstoffatom‐Übertragungsreaktion[AlVO4]+.+CO→[AlVO3]+.+CO2. IHe‐ und CO‐Komplexe dieser heteronuklearen Metalloxidkationen zeigen, dass [AlVO3]+. im Unterschied zu [AlVO4]+. keine endständige Al−Ot‐Gruppe mehr enthält, sehr wohl aber noch die V=Ot‐Gruppe aufweist. Somit entspricht die Al−Ot‐Einheit, in Einklang mit der theoretischen Voraussage, dem aktiven Zentrum des Redox‐Systems [AlVOx]+./CO/N2O (x=3, 4)

Site-specific vibrational spectral signatures of water molecules in the magic H₃O⁺(H₂O)₂₀ and Cs⁺(H₂O)₂₀ clusters

Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, alongwith isotopic substitution, to quench water clusters attached to the H3O+ and Cs+ ions into structures that yield well-resolved vibrational bands over the entire 215- to 3,800-cm−1 range. The magic H3O+ (H2O)20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface

Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

Gas-phase infrared spectra of several ionized nitrogen substituted polycyclic aromatic hydrocarbons (PANHs) have been recorded in the 600-1600 cm(-1) region via IR multiple-photon dissociation (IRMPD) spectroscopy. The UV photoionized PANH ions are trapped and isolated in a quadrupole ion trap where they are irradiated with an IR free electron laser. The PANHs were studied in their radical cation (PANH(+)) and protonated (H+ PANH) forms, and include quinoline, isoquinoline, phenanthridine, benzo[h] quinoline, acridine, and dibenzo[f,h] quinoline. Experimental IRMPD spectra were interpreted with the aid of density functional theory methods. The PANH(+) IR spectra are found to resemble those of their respective non-nitrogenated PAH cations. The IR spectra of H+ PANHs are significantly different owing to the NH inplane bending vibration, which generally couples very well with the aromatic CH bending and CC stretching modes. Implications of the NPAH (+, H+) laboratory spectra are discussed for the astrophysical IR emissions and, in particular, for the band at 6.2 mu m

Infrared spectra of isolated protonated polycyclic aromatic hydrocarbon molecules

Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H(+)PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H(+)PAHs contribute as carriers of the UIR bands

Experimental Identification of the Active Site in the Heteronuclear Redox Couples [AlVO_x]^+./CO/N₂O (x=3, 4) by Gas‐Phase IR Spectroscopy

Cryogenic ion vibrational spectroscopy was used in combination with electronic structure calculations to identify the active site in the oxygen atom transfer reaction [AlVO4]+.+CO→[AlVO3]+.+CO2. Infrared photodissociation spectra of messenger‐tagged heteronuclear clusters demonstrate that in contrast to [AlVO4]+., [AlVO3]+. is devoid of a terminal Al−Ot unit while the terminal V=Ot group remains intact. Thus it is the Al−Ot moiety that forms the active site in the [AlVOx]+./CO/N2O (x=3, 4) redox couples, which is in line with theoretical predictions

Spectroscopic Identification of a Bidentate Binding Motif in the Anionic Magnesium-CO₂ Complex ([ClMgCO₂]⁻)

A magnesium complex incorporating a novel metal-CO2 binding motif is spectroscopically identified. Here we show with the help of infrared photodissociation spectroscopy that the complex exists solely in the [ClMg(η2-O2C)]− form. This bidentate double oxygen metal–CO2 coordination has previously not been observed in neutral nor in charged unimetallic complexes. The antisymmetric CO2 stretching mode in [ClMg(η2-O2C)]− is found at 1128 cm−1, which is considerably redshifted from the corresponding mode in bare CO2 at 2349 cm−1, suggesting that the CO2 moiety has a considerable negative charge (∼1.8 e− ). We also employed electronic structure calculations and kinetic analysis to support the interpretation of the experimental results

Spectroscopic Identification of a Bidentate Binding Motif in the Anionic Magnesium-CO₂ Complex ([ClMgCO₂]⁻)

A magnesium complex incorporating a novel metal-CO2 binding motif is spectroscopically identified. Here we show with the help of infrared photodissociation spectroscopy that the complex exists solely in the [ClMg(η2-O2C)]− form. This bidentate double oxygen metal–CO2 coordination has previously not been observed in neutral nor in charged unimetallic complexes. The antisymmetric CO2 stretching mode in [ClMg(η2-O2C)]− is found at 1128 cm−1, which is considerably redshifted from the corresponding mode in bare CO2 at 2349 cm−1, suggesting that the CO2 moiety has a considerable negative charge (∼1.8 e− ). We also employed electronic structure calculations and kinetic analysis to support the interpretation of the experimental results

Spectroscopic snapshots of the proton-transfer mechanism in water

The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D+(D2O)n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium “Eigen” cluster [D3O+(D2O)3] with increasingly strong H-bond acceptor molecules (D2, N2, CO, and D2O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule