The Role of Residual Charges in the Interaction between NpO₂⁺ and Th⁴⁺ Cations: Spectrophotometric and Computational Studies

Abstract

Spectrophotometric investigations have been carried out in order to understand the interaction of NpO 2 + (8.5 ϫ 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 temperature (293-343 K). These studies show the appearance of the signature peak of Np IV (ca. 964 nm) in addition to those responsible 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[a]

Effect of hydration of a room temperature ionic liquid on the cation–cation interaction of UO22+ and NpO2+ ions

“…991 nm) or aqueous medium (ca. 992 nm) 11,12 but the overall shift in the Np(V) peak before and after U(VI) addition was similar (16 nm) to that of the wet RTIL.…”

“…The CCI of Np(V) with U(VI) was predominantly studied in solutions with high background electrolyte. 11,12 In our earlier work, we have seen that that the CCI between the Np(V)-M n+ (M: U(VI) or Th(IV)) is predominantly electrostatic in nature and hence, high background electrolyte is generally required to have any appreciable interaction. The RTIL as a solvent can act as a medium with low dielectric constant and with sufficiently high electrolytic concentration.…”

“…31,35,37 Although there are several reports available on the U(VI) ion stability, complexation and speciation in RTILs, [38][39][40] there is no report on the Np(v) speciation in a hydrophobic RTIL except one recently published by Dong et al 41 The unusual stability of the different oxidation states of Np in a hydrophilic RTIL (namely Hbet.NTf 2 ) was reported by Long et al 42 A hydrolysis-stable complex of Np(IV), i.e., [BuMeIm] 2 [NpCl 6 ] (where BuMeIm + is 1-n-butyl-3-methylimidazolium) in a hydrophobic RTIL 43 was reported by Nikitenko et al 43 Although the Np(V) f-f transition peak position and the associated e value are known for the aqueous media, only one report is available on the complexation of Np(V) with nitrate in RTIL. 41 Our earlier studies for CCI of Np(V) with the other actinides such as U(VI) 12 or Th(IV) 11 suggested that the CCI of Np(V) with other actinides, such as U(VI) was weak and electrostatic in nature and hence, should be very much affected by the medium. Xian et al have also shown that the strength of the CCI between Np(V) and U(VI) increases with increasing ionic strength of the aqueous medium.…”

“…6 The pentavalent actinyl ions are known for their unique interaction with other actinyl ions termed as cation-cation interaction (CCI) and the literature on the CCI is mainly dominated by Np(V). [9][10][11][12][13][14] These CCIs can alter the structural motifs (in solid compounds) and can also influence the chemical or redox properties of the involved actinyls. 15,16 The aqueous chemistry of Np(V) and its CCI in the acidic/alkaline or high ionic strength solutions was studied in the past 12,13,17 The CCI between Np(V) and UO 2 2+ (U(VI)) is the most studied system, where the axial oxygen of Np(V) acts as an electron donor to the neighbouring U(VI) units leading to an electrostatic interaction.…”

“…19 Theoretical studies suggested that the CCIs cannot overcome the electrostatic repulsion between the two AnO 2 2+ subunits and hence, generally the formation of such complexes is not favoured in aqueous solutions. 16,18,20 Most of the actinyl CCI studies are reported in the solid state 9 and in the aqueous solution of either higher acidity or ionic strength, 11,12 but to the best of our knowledge, no report is available for this interesting interaction between the actinyl ions in room-temperature ionic liquids (RTILs).…”