Ground and excited states analysis of alkali metal ethylenediamine and crown ether complexes

High-level electronic structure calculations are carried out to obtain optimized geometries and excitation energies of neutral lithium, sodium, and potassium complexes with two ethylenediamine and one or two crown ether molecules. Three different sizes of crowns are employed (12-crown-4, 15-crown-5, 18-crown-6). The ground state of all complexes contains an electron in an s-type orbital. For the mono-crown ether complexes, this orbital is the polarized valence s-orbital of the metal, but for the other systems this orbital is a peripheral diffuse orbital. The nature of the low-lying electronic states is found to be different for each of these species. Specifically, the metal ethylenediamine complexes follow the previously discovered shell model of metal ammonia complexes (1s, 1p, 1d, 2s, 1f), but both mono- and sandwich di-crown ether complexes bear a different shell model partially due to their lower (cylindrical) symmetry and the stabilization of the 2s-type orbital. Li(15-crown-5) is the only complex with the metal in the middle of the crown ether and adopts closely the shell model of metal ammonia complexes. Our findings suggest that the electronic band structure of electrides (metal crown ether sandwich aggregates) and expanded metals (metal ammonia aggregates) should be different despite the similar nature of these systems (bearing diffuse electrons around a metal complex).

Photochemical activation of carbon dioxide in Mg+(CO2)(H2O)0,1

We combine multi-reference ab initio calculations with UV-VIS action spectroscopy to study photochemical activation of CO2 on a singly charged magnesium ion, [MgCO2(H2O)0,1]+, as a model system for the metal/ligand interactions relevant in CO2 photochemistry. For the non-hydrated species, two separated Mg+ 3s-3p bands are observed within 5.0 eV. The low-energy band splits upon hydration with one water molecule. [Mg(CO2)]+ decomposes highly state-selectively, predominantly via multiphoton processes. Within the low-energy band, CO2 is exclusively lost within the excited state manifold. For the high-energy band, an additional pathway becomes accessible: the CO2 ligand is activated via a charge transfer, with photochemistry taking place on the CO2 - moiety eventually leading to a loss of CO after absorption of a second photon. Upon hydration, already excitation into the first and second excited state leads to CO2 activation in the excited state minimum; however, CO2 predominantly evaporates upon fluorescence or absorption of another photon.

Geometric and electronic structure analysis of calcium water complexes with one and two solvation shells

The stability of calcium water complexes is investigated quantum mechanically. Ground and excited electronic states are studied for hexa-, octa-, and octakaideca-coordinated complexes, where calcium valence electrons move to outer diffuse orbitals.

Superatomic nature of alkaline earth metal–water complexes: the cases of Be(H2O)0,+4 and Mg(H2O)0,+6

Beryllium– and magnesium–water complexes are shown to accommodate peripheral electrons around their Be²⁺(H₂O)₄ and Mg²⁺(H₂O)₆ cores in hydrogenic type orbitals.

Spectroscopy and reactivity of size-selected Mg[sup +]-ammonia clusters

Photodissociation spectra for mass-selected Mg(+)(NH(3))(n) clusters for n=1 to 7 are reported over the photon energy range from 7000 to 38 500 cm(-1). The singly solvated cluster, which dissociates primarily via a N-H bond cleavage, exhibits a resolved vibrational structure corresponding to two progressions in the intracluster Mg(+)-NH(3) modes. The addition of the second, third, and fourth solvent molecules results in monotonic redshifts that appear to halt near 8500 cm(-1), where a sharp feature in the electronic spectrum is correlated with the formation of a Mg(+)(NH(3))(4) complex with T(d) symmetry and the closing of the first solvation shell. The spectra for the clusters with 5 to 7 solvent molecules strongly resemble that for the tetramer, suggesting that these solvent molecules occupy a second solvation shell. The wavelength-dependent branching-ratio measurements show that increasing the photon energies generally result in the loss of additional solvent molecules but that enhancements for a specific solvent number loss may reveal special stability for the resultant fragments. The majority of the experimental evidence suggests that the decay of these clusters occurs via the internal conversion of the initially excited electronic states to the ground state, followed by dissociation. In the case of the monomer, the selective cleavage of a N-H bond in the solvent suggests that this internal-conversion process may populate regions of the ground-state surface in the vicinity of an insertion complex H-Mg(+)-NH(2), whose existence is predicted by ab initio calculations.