Add time:08/17/2019         Source:sciencedirect.com

The mode of coordination, mono-, bidentate, sulfate in UO2(SO4) and UO2(SO4)22−, and the relative energy of the various isomers have been studied at the DFT and MP2 levels using DFT-optimized geometry in a CPCM solvent model. The U–S distances in the mono- and bidentate coordination, 3.63 and 3.08 Å, respectively, agree very well with experimental observations from solutions. The U–Osulfate distance is significantly different in complexes with mono- and bidentate coordination, 2.23, vs. 2.37 Å, an observation difficult to deduce from the experimental studies. The experimental distance between uranium and the equatorial oxygen atoms, U–Oeq, is very close to the calculated average distance in a five-coordination model, but significantly longer in a six-coordination model, indicating a preference for the former; this finding is supported by energy calculations, where the five-coordinated isomers for the UO2(SO4) have the lowest Gibbs energy. For UO2(SO4)22−, the calculated Gibbs energy of reaction indicates that the six-coordinated isomer is slightly more stable than the five-coordinated one; however, the difference is small and less than the expected uncertainty in calculations of this type. Bidentate coordination of the sulfate group is always preferred over monodentate coordination. However, all differences in Gibbs energy between the different isomers is small, indicating that the mode of coordination may change with the composition of test solutions used, as observed experimentally. The U–O–Smono angle is close to 143° in complexes with a monodentate sulfate group; this is traced to steric effects, which overcome the electronic preference for a linear U–O–S bond. This study demonstrates the significant increase in chemical information that may be obtained by combining experimental data on structures and thermodynamics with quantum chemical methods.

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