Abstract
The investigation of copper nitrate cluster anions Cu(ii)n(NO3)2n+1-, n ≤ 4, in the gas phase using ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy provides detailed insight into the electronic structure of the copper salt and its intriguing photochemistry. In the experimentally studied region up to 5.5 eV, the spectra of copper(ii) nitrate exhibit a 3d-3d band in the vis/NIR and well-separated bands in the UV. The latter bands originate from Ligand-to-Metal Charge Transfer (LMCT) as well as n-π* transitions in the nitrate ligands. The clusters predominantly decompose by loss of neutral copper nitrate in the electronic ground state after internal conversion or via the photochemical loss of a neutral NO3 ligand after a LMCT. These two decomposition channels are in direct competition on the ground state potential energy surface for the smallest copper nitrate cluster, Cu(ii)(NO3)3-. Here, copper nitrate evaporation is thermochemically less favorable. Population of π* orbitals in the nitrate ligands may lead to N-O bond photolysis. This is observed in the UV region with a small quantum efficiency, with photochemical loss of either nitrogen dioxide or an oxygen atom.
Highlights
The photochemistry of systems can be challenging to describe by quantum chemical methods since excited state calculations are still much more demanding than calculations on the electronic ground state.[26,27]
We recently investigated photochemical hydrogen evolution from hydrated magnesium,[55,56] the effect of salt environments on the reactions and photolysis of organic substances,[57,58,59] as well as the evolution of the hydration environment of a single electron[60] or a carbon dioxide radical anion[61] using action spectroscopy
Internal conversion dominates after exciting 3d–3d excitations in the visible region and Ligand-to-Metal Charge Transfer (LMCT) excitations in the UV, which allowed the photochemical loss of the neutral ligand.[66]
Summary
Copper and copper oxide nanoparticles are used in industry for many oxidation[1,2,3] and reduction processes[4,5] due to their favorable catalytic properties.[6,7] As a coinage metal, copper exhibits a very distinct chemistry acting as an electron donor.[8,9] It is useful in methanol synthesis,[10,11] carbon dioxide activation[12,13] and hydrogen storage applications.[14,15,16] Copper salts are typically used in the production of heterogeneous catalysts via a calcination process.[17,18,19,20] Copper nitrate is the most widely used copper salt due to its stability, high availability and comparatively low cost.[21]From a fundamental point of view, the photochemistry of nitrate is of high interest in its own right since it plays a key role in our atmosphere as trace compound.[22].
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