Signatures of Size-Dependent Structural Patterns in Hydrated Copper(I) Clusters, Cu⁺(H₂O)_n=1–10

Abstract

The isomers of a hydrated Cu(I) ion with n = 1-10 water molecules were investigated by using ab initio quantum chemistry and an automated isomer-search algorithm. The electronic structure and vibrational spectra of the hundreds of resulting isomers were used to analyze the source of the observed bonding patterns. A structural evolution from dominantly two-coordinate structures (n = 1-4) toward a mixture of two- and three-coordinate structures was observed at n = 5-6, where the stability provided by expanded hydrogen-bonding was competitive with the dominantly electrostatic interaction between the water ligand and remaining binding sites of the metal ion. Further hydration (n = 7-10) led to a mixture of three- and four-coordinate structures. The metal ion was found, through spectroscopic signatures, to appreciably perturb the O-H bonds of even third-shell water molecules, which highlighted the ability of this nominally simple ion to partially activate the surrounding water network.

Helium nanodroplets doped with copper and water

“…The results presented in Figure 6 are markedly different from those reported earlier [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Michl and coworkers applied secondary-ion mass spectrometry of copper covered with a frozen water film [19].…”

“…This rather unusual feature has been traced to the peculiar structure of (H 2 O) 2 Cu + whose lowest-energy structure has the H 2 O molecules adsorbed on opposite sides of the Cu + ion with the O atoms facing Cu + . Most theoretical studies agree that Cu + in (H 2 O) 3 Cu + and (H 2 O) 4 Cu + is two-fold coordinated, but the increase to three-or even four-fold coordination in ions containing as many 10 H 2 O is less clear [21][22][23][24][25].…”

“…We also see an abrupt drop of the abundance at (H 2 O) 6 Cu + which suggests a sudden decrease in the dissociation energy. The only theoretical study of ions in this size range [25] reported relative energies of structural isomers relative to the ground state structure; dissociation energies of (H 2 O) n Cu + ions in their ground state were not reported. However, it was noted that the coordination of Cu + increased from two or three for n = 5, 6 to three or four for n ≥ 7.…”

“…(H 2 O) 2 Cu + is remarkably stable; it's dissociation energy (1.7 eV) exceeds that of (H 2 O)Cu + by about 0.2 eV [19]. Several theoretical studies have shown that Cu + is, indeed, two-fold coordinated; the oxygen atoms of the first two H 2 O bind to opposite sides of Cu + [21][22][23][24][25]. The interaction is nearly evenly divided among electrostatic, polarization, and charge transfer; the Cu Mulliken charge is only +0.85 e [25].…”

“…Several theoretical studies have shown that Cu + is, indeed, two-fold coordinated; the oxygen atoms of the first two H 2 O bind to opposite sides of Cu + [21][22][23][24][25]. The interaction is nearly evenly divided among electrostatic, polarization, and charge transfer; the Cu Mulliken charge is only +0.85 e [25]. Properties of (H 2 O) n Cu + have also been explored by vibrational spectroscopy [26][27][28].…”