Infrared Multiple Photon Dissociation Spectroscopy Confirms Reversible Water Activation in Mn+(H2O)n, n ≤ 8

Controlled activation of water molecules is the key to efficient water splitting. Hydrated singly charged manganese ions Mn+(H2O)n exhibit a size-dependent insertion reaction, which is probed by infrared multiple photon dissociation spectroscopy (IRMPD) and FT-ICR mass spectrometry. The noninserted isomer of Mn+(H2O)4 is formed directly in the laser vaporization ion source, while its inserted counterpart HMnOH+(H2O)3 is selectively prepared by gentle removal of water molecules from larger clusters. The IRMPD spectra in the O–H stretch region of both systems are markedly different, and correlate very well with quantum chemical calculations of the respective species at the CCSD(T)/aug-cc-pVDZ//BHandHLYP/aug-cc-pVDZ level of theory. The calculated potential energy surface for water loss from HMnOH+(H2O)3 shows that this cluster ion is metastable. During IRMPD, the system rearranges back to the noninserted Mn+(H2O)3 structure, indicating that the inserted structure requires stabilization by hydration. The studied system serves as an atomically defined single-atom redox-center for reversible metal insertion into the O–H bond, a key step in metal-centered water activation.


Mn + (H 2 O) + ℎ → Mn + (H 2 O) − + (H 2 O)
(1) Typical irradiation times are between 0.2-20.0 s at 1000 Hz repetition rate. The normalized IRMPD Yield is calculated from the precursor ion and BIRD-corrected fragment ion intensities along with considerations given to the laser power. First, ion intensities are normalized so that the precursor ion intensity is 100%. In the next step, the fragment intensities are BIRD corrected by subtracting the fragment intensities form a control experiment without laser irradiation, but identical residence time in the ICR cell. IRMPD yields are then calculated as (sum over BIRD corrected fragment intensities)/(sum over BIRD corrected fragment intensities and precursor intensity)/(laser power) and re-normalized for graphical display so that the maximum IRMPD yield in the spectrum is 100%.
The laser power is measured after every mass spectrum to account for small fluctuations. The laser power drops in the range 3500 -3520 cm -1 . Due to the complex kinetics of the IRMPD process, this may lead to small artifacts in the IRMPD yield at these wavelengths even after power correction.
In conjunction with the IRMPD measurements, BIRD experiments were also performed on the Mn + (H2O)8 complex. Figure S1a shows a typical mass spectrum recorded during an experimental run, presenting Mn + (H2O)n clusters, n = 4-15. Figure S1a-e presents a sequence of mass spectra used to record the IRMPD spectrum presented in Figure 1b in the main article.
First, the Mn + (H2O)8 complex is mass-selected ( Figure S1b), and is heated by room temperature black-body radiation from the ICR cell walls for 20 s ( Figure S1c). The [Mn,(H2O)4] + complex is mass-selected ( Figure S1d) and subject to IR radiation at 3680 cm -1 , leading to water loss, Figure S1e. To construct the IRMPD spectrum in Figure 1b, the whole process is repeated, changing the irradiation wavenumber in Figure S1e. A series of mass spectra like the S4 one in Figure 1e, measured with the irradiation wavenumber changed sequentially, is obtained to generate IRMPD spectra.
In addition to using BIRD to dissociate the Mn + (H2O)8 complex, IR radiation from the OPO laser system was also employed. The IRMPD spectrum of Mn + (H2O)8, Figure 1c, shows a strong broad band at ca. 3200 cm -1 . This band was used to photodissociate the Mn + (H2O)8 complex, S5 Figure S1: Representative mass spectra outlining the experimental procedure used to construct BIRD+IRMPD spectra. (a) mass spectrum obtained from the ion source, (b) mass-selected Mn + (H2O)8 cluster, (c) mass distribution after 20 s irradiation with black-body infrared radiation, (d) mass-selected Mn + (H2O)4 cluster, and (e) mass spectrum recorded after irradiation with infrared light at 3680 cm -1 for 0.3 s. Arrows indicate that mass spectra, a-e, were measured sequentially. In d-f), simulated infrared spectra were modelled at the BHandHLYP/aug-cc-pVDZ level with a scaling factor of 0.96.

S7
Computational Data Figure S3. Interpolation between septet and quintet Mn + (H2O)2 minima optimized at the CCSD/aug-cc-pVDZ level at different computational levels using the aug-cc-pVDZ basis set. State average of one quintet and one septet state was used for multi-reference calculations. Table S2 -Relative stability of Mn + (H2O)2 in septet spin multiplicity compared to the quintet analogue (in kJ mol −1 ), negative values indicate that the ion in septet spin multiplicity is more stable. The structures were optimized in both spin multiplicities at the respective level using the aug-cc-pVDZ basis set, energies include zero-point correction. For the CCSD(T) value, the CCSD zero-point correction was used.  Table S3 -Relative energies of Mn + (H2O)2 and HMnOH + (H2O) isomers as optimized at different levels of theory using the aug-cc-pVDZ basis set (in kJ mol −1 ). See Figure S4 for the respective isomers.