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

Laser ionization of AnC(4) alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An(m)C(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, ThmCn+ cluster ions with m = 3-13 and n = 5-30 were also produced, with an intriguing high intensity of Th13Cn+ 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 AnC(4)(+) ions are consistent with enhanced stability due to strong metal-C-2 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 isparities for the An(2)C(3)(+) and An(2)C(4)(+) 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+.