Spectroscopic study of neodymium soaps in 1-pentanol

Article abstract of DOI:10.1016/S0925-8388(00)00646-0

The optical absorption spectra of neodymium(III) butanoate, pentanoate, hexanoate, heptanoate and octanoate in 1-pentanol have been measured. This solvent has been chosen, because the neodymium(III) soaps are sufficiently soluble in it. A set of free-ion parameters is obtained by fitting the calculated energy levels against the experimental levels. The experimental dipole strengths of the transitions are determined from the absorption spectra and parameterised in terms of the Judd-Ofelt intensity parameters f!A ( A = 2, 4, 6). The f!4 values go through a maximum for neodymium(III) hexanoate, whereas the Ω₂ parameter increases in the order butanoate≤pentanoate≤hexanoate≤heptanoate≤octanoate. The Ω₆ parameter shows only an increase between the butanoate and the pentanoate complex, whereas this parameter remains fairly constant for the higher homologues of the neodymium(III) soaps.

Full text of DOI:10.1016/S0925-8388(00)00646-0

Journal of Alloys and Compounds 303–304 (2000) 387–392
L
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Spectroscopic study of neodymium soaps in 1-pentanolq
*
¨
K. Binnemans , P. Martello, I. Couwenberg, H. De Leebeeck, C. Gorller-Walrand
K.U. Leuven, Department of Chemistry, Coordination Chemistry Division, Celestijnenlaan 200F, B-3001 Heverlee (Leuven), Belgium
Received 4 January 1999; accepted 9 June 1999
Abstract
The optical absorption spectra of neodymium(III) butanoate, pentanoate, hexanoate, heptanoate and octanoate in 1-pentanol have been
measured. This solvent has been chosen, because the neodymium(III) soaps are sufficiently soluble in it. A set of free-ion parameters is
obtained by fitting the calculated energy levels against the experimental levels. The experimental dipole strengths of the transitions are
determined from the absorption spectra and parameterised in terms of the Judd–Ofelt intensity parameters Vl (l52, 4, 6). The V4 values
go through a maximum for neodymium(III) hexanoate, whereas the V2 parameter increases in the order butanoate,pentanoate,
hexanoate,heptanoate,octanoate. The V6 parameter shows only an increase between the butanoate and the pentanoate complex, whereas
this parameter remains fairly constant for the higher homologues of the neodymium(III) soaps.
reserved.
2000 Elsevier Science S.A. All rights
Keywords: Lanthanides; Rare earths; Optical properties; Spectroscopy; Judd–Ofelt theory; Metal soaps
1. Introduction
In this paper, we describe the spectroscopic properties of
neodymium(III) soaps in 1-pentanol. The main aim of this
study is to investigate the relation between the chain length
of the alkanoate group and the intensities of the f–f
transitions. We chose 1-pentanol as the solvent because of
the relative good solubility of the neodymium(III) soaps in
1-pentanol. However, since the solubility of the metal
soaps is rapidly decreasing with increasing chain length,
we restricted ourselves in this study to neodymium(III)
soaps with a chain length shorter than nine carbon atoms.
The spectra are analysed within the framework of the
Judd–Ofelt theory.
The alkali salts of the fatty acids (5carboxylic acids
with long alkyl chains) are well known as soaps. These
soaps are water soluble. If the alkali ion in the higher
homologues is replaced by an alkaline earth or by a
transition metal ion, the resulting compounds are water
insoluble, but more soluble in non-polar organic solvents.
The alkaline earth and the transition metal salts of the fatty
acids are the so-called metal soaps [1,2]. The lanthanides,
or the rare earths in general, are also able to form metal
soaps. These compounds have the general formula
Ln(CnH2n11COO)3 [3,4]. Although in the literature several
studies on lanthanide soaps in solution can be found,
among them Refs. [5–11], hardly any reports on the
spectroscopic properties in solution are available [12,13].
A detailed knowledge of the electronic properties of these
lanthanide complexes is desirable, because they can have
several possible applications, such as catalyst for the
polymerisation of 1,4-butadiene (the cis-polymer is pref-
erentially formed) [14,15].
2. Experimental details
The neodymium(III) soaps were prepared by
a
metathesis reaction between neodymium nitrate and the
sodium salt of a fatty acid (1:3 molar ratio) in an water/
ethanol solution. The precipitate was filtered off, washed
with water, ethanol and acetone, subsequently crystallised
twice from an toluene/methanol (1:1, v/v) mixture and
dried in a vacuum oven for 24 h at 408C. The crystallised
neodymium soaps were filtered off, washed with a little
cold methanol and dried for 24 h under vacuum. With the
neodymium(III) butanoate, one has to be careful not to use
qPaper presented at the 22nd Rare Earth Research Conference, July
10–15, 1999, Argonne, IL, USA (Poster III-25).
*Corresponding author. Tel.: 132-16-327-446; fax: 132-16-327-992.
E-mail address: koen.binnemans@chem.kuleuven.ac.be (K. Bin-
nemans)
0925-8388/00/$ – see front matter
PII: S0925-8388(00)00646-0
2000 Elsevier Science S.A. All rights reserved.

388
K. Binnemans et al. / Journal of Alloys and Compounds 303–304 (2000) 387–392
too much washing solvent because of the high solubility of
this metal soap.
The purity of the soaps was determined by CHN
elemental analysis (on a CE Instruments EA-1110 elemen-
tal analyser) and by measuring the melting point (on a
Mettler-Toledo DSC821e differential scanning calorime-
extraction of the free-ion parameters is based on mini-
mising the sum of the squared differences between the
experimental and calculated energy levels. For the calcu-
lated energy levels, we made use of a parametric free-ion
Hamiltonian. The free-ion Hamiltonian is characterised by
a set of three electron repulsion parameters (F2, F4, F6),
by the spin-orbit coupling constant z4f, the Trees configu-
ration interaction parameters (a, b, g ), the three-body
configuration interaction parameters (T 2, T 3, T 4, T 6, T 7,
T 8) and parameters which describe magnetic interactions
(M0, M2, M4, P2, P4, P6). A further parameter EAV G takes
into account the kinetic energy of the electrons and their
interactions with the nucleus. It shifts only the barycentre
of the whole 4f3 configuration. So one can write [18,19]:
¨
ter). We purchased 1-pentanol (99%) from Riedel-de-Haen.
Absorption spectra were recorded at room temperature
using a Shimadzu UV-3100 spectrophotometer.
3. Results and discussion
The absorption spectra of the neodymium(III) soaps
were recorded in 1-pentanol at room temperature. The
concentration of the solutions used in this study was 0.01
M. For the higher homologues it proved very difficult to
obtain more concentrated solutions at ambient temperature.
All the solutions had the typical blue-violet colour of the
trivalent neodymium ion [16]. Because of the low con-
centration of the Nd(III) ion, we used quartz cells with an
path length of 10 cm. An advantage of 1-pentanol is its
broad spectral window, with a good transparency in the
ultraviolet. The adsorption spectrum of neodymium(III)
hexanoate in 1-pentanol is shown in Fig. 1.
H
free ion 5 EAVG 1
O
Fkfk 1 z4f ASO 1 aL L 1 1
s d
k
1 bG(G2) 1gG(R7) 1
O
Titi 1
O
Pkpk
i
k
1
O
Mlml
(1)
l
where i52, 3, 4, 6, 7, 8, k52, 4, 6 and l50, 2, 4, whereas
fk and ASO represent the angular part of the electrostatic
and spin-orbit interaction respectively. L is the total orbital
angular momentum. G(G2) and G(R7) are the Casimir
operators for the groups G2and R7. The ti are the three-
particle operators. pk and ml represent the operators for the
magnetic corrections. The starting values of the free-ion
parameters were taken from Carnall et al. [20] for
LaF3:Nd31. The parameters EAVG, F2, F4, F6, a, b, g, j4f
were freely varied. The other parameters were constraint at
The energy levels of the trivalent neodymium ion (4f3
configuration) could be determined from the barycentres of
the absorption bands [17]. The experimental energy levels
are used to extract a set of free-ion parameters. This set of
parameters is the start point for the calculated matrix
element, necessary for the intensity calculations. The
Fig. 1. Absorption spectrum of neodymium(III) hexanoate in 1-pentanol (ambient temperature).

K. Binnemans et al. / Journal of Alloys and Compounds 303–304 (2000) 387–392
389
Table 1
Free-ion parameters (in cm21) of neodymium(III) soaps in 1-pentanola
Parameter
NdC4
NdC5
NdC6
NdC7
NdC8
EAV G
F2
F4
F6
a
24 135610
72 3646174
52 1866380
35 5006210
17.061.1
2591636
1497655
87266
[298]
[35]
[59]
[2285]
[332]
24 136610
72 3086115
52 3656378
35 3886132
18.161.1
2617632
1519655
868611
[298]
[35]
[59]
[2285]
[332]
[305]
24 176610
72 4546125
51 9266414
35 2716145
16.861.1
2564634
1581656
87066
[298]
[35]
[59]
[2285]
[332]
24 129610
72 3086111
52 2666366
35 3516128
17.861.1
2607632
1546653
86865
[298]
[35]
[59]
[2285]
[332]
24 150610
72 522698
52 3156323
35 3196133
18.460.9
2655628
1600647
87165
[298]
[35]
[59]
[2285]
[332]
[305]
b
g
j4f
T 2
T 3
T 4
T 6
T 7
T 8
[305]
[2.11]
[192]
[305]
[2.11]
[192]
[305]
[2.11]
[192]
Mtot
[2.11]
[192]
[2.11]
[192]
b
Ptot
b
s
38
41
44
39
35
a NdC4, neodymium(III) butanoate; NdC5, neodymium(III) pentanoate; NdC6, neodymium(III) hexanoate; NdC7, neodymium(III) heptanoate; NdC8,
neodymium(III) octanoate. The values in square brackets were constraint during the fitting procedure.
b The Ml and Pk parameters were in the following ratios: M2 /M050.56, M4 /M050.38, P4 /P250.50, P6 /P250.10.
the values for LaF3:Nd31. The Ml and Pk parameters were
in the following ratios: M2 /M050.56, M4 /M050.38, P4 /
P250.50, P6 /P250.10. In Table 1, the free-ion parame-
ters sets for the Nd(III) alkanoates are given. In Table 2,
the experimental and calculated free-ion energy levels of
Nd(III) hexanoate are listed.
The transitions observed in the absorption spectra of
trivalent lanthanide ions are intraconfigurational f–f transi-
tions. The majority of these transitions are induced electric
dipole transitions, although a few magnetic dipole transi-
tions are known. The intensities of the transitions can be
characterised by the dipole strength D [21]:
for 1-pentanol). Finally, the kJiU (l)iJ9l are reduced matrix
elements. The Vl parameters can be determined by a
standard-least squares fitting method.
The intensity calculations for neodymium(III) hexanoate
in 1-pentanol are given in Table 3. The Judd–Ofelt theory
is able to give an adequate description of the intensities of
the f–f transitions in the trivalent neodymium ion. For the
other neodymium(III) soaps, the calculations are analo-
gous. The intensity parameters for the neodymium(III)
soaps are listed in Table 4. For all the systems, the order of
the parameter values is: V4 ,V2 ,V6. There is no increase
or decrease of the value of the V4 parameter as a function
of the chain length of the alkanoate group, but the values
go through a maximum value for neodymium(III) hexa-
noate. However, the value of the V2 parameter slightly
increases with increasing chain length, from (5.6760.38)?
10220 cm2 for neodymium(III) butanoate to (6.5260.46)?
10220 cm2 for neodymium(III) octanoate. The greatest
increase is observed between the butanoate and the penta-
noate complex. For the V6 parameter, an increase is
observed between the butanoate and the pentanoate com-
plex, but this parameter remains fairly constant when
increasing the chain length of the alkanoate group. The
trends in the parameters are visualised in Fig. 2. The
parameter values are not low, but the values are not
extremely high either. This can be attributed to the lack of
strong polarisable groups (e.g. aromatic groups) in the
ligands. Although the chain length has some influence on
the spectral intensities, the optical properties are in the first
place determined by the nature of the solvent. The slight
increase of the V2 parameter with increasing chain length
can be attributed to a higher polarisability of a compound
with a long alkyl chain. The trends of the V4 and V6
parameters are more difficult to rationalise.
¯
¯
n
1
A n
s d
]
] ¯
dn
D 5
E
(2)
108.9Cd
where C is the concentration of the neodymium ion
(mol l21), d is the optical path length (cm), A is the
¯
absorbance (A 5 2 log(I0 /I)) and n is the wavenumber
(cm21). The dipole strength is expressed in D2 (Debye2).
According to the Judd–Ofelt theory [22,23], the intensities
of induced electric dipole transitions can be described in
terms of three phenomenological intensity parameters Vl
(l52, 4 and 6):
2
1036
2J 1 1
s
n2 1 2
9n
d
2
VlukJiU (l)iJ9lu2
(3)
]
D 5
e
O
l52,4,6
The factor 1036 convert D2 units into esu?cm. The
elementary charge e is 4.803310210 esu. The degeneracy
of the ground state is equal to 2J11 (i.e. 10 for Nd31).
The factor (n2 1 2)2 /9n takes into account that the
neodymium ions are not in a vacuum, but in a dielectric
medium (n is the refractive of the solvent; i.e. n51.4101

390
K. Binnemans et al. / Journal of Alloys and Compounds 303–304 (2000) 387–392
Table 2
From a study of the spectroscopic properties, it is
difficult to determine the first coordination sphere around
the neodymium(III) ion. In the solid state, these type of
compounds form a double layer structure [24,25], but it
can be expected that this layer structure is lost after
dissolution in the solvent. The only fact we can sure about
is that in solution, the lanthanide carboxylate ratio is 1:3.
Besides the carboxylate groups, pentanol molecules will be
present in the first coordination sphere, so that the coordi-
nation number of the neodymium(III) ion is 8 or 9. The
number of pentanol molecules will depend on the coordi-
nation mode of the carboxylate ions (monodentate or
bidentate). The possibility of an equilibrium between
carboxylate groups with a monodentate and a bidentate
coordination cannot be excluded. The small variation of
the Vl parameters with increasing chain length and the
similarity of the band shapes of the transitions in the
absorption spectrum indicate that the coordination of the
neodymium(III) ion changes little or not with the lengths
of the alkyl chain of the fatty acid.
Experimental and calculated free-ion levels of Nd(III) hexanoate in
1-pentanola
Level
Eexp (cm21
)
Ecalc (cm21
)
4I9 / 2
–
–
–
–
103
1929
3877
4I11 / 2
4I13 / 2
4I15 / 2
4F3 / 2
4F5 / 2
5906
11 514
12 485
–
13 415
13 608
14 703
15 908
17 195
–
19 042
–
–
21 026
21 259
–
11 514
12 521
12 603
13 467
13 559
14 707
15 874
17 239
17 274
19 068
19 440
19 465
21 020
21 295
21 392
21 444
23 321
23 826
26 225
28 180
28 342
28 709
29 287
30 001
30 409
30 622
31 449
32 802
33 373
34 139
34 259
38 361
39 729
47 562
48 532
66 034
67 236
4
(2H, F)9 / 2
4F7 / 2
4S3 / 2
4F9 / 2
2H(2)11 / 2
4G5 / 2
2
(4G, G(1))7 / 2
4G7 / 2
2K13 / 2
4G9 / 2
2
(2G(1), G(2))9 / 2
(2D(1), P)3 / 2
2
2K15 / 2
4G11 / 2
2P1 / 2
–
23 334
23 818
26 209
28 086
28 422
28 735
29 291
–
2D(1)5 / 2
2
(2P, D(1))3 / 2
4D3 / 2
4. Conclusions
4D5 / 2
4D1 / 2
The spectroscopic properties of the neodymium(III)
soaps in 1-pentanol were investigated. The intensities of
the f–f transitions of Nd(III) were parameterised within the
framework of the Judd–Ofelt theory. Although some
influence of the chain length on the Vl intensity parame-
ters is observed, this influence is small. The values of the
V4 parameter go through a maximum for neodymium(III)
hexanoate. An increase of the parameter values with
increasing chain lengths is observed for the V2 and the V6
parameters. The transitions are not very intense in com-
parison to systems with highly polarisable ligands (i.e.
ligands containing phenyl groups). For all the
neodymium(III) soaps, the order of the parameters is
V4 ,V2 ,V6.
2I11 / 2
2L15 / 2
4D7 / 2
30 352
–
–
2I13 / 2
2L17 / 2
2H(1)9 / 2
2D(2)3 / 2
2H(1)11 / 2
–
33 391
–
34 276
38 426
39 663
–
–
–
–
(2D(2), D)5 / 2
4
(2F(2), F(1))5 / 2
2
(2F(2), F(1))7 / 2
2
(2G(2), G(1))9 / 2
2
(2G(2), G(1))9 / 2
2
2F(1)7 / 2
(2F(1), F(2))5 / 2
2
a The free-ion parameters used for the calculation can be found in
Table 1.
Table 3
Experimental and calculated dipole strengths for the transitions in the absorption spectrum of neodymium(III) hexanoate in 1-pentanola
21
¯
Transition
n (cm
)
Dexp
Dcalc
D
exp 2Dcalc
Dcalc /Dexp
←4I9 / 2
(1026 Debye2)
(1026 Debye2)
(1026 Debye2)
4F3 / 2
11 510
12 490
13 340
14 700
15 910
17 200
19 250
21 390
23 330
28 320
382
1549
1501
104
25
2633
898
206
20
426
1502
1566
110
27
2647
715
149
50
244
147
265
26
1.12
0.97
1.04
1.06
1.08
1.01
0.80
0.72
1.67
1.09
4
2H9 / 2, F5 / 2
4F7 / 2, S3 / 2
4
4F9 / 2
2H11 / 2
22
4G5 / 2, G7 / 2
214
1183
157
230
265
2
4G7 / 2, K13 / 2, G9 / 2
2
4
2K15 / 2, G11 / 2, D3 / 2, G9 / 2
4
2
2
2P1 / 2
2I11 / 2, D1 / 2, D3 / 2, D5 / 2
733
798
4
4
4
a The Judd–Ofelt parameters used for the intensity calculation are: V2 5(6.2460.42)?10220 cm2, V4 5(4.4860.49)?10220 cm2 and V6 5(7.9060.37)?
4
10220 cm2. The R.M.S. value is 85?1026 Debye2. All the transitions start from the I9 / 2 ground state.

K. Binnemans et al. / Journal of Alloys and Compounds 303–304 (2000) 387–392
391
Table 4
Judd–Ofelt intensity parameters for neodymium(III) soaps in 1-pentanol
Compound
V2 (10220 cm2)
V4 (10220 cm2)
V6 (10220 cm2)
Neodymium(III) butanoate
Neodymium(III) pentanoate
Neodymium(III) hexanoate
Neodymium(III) heptanoate
Neodymium(III) octanoate
5.6760.38
6.2060.41
6.2460.42
6.4360.52
6.5260.46
3.7560.44
4.3460.48
4.4860.49
4.2260.61
3.9660.54
6.9360.34
7.7660.37
7.9060.37
7.8660.46
8.0660.41
Fig. 2. Dependence of the values of the Vl intensity parameters of neodymium(III) soaps in 1-pentanol on the chain length of the alkanoate group.
[7] K.N. Mehrotra, M. Chauhan, R.K. Shukla, J. Appl. Polym. Sci. 55
Acknowledgements
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