Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations, Al+ (H2 O)n , n=1-10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s-3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1 . On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+ (H2 O)n-1 and are transparent in the investigated energy range. For n=4-8, a mixture of Al+ (H2 O)n and HAlOH+ (H2 O)n-1 is present in the experiment.

Auf zur Wasserstoffentwicklung: Das Infrarot-Spektrum von hydratisiertem Aluminiumhydrid-Hydroxid HAlOH+(H2O)n-1, n=9-14

Hydratisierte Al+‐Ionen eliminieren H2 in einem Bereich von 11 bis 24 Wassermolekülen. Wir untersuchten die Struktur von HAlOH+(H2O)n−1, n=9–14, durch IR‐Mehrfachphotonendissoziationsspektroskopie bei 1400–2250 cm−1. Aufgrund quantenchemischer Rechnungen ordnen wir die Merkmale bei 1940 und 1850 cm−1 der Al‐H‐Streckschwingung in fünf‐ bzw. sechsfach koordinierten AlIII‐Komplexen zu. Es werden Wasserstoffbrücken in Richtung des Hydrids beobachtet, beginnend bei n=12. Die Frequenz der Al‐H‐Streckschwingung ist sehr empfindlich gegenüber der Struktur des Netzwerks aus Wasserstoffbrücken, und die große Anzahl von Isomeren führt zu einer deutlichen Verbreiterung und Rotverschiebung der Absorptionen der wasserstoffbrückengebundenen Al‐H‐Streckschwingung. Das Hydrid kann sogar als doppelter Wasserstoffbrücken‐Akzeptor wirken und die Al‐H‐Streckschwingung zu Frequenzen verschieben, die unter denen der Wasser‐Biegemoden liegen. Das Einsetzen der Wasserstoffbrückenbindung und das Verschwinden der freien Al‐H‐Streckschwingung fallen mit dem Einsetzen der Wasserstoffentwicklung zusammen.

Getting Ready for the Hydrogen Evolution Reaction: The Infrared Spectrum of Hydrated Aluminum Hydride-Hydroxide HAlOH+ (H2 O)n-1 , n=9-14

Hydrated singly charged aluminum ions eliminate molecular hydrogen in a size regime from 11 to 24 water molecules. Here we probe the structure of HAlOH+ (H2 O)n-1 , n=9-14, by infrared multiple photon spectroscopy in the region of 1400-2250 cm-1 . Based on quantum chemical calculations, we assign the features at 1940 cm-1 and 1850 cm-1 to the Al-H stretch in five- and six-coordinate aluminum(III) complexes, respectively. Hydrogen bonding towards the hydride is observed, starting at n=12. The frequency of the Al-H stretch is very sensitive to the structure of the hydrogen bonding network, and the large number of isomers leads to significant broadening and red-shifting of the absorption of the hydrogen-bonded Al-H stretch. The hydride can even act as a double hydrogen bond acceptor, shifting the Al-H stretch to frequencies below those of the water bending mode. The onset of hydrogen bonding and disappearance of the free Al-H stretch coincides with the onset of hydrogen evolution.

Exceptional isotopic-substitution effect: breakdown of collective proton tunneling in hexagonal ice due to partial deuteration

Multiple proton transfer controls many chemical reactions in hydrogen-bonded networks. However, in contrast to well-understood single proton transfer, the mechanisms of correlated proton transfer and of correlated proton tunneling in particular have remained largely elusive. Herein, fully quantized ab initio simulations are used to investigate H/D isotopic-substitution effects on the mechanism of the collective tunneling of six protons within proton-ordered cyclic water hexamers that are contained in proton-disordered ice, a prototypical hydrogen-bonded network. At the transition state, isotopic substitution leads to a Zundel-like complex, [HO⋅⋅⋅D⋅⋅⋅OH], which localizes ionic defects and thus inhibits perfectly correlated proton tunneling. These insights into fundamental aspects of collective proton tunneling not only rationalize recent neutron-scattering experiments, but also stimulate investigations into multiple proton transfer in hydrogen-bonded networks much beyond ice.

Computational study of the one- and two-dimensional infrared spectra of a proton-transfer mode in a hydrogen-bonded complex dissolved in a polar nanocluster

The signatures of nanosolvation on the one- and two-dimensional (1D and 2D) IR spectra of a proton-transfer mode in a hydrogen-bonded complex dissolved in polar solvent molecule nanoclusters of varying size are elucidated by using mixed quantum-classical molecular dynamics simulations. For this particular system, increasing the number of solvent molecules successively from N=7 to N=9 initiates the transition of the system from a cluster state to a bulk-like state. Both the 1D and 2D IR spectra reflect this transition through pronounced changes in their peak intensities and numbers, but the time-resolved 2D IR spectra also manifest spectral features that uniquely identify the onset of the cluster-to-bulk transition. In particular, it is observed that in the 1D IR spectra, the relative intensities of the peaks change such that the number of peaks decreases from three to two as the size of the cluster increases from N=7 to N=9. In the 2D IR spectra, off-diagonal peaks are observed in the N=7 and N=8 cases at zero waiting time, but not in the N=9 case. It is known that there are no off-diagonal peaks in the 2D IR spectrum of the bulk version of this system at zero waiting time, so the disappearance of these peaks is a unique signature of the onset of bulk-like behavior. Through an examination of the trajectories of various properties of the complex and solvent, it is possible to relate the emergence of these off-diagonal peaks to an interplay between the vibrations of the complex and the solvent polarization dynamics.

Reduction of Acetonitrile by Hydrated Magnesium Cations Mg+ (H2 O)n (n≈20-60) in the Gas Phase

Ion-molecule reactions of Mg⁺ (H₂ O)_n (n≈20-60) with CH₃ CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH₃ CN initiates the formation of MgOH⁺ (H₂ O)_n-1 together with CH₃ CHN^. or CH₃ CNH^. , which is similar to the reaction of hydrated electrons (H₂ O)_n ^- with CH₃ CN. In subsequent reaction steps, three more CH₃ CN molecules are taken up by the clusters, to form MgOH⁺ (CH₃ CN)₃ after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH₃ CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(CN)-like orbital of the bent CH₃ CN^.- , which undergoes spontaneous proton transfer to form CH₃ CNH^. or CH₃ CHN^. , with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH₃ CNH^. or CH₃ CHN^. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH₃ CN molecules taken up by the resulting MgOH⁺ (H₂ O)_n clusters coordinate directly to the first solvation shell of the MgOH⁺ core, as revealed by DFT calculations.

Micro-Hydration of the MgNO3+ Cation in the Gas Phase

Coordination complexes of the magnesium nitrate cation with water [MgNO(3)(H(2)O)(n)](+) up to n=7 are investigated by experiment and theory. The fragmentation patterns of [MgNO(3)(H(2)O)(n)](+) clusters generated via electrospray ionization indicate a considerable change in stability between n=3 and 4. Further, ion-molecule reactions of mass-selected [MgNO(3)(H(2)O)(n)](+) cations with D(2)O reveal the occurrence of consecutive replacement of water ligands by heavy water, and in this respect the complexes with n=4 and 5 are somewhat more reactive than their smaller homologs with n=1-3 as well as the larger clusters with n=6 and 7. For the latter two ions, the theory suggests the existence of isomers, such as complexes with monodentate nitrato ligands as well as solvent-separated ion pairs with a common solvation shell. The reactions observed and the ion thermochemistry are discussed in the context of ab initio calculations, which also reveal the structures of the various hydrated cation complexes.