Thorium and Uranium Carbide Cluster Cations in the Gas Phase: Similarities and Differences between Thorium and Uranium

Abstract

Laser ionization of AnC₄ alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An_<i>m</i>C_<i>n</i>⁺, with <i>m</i> = 1, <i>n</i> = 2–14, and <i>m</i> = 2, <i>n</i> = 3–18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, Th_<i>m</i>C_<i>n</i>⁺ cluster ions with <i>m</i> = 3–13 and <i>n</i> = 5–30 were also produced, with an intriguing high intensity of Th₁₃C_<i>n</i>⁺ cations. The AnC₁₃⁺ ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnC_<i>n</i>⁺ ions with increasing values of <i>n</i>. High abundances of AnC₂⁺ and AnC₄⁺ ions are consistent with enhanced stability due to strong metal–C₂ bonds. Among the most abundant bimetallic ions was Th₂C₃⁺ for thorium; in contrast, U₂C₄⁺ was the most intense bimetallic for uranium, with essentially no U₂C₃⁺ appearing. Density functional theory computations were performed to illuminate this distinction between thorium and uranium. The computational results revealed structural and energetic disparities for the An₂C₃⁺ and An₂C₄⁺ cluster ions, which elucidate the observed differing abundances of the bimetallic carbide ions. Particularly noteworthy is that the Th atoms are essentially equivalent in Th₂C₃⁺, whereas there is a large asymmetry between the U atoms in U₂C₃⁺