Normal mode analysis of zinc caprate (Zn)1/2O2C(CH2)8CH3

Article abstract of DOI:10.1016/j.saa.2007.06.043

A normal mode analysis was made for zinc caprate with using the Wilson GF matrix method. Based on the analysis and infrared spectra, structural transition behavior of zinc caprate was discussed.

Full text of DOI:10.1016/j.saa.2007.06.043

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 195–197
Normal mode analysis of zinc caprate (Zn)1/2O2C(CH2)8CH3
Shintaro Suzuki, Tsutomu Ishioka∗
Department of Chemistry, Faculty of Science, Toyama University, Gofuku, Toyama 930-8555, Japan
Received 2 March 2007; accepted 23 June 2007
Abstract
A normal mode analysis was made for zinc caprate with using the Wilson GF matrix method. Based on the analysis and infrared spectra,
structural transition behavior of zinc caprate was discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Zinc caprate; Infrared spectra; Normal mode analysis
1. Introduction
which consists of ethanol and water (50:50, v/v). Zinc chloride
aqueous solution was titrating slowly into the above solution
and stirred for 2–3 h. The resultant solution was precipitated in
diethyl ether over night. These procedures were repeated twice.
Based on only this synthesis, enough thermal data could not
be obtained. And therefore, the synthesized result was purified
by washing with water, ethanol and acetone. The product was
recrystallized from benzene. These procedures were repeated
two times. Then, three endothermic peaks appear in the thermal
analysis [3]. Thermal behavior was measured by a Rigaku DSC
PTC-10A. Infrared spectra were observed with a JASCO R-800
spectrophotometer. Temperature dependence of infrared spectra
( 1 ◦C accuracy) was measured with a JASCO HC-12 heating
unit with a heating rate of 10 K/h. Temperature was measured
by a sealed CA thermocouple with an ADVANTEST TR2114
digital multi-thermometer.
Structure and thermal transition behavior of metal salt of sat-
urated fatty acid (soap) depend on its chain length. For example,
potassium soaps take the monoclinic A form crystal for C4 to
C12whereCisthecarbonnumberandthetriclinicBformcrystal
for C12 to C18 at room temperature [1]. A kind of metal affects
the thermal transition behavior, e.g., potassium stearate has eight
thermal transitions in the temperature region from 50 to 300 ◦C
[2] but zinc stearate has only one transition around 130 ◦C [3].
We have studied two zinc soaps, i.e., zinc palmitate (C16) [4]
and stearate (C18) [5]. Both zinc soaps have one thermal transi-
tion around 130 ◦C. At the transition, both chain molecules go
into liquid like conformation and some structural disorder occurs
around the COO group. A normal mode analysis of long chain
compound is rather difficult by using Gaussian program, since
its assignment is complicated without using potential energy
distribution. Empirical calculation is still valuable for such as
metal salts of saturated fatty acids. In this study, we treated zinc
caprate (C10) in order to clarify its structure and thermal tran-
sition behavior with using infrared spectroscopy and a normal
mode analysis.
3. Results and discussion
In order to investigate the molecular conformation of zinc
caprate at room temperature and assign the intramolecular
modes, a normal mode analysis was made by the Wilson GF
matrix method using an IBM RS/6000-580 computer. The
details have been reported elsewhere [6]. The molecule was
assumed to have all-trans conformation and belongs to the Cs
point group symmetry with the A’ and A” species. A tetrahe-
dral geometry was assumed for C–C–C, C–C–H and H–C–H
angles with C–C and C–H bond lengths equal to 1.54 and
2. Experimental
Zinc caprate specimen was synthesized as follows. Sodium
ꢀ
caprate ( Company, 99% purity) was dissolved into a solvent
˚
1.093 A, respectively. The C–C–O and O–C–O angles were
◦
˚
∗
assumed to be 120 with a C–O length equal to 1.27 A. The
valence force constants were estimated as follows. Zinc caprate
Corresponding author. Tel.: +81 764 45 6610; fax: +81 764 45 6549.
E-mail address: ishioka@sci.u-toyama.ac.jp (T. Ishioka).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.06.043

196
S. Suzuki, T. Ishioka / Spectrochimica Acta Part A 70 (2008) 195–197
cating the band is assigned to C␣H2. For CO2(CD2)14CD3,
1433 cm−1 band disappeared. This band is assigned to CH2
bend. The 1400 cm−1 band remained for CO2(CD2)16CD3.
Therefore, this band is assigned to the COO symmetric stretch.
By the spectra of the deutrated homologous compounds, we
revised the assignments of the bands of zinc caprate as the
wavenumber of the COO symmetric stretch from 1464 to
1417 cm−1. Here, the force constants were changed for the C–O
˚
stretch and the O–C–O interction from 9.420 mdyn/A to 9.532
and from 1.640 to 2.793, respectively. The observed infrared
spectrum of zinc caprate at room temperature is shown in Fig. 2.
Their assignments are also shown in Fig. 1. The agreement
between the observed and calculated frequencies were satisfac-
tory, indicating that the molecule has an all-trans conformation
at room temperature. In the long-chain compounds, the alkyl
chains usually packed in a small periodic cell. This small peri-
odic structure is called as subcell. There are two types of subcell
which have parallel and perpendicular forms of lateral chain
packing. In the perpendicular case, we observe the correlation
split in the subcell for the CH2 bending and the CH2 rocking
regions, but in the parallel case we do not observe it. In this
case of zinc caprate, we did not observe the split in the CH2
bending and rocking regions, and therefore the subcell is paral-
lel type. Some saturated fatty acids have a polymorphism, e.g.,
for stearic acid, the B form has a gauche conformation at the
C␣–C␤ position [10]. But in this case of zinc palmitate, no such
a polymorphism was found at the present stage.
Zinc caprate has three thermal transition points at 99, 105
and 134 ◦C and four phase I, II, III and IV from lower temper-
ature side [3]. The phases I, II and III are crystalline phases,
and phase IV is liquid phase. In order to clarify the structural
change at these transitions, we measured the temperature depen-
dence of infrared spectra (Fig. 3). As a result, the bands in the
methylene progressive region from 700 to 1400 cm−1 which are
characteristic of the all-trans conformation gradually decrease
their intensity in phases III and IV, indicating the alkyl chains go
into liquid like conformation. ꢀH and ꢀS of zinc caprate from
phase III to phase IV are 11.71 kcal/mol and 28.77 kcal/mol K at
407 K, respectively. In the case of n-decane having the same car-
bon number of 10, ꢀH is 6.86 kcal/mol and ꢀS 28.23 kcal/mol K
at the melting point of 241 K. Between these two compounds,
ꢀS is the similar value, but ꢀH of zinc caprate is almost twice
larger than that of n-decane. This indicates that in zinc caprate,
Fig. 1. InfraredspectraofCO2(CH2)14CH3, CO2CD2(CH2)13CH3 (2,2-D2)and
CO2(CD2)14CD3 (D31) from 400 to 4000 cm−1 region at room temperature.
molecule consists of the alkyl chain and carboxylate group. The
force constants of the alkyl chain have been well established by
Schachtschneider and Snyder [7]. We used his values of calcu-
lation V which were obtained for a series of normal alkanes.
The contribution of ␣-methylene group to the spectra should
be considered for zinc palmitate as in the case of normal fatty
acids. We transferred the constants of Umemura [8]. All tor-
sional constants used were also Umemura’s value. The constants
of the carboxylate group were evaluated for zinc palmitate and
zinc stearate by us [5] but it was found that these constants
have a slight mistakes [9]. We have reported the assignment
of the bands around 1400 and 1464 cm−1 for zinc palmitate.
The spectral pattern of zinc palmitate and zinc caprate is almost
the same except the mthylene progressive bands. We measured
the infrared and Raman spectra of zinc palmitate, and consid-
ered the band around 1400 cm−1 as the overlapped band of the
COO symmetric stretch and C␣H2 bend and the 1464 cm−1
was assigned to the CH2 bend. However, there remain a ques-
tion, i.e., the band width of at about 1400 cm−1 was narrow
but 1464 cm−1 was broad. In order to clarify the question. We
carry out infrared measurements and normal coordinate analy-
sis again. Fig. 1 shows the infrared spectra of zinc palmitates of
CO2(CH2)14CH3, CO2CD2(CH2)13CH3 and CO2(CD2)14CD3.
IntheCO2CD2(CH2)13CH3, 1397 cm−1 banddisappeared, indi-
Fig. 2. Infrared spectrum of zinc caprate from 400 to 3250 cm−1 at room temperature where the symbols are the same as ref. [6].

S. Suzuki, T. Ishioka / Spectrochimica Acta Part A 70 (2008) 195–197
197
ues. In this case of the coordination structure of COO groups to
Zn atom is decided as bridging bidentate, since the difference
of the carboxylate COO antisymmetric and symmetric stretch
is 140 cm−1. The carboxylate antisymmetric stretch ␯a(COO)
band at 1539 cm−1 split in phases III and IV. This means not only
coordination structural change but also slight structural distor-
tion around the COO group occurs in these phases as confirmed
by XAFS study [5]. Annearing effect occurred for zinc palmitate
[4] and zinc stearete [12]. Annealing means that increasing sam-
ple’s temperature up to 150 ◦C, and cooling to room temperture
slowly. In the case of zinc palmitate and stearate, the satellite
bands were observed around the CH2 rocking band which are
ascribed to TGT conformation at the COO end where T means
trans and G means gauche conformation for this annealing pro-
cedure. But in this case of zinc caprate, no annealing effect was
observed. A series of zinc soaps were studied by us for even car-
bon numbers from 10 to 18. Over the carbon number larger than
12, a single thermal transition behavior was observed, but less
than the number 12, multiple thermal structural change occur.
This may be explained that crystal structure is different at the
boundary of the carbon number 12, as in the case of potassium
soaps [1].
References
[1] D.M. Small, Handbook of Lipid Research, vol. 4, Plenum Press, New York,
1986.
[2] A. Cingolani, G. Spinoro, M. Sanesi, P. Franzosini, Z. Naturforsch. A 35
(1980) 757.
[3] I. Konkoly-Thege, I. Ruff, S.O. Adeosun, S.J. Sime, Thermochim. Acta 24
(1978) 89.
Fig. 3. Temperature dependence of infrared spectra of zinc caprate from 40 to
180 ◦C.
[4] T. Ishihoka, M. Kameda, A. Shimizu, I. Kanesaka, Asian J. Spectrosc. 4
(2000) 145.
[5] T. Ishioka, K. Maeda, I. Watanabe, S. Kawauchi, M. Harada, Spectrochim.
Acta A56 (2000) 1731.
[6] T. Ishioka, H. Wakisaka, T. Saito, I. Kanesaka, J. Phys. Chem. 102 (1998)
5239.
[7] J.H. Schachtschneider, R.G. Snyder, Spectrochim. Acta 19 (1963) 117.
[8] J. Umemura, J. Chem. Phys. 68 (1978) 42.
[9] T. Ishioka, M. Yamashita, unpublished study.
[10] M. Gotoh, E. Asada, Bull. Chem. Soc. Jpn. 51 (1978) 2456.
[11] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Part B, fifth ed., Wiley/Interscience, 1997.
[12] T. Ishioka, A. Kiritani, T. Kojima, Spectrochim. Acta 66 (2007) 1048–1051.
the disorderliness of the alkyl chain at the melting is almost the
same, but the COO group bonding to the Zn atoms stabilize the
chain melt at higher temperature and large ꢀH was required for
the melt. The coordination structure of the carboxylate groups
to a metal is sometimes decided from the frequency difference
(ꢀ) between the antisymmettric and the symmetric COO stretch
bands [11]. Unidentate structure exhibit the ꢀ values which are
much greater than the ionic structure of 160–200 cm−1. Chelat-
ing structure exhibit ꢀ values which are significantly less than
the ionic values. The ꢀ values for bridging strurcture are greater
than those of chelating structure, and close to the ionic val-