Hydrogenating atomic coinage metal cations (group 11) with H2 on charged He droplets (T<10 K) doped with metal Atoms: Early stages, ligation energies, core structures, group variations, and relativistic effects

Abstract

Experimental mass-spectrometric results are reported which demonstrate that ionized helium nanodroplets (at T < 10 K) doped with the coinage metal atoms M = Cu, Ag or Au, and then exposed to molecular hydrogen, provide an abundance of singly and multiply hydrogenated coinage metal cations M⁺(H₂)_n and MH⁺(H₂)_n with n up to 25 and beyond. We explore here the information provided by observed distributions in the measured intensities of these ligated ions with n = 1–6 and use ab initio quantum chemical calculations to extract fundamental insights into variations in ligation energies and structures with increasing H₂ ligation and going down the Periodic Table (M⁺ = Cu⁺, Ag⁺ and Au⁺) with an eye on the role of relativistic contributions to the ligation energies expected with the increasing nuclear charge of M⁺. “Magic” numbers of H₂ additions with relatively high intensity are apparent among the first 5, especially the first 2, H₂ additions. Very substantial relativistic enhancements of the third over the second-row ligation energies were computed, especially for Au⁺: 83 % and 60 % respectively, for the first two H₂ ligands intimately bonded to Au⁺, and 19 % and 67 % respectively, for the addition of the first two H₂ ligands to AuH⁺. Relativistic enhancements for the addition of a third H₂ ligand and beyond were found to be essentially negligible.