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.



Experimental Details
The experiments are performed on a Bruker Spectrospin 4.7 Tesla Fourier-Transformation Ion Cyclotron Resonance Mass Spectrometer (FT-ICR MS). [1]The mass spectrometer is equipped with a Bruker infinity cell [2] along with a laser vaporization source, [3] where a solid disk of aluminum is vaporized by a frequency doubled Litron Nano S 60-30 Nd:YAG laser (532 nm, 5 mJ/pulse, 30 Hz).The plasma containing Al + is entrained in a pulse of Helium seeded with water vapor created via a homebuilt piezoelectric valve.The ensuing pulse is cooled via supersonic expansion, whereby hydrated aluminum complexes, Al + (H2O)n, are created.The clusters are transferred into the ICR-Cell, where they are mass selected and stored within the 4.7 T magnetic field [4] under ultra-high vacuum conditions (p ≈ 5 x 10 -10 mbar).
The mass-selected ion of interest is irradiated with infrared light provided by an EKPLSA NT273-XIR OPO covering the range from 1400-2240 cm -1 .Typical irradiation times are 0.2 s at 1000 Hz repetition rate.The normalized IRMPD Yield is calculated from the precursor ion and fragment ion intensities along with laser power are considered.The laser power is measured after every mass spectrum to account for any fluctuations.Especially for larger clusters, dissociation due to black-body infrared radiative dissociation (BIRD) from the cell walls can occur.To prevent this, the ICR cell is surrounded by a copper jacket, whereby the ions can be cooled with liquid nitrogen to a temperature of ca.85 K, [5] minimizing the effects due to BIRD. [6,7]The remaining BIRD fragmentation can be compensated by subtraction of the measured fragment ion intensities with a reference mass spectrum where the ions were trapped without irradiation from the OPO.In concordance with previous investigations, we expect these clusters to be thermalized within the ICR cell, since IR exchange in the 300-500 cm -1 range is efficient, and there are many IR photons present at 85 K. [7]

Computational Details
All geometries of HAlOH + (H2O)n-1 were optimized at M06/6-311++G** level of theory and confirmed as local minima with no imaginary frequency evaluated at the same level of theory.
Initially, the penta-and hexa-coordinate ionic cores (i.e., HAlOH + (H2O)3 and HAlOH + (H2O)4) were optimized.From these smallest cores (n = 4 and 5, respectively), a water molecule was then placed at different locations in the second solvation shell in a way that maximizes the number of hydrogen bonds among water molecules, followed by further geometry optimizations.This procedure was repeated systematically by placing additional water molecules to low-lying geometries of the n-1 cluster, to build plausible initial geometries for clusters up to n = 14 for both coordinate modes.All optimized geometries at each size 9 ≤ n ≤ 14 were classified into three categories: conformers with the hydride of the Al-H bond forming (i) zero (n-5c and n-6c), (ii) one (n-5c-HB and n-6c-HB) or (iii) two (n-5c-HB2 and n-6c-HB2) hydrogen bonds.To obtain vibrational spectra of these conformers, their geometries were re-optimized, followed by harmonic frequency analyses at the B3LYP/6-311++G** level of theory.The Al-H bond lengths (r AlH) and hydrogen bond lengths between the hydride and water molecules (rHB1 and rHB2) (obtained from M06) along with the Al-H stretching frequency (obtained from B3LYP with an anharmonic scaling of 0.982) are listed in Tables S1-S3.a) as a representative, the top panel represents the cluster distribution from the ion source, followed by the mass-selected ion of interest, n = 9.Mass-selection was achieved in the ICR cell via resonant excitation and ejection of all other unwanted ions. [4]To avoid unnecessarily long delays prior to IR irradiation and to minimize potential residual excitation of the ion of interest, we did not employ broad-band excitation, and we did not eject some low-intensity peaks that do not interfere with the experiment.Each mass-selected ion was irradiated for 0.2 s at 1614 cm -1 .For n = 9, 10 and 11, a) and c), IRMPD leads to loss of intact water molecules, whereas for n = 12, 14 and 13, b) and d), loss of H2+xH2O, x = 2,3, is also observed.The linear onset of the decay of the reactant ion in the semilogarithmic plot, without exhibiting an induction delay, indicates a single-photon process.The upward curvature at later times is assigned to incomplete overlap of the laser beam with the ion cloud.Absorption of multiple photons would lead to a downward curvature.

Figure S1 .
Figure S1.Selected six-coordinate conformers of HAlOH + (H2O)n-1 (n = 11) optimized at the M06/6-311++G** level.Relative zero-point corrected energies ∆E0 are in kJ mol −1 , referenced to the structures in Figure 2. Theoretical IR spectra (colored lines) were obtained from harmonic vibration analyses including an anharmonic scaling factor of 0.982 for geometries optimized at the B3LYP/6-311++G** level.Calculated lines are broadened with Gaussian functions with 20 cm -1 FWHM.The experimental IRMPD spectrum is shown by the gray shaded area.

Figure
Figure S3.a) Experimental IRMPD spectrum of n = 13 in the O-H stretch region.b) Simulated IR spectra obtained from harmonic vibration analyses including an anharmonic scaling factor of 0.96 for geometries optimized at the B3LYP/6-311++G** level.The hydrogen-bonded hydride O-H stretch is indicated by an asterisk (*).

Figure S4 :
Figure S4: IRMPD kinetics of HAlOH + (H2O)7 a) 1614 cm -1 and b) 1944 cm -1 scaled to the total energy provided to the ion cloud to compensate for the wavelength-dependent laser power.

Table S2 .
Summary of Al-H bond lengths rAlH, hydrogen-bond lengths between the hydride of Al-H and water molecules rHB1, relative zero-point corrected energies ∆E0 referenced to Figure2(calculated at the M06/6-311++G** level of theory) and Al-H stretching frequencies (calculated at the B3LYP/6-311++G** level of theory with an anharmonic scaling of 0.982) for all optimized hexa-coordinate geometries featuring a single hydrogen bond to Al-H (n-6c-HB1).

Table S3 .
Summary of Al-H bond lengths rAlH, hydrogen-bond lengths between the hydride of Al-H and water molecules rHB2, relative zero-point corrected energies ∆E0 referenced to Figure2(calculated at the M06/6-311++G** level of theory) and Al-H stretch frequencies (calculated at the B3LYP/6-311++G** level of theory with an anharmonic scaling of 0.982) for all optimized hexa-coordinate geometries featuring two hydrogen bonds to Al-H (n-6c-HB2).

Table S4 .
Hydration energy of the lowest energy structure reported in Figure2and average hydration energy of