Photochemistry and spectroscopy of small hydrated magnesium clusters Mg+(H2O)n, n = 1–5

Abstract

Hydrated singly charged magnesium ions Mg(HO), n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D states (correlating with the 3s-3p transitions of Mg) are observed for all cluster sizes, as well as a second absorption band at 4-5 eV for n = 3-5. Due to the lifted degeneracy of the 3p energy levels of Mg, the absorptions are broad and red shifted with increasing coordination number of the Mg center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of HO, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

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

“…The excitation at 4.93 eV (MRCI(7,9)/ def2TZVP//CCSD/aug-cc-pVDZ) corresponds to excitation into the significantly perturbed 3p z orbital of Mg + , which is collinear with the CO 2 ligand. This is consistent with previous interpretation and the similar case of [Mg(H 2 O)] + [47,49,[54][55][56].…”

“…Furthermore, formation of [Mg(OH)] + as the second most intense channel likely occurs sequentially by absorption of the predominant [Mg(H 2 O)] + fragment as it has an intense absorption band in this range and loses an H atom [47,49]. Additionally, H loss from the comparable [Mg(H 2 O) 2 ] + case happens via a multiphoton-process, in line with our interpretation [47,49].…”

“…The D 1 minimum with linear CO 2 has a dramatically different structure compared to the FC point. After promoting the electron from the 3s to the 3p orbital of Mg + , the ion linearizes analogously to [Mg(H 2 O) 2 ] + [47,49] and the water molecule rotates by 90° to maximize the interaction of the positively charged hydrogen atoms with the electron in the 3p orbital of Mg + , gaining 0.53 eV of internal energy. Contrary to the case without the water ligand, the bent minimum with an activated CO 2 -ligand is now accessible with the energy available after excitation.…”

“…Due to their intriguing charge-transfer chemistry in the ground state, hydrated magnesium ions Mg + (H 2 O) n have 1 Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria been studied extensively by theory and experiment [38][39][40][41][42][43][44][45][46]. Photodissociation spectroscopy of hydrated magnesium ions, Mg + (H 2 O) n , provides a suitable model system and benchmark tool for theoretical calculations to investigate the hydrogen production on metal centers [47][48][49][50][51]. Theoretical investigations of Mg + complexes go back to 1991 [52,53].…”

Asymmetric Solvation of the Zinc Dimer Cation Revealed by Infrared Multiple Photon Dissociation Spectroscopy of Zn2+(H2O)n (n = 1–20)

“…Studying hydrated metal ions in the gas phase provides the opportunity to investigate solvation at a molecular level and to study various chemical processes such as hydrogen production via catalysis at metal centers [32] and corrosion effects [33] in a welldefined environment. Our group has an extended history in studying these systems [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Infrared action spectroscopy, coupled with quantum chemical calculations, is a powerful technique used to investigate the structures of gas-phase molecular complexes, [52][53][54][55][56][57][58][59][60] in particular hydrated metal ions [61][62][63][64][65][66][67][68][69].…”