Vibrational spectra and structures of zinc carboxylates II. Anhydrous zinc acetate and zinc stearate

Article abstract of DOI:10.1016/S1386-1425(98)00061-4

A normal mode analysis was carried out for a monoclinic anhydrous zinc acetate crystal in which the acetate groups had bridging bidentate coordination forms, and spectral assignments were made. Based on the assignments, a relation between the coordination structure of the carboxylate groups around the zinc atom and the vibrational frequencies of the carboxylate rocking mode was found. This relation was applied to zinc stearate to determine its coordination form, and we found that zinc stearate had a bridging bidentate form.

Full text of DOI:10.1016/S1386-1425(98)00061-4

Spectrochimica Acta Part A 54 (1998) 1811–1818
Vibrational spectra and structures of zinc carboxylates II.
Anhydrous zinc acetate and zinc stearate
Tsutomu Ishioka *, Youko Shibata, Mizuki Takahashi, Isao Kanesaka
Department of Chemistry, Faculty of Science, Toyama Uni6ersity, Gofuku, Toyama 930-8555, Japan
Received 16 October 1997; received in revised form 8 March 1998; accepted 8 March 1998
Abstract
A normal mode analysis was carried out for a monoclinic anhydrous zinc acetate crystal in which the acetate
groups had bridging bidentate coordination forms, and spectral assignments were made. Based on the assignments,
a relation between the coordination structure of the carboxylate groups around the zinc atom and the vibrational
frequencies of the carboxylate rocking mode was found. This relation was applied to zinc stearate to determine its
coordination form, and we found that zinc stearate had a bridging bidentate form. © 1998 Elsevier Science B.V. All
rights reserved.
Keywords: Anhydrous zinc acetate; Zinc stearate; Vibrational spectra
1
. Introduction
mode analysis on anhydrous zinc acetate crystal
and assigned the vibrations of intra- and inter-
molecular ones. A qualitative vibrational analy-
sis has been reported by Johnson et al. [2]. They
considered that the coordination structure was a
chelating bidentate form, but the structure has
been revealed as a bridging bidentate one [1].
A similar crystal structure has been reported
for zinc propionate (CH CH CO ) Zn [3]. Its
Monoclinic anhydrous zinc acetate has a crys-
tal structure in which a zinc atom is tetrahe-
drally coordinated by the four oxygens of the
four bridging bidentate carboxylate groups in a
syn-anti arrangement [1]. They form two-dimen-
sional sheets along the bc plane. The space
6
2
group is C2/c or C . The averaged Zn–O dis-
h
3
2
2 2
˚
tance is 1.957 A and the C–O distance is 1.252
crystal structure belongs to space group P2 /c in
1
˚
A. There are the four symmetry species of Au
which the zinc atoms are linked by propionate
groups in a syn-anti arrangement in the bc
and Bu of infrared active vibrations and Ag and
Bg of Raman active ones. In this study, we
made a factor group analysis and a normal
˚
plane. The averaged Zn–O distance is 1.953 A
˚
and the C–O distance is 1.25 A. Hence, this
coordination structure is very similar to that in
monoclinic anhydrous zinc acetate. Also, zinc
propionate is one of the simplest zinc soaps.
The crystal structures of metal soaps are known
*
Corresponding author. Tel.: +81 764 456610; fax: +81
7
1
64 456549; e-mail: ishioka@sci.toyama-u.ac.jp
386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved.
PII S1386-1425(98)00061-4

1
812
T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
only for a few compounds, i.e. two crystal
forms of potassium palmitate and its high tem-
perature phase [4–6], copper caprate and capry-
late [7,8] and strontium caprate hydrate [9].
However, the crystal structure of rather com-
mon longer chain soaps such as zinc stearate
have not been reported until now. The coordi-
nation structure of the carboxylate groups
around a zinc atom in zinc stearate is not
known.
3. Results and discussion
3.1. Anhydrous zinc acetate
3.1.1. Crystal structure
Anhydrous zinc acetate takes different struc-
tures by different preparation methods. Before
vibrational analysis, we need to check whether a
specimen have a known crystal structure. Two
crystal structures of anhydrous zinc acetate have
been known. One is monoclinic form in which a
zinc atom is tetrahedrally coordinated by the four
oxygens of the four bridging bidentate carboxy-
late groups, and they form two-dimensional sheets
along the bc plane [1]. Another is orthorhombic
form in which a zinc atom is coordinated as
described above, but they form three-dimensional
network [10]. The former crystal was crystallized
from dry-ethanol solution of zinc acetate dihy-
drate, but the preparation method of the latter
crystal was not reported. Fig. 1 shows three ob-
served X-ray diffraction patterns of (a–c) and one
simulated pattern of (d): (a) is a pattern of the
specimen dried at 110°C over 1 night; (b) is that
dried at 120°C over 1 week; (c) is crystallized
from dry-ethanol solution; (d) is simulated based
on the monoclinic structure. For the simulation,
the 2q values were obtained from the reported
cell-dimensions and the intensities from the re-
In this study, we discuss the coordination ge-
ometry in zinc stearate based on the vibrational
analysis of anhydrous zinc acetate.
2
. Experimental
Anhydrous zinc acetate powder was recrystal-
lized from dry-ethanol solution of zinc acetate
dihydrate (Aldrich, 99.999%). Absence of water
molecules was confirmed by an infrared spec-
trum. The specimen was unstable under mois-
ture and readily changed to the dihydrate.
Other anhydrous zinc acetate powders were ob-
tained by drying zinc acetate dihydrate at
1
10°C over a night and at 120°C over 1 week.
Dehydration of the two water molecules by
drying was confirmed by a Rigaku TG-DTA
8
101BH.
2
Zinc stearate was synthesized from sodium
ported ꢀF ꢀ values where F ’s below 50 were ne-
0
o
stearate (Sigma, 99%) in dry ethanol solution
by titrating equimolar ZnCl2 aq. slowly. The
solution was precipitated in diethylether. The
product was filtered and dried at 60°C under
vacuum. These procedures were repeated 5×.
The purity was confirmed by the IR spectrum
and the elemental analysis.
glected. Since intensity corrections were not made,
discrepancies between observed and calculated in-
tensities in the high 2q region were recognized.
The observed patterns (a) and (b) are clearly
different from (c), but (c) coincides with (d).
Hence, we confirmed to having obtained a powder
specimen (c) having the monoclinic structure. The
structures of (a) and (b) may be ascribed to the
orthorhombic one but we did not confirm it.
X-ray powder diffractions were measured by
a SHIMAZU XD-3A diffractometer with a Ni
˚
filtered Cu–K line of 1.5418 A.
h
Infrared spectra were measured by a JASCO
IR-810 spectrophotometer at room temperature.
Raman spectra were obtained at 77 K by a
JASCO R-800 double monochromator with an
3.1.2. Normal mode analysis
A normal mode analysis was made for the
monoclinic specimen as follows. The Cartesian
coordinates were evaluated according to the crys-
+
Ar laser 514.5 nm excitation line and at room
tal structure. The CH group was assumed as a
3
temperature by a JASCO RFT-200 FT-Raman
spectrometer with an Nd: YAG laser 1064 nm
excitation line.
unit atom. The unit cell contains eight asymmetric
units. We calculated the frequencies for a primi-
tive cell containing four asymmetry units of

T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
1813
Fig. 1. X-ray powder diffraction patterns of (a) anhydrous zinc acetate obtained by drying zinc acetate dihydrate at 110°C over 1
night; (b) by drying at 120°C over 1 week; (c) by crystallized from dry-ethanol solution of zinc acetate dihydrate and (d) simulated
based on the crystal structure of monoclinic anhydrous zinc acetate.
−
1
Fig. 2. Infrared spectrum of monoclinic anhydrous zinc acetate at room temperature in the 400–4000 cm
due to grating change.
region. The asterisk is
−
1
Zn(OOCCH ) where Zn and O were assumed to
the 400–4000 cm
around 3400 cm
region where a broad band
was confirmed to be ascribed
3
2
−
1
be covalently bonded. The crystal belongs to the
6
space group C2/c or C . The asymmetric unit
to moisture in the KBr disc. Fig. 3 shows Raman
spectrum of anhydrous zinc acetate in the 200–
2000 and 2500–3500 cm
2
h
belongs to the C point group and has 11A and
2
−
1
1
0B vibrations. The site symmetry is C . Hence,
region. The valence
2
we have (22+4R)Ag, (22+3T)Au, (20+8R)Bg
and (20+6T)Bu crystal vibrations where R and T
indicate rotational and translational lattice
modes, respectively. The Ag and Bg species are
Raman active and the Au and Bu species are
infrared active. Fig. 2 shows infrared spectrum in
force constants were transferred from the values
of zinc acetate dihydrate [11] and were adjusted to
fit both the calculated and the observed frequen-
cies for the COO stretches, the CC stretch, the
OCO bend, the CCO bend and the COO out-of-
plane, as listed in Table 1. We were able to fit the

1
814
T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
−
1
and 2500–3500 cm−1 regions.
Fig. 3. Raman spectrum of monoclinic anhydrous zinc acetate at 77 K in the 200–2000 cm
frequencies by adjusting a few force constants at
an interval of 0.01 for one frequency, e.g. we
adjusted K(O–C), H (OCO) and K(C–CH ) for
3.2. Zinc stearate
The monoclinic anhydrous zinc acetate has the
COO modes as follows. The antisymmetric stretch
h
3
the frequency of the COO stretches, and y(COO)
for the COO out-of-plane bend, by referring the
result of the potential energy distributions in the
zinc acetate dihydrate. The non-bonded force field
was assumed as Buckingham type and was trans-
ferred from that in our normal mode analyses of
CH CO ·NH and BaCl ·2H O crystals [12,13]
−
1
at 1565 cm
(IR), the symmetric stretch at 1450
−
1
−1
cm
697 cm
of-plane at 612 cm
(IR) and 1471 cm
(Raman), the bend at
−
1
−1
(IR) and 699 cm
(Raman), the out-
−
1
(IR and Raman), and the
(IR) and 526 cm
−
1
−1
rock at 522 cm
(Raman).
On the other hand, zinc acetate dihydrate has the
3
2
4
2
2
as initial values, and then adjusted by the least
squares method. The cut-off distance was 5 A.
The calculation was made using an IBM RS/
modes as follows [11]. The antisymmetric stretch
˚
−1
at 1558 cm
(IR), the symmetric stretch at 1445
−
1
−1
cm
696 cm
of-plane at 622 cm
(IR) and 1460 cm
(Raman), the bend at
−
1
−1
6
000–580 computer at this university. The assign-
(IR) and 698 cm
(Raman), the out-
−
1
−1
ments were made with L vectors. The calculated
(IR) and 627 cm
(Ra-
x
−
1
frequencies agreed well with the observed ones, as
listed in Table 2.
man), and the rock at 473 cm (Raman). In zinc
acetate dihydrate and anhydrous zinc acetate, the
carboxylate groups have different coordination
forms, i.e. the chelating bidentate and the bridg-
ing bidentate forms, respectively. The difference
3
.1.3. Spectral assignments
The assignments regarding the COO group are
−
1
reflects the frequency difference of about 50 cm
the same as those in the case of zinc acetate
dihydrate and those reported by Johnson et al. [2].
In the infrared spectrum, 3T (Au) and 6T (Bu)
and in the Raman spectrum, 4R (Ag) and 8R (Bg)
should be appeared. By referring our previous
normal mode analysis of ammonium acetate crys-
tal and its deuterated compounds [12], these lat-
tice modes are expected to appear below about
in the rock. Based on this result, we examined the
coordination form of zinc stearate as a typical
long-chain soap.
3.2.1. Spectral assignments
Figs. 4 and 5 show the infrared spectrum of
−
1
zinc stearate in the 400–1800 cm
man spectrum in the 100–2000 cm
spectively. In the infrared spectrum, fine
and the Ra-
region, re-
−
1
−1
1
00 cm . With using the L vectors, we assigned
x
these modes and listed in Table 2. Some dis-
crepancies of the assignments in the low frequency
region were found between ours and those of
Johnson et al. but precise argument was not possi-
ble since the observed bands were very few.
−
1
structures appeared in the 700–1400 cm
region
which were assigned to the methylene progressive
bands. We observed three intense bands in the
−
1
−1
1300–1600 cm
region. The 1540 cm
band is

T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
1815
Table 1
Valence and non-bonded force constants of Zn(CH CO )
3
2 2
Force constantsa
Force constants
˚
˚
Initial
Final
Bond distance (A)
Initial
Final
Bond distance (A)
K(Zn···Zn)
K(Zn–O)
0.340
1.500
0.340
1.300
1.293
1.275
1.255
0.154
0.100
0.200
0.164
0.113
0.100
0.050
0.050
0.500
0.497
0.324
0.314
0.303
0.301
0.300
8.000
8.000
8.000
8.000
0.005
0.004
0.003
0.003
0.002
4.651
1.951
1.953
1.958
1.965
2.785
2.953
2.708
2.780
2.920
2.964
3.268
3.410
2.175
2.189
3.019
3.084
3.154
3.164
3.172
0.002
0.001
0.730
0.350
0.350
0.350
0.010
0.010
0.010
0.010
0.010
0.100
4.850
4.850
0.002
0.001
0.350
0.350
0.350
0.350
0.010
0.010
0.010
0.010
0.010
0.070
5.000
5.000
0.100
0.082
0.053
0.050
0.100
0.500
1.250
0.100
0.610
1.500
0.160
1.246
0.280
0.400
3.565
3.626
2.351
2.366
2.390
2.392
3.165
3.363
3.573
3.765
3.947
3.606
1.293
1.275
1.255
K(O···CH3)
K(Zn···O)
K(Zn···C)
0.154
.100
0.500
0
0.164
0.113
0.100
K(Zn···CH3)
K(O···O)
0.050
.050
0.940
K(C···C)
K(C–CH3)
0
0.497
0.324
0.314
0.303
0.301
0.300
K(CH ···CH ) 0.800
3.281
3.533
4.072
4.155
3
3
0.082
0.053
0.050
0.100
0.620
1.750
0.100
0.205
1.640
0.160
1.246
0.280
0.515
H(O–Zn–O)
H (CCO)
i
K(O–C)
K(O···C)
9.420
H (OCO)
h
9
9
9
.420
.420
.420
H(COZn)
y(COO)
F (COO)
r
0.005
3.446
3.459
3.501
3.509
3.554
FRr(COO)
Frh
Fri(COO)
FRi(COO)
0.004
0.003
0.003
0.002
a
˚
˚
2
˚
Stretching constants (K) are in units of mdyn/A, bendings (H) are of mdyn A/rad , and out-of-plane (y) is of mdyn A.
assigned to the carboxylate antisymmetric stretch
[17,18]. In the case of the polymethylene chain,
these two bands were caused by Fermi resonance
−
1
and 1465 cm
to the CH bending by referring
2
to our assignments of potassium soaps [14–16].
between the fundamental CH2 bending of the
−
1
−1
The 1399 cm
band may be assigned to the
phase difference ƒ=0 at 1442 cm
and the
C H bending and/or the carboxylate symmetric
overtone of the CH rocking of ƒ=y at 722
cm . In this case of zinc stearate, the frequency
h
2
2
−
1
stretch, both of which should be observed in the
infrared spectrum. In the Raman spectrum, five
intense bands appeared at 1067, 1134, 1299, 1443
of the fundamental CH bending might be some-
2
what higher. In the long-chain compounds, the
alkyl chains usually packed in a small periodic
structure within the real unit cell. This small
periodic structure is called as subcell. There are
typically two types of subcell which have parallel
and perpendicular forms of lateral chain packing.
In the perpendicular case, we observe the correla-
−
1
−1
and 1462 cm . The 1067 and 1134 cm
bands
were assigned to the antisymmetric and symmetric
−
1
CC stretches, respectively [17]. The 1299 cm
to
the CH2 twist. The CH2 bending region was
−
1
rather complicated. The 1443 and 1462 cm
bands were assigned by referring the well-estab-
lished vibrations of the polymethylene chain
tion split in the subcell for the CH bending and
2

1
816
T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
Table 2
Observed and calculated frequencies (cm ) of Zn(CH CO )
−
1
3
2 2
wobs.
wcalc.
Assignment
wobs.
wcalc.
Assignmenta
IR (u mode)
3
000
w (CH )
522
519 Bu
511 Au
369 Au
361 Bu
348 Au
341 Bu
316 Au
300 Bu
299 Au
292 Bu
240 Bu
229 Au
222 Au
207 Au
204 Bu
172 Bu
150 Au
137 Au
135 Bu
122 Au
122 Bu
109 Au
108 Bu
85 Au
k(COO)
k(COO)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
l(OZnO)
l(OZnO)
l(OZnO)
l(OZnO)
T
l(OZnO)
T
l(OZnO)
T
T
T
T
T
T
T
as
3
5
22
2940
1565
1565
1565
1565
1450
1450
1450
1450
1415
1343
1050
1032
w (CH )
s 3
1566 Au
1566 Bu
1560 Au
1560 Bu
1450 Au
1450 Bu
1444 Au
1443 Bu
wa(COO)
wa(COO)
wa(COO)
wa(COO)
ws(COO)
ws(COO)
ws(COO)
ws(COO)
l (CH )
as
3
l (CH )
s
3
CH3 rock
CH3 rock
R(CC)
R(CC)
R(CC)
9
9
9
9
6
6
6
6
6
6
6
6
5
5
55
55
55
55
97
97
97
97
12
12
12
12
22
22
958 Au
958 Bu
950 Bu
946 Au
717 Au
717 Bu
704 Au
698 Bu
617 Au
606 Bu
605 Au
603 Bu
542 Bu
534 Au
R(CC)
h(COO)
h(COO)
h(COO)
h(COO)
y(COO)
y(COO)
y(COO)
y(COO)
k(COO)
k(COO)
68 Bu
59 Au
51 Bu
47 Au
27 Bu
Raman (g mode)
3
2
2
023
995
941
w (CH )
362 Ag
348 Bg
343 Ag
319 Bg
301 Ag
300 Bg
298 Ag
242 Ag
228 Bg
218 Bg
213 Ag
207 Bg
174 Ag
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
w(ZnO4)
l(OZnO)
l(OZnO)
as
3
w (CH )
as
3
w (CH )
s
3
1
1
1
1
567 Bg
566 Ag
560 Ag
560 Bg
wa(COO)
wa(COO)
wa(COO)
wa(COO)
ws(COO)
ws(COO)
ws(COO)
ws(COO)
l (CH )
321
217
1471
1471
1471
1471
1433
1415
1354
1450 Ag
1450 Bg
1444 Bg
1443 Ag
as
3
l (CH )
as
3
l (CH )
s
3
9
9
9
9
6
6
58
58
58
58
99
99
958 Ag
958 Bg
954 Ag
950 Bg
719 Ag
719 Bg
R(CC)
R(CC)
R(CC)
R(CC)
h(COO)
h(COO)
146 Ag
142 Bg
134 Ag
132 Bg
119 Bg
115 Ag
l(OZnO)
R
l(OZnO)
R
R
l(OZnO)

T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
1817
Table 2 (Continued)
wobs. wcalc.
Assignment
wobs.
wcalc.
Assignmenta
699
699
612
612
612
612
526
526
526
526
704 Bg
699 Ag
616 Ag
614 Bg
611 Ag
603 Bg
544 Ag
533 Bg
521 Ag
509 Bg
h(COO)
h(COO)
y(COO)
y(COO)
y(COO)
y(COO)
k(COO)
k(COO)
k(COO)
k(COO)
w(ZnO4)
102 Bg
82 Bg
76 Ag
63 Ag
53 Bg
52 Ag
37 Bg
36 Ag
25 Bg
22 Ag
R
R
l(OZnO)
R
R
R
R
R
R
R
368 Bg
Observed, obs.; calculated, calc.
a
The symbols are the same as part 1 of this series. T and R indicate translational and rotational lattice vibrations, respectively.
the CH rocking regions, but in the parallel case
3.2.2. Coordination structure
2
we do not observe it. In this case of zinc stearate,
The carboxylate antisymmetric and symmetric
stretch of zinc stearate appeared apart from those
we did not observe the split in the CH bending
2
−
1
and rocking regions, and therefore the subcell was
of zinc acetates ꢀ10 cm . Hence, the force field
around the zinc atom in zinc stearate is somewhat
different from those in zinc acetates. However, the
rock which is the key band to determine the
coordination structure, appeared at almost the
same frequency with that of the monoclinic zinc
acetate anhydride. Hence, we considered the coor-
dination structure in the zinc stearate may be the
same as the monoclinic anhydrous zinc acetate,
i.e. the bridging bidentate.
−
1
parallel type. We found the 1403 cm
band in
the Raman spectrum. This was not the split com-
ponent of the CH bending in a perpendicular
2
−
1
type subcell which should appear at 1416 cm
.
Usually, the intensity of the C H bend is very
h
2
weak in the Raman spectrum. Hence, the Raman
−
1
1
403 cm
band was ascribed to the carboxylate
−
1
symmetric stretch. Therefore, the 1399 cm
in-
frared band is considered to be an overlapped
bands of the carboxylate symmetric stretch and
the C H bend. In the infrared spectrum, the
An EXAFS study pointed out that the averaged
Zn–O (carboxylate) length reflects the coordina-
tion structure around the zinc atom [19]. Zinc
acetate dihydrate has the averaged length of 2.188
h
2
−
1
carboxylate bend was observed at 744 cm . The
carboxylate out-of-plane and the rock appeared at
−
1
˚
5
79 and 548 cm , respectively. These assign-
(4) A, monoclinic anhydrous zinc acetate 1.957
ments were consistent with our ones for potas-
sium soaps.
(2), orthorhombic anhydrous zinc acetate 1.934
(11), zinc propionate 1.953. Among these, only
−
1
Fig. 4. Infrared spectrum of zinc stearate at room temperature in the 400–1800 cm
region.

1
818
T. Ishioka et al. / Spectrochimica Acta Part A 54 (1998) 1811–1818
−
1
Fig. 5. Raman spectrum of zinc stearate at room temperature in the 100–2000 cm
region.
˚
.95 A by an EXAFS experiment [20]. This
[5] E.L.V. Lewis, T.R. Lomer, Acta Crystallogr. B25 (1969)
02.
1
7
value closely agreed with the values of anhy-
drous zinc acetates and zinc propionate which
have the bridging bidentate coordination forms.
Hence, we conclude that the coordination struc-
ture around the zinc atom in zinc stearate is the
bridging bidentate form.
[
[
6] D.M. Glover, Acta Crystallogr. A37 (1981) 251.
7] T.R. Lomer, K. Perera, Acta Crystallogr. B30 (1974)
2
912.
[8] T.R. Lomer, K. Perera, Acta Crystallogr. B30 (1974)
913.
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