Actinide sulfides in the gas phase: experimental and theoretical studies of the thermochemistry of AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm)

Abstract

The gas-phase thermochemistry of actinide monosulfides, AnS, was investigated experimentally and theoretically. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the reactivity of An(+) and AnO(+) (An = Th, Pa, U, Np, Pu, Am and Cm) with CS(2) and COS, as well as the reactivity of the produced AnS(+) with oxidants (COS, CO(2), CH(2)O and NO). From these experiments, An(+)-S bond dissociation energies could be bracketed. Density functional theory studies of the energetics of neutral and monocationic AnS (An = Ac, Th, Pa, U, Np, Pu, Am and Cm) provided values for bond dissociation energies and ionization energies; the computed energetics of neutral and monocationic AnO were also obtained for comparison. The theoretical data, together with comparisons with known An(+)-O bond dissociation energies and M(+)-S and M(+)-O dissociation energies for the early transition metals, allowed for the refining of the An(+)-S bond dissociation energy ranges obtained from experiment. Examination of the reactivity of AnS(+) with dienes, coupled to comparisons with reactivities of the AnO(+) analogues, systematic considerations and the theoretical results, allowed for the estimation of the ionization energies of the AnS; the bond dissociation energies of neutral AnS were consequently derived. Estimates for the case of AcS were also made, based on correlations of the data for the other An and the electronic energetics of neutral and ionic An. The nature of the bonding in the elementary molecular actinide chalcogenides (oxides and sulfides) is discussed, based on both the experimental data and the computed electronic structures. DFT calculations of ionization energies for the actinide atoms and the diatomic sulfides and oxides are relatively reliable, but the calculation of bond dissociation energies is not uniformly satisfactory, either with DFT or CCSD(T). A key conclusion from both the experimental and theoretical results is that the 5f electrons do not substantially participate in actinide-sulfur bonding. We emphasize that actinides form strikingly strong bonds with both oxygen and sulfur.

Relativistic Pseudopotentials and Their Applications

“…Kovács et al [106] evaluated trends in the bond distances and dissociation enthalpies of AnO and AnO 2 (An = Th-Lr) at the DFT level and demonstrated the very good performance of the B3LYP exchange-correlation functional in conjunction with SPPs. Pereira et al [107] published SPP DFT studies on neutral and monocationic AnS (An = Ac-Cm), where it was found that both B3LYP and MPW1PW91 produced ionization energies in good agreement with the experimental values, i.e., the mean difference amounts to 22 kJ/mol with the larger deviations occurring for Np (65 kJ/mol) and Cm (35 kJ/mol). For ThS and CmS, DFT yields binding energies, which are within 15 kJ/mol of the SPP CCSD(T) values.…”

The pure rotational spectrum of thorium monosulfide, ThS

“…The ThS bond dissociation energy and the ionization energy were derived from these measurements. In addition, Liang et al [3] and Pereira et al [4] predicted the properties of ThS using Density Functional Theory (DFT) methods. These calculations yielded results that were in good agreement with their experimental results.…”

“…The spectroscopy and reaction dynamics of gas phase ThS have been examined in recent years [1][2][3][4]. Pereira et al [4] used mass spectrometry to examine the reactions of both ThS and ThS + with NO, CO 2 , OCS and CH 2 O. The ThS bond dissociation energy and the ionization energy were derived from these measurements.…”

Synthesis, structure and bonding of actinide disulphide dications in the gas phase

“…[23][24][25] In a previous study of An sulphides, we were able to produce AnS 2 + ions for An = Th, Pa, U, and Np by sequential reactions of An + ions with OCS. 26 Earlier experiments by Gibson had shown that AnS 2 + could be obtained using thiols as reagents for An = U, Np, Pu, and Am.…”

“…Experimental details 3 Gas-phase experiments were performed by Fourier transform ion-cyclotron resonance mass spectrometry (FTICR-MS), using procedures described in detail in our previous studies of actinide sulphides, 18,26 as well as in the Supporting Information. The An 2+ ions were produced by direct laser desorption/ionization of small pieces of alloys that contained ~20% of Th or ~5% of the remaining actinide metals (Np, Pu, Am) in a Pt matrix.…”