Chemistry Reference
In-Depth Information
of Zr(NO
3
)
4
into the organic phase, which was attributed to interactions between
small reverse micelle-like particles containing two to three TBP molecules. The
particles interact through attractive forces between their polar cores with a poten-
tial energy exceeding 2
k
B
T
. The interparticle attractions lead to the third-phase
formation.
2.6 MoDeLInG
Auwer et al. used X-ray Absorption Spectroscopy (XAS) to study various U, Np, and
Pu nitrato coordination complexes of the type AnO
2
(NO
3
)
2
∙TBP (
143
). No significant
variation in the actinide environment was noticed across the series UO
2
2+
, NpO
2
2+
,
and PuO
2
2+
. Relativistic molecular orbital calculations for (UO
2
(NO
3
)
2
(TBPO)
2
),
(UO
2
(NO
3
)
2
(TBP)
2
), (UO
2
(NO
3
)
2
(TMP)
2
) (TMP is trimethylphosphate), and
(UO
2
(NO
3
)
2
(H
2
O)
2
) using the discrete-variational Dirac-Slater molecular orbital
method showed that the bonds between uranium and the ligands of these complexes
have a degree of covalent character (
144
). There is a close relationship between the
ligand-displacement ability in complexes of the type (UO
2
X
2
L
2
) (X = Cl, NO
3
) and
effective charges of atoms to be coordinated in each ligand. The strength of bond
between ligands and the central uranium atom is calculated by Mulliken population
analysis. From these calculated results, it was shown that the overlap population on
TBP with a central uranium atom is larger than that on TMP.
Rabbe et al. applied the molecular orbital approach to establish structure-activity
relationships on a database of 22 monoamides used as U(VI) nitrate extractants (
145
).
Semiempirical calculations on the monoamides were carried out using the AM1
self-consistent field method. A quantitative relationship was established between the
U(VI) nitrate distribution ratio and a charge parameter of the monoamide extractant.
Further, it was found that predominant factors determining the extracting ability of
a monoamide were of three kinds: (1) electron density of the coordinating atoms or
groups, which should be as high as possible; (2) steric effects, which should be as low
as possible; and (3) lipophilicity of the ligands, which should be above a minimum
threshold value (
146
). Molecular mechanics calculations were done on UO
2
(NO
3
)
2
A
2
complexes in order to determine the influence of steric effects on the formation of
these compounds. Calculations of monoamide lipophilicity using Rekker's method
showed that all the molecules of the database were lipophilic enough and, conse-
quently, this parameter was not significantly important for the extraction of ura-
nyl nitrate by these monoamides. In this context, assuming that the alkyl group
(attached to N atom) does not influence the charge parameter, Pathak et al. used
the reported values of
N,N
-dibutylacetamide,
N,N
-dibutylpropionamide,
N,N
-
dibutylisobutyramide, and
N,N
-dibutylpivalamide for di(2-ethylhexyl) derivatives of
acetamide (D2EHAA), propionamide (D2EHPRA), isobutyramide (D2EHIBA), and
pivalamide (D2EHPVA), respectively (
66, 67
). It was observed that log
D
U
, log
D
Th
,
as well as
K
H
varied linearly with the total electron density of the selected amides,
irrespective of acidity. It appeared that the charge density accounts both for elec-
tronic as well as steric factors relevant to the branching of the alkyl substituents on
the C
α
atom. Dramatic changes in the actinide coordination sphere appeared when
the An(VI) metal was reduced to An(IV).
