Bonding, Thermodynamics, and Spectroscopy of the Metal Borides UB^0/+/– and WB^0/+/–

Abstract

The bonding and spectroscopy of the UB0/+/– and WB0/+/– molecules were examined by performing high-level electronic structure calculation on their low-lying electronic states. The calculations were performed at the SO-CASPT2 level to obtain the low-lying excited states and at the FPD level to calculate the adiabatic electronic affinities (AEA), ionization energies (IE), and bond dissociation energies (BDE). Compared to UC and UN, UB has a much denser manifold of states below 1.7 eV. The ground state of UB is predicted to be 8I5/2, and that of WB is 6Π7/2. The calculated IEs of UB and WB are 6.241 and 7.314 eV, respectively, and the corresponding AEAs are 1.160 and 1.422 eV. The BDE of UB is predicted to be 223.1 kJ/mol, which is considerably lower than those predicted for UC and UN and ∼35 kJ/mol lower than the BDE of WB. NBO calculations show that the U and B are connected by two 1-electron π bonds and one 1-electron σ bond with substantial ionic character and a bond order of 1.5. There are three unpaired electrons in the 5f on U. WB has less ionic character than UB with a doubly occupied π bond and a singly occupied σ bond for a bond order of ∼1.5. The results show that the U in UB behaves more like an actinide and the W in WB more like a transition metal.

Probing the Chemical Bond between Lanthanides and Carbon: CeC, PrC, NdC, LuC, and TmC2

“…This is similar to that found for diatomic UC, where the 2σ orbital is 50.3% U, 49.7% C in character and the 1π orbital is 40.1% U, 59.9% C in character . Similar degrees of f-block metal participation in the bonding orbitals are also found for the pentavalent NPrO molecule and in diatomic UB. , For the 1π bonding MO, the 5dπ Ln , 4fπ Ln , and 2pπ C orbital contributions for La, Ce, Pr, and Nd are quite similar. In contrast, the 2σ bonding MO displays a systematic variation in the lanthanide 5dσ and 4fσ orbital contributions.…”

“…96 Similar degrees of f-block metal participation in the bonding orbitals are also found for the pentavalent N�Pr�O molecule and in diatomic UB. 97,98 For the 1π bonding MO, the 5dπ Ln , 4fπ Ln , Lanthanum carbide has the greatest 5dσ and least 4fσ orbital contribution to the 2σ bonding MO. The amount of 5dσ character then decreases and the amount of 4fσ character increases as one moves through the series of early lanthanide carbides LaC, CeC, PrC, and NdC, reflecting the increasing stabilization of the 4f orbitals as one traverses the early lanthanide series.…”

Energetic and Electronic Properties of UO0/± and UF0/±

“…For UF, the spin−orbit contribution is the most important, and it will be discussed in the electronic structure analysis. Comparing the AEAs of UB (1.160 eV), 27 UC (1.493 eV), 28 and UN (1.402 eV), 29 the AEAs for UO and UF follow the decreasing trend predicted for UC to UF. The AEA of UF is quite low relative to the other diatomics with first-row atoms bound to U.…”

“…The same trend is observed for UF, UF + , and UF − as well as for UN, UN + , and UN − . 29 For UC 28 and UB, 27 the bond distance slightly shortens on ionization (from 1.854 to 1.825 Å and from 2.296 to 2.290 Å, respectively) and elongates (to 1.886 Å and 2.337 Å, respectively) on electron absorption. The harmonic vibrational frequencies increase for UC + and UC − 10 to 15 cm −1 larger than for UC.…”

“…Bond dissociation energies (BDEs), adiabatic electron affinities (AEAs), and adiabatic ionization energies (IEs) have been predicted for these six molecules using the Feller−Peterson−Dixon composite approach. Comparisons to previous work on UB, 27 UC, 28 and UN 29 were performed to identify possible trends in the series.…”