Thorium and uranium carbide cluster cations in the gas phase: Similarities and differences between thorium and uranium

Laser ionization of AnC₄ alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An _mC_n⁺, with m = 1, n = 2-14, and m = 2, n = 3-18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, Th_mC_n⁺ cluster ions with m = 3-13 and n = 5-30 were also produced, with an intriguing high intensity of Th₁₃C_n⁺ cations. The AnC ₁₃⁺ ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnC_n ⁺ ions with increasing values of n. High abundances of AnC ₂⁺ 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 U₂C ₃⁺ 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 ⁺.