Protactinium and Actinium Monohydrides: A Theoretical Study on Their Spectroscopic and Thermodynamic Properties

Abstract

Spectroscopic and thermodynamics properties including bond dissociation energies (BDEs), adiabatic electron affinities (AEAs), and ionization energies (IEs) have been predicted for AcH and PaH using the Feller–Peterson–Dixon composite approach. Comparisons with previous studies on ThH and UH were performed to identify possible trends in the actinide series. Multireference CASPT2 calculations were used to predict the spin-orbit effects and obtain potential energy curves for the low-lying Ω states around the equilibrium distance as well as the vertical detachment energies (VDEs) from AcH– and PaH– to excited states of the neutral species. The calculated AEA for AnH (An = Ac, Th, Pa, U) showed that the AEA increases from AcH (0.425 eV) to ThH (0.820 eV) and decreases to PaH (0.781 eV) and to UH (0.457 eV), whereas the IE values are 5.887 eV (AcH), 6.181 eV (ThH), 6.204 eV (PaH), and 6.182 eV (UH). The ground state of AcH, AcH–, PaH, and PaH– are predicted to be1Σ+ 0,2Π3/2, 3H4, and 4I9/2, respectively. The BDEs for AcH and PaH are 276.4 and 237.2 kJ/mol, and those for AcH– and PaH– are 242.8 and 239.8 kJ/mol, respectively. The natural bond analysis shows a significant ionic character, An+H–, in the bonding of the neutral hydrides.

Energetic and Electronic Properties of NpH0/+/– and PuH0/+/–

“…To obtain further insights into the electronic structure of the actinide monohydrides, we examined various properties based on the NBOs and NPA charges (Table ). The results show that NpH and PuH have a mainly ionic character as do the other early actinide hydrides. ,, For NpH, the metal has a positive charge of 0.71 e consistent with the assignment of a Np +0.71 H –0.71 ionic bond. These charges are similar to those obtained for UH (+0.71 on U) and PaH (+0.69 on Pa).…”

“…The interaction of Np and Pu with H to form AnH is described using high-level electronic structure methods. Together with our prior work on the Ac, Th, Pa, and U monohydrides, ,, further insights into the role of 5f and 6d electrons in the bonding formation and thermodynamics of these molecules in the +1/0/–1 charge states are provided. The FPD approach was used to predict electron attachment/detachment properties (AEA, VDE, and IE) and BDEs.…”

“…The same result was obtained for the earlier actinide hydrides with the exception of PaH, which showed similar SO corrections for both AEA and IE properties. 2 The IE for NpH is comparable to the atomic IE (6.26554 eV), whereas the IE for PuH is larger by about 0.3 eV as compared to the Pu atom (6.02576 eV). 51 On the basis of our prior work on using this same approach for the IE(UH) 3 where we obtain good agreement with experiment, we estimate an error bar of ±5 kJ/mol for the IEs.…”

“…Obtaining atomic properties of the actinides from direct calculations is also difficult, but a way to probe the electronic structure of an actinide is to examine the An−H interaction. Our prior calculations at the SO-CASPT2 level on the early actinide monohydrides, AcH (7s 2 ), 2 ThH (6d 1 7s 2 ), 3,4 PaH (5f 2 7s 2 ), 2 and UH (5f 3 7s 2 ), 5 showed that there are 10, 26, 63, and 51 accessible states within 1.6 eV for the Ac, Th, Pa, and U hydrides, respectively, as the introduction of valence 5f electrons increases the number of excited states but not in a simple fashion. At the Feller−Peterson−Dixon (FPD) level, 6−9 based on combining CCSD(T) calculations extrapolated to the complete basis set limit, adiabatic electron affinities (AEA) were predicted to increase from AcH (0.42 eV) to ThH (0.82 eV) and then to decrease for PaH (0.78 eV) to UH (0.46 eV).…”

Bonding, Thermodynamics, and Spectroscopy of the Metal Borides UB0/+/– and WB0/+/–

“…These simple borides show that fractional bonding persists in the borides of heavy metals. The behavior of U is in contrast to that of Th and Ac which behave like transition metals in many cases, whereas Pa in such small diatomics is often behaving as an actinide. − …”