Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2(H2O)n].+, n=0–8

Abstract We investigate activation of carbon dioxide by singly charged hydrated magnesium cations Mg .+(H2O)n, through infrared multiple photon dissociation (IRMPD) spectroscopy combined with quantum chemical calculations. The spectra of [MgCO2(H2O)n].+ in the 1250–4000 cm−1 region show a sharp transition from n=2 to n=3 for the position of the CO2 antisymmetric stretching mode. This is evidence for the activation of CO2 via charge transfer from Mg .+ to CO2 for n≥3, while smaller clusters feature linear CO2 coordinated end‐on to the metal center. Starting with n=5, we see a further conformational change, with CO2 .− coordination to Mg2+ gradually shifting from bidentate to monodentate, consistent with preferential hexa‐coordination of Mg2+. Our results reveal in detail how hydration promotes CO2 activation by charge transfer at metal centers.


Energetics of dissociation channels
. Dissociation energies of [MgCO2(H2O)n] •+ for loss of H2O and CO2, respectively. Calculated at the M06L/aug-cc-pVDZ level of theory. All energies are given in kJ/mol relative to the most stable isomer.

Experimental Setup
Either a Continuum Surelite II (10 Hz) or a Litron Nano S60-30 (30 Hz) is used as vaporization laser for an isotopically enriched 24 Mg (99.9%) target. The pick-up gas consisting of helium seeded with CO2 is supersonically expanded, passed through a skimmer and then through a set of electrostatic lenses which guide the ions through differential pumping stages 1 into the ICR infinity cell. 2 In the ICR cell, the ions are trapped in an electromagnetic field under ultra-high vacuum conditions (~10 -10 mbar) in the center of a 4.7 T superconducting magnet as explained in detail by Marshall et al. 3 The ions are then resonantly excited and their cyclotron frequency is measured. 3 For each data point, 20 to 50 spectra are accumulated and averaged to obtain a higher signal-to-noise ratio.
For spectroscopy, ions are mass selected and irradiated by infrared (IR) radiation from a 1000 Hz diode pumped EKSPLA NT273-XIR or EKSPLA NT277 laser system. Each data point in the absorption spectra corresponds to a full mass spectrum, measured after irradiation with a preset irradiation time (0.6-20 s). The IR/OPO laser system EKSPLA NT273-XIR operates between 4476 and 12000 nm whereas the EKSPLA NT277 operates from 2500 to 4475 nm.
The measurements for n ≥ 4 were recorded at 1250-2234 cm -1 . Because the antisymmetric stretching mode of linear CO2 lies above 2234 cm -1 , measurements were recorded at 1250-S2 4000 cm -1 for n = 0-3. Immediately after each mass spectrum, the laser power is measured before the laser is tuned to the next wavelength. The wavelength is calibrated using a HighFinesse Laser Spectrum Analyzer IR-III. For the correction of the photon loss by the CaF2 window, the transmission curve provided by ThorLabs was used. 4 Photodissociation of the complex can occur via vibrational resonant excitation during laser irradiation and/or via BIRD.
To account properly for the influence of BIRD, every tenth to thirtieth measurement is performed without irradiation to gain information on the relative abundance of the BIRD fragments and the precursor ion. These fragment ion signals due to BIRD IBIRD are subtracted from the mass spectra with irradiation of the laser I0: Ikorr = I0 − IBIRD.
For larger clusters, BIRD has a stronger influence. At room temperature, after a trapping time of 1 s, roughly 75% of the MgCO2(H2O)8 + cluster loose one water molecule, forming Mg(CO2)(H2O)7 + . To yield the corrected IR spectrum of n = 7 at room temperature, the cluster with n = 8 water molecules was isolated to maximize n = 7 as precursor. Then the remaining depletion of n = 8 (due to radiation) is added to the relative abundance of the n = 7 precursor ion.