Comparative EXAFS Investigation of Uranium(VI) and -(IV) Aquo Chloro Complexes in Solution Using a Newly Developed Spectroelectrochemical Cell

Abstract

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U L(III)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5(2+), UO2(H2O)4Cl+, UO2(H2O)3Cl2(0), and UO2(H2O)2Cl3- with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9(4+), U(H2O)8Cl3+, U(H2O)(6-7)Cl2(2+), and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.76+/-0.02 A, Oeq = 2.41 +/- 0.02 A, and U-Cl = 2.71 +/- 0.02 A; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 A and U-Cl = 2.71 +/- 0.02 A.

Theoretical Study of the Excited States and Luminescent Properties of (H2O)nUO2Cl2 (n = 1–3)

“…The CCSD­(T) optimized ground-state geometrical parameters of the (H 2 O) n UO 2 Cl 2 ( n = 1–3) complexes with constraint of C 2 v symmetry are summarized in Table (see structures in Figure ), where the UO 2 Cl 2 geometrical parameters are also included for comparison. The calculated U–O ax bond lengths are in a range of 175–177 pm in these water-solvated UO 2 Cl 2 complexes, which are in good agreement with the reported experimental and theoretical results of 176 pm. , …”

Theoretical Study of the Excited States and Luminescent Properties of (H2O)nUO2Cl2 (n = 1–3)

“…The CCSD­(T) optimized ground-state geometrical parameters of the (H 2 O) n UO 2 Cl 2 ( n = 1–3) complexes with constraint of C 2 v symmetry are summarized in Table (see structures in Figure ), where the UO 2 Cl 2 geometrical parameters are also included for comparison. The calculated U–O ax bond lengths are in a range of 175–177 pm in these water-solvated UO 2 Cl 2 complexes, which are in good agreement with the reported experimental and theoretical results of 176 pm. , …”

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+

“…The U–O ax and U–O water distances were slightly elongated in S 0 of aquo–chloro complexes compared to that of the pure aquo complex (0.01 and 0.04 Å in [UO 2 Cl­(H 2 O) 4 ] + compared to 0.01 and 0.05 Å in [UO 2 Cl 2 (H 2 O) 3 ] 0 ). Correspondingly, calculated U–Cl distances were 2.67 and 2.73 Å, respectively, which excellently matched the EXAFS results obtained by Hennig et al…”

Structural Correspondence between Uranyl Chloride Complexes in Solution and Their Stability Constants

“…This result supports a previously published optical spectroscopy study in which the number of coordinating chloride ions was seen to saturate at concentrations above 6 M Cl – . This result rules out the simple chloride substitution mechanism modeled for this reaction, at least under the conditions of our experiments, and is inconsistent with one favored model chosen to fit EXAFS data on a similar series of solutions but consistent with another …”

“…The U–O interaction at 2.41 Å has been previously seen in EXAFS studies, where it is attributed to about 5 waters in the equatorial coordination sphere perpendicular to the linear dioxo moiety. ,− Consistent with our current analysis, an integration of the equatorial water peak associated with an aqueous uranyl ion previously reported from HEXS data was integrated to yield 47(2) electrons, corresponding to 4.7 waters . Although calculations tend to favor predominately 5-fold equatorial waters, − this slightly reduced coordination has been reproduced with calculations that included second-coordination sphere waters explicitly …”

Theoretical Study of the Excited States and Luminescent Properties of (H2O)nUO2Cl2 (n = 1–3)

“…The CCSD­(T) optimized ground-state geometrical parameters of the (H 2 O) n UO 2 Cl 2 ( n = 1–3) complexes with constraint of C 2 v symmetry are summarized in Table (see structures in Figure ), where the UO 2 Cl 2 geometrical parameters are also included for comparison. The calculated U–O ax bond lengths are in a range of 175–177 pm in these water-solvated UO 2 Cl 2 complexes, which are in good agreement with the reported experimental and theoretical results of 176 pm. , …”

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+

“…The U–O ax and U–O water distances were slightly elongated in S 0 of aquo–chloro complexes compared to that of the pure aquo complex (0.01 and 0.04 Å in [UO 2 Cl­(H 2 O) 4 ] + compared to 0.01 and 0.05 Å in [UO 2 Cl 2 (H 2 O) 3 ] 0 ). Correspondingly, calculated U–Cl distances were 2.67 and 2.73 Å, respectively, which excellently matched the EXAFS results obtained by Hennig et al…”

Structural Correspondence between Uranyl Chloride Complexes in Solution and Their Stability Constants

“…This result supports a previously published optical spectroscopy study in which the number of coordinating chloride ions was seen to saturate at concentrations above 6 M Cl – . This result rules out the simple chloride substitution mechanism modeled for this reaction, at least under the conditions of our experiments, and is inconsistent with one favored model chosen to fit EXAFS data on a similar series of solutions but consistent with another …”

“…The U–O interaction at 2.41 Å has been previously seen in EXAFS studies, where it is attributed to about 5 waters in the equatorial coordination sphere perpendicular to the linear dioxo moiety. ,− Consistent with our current analysis, an integration of the equatorial water peak associated with an aqueous uranyl ion previously reported from HEXS data was integrated to yield 47(2) electrons, corresponding to 4.7 waters . Although calculations tend to favor predominately 5-fold equatorial waters, − this slightly reduced coordination has been reproduced with calculations that included second-coordination sphere waters explicitly …”

Theoretical Study of the Excited States and Luminescent Properties of (H2O)nUO2Cl2 (n = 1–3)

“…The CCSD­(T) optimized ground-state geometrical parameters of the (H 2 O) n UO 2 Cl 2 ( n = 1–3) complexes with constraint of C 2 v symmetry are summarized in Table (see structures in Figure ), where the UO 2 Cl 2 geometrical parameters are also included for comparison. The calculated U–O ax bond lengths are in a range of 175–177 pm in these water-solvated UO 2 Cl 2 complexes, which are in good agreement with the reported experimental and theoretical results of 176 pm. , …”

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+

“…The U–O ax and U–O water distances were slightly elongated in S 0 of aquo–chloro complexes compared to that of the pure aquo complex (0.01 and 0.04 Å in [UO 2 Cl­(H 2 O) 4 ] + compared to 0.01 and 0.05 Å in [UO 2 Cl 2 (H 2 O) 3 ] 0 ). Correspondingly, calculated U–Cl distances were 2.67 and 2.73 Å, respectively, which excellently matched the EXAFS results obtained by Hennig et al…”

Structural Correspondence between Uranyl Chloride Complexes in Solution and Their Stability Constants

“…This result supports a previously published optical spectroscopy study in which the number of coordinating chloride ions was seen to saturate at concentrations above 6 M Cl – . This result rules out the simple chloride substitution mechanism modeled for this reaction, at least under the conditions of our experiments, and is inconsistent with one favored model chosen to fit EXAFS data on a similar series of solutions but consistent with another …”

“…The U–O interaction at 2.41 Å has been previously seen in EXAFS studies, where it is attributed to about 5 waters in the equatorial coordination sphere perpendicular to the linear dioxo moiety. ,− Consistent with our current analysis, an integration of the equatorial water peak associated with an aqueous uranyl ion previously reported from HEXS data was integrated to yield 47(2) electrons, corresponding to 4.7 waters . Although calculations tend to favor predominately 5-fold equatorial waters, − this slightly reduced coordination has been reproduced with calculations that included second-coordination sphere waters explicitly …”