A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Coordination and Activation of N2 at Low-Valent Magnesium using a Heterobimetallic Approach: Synthesis and Reactivity of a Masked Dimagnesium Diradical

The activation of dinitrogen (N2 ) by transition metals is central to the highly energy intensive, heterogeneous Haber-Bosch process. Considerable progress has been made towards more sustainable homogeneous activations of N2 with d- and f-block metals, though little success has been had with main group metals. Here we report that the reduction of a bulky magnesium(II) amide [(TCHP NON)Mg] (TCHP NON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5 % w/w K/KI yields the magnesium-N2 complex [{K(TCHP NON)Mg}2 (μ-N2 )]. DFT calculations and experimental data show that the dinitrogen unit in the complex has been reduced to the N2 2- dianion, via a transient anionic magnesium(I) radical. The compound readily reductively activates CO, H2 and C2 H4 , in reactions in which it acts as a masked dimagnesium(I) diradical.

Interactions of Si+(2PJ) and Ge+ (2PJ) with rare gas atoms (He-Rn): interaction potentials, spectroscopy, and ion transport coefficients

Accurate interatomic potentials were calculated for the interaction of a singly-charged silicon cation, Si⁺, with a single rare gas atom, RG (RG = Kr-Rn), as well as a singly-charged germanium cation, Ge⁺, with a single rare gas atom, RG (RG = He-Rn). The RCCSD(T) method and basis sets of quadruple-ζ and quintuple-ζ quality were employed; each interaction energy is counterpoise corrected and extrapolated to the basis set limit. The lowest electronic term (²P) of each cation was considered, and the interatomic potentials calculated for the diatomic terms that arise from these: ²Π and ²Σ⁺. Additionally, the interatomic potentials for the respective spin-orbit levels were calculated, and the effect on the spectroscopic parameters was examined. Variations in several spectroscopic parameters with the increasing atomic number of RG were examined. The presence of incipient chemical interaction was also examined via Birge-Sponer-like plots and various population analyses across the series. In the cases of heavier RG, these were consistent with a small amount of electron transfer from the heavier RG atom to the cation, rationalizing the spin-orbit splittings. This was also supported by the observed larger-than-expected spin-orbit splittings for the Si⁺-RG complexes. Finally, each set of RCCSD(T) potentials including spin-orbit coupling was employed to calculate transport coefficients for the cation moving through a bath of the RG. The calculated ion mobilities showed significant differences for the two atomic spin-orbit states, arising from subtle changes in the interaction potentials.

A study of the reactions of Ni+ and NiO+ ions relevant to planetary upper atmospheres

The reactions between Ni+(2D) and O3, O2, N2, CO2 and H2O were studied at 294 K using the pulsed laser ablation at 532 nm of a nickel metal target in a fast flow tube, with mass spectrometric detection of Ni+ and NiO+. The rate coefficient for the reaction of Ni+ with O3 is k(294 K) = (9.7 ± 2.1) × 10-10 cm3 molecule-1 s-1; the reaction proceeds at the ion-permanent dipole enhanced Langevin capture rate with a predicted T-0.16 dependence. Electronic structure theory calculations were combined with Rice-Ramsperger-Kassel-Markus theory to extrapolate the measured recombination rate coefficients to the temperature and pressure conditions of planetary upper atmospheres. The following low-pressure limiting rate coefficients were obtained for T = 120-400 K and He bath gas (in cm6 molecule-2 s-1, uncertainty ±σ at 180 K): log10(k, Ni+ + N2) = -27.5009 + 1.0667log10(T) - 0.74741(log10(T))2, σ = 29%; log10(k, Ni+ + O2) = -27.8098 + 1.3065log10(T) - 0.81136(log10(T))2, σ = 32%; log10(k, Ni+ + CO2) = -29.805 + 4.2282log10(T) - 1.4303(log10(T))2, σ = 28%; log10(k, Ni+ + H2O) = -24.318 + 0.20448log10(T) - 0.66676(log10(T))2, σ = 28%). Other rate coefficients measured (at 294 K, in cm3 molecule-1 s-1) were: k(NiO+ + O) = (1.7 ± 1.2) × 10-10; k(NiO+ + CO) = (7.4 ± 1.3) × 10-11; k(NiO+ + O3) = (2.7 ± 1.0) × 10-10 with (29 ± 21)% forming Ni+ as opposed to NiO2+; k(NiO2+ + O3) = (2.9 ± 1.4) × 10-10, with (16 ± 9)% forming NiO+ as opposed to ONiO2+; and k(Ni+·N2 + O) = (7 ± 4) × 10-12. The chemistry of Ni+ and NiO+ in the upper atmospheres of Earth and Mars is then discussed.

Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2 (H2 O)n ].+ , n=0-8

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.

Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20-70: spontaneous formation of a hydrated electron?

Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n-1]+ to [Mg(H2O)n]+ around n = 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 20-70 measured for photon energies of 1.0-5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7-1.8 eV and 2.1-2.5 eV, depending on cluster size. The spectra shift slowly from n = 20 to n = 50, but no significant change is observed for n = 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e- pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

A study of the reactions of Al+ ions with O3, N2, O2, CO2 and H2O: influence on Al+ chemistry in planetary ionospheres

The reactions between Al+(31S) and O3, O2, N2, CO2 and H2O were studied using the pulsed laser ablation at 532 nm of an aluminium metal target in a fast flow tube, with mass spectrometric detection of Al+ and AlO+. The rate coefficient for the reaction of Al+ with O3 is k(293 K) = (1.4 ± 0.1) × 10-9 cm3 molecule-1 s-1; the reaction proceeds at the ion-dipole enhanced Langevin capture frequency with a predicted T-0.16 dependence. For the recombination reactions, electronic structure theory calculations were combined with Rice-Ramsperger-Kassel-Markus theory to extrapolate the measured rate coefficients to the temperature and pressure conditions of planetary ionospheres. The following low-pressure limiting rate coefficients were obtained for T = 120-400 K and He bath gas (in cm6 molecule-2 s-1, uncertainty ±σ at 180 K): log10(k, Al+ + N2) = -27.9739 + 0.05036 log10(T) - 0.60987(log10(T))2, σ = 12%; log10(k, Al+ + CO2) = -33.6387 + 7.0522 log10(T) - 2.1467(log10(T))2, σ =13%; log10(k, Al+ + H2O) = -24.7835 + 0.018833 log10(T) - 0.6436(log10(T))2, σ = 27%. The Al+ + O2 reaction was not observed, consistent with a D°(Al+-O2) bond strength of only 12 kJ mol-1. Two reactions of AlO+ were also studied: k(AlO+ + O3, 293 K) = (1.3 ± 0.6) × 10-9 cm3 molecule-1 s-1, with (63 ± 9)% forming Al+ as opposed to OAlO+; and k(AlO+ + H2O, 293 K) = (9 ± 4) × 10-10 cm3 molecule-1 s-1. The chemistry of Al+ in the ionospheres of Earth and Mars is then discussed.

CO2/O2 Exchange in Magnesium-Water Clusters Mg+(H2O) n

Hydrated singly charged metal ions doped with carbon dioxide, Mg2+(CO2)-(H2O) n, in the gas phase are valuable model systems for the electrochemical activation of CO2. Here, we study these systems by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry combined with ab initio calculations. We show that the exchange reaction of CO2 with O2 proceeds fast with bare Mg+(CO2), with a rate coefficient kabs = 1.2 × 10-10 cm3 s-1, while hydrated species exhibit a lower rate in the range of kabs = (1.2-2.4) × 10-11 cm3 s-1 for this strongly exothermic reaction. Water makes the exchange reaction more exothermic but, at the same time, considerably slower. The results are rationalized with a need for proper orientation of the reactants in the hydrated system, with formation of a Mg2+(CO4)-(H2O) n intermediate while the activation energy is negligible. According to our nanocalorimetric analysis, the exchange reaction of the hydrated ion is exothermic by -1.7 ± 0.5 eV, in agreement with quantum chemical calculations.

Meteoric Metal Chemistry in the Martian Atmosphere

Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN mission show that a persistent layer of Mg⁺ ions occurs around 90 km in the Martian atmosphere but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global meteoric ablation rate of 0.06 t sol^-1, out of a cosmic dust input of 2.7 ± 1.6 t sol^-1. The absence of detectable Mg at 90 km requires that at least 50% of the ablating Mg atoms ionize through hyperthermal collisions with CO₂ molecules. Dissociative recombination of MgO⁺.(CO₂)_n cluster ions with electrons to produce MgCO₃ directly, rather than MgO, also avoids a buildup of Mg to detectable levels. The meteoric injection rate of Mg, Fe, and other metals-constrained by the IUVS measurements-enables the production rate of metal carbonate molecules (principally MgCO₃ and FeCO₃) to be determined. These molecules have very large electric dipole moments (11.6 and 9.2 Debye, respectively) and thus form clusters with up to six H₂O molecules at temperatures below 150 K. These clusters should then coagulate efficiently, building up metal carbonate-rich ice particles which can act as nucleating particles for the formation of CO₂-ice clouds. Observable mesospheric clouds are predicted to occur between 65 and 80 km at temperatures below 95 K and above 85 km at temperatures about 5 K colder.

A new model for magnesium chemistry in the upper atmosphere

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Mg-containing species, which are important for the chemistry of meteor-ablated magnesium in the upper mesosphere/lower thermosphere region. The study is motivated by the very recent observation of the global atomic Mg layer around 90 km, using satellite-born UV-visible spectroscopy. In the laboratory, Mg atoms were produced thermally in the upstream section of a fast flow tube and then converted to the molecular species MgO, MgO(2), OMgO(2), and MgCO(3) by the addition of appropriate reagents. Atomic O was added further downstream, and Mg was detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined at 300 K: k(MgO + O → Mg + O(2)) = (6.2 ± 1.1) × 10(-10); k(MgO(2) + O → MgO + O(2)) = (8.4 ± 2.8) × 10(-11); k(MgCO(3) + O → MgO(2) + CO(2)) ≥ 4.9 × 10(-12); and k(MgO + CO → Mg + CO(2)) = (1.1 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1). Electronic structure calculations of the relevant potential energy surfaces combined with RRKM theory were performed to interpret the experimental results and also to explore the likely reaction pathways that convert MgCO(3) and OMgO(2) into long-lived reservoir species such as Mg(OH)(2). Although no reaction was observed in the laboratory between OMgO(2) and O, this is most likely due to the rapid recombination of O(2) with the product MgO(2) to form the relatively stable O(2)MgO(2). Indeed, one significant finding is the role of O(2) in the mesosphere, where it initiates holding cycles by recombining with radical species such as MgO(2) and MgOH. A new atmospheric model was then constructed which combines these results together with recent work on magnesium ion-molecule chemistry. The model is able to reproduce satisfactorily some of the key features of the Mg and Mg(+) layers, including the heights of the layers, the seasonal variations of their column abundances, and the unusually large Mg(+)/Mg ratio.