Structural and Energetic Data

Isoamethyrin is the only expanded porphyrin ligand here investigated for which there has been an experimentally characterised uranyl, neptunyl and plutonyl complex. These are also the only complexes investigated here for which significant spin contamination was not an issue. Thus the data contained in this section was obtained using the spin-unconstrained approach, with the topological and integrated properties obtained using the spin-constrained approach at the geometry optimised with the spin unconstrained approach included for comparison.

Due to aforementioned difficulties in reproducing the experimental structure when a simplified ligand is used (Chapter 4, Chapter 5), peripheral alkyl substituents are included for each complex here, and for the sake of consistency with Chapters 4 and 5, complexes will be referred to as AnO2-isoamethyrin(1.0.1.0.0.0)′, where the prime indicates the presence of peripheral alkyl groups. The only geometrical differences between different complexes are slight variations in bond length, with no significant

181 structural differences to be found. Because of this, Figure 6.1 shows only the optimised geometry for NpO2-isoamethyrin(1.0.1.0.0.0)′ optimised using the PBE xc-functional in the gas phase (for the uranyl complex, see Chapter 5).

Table 6.8 shows average and individual An-N and An-O bond lengths for uranyl, neptunyl and plutonyl complexes with isoamethyrin for complexes optimised using the PBE and B3LYP xc-functionals in the gas phase. Crystallographic data is available for the uranyl(VI) isoamethyrin complex^133,190, revealing an average U-N bond length of 2.66 Å and a U-O bond length of 1.760 Å. Both xc-functionals employed here replicate the average U-N bond length to within a few hundredths of an angstrom, likewise the U-O bond. Crystallographic data for the neptunyl(V) complex¹⁹⁰ reveals two distinct U-O bond lengths of 1.762 and 1.826 Å, and Np-N bond lengths of 2.649 to 2.880 Å, with the existence of a short contact, typical of hydrogen bonding, seen between the oxygen ion involved in the longer O bond and a solvent molecule. An average Np-N bond length of 2.772 Å, somewhat greater than the average experimental U-Np-N bond length in the uranyl(VI) complex of 2.66Å, although the different charge state of the neptunyl complex means a comparison to calculated values cannot be made.

U Np Pu

PBE B3LYP PBE B3LYP PBE B3LYP

𝑟_An−N

̅̅̅̅̅̅̅̅ 2.688 2.705 2.687 2.698 2.699 2.716

𝑟_An−O 1.787 1.766 1.779 1.746 1.764 1.734

Δ𝑟_An−N2 0.076 0.070 0.070 0.055 0.067 0.057

Table 6.8: Average An-N and An-O bond lengths and Δ𝑟_An−O, the difference between the coordinated and uncoordinated An-O bond length, all in Å for AnO2-isoamethyrin(1.0.1.0.0.0)′

(An = U, Np, Pu) calculated using the PBE and B3LYP xc-functionals.

It is apparent that as one moves from U to Np, An-N bonds are very similar in length, with at most, a difference of a few thousandths of an Angstrom on average, while Np-O bonds are shorter than U-Np-O bonds by ~0.2 Å at most. These changes occur with both functionals. Pu-N bonds are longer than Np-N and U-N bonds by a few hundredths to a few thousandths of an Angstrom on average, depending on which model chemistry is used, and Pu-O bonds are shorter than Np-O bonds by a few hundredths of an Angstrom, again independent of xc-functional or solvation state. All An-O bonds are longer than their uncoordinated counterparts.

182 Figure 6.1: Optimised structure for NpO2-isoamethyrin(1.0.1.0.0.0)′ generated from data obtained using the PBE xc-functional in the gas phase.

Considering the binding energies given in Table 6.9, the uranyl complex is found to be more stable than the neptunyl complex by ~ 0.4 – 0.5 eV, dependent on xc-functional.

The neptunyl complex is slightly more stable than the plutonyl complex, by only 0.02 eV when the PBE xc-functional is used, and 0.1 eV when the B3LYP xc-functional is employed. The deformation adjusted binding energy shows decreasing stability in the order U > Np > Pu. The deformation energies are given in Table 6.10.

The distortion of the ligand upon complexation appears to be more or less independent of the actinide centre, and ligand deformation energy is calculated to be almost constant from U to Pu, with at most a few hundredths of an eV difference, and an overall small decrease from U to Pu. The actinyl deformation energy, which decreases by up to 0.1 eV from the neptunyl complex to the plutonyl complex, consistent with the lengthening of An-N bonds from Np to Pu, possibly suggesting that the plutonylunit is less affected by complexation with the isoamethyrin(1.0.1.0.0.0)′ ligand than the neptunyl unit.

Again, the uranyl deformation energy is in all cases higher than the neptunyl deformation energy.

When the total binding energy is considered, the uranyl complex is more stable than the neptunyl complex by ~ 0.4 – 0.5 eV, dependent on xc-functional. The neptunyl complex is slightly more stable than the plutonyl complex, by only 0.02 eV when the PBE xc-functional is used, and 0.1 eV when the B3LYP xc-functional is employed. The deformation adjusted binding energy shows decreasing stability in the order U > Np >

Pu.

183

Table 6.11 contains the frequencies of the actinyl stretching modes for uranyl, neptunyl and plutonyl isoamethyrin(1.0.1.0.0.0)′. As mentioned in Chapters 4 and 5, the peripherally substituted isoamethyrin complexes have many more atoms than the simplified complexes studied and as such, calculations, particularly calculations where the hybrid B3LYP xc-functional is employed, are very computationally expensive. In this case, it was decided that numerical force calculations on the peripherally substituted complexes optimised with the B3LYP xc-functional could be omitted, meaning that here it is only the vibrational frequencies of the structures optimised with the PBE xc-functional which are reported.

As one moves from uranyl to the neptunyl complex, the frequency of the symmetric stretching mode decreases by ~15 cm^-1in the case of structures optimised in the gas phase. The asymmetric stretching mode is found to be ~5 cm^-1 greater in the neptunyl complex than the uranyl complex. The frequencies for the uranyl complex are shifted

~160-170 cm^-1 compared to uncoordinated uranyl, with the asymmetric mode being more significantly shifted. The shifts of the neptunyl stretching modes are somewhat smaller, ~140-155 cm^-1 with the asymmetric mode again being the most significantly affected. Both plutonyl stretching modes are shifted by ~145 cm^-1.