Gas-phase lanthanide chlorideclusters: relationships among ESI abundances and DFT structures and energetics

Abstract

Anionic lanthanide chloride clusters, Ln_nCl_3n+1⁻, were produced by electrospray ionization (ESI) of LnCl₃ in isopropanol, where Ln = La–Lu (except Pm); the clusters were characterized using a quadrupole ion trap mass spectrometer. High-abundance “magic number” clusters were apparent at n = 4 for the early Ln (La–Sm), and at n = 5 for the late Ln (Dy–Lu). Density functional theory computations of La_nCl_3n+1⁻ and Lu_nCl_3n+1⁻ clusters (n = 1–6) indicate that the clusters with n = 4–6 are rings with a central chlorine atom. Computed structures show six-coordinate Ln in distorted octahedral sites in “magic number” La₄Cl₁₃⁻ and Lu₅Cl₁₆⁻, which have particularly large dissociation energies. For lanthanum, larger anionic chloride clusters with multiple charges of down to −5 were observed; their fragmentation by collision-induced dissociation in the ion trap revealed La₄Cl₁₃⁻ as a common product. Gas-phase hydrolysis to Ln_nCl3n₊₁₋y(OH)_y⁻ (y = 1, 2) was prevalent for the late lanthanides, but only for small clusters, n = 2 or 3; larger clusters were evidently resistant to gas-phase hydrolysis. ESI of selected LnBr₃ and LnI₃ resulted in Ln_nX_3n+1⁻ clusters (X = Br, I)—in contrast to Ln_nCl_3n+1⁻ clusters, the only observed (minor) high-abundance clusters were La₄Br₁₃⁻ and Ce₄Br₁₃⁻.