The electronic structure of ligands with phosphoryl and carbonyl binding sites and their complexation behavior with uranyl nitrate were investigated using density functional theory (DFT). The quantum chemical calculations indicate that the electronic charges on both phosphoryl and carbonyl groups are more polarized toward oxygen atoms in isolated ligands. This effect is predominant in the case of complexes of the former. Both PO and CO groups are positively charged with the exception in methylisobutylketone (MIBK), where the C=O group is virtually neutral. The fragment molecular orbital analysis suggests that during complexation, a certain amount of charge transfer occurs from the filled pπ-orbitals [π x (CO/PO) and π y (CO/PO) ] of the ligand to 5f, 6d, and 7s orbitals of the uranium atom (fσ* and dsσ*). The NBO analysis reaffirms the charge transfer mechanism. The observed red shift in ν­(CO) and ν­(PO) identified in the simulated infrared spectrum of the corresponding complexes implies a moderate weakening of both carbonyl and phosphoryl bonds upon complexation. The atoms in molecules (AIM) analysis suggests a stronger phosphoryl binding compared to carbonyl interactions and an ionic U–O bond. The estimated complexation energies are considerable for phosphoryl ligands compared to those of the carbonyl analogue, with a reasonably large value derived for tri-n-butyl phosphate (TBP). The energy decomposition analysis marked significant stabilizing orbital interactions for phosphoryl ligands. The contributions of estimated dispersion energies are considerable in all complexes and extensively depend on the alkyl unit.

It is well known that a nuclear reactor generates various fission products including radioactive fission gases made of isotopes of Xe and Kr. The separation of Xe and Kr isotopes and their entrapment are important tasks for efficient operation of nuclear reactors and fuel-reprocessing plants. Two-dimensional materials are known to have a large surface-to-volume ratio; this makes them prospective candidates as gas adsorbent materials. Motivated by this, we carry out ab initio density functional theorybased calculations to explore the reactivity and selectivity of a monolayer of pristine and 3d transition metal (TM)-functionalized MoS 2 toward the fission gas atoms Xe and Kr. To this end, we first study and analyze the adsorption of the TM adatoms on the monolayer of MoS 2 at different inequivalent crystallographic sites. Further, we calculate partial atomic charges and atom-projected density of states of the functionalized composite systems to understand the bonding mechanism of TM adatoms with the monolayer of MoS 2 . We predict the coexistence of both ionic and covalent bonding between the TM atoms and the surface atoms. Subsequently, we probe the adsorption of Xe and Kr atoms on both the pristine and the TM-functionalized MoS 2 surfaces. Our calculated results indicate that the adsorption energies for Xe (Kr) gas atoms on the functionalized MoS 2 are enhanced up to a maximum of 2.77 (2.86) times of adsorption energy found in case of the pristine surface. We find that the adsorption energy of Xe and Kr gas atoms over different TM atom-functionalized MoS 2 follows the order Ti > V > Co > Ni > Fe > Sc > Mn > Cr > Cu. Charge density difference analyses indicate that the polarization of Xe/Kr adatoms is enhanced in the case of functionalized surfaces, which leads to the strengthened interaction between the noble gas (NG) atoms and the functionalized surfaces, compared to the adsorption of NG on the pristine MoS 2 surface. This polarization has direct one-to-one correspondence with the adsorption energy of these gas atoms on the surface. Moreover, our calculated results suggest that both the pristine and the functionalized MoS 2 surfaces show selectivity for Xe atom over Kr atom. The present study predicts that the TM atom-functionalized MoS 2 surfaces may have great potential for adsorption and selective separation of Xe and Kr atoms, which may have important implication in the field of spent nuclear fuel management.