On the Molecular Structure of Uranium Dicarbide: T-Shape versus Linear Isomers

Abstract

A theoretical study of the molecular structure of uranium dicarbide has been carried out employing DFT, coupled cluster, and multiconfigurational methods. A triangular species, corresponding to a (5)A(2) electronic state, has been found to be the most stable UC(2) species. A triplet linear CUC species, which has been observed in recent infrared spectroscopy experiments, lies much higher in energy. A topological analysis of the electronic density has also been carried out. The triangular species is shown to be in fact a T-shape structure with a U-C interaction which can be considered to be a closed-shell interaction.

Molecular structure of uranium carbides: Isomers of UC3

“…The threemembered ring UC 2 has very similar geometrical parameters as the 5 A 2 state of uranium dicarbide at the same levels of theory, 9 whereas the C 1 -U bond distance approaches the bond distance found in linear CUC, namely, 1.834 Å and 1.820 Å at the B3LYP and CCSD levels, respectively. The predicted most intense line in the IR spectrum of this species corresponds to 521 cm −1 (B3LYP) or 529 cm −1 (CCSD).…”

“…This is in contrast with the behavior observed for the UC 2 system, because CUC is a true minimum, whereas UCC was shown to be in fact a transition state with imaginary π vibrational frequencies. 9 It is worth pointing out that the UCCC species has a large dipole moment, partly due to the arrangement of the atoms in this structure.…”

“…It should be pointed out that in the experiments by Wang et al 7,8 the linear CUC isomer is apparently observed, but the UC 2 cyclic isomer is considerably more stable. 9,10 Therefore, the presence of cyclic UC 2 cannot be ruled out. In fact the predicted dissociation energy of cyclic UC 2 9 agrees rather well with the value derived from previous mass spectrometry studies.…”

“…9 In first place molecular geometries have been optimized at the DFT level, employing the B3LYP exchange-correlation functional. 11,12 Subsequently, geometries have been obtained at the CCSD (coupled-cluster single and double excitation model) level.…”

“…Despite this result, the species observed in the IR experiments was concluded to be a symmetric linear CUC isomer with a 3 u + electronic state, which is predicted to lie about 238 kJ/mol 9 or 280 kJ/mol 10 higher in energy than the triangular isomer, according to the most reliable theoretical methods employed. Another possible isomer, a linear UCC species, is shown by theoretical calculations 9 to be in fact a a) Author to whom correspondence should be addressed. Electronic mail:…”