FULL PAPER DOI:10.1002/ejic.201402211 The Role of Residual Charges in the Interaction between NpO 2 + and Th 4+ Cations: Spectrophotometric and Computational Studies Parveen Kumar Verma, [a] Priyanath Pathak,* [a] Arunasis Bhattacharyya, [a] Dattaprasad Ravaji Prabhu, [a] Tapan Kumar Ghanty, [b] and Prasanta Kumar Mohapatra [a] Keywords: Thorium / Uranium / Neptunium / Thermodynamics / Spectrophotometry / Density functional theory Spectrophotometric investigations have been carried out in order to understand the interaction of NpO 2 + (8.5  10 –5 M) with Th 4+ under varying conditions of nitric acid (1–6 M HNO 3 ), thorium concentrations (up to 0.86 M), and tempera- ture (293–343 K). These studies show the appearance of the signature peak of Np IV (ca. 964 nm) in addition to those re- sponsible for the NpO 2 + species (980 nm) and the NpO 2 + – Th 4+ complex (ca. 994 nm). The intensity of the Np IV peak increases with increasing temperature, while that of the Np V Introduction The flexibility in the electronic structures of higher-va- lent actinide cations results in a fascinating and rich coordi- nation chemistry. This is particularly true for the dioxido penta- and hexavalent cations of the lighter actinides such as U, Np, Pu, and Am. [1] Even though the oxido moiety is assumed to be chemically inert in these cations, vast litera- ture exists that shows that the pentavalent Np cation (NpO 2 + ) displays a tendency of binding with charged metal ions. [2] Such interactions of Np V with polyvalent metal cat- ions are reported to be more probable in noncomplexing perchlorate media. [3] Similar interactions have been re- ported for other pentavalent actinide cations such as UO 2 + , PuO 2 + , and AmO 2 + with several other cations. [4–6] Interest- ingly, Madic et al. reported similar interactions in the case of pentavalent cis-dioxidovanadium(V), VO 2 + , with oxido- vanadium(IV), VO 2+ . [7] Major studies have been reported for NpO 2 + cations in solutions as well as in roughly half of the reported inorganic Np V solid compounds. These inter- actions have significant influence on Np V structural chemis- try and on the electronic properties. [8] Generally, these inter- [a] Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India E-mail: ppathak@barc.gov.in www.barc.gov.in [b] Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400085, India Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402211. Eur. J. Inorg. Chem. 2014, 3547–3554 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3547 peak decreases. The Th interaction is noticed at higher Th IV concentrations, which explains the reason for neglecting the Np V –Np IV interactions. The variation of the stability constant data with temperature suggests both exothermic (at low acid- ity) and endothermic nature of interactions. Density func- tional theory (DFT) calculations have shown that the NpO 2 + – Th 4+ interaction is stronger than that of the NpO 2 + –UO 2 2+ one, which has been attributed to high positive charge on the Th 4+ ions. actions are explained in terms of the bonding of the -yl atom of an actinyl ion with another actinyl cation, which has been attributed to the presence of a residual negative charge of –0.6 on the -yl oxygen atoms of the NpO 2 + moi- ety. [9] The residual negative charge on the oxygen atoms of neptunyl(VI) is reported to be –0.17. [10] The higher residual charges on the oxygen atoms of neptunyl(V) cations make it more active for interactions as compared to those of actinyl(VI) cations. Choppin and Rao reported that the -yl oxygen atoms of actinyl cations (both penta- and hexava- lent) possess an almost similar negative charge, i.e. –0.6 sug- gesting thereby that attractive forces between the negatively charged oxygen atoms and any metal cation would be iden- tical for the oxidation states. [8,9,11–15] Generally, these stud- ies have indicated the formation of inner-sphere T-shaped solution phase geometry. A recent work shows cation–cat- ion interactions between Np VI units by boric acid flux reac- tion of NpO 2 (ClO 4 ) 2 with NaClO 4 , which forms a layered structure of double neptunyl(VI) borate sheets bridged by another Np VI site through cation–cation interactions. [15] Experimental conditions such as ionic strength, metal con- centrations, nature of acid and its concentration, and tem- perature show significant influence on cation–cation inter- actions (CCIs) in aqueous media. [16–18] The majority of these studies have been performed in perchlorate media with the aim of determining the stability constant values and structure of these complexes. There is a need to extend these studies in a nitric acid medium, es- sentially because of its use in dissolution and reprocessing