Energetic and Electronic Properties of NpH^0/+/– and PuH^0/+/–

High-level correlated molecular orbital theory calculations have been performed to predict the thermodynamic and electronic properties of diatomic NpH^0/+/– and PuH^0/+/–. The excited states up to ∼10,000 cm^–1 were predicted for these molecules at the multireference SO-CASPT2 level. The inclusion of spin–orbit effects is fundamental to predict the low-lying state ordering. NpH is predicted to have a ⁵Π₀ ground state, and PuH has a ⁶Π_1/2 ground state at the SO-CASPT2 level. The adiabatic electron affinities (AEAs) and ionization energies (IEs) of NpH and PuH were calculated to be 0.389 and 6.156 and 0.396 and 6.296 eV, respectively, using the Feller–Peterson–Dixon approach. The AEA increases going from AcH (0.425 eV) to ThH (0.820 eV) and decreases from ThH to PuH. The IEs of Pa-Np hydrides are close to ∼6.2 eV followed by an increase of 0.14 eV to PuH (6.296 eV). The An–H bond dissociation energy (BDE) decreases from 276.4 (AcH) to 107.1 (PuH) kJ/mol; the BDE­(NpH) is ∼80 kJ/mol higher than that of PuH. Natural bond orbital calculations show that the bond character for these molecules is mainly ionic, An⁺H^–. The additional electron in NpH^– and PuH^– populates the 6d orbital, and NpH⁺ and PuH⁺ are formed by the removal of a 7s electron. The current work in conjunction with prior work on the AcH to UH in different charge states provides insights into how these properties change across the actinide series.