Computational studies of molecular actinide and lanthanide complexes.

This thesis reports computational DFT studies of three families of complexes (see below). Before presenting the results from these studies, the first chapter introduces the lanthanides and actinides and investigates their similarities and differences through a discussion of selected compounds of each of the series of metals. The second chapter introduces the electronic structure methods that were used in this research and mentions some of the relevant computational utilities. Chapter 3 discusses DFT studies of mixed water/hydroxide uranyl systems, U02(H20)a-:r- OH)x (2-x)+ for x = 0 -> 5 with a = 5, and some a = 3, 4. The reasons for the observed lengthening, and resultant weakening, of U-Oy/ as x increases are investigated. These studies show that this lengthening appears to be predominantly due to a reduction in ionic character stemming from charge build up on the U centre, and not from hydroxide / Oyi competition for U 6d, as has been previously suggested. Time-Dependent DFT is used in chapter 4 to simulate the electronic spectra of U02(NCN)2 and U02(NPN)2 in order to investigate why these complexes are coloured red rather than the normally observed green/yellow of uranyl systems. This chapter involves some preliminary benchmarking calculations on three other uranyl complexes, U02Cl2(TBP)2 , UC>2Cl2(THF)2 , U02(NC>3)2(TBP)2 , as there is as yet no literature suggesting suitable exchange potentials and/or basis sets for TD-DFT calculations on actinides. The conclusion was reached that the unusual colour of the nitrogen donor complexes is due to a small HOMO-LUMO gap resulting from the relatively low energy nitrogen based ligand MOs compared with usual uranyl ligands such as O and CI. Chapter 5 systematically investigates M(N(EPR.2)2)3 , M = Ln (Ln = La, Ce, Pr, Pm, Eu), An (An = U, Np, Pu, Am, Cm) E = O, S, Se, Te R = H to investigate the ligands' suitability for extraction of An(III) from Ln(III), and also to test the suitability of La and U as models for Eu and Am/Cm respectively. The results lead me to conclude that chalcogen donor ligands are extremely promising for successful separation of Cm from Am and Eu, but good separation factors of Am from Eu seem unlikely. Furthermore I conclude that La and U are not suitable models for Eu and Am/Cm as the 2+ capability of Eu seems present in with S, Se, Te U shows considerable covalency in M-E with the heavier chalcogens and there is no evidence of covalency in any Am-E or Cm-E