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SOLUTE STRUCTURE OF COPPER(II)ACETATE SOLUTIONS IN LIQUID AND GLASSY STATES
M. Nomura, T. Yamaguchi
To cite this version:
M. Nomura, T. Yamaguchi. SOLUTE STRUCTURE OF COPPER(II)ACETATE SOLUTIONS IN LIQUID AND GLASSY STATES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-619-C8-622.
�10.1051/jphyscol:19868116�. �jpa-00226014�
Colloque C8, supplgment au n o 12, Tome 47, dbcembre 1 9 8 6
SOLUTE STRUCTURE OF COPPER(I1)ACETATE SOLUTIONS IN LIQUID AND GLASSY STATES
M. NOMURA and T. YAMAGUCHI*
The Photon Factory, National Laboratory for High Energy Physics, Oho-machi, Tsukuba-gun, Ibaraki 305, Japan
" ~ e p a r t m e n t of Electronic Chemistry, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan
Abstract Structures of solutes in copper(I1)acetate aqueous and ethanolic solutions in both liquid and glassy states are determined by means of EXAFS and XANES. The solute in the aqueous solution has monomeric structure in both states. It has four short Cu-0 bonds (195pm) and two longer bonds (228pm) in glassy water-glycerol mixed solution. On the other hand, it has dimeric structure similar to its crystalline state in both states of ethanolic solutions. Weaker interactions such as Jahn-Teller elongation and nonbonded interatomic pairs are clarified by vitrifying the solutions and information on non-uniform Cu-0 bond lengths are extracted by comparing the EXAFS spectra of solutions in the liquid and glassy states.
I. INTRODUCTION
The solute structure in glassy solutions is of great interest in connection with structural investigations of the solutions in the liquid state. Also it is important to check if a solute in the glassy state has a sipilar structure as in the liquid state in order to compare information derived from frozen solutions with that on the liquid state. However, there is no EXAFS study on the solute structure in glassy solutions.
We report here the solute structure of copper(1I)acetate aqueous and ethanolic solutions in the glassy state compared with that in the liquid state. This compound has well known binucler structure in the crystalline state. However, there is no report on the structure of solute in solutions except deduced from magnetic susceptibility data /I/. XANES was used as an indicator of a structural change during vitrification, because it is expected to be sensitive to the change of electronic state and the symmetry around the X-ray absorbing atom. Some weak interactions are clarified from EXAFS spectra of glassy solutions.
11. EXPERIMENTAL
Copper(I1)acetate monohydrate (CU~(CH~COO)~- 2H20, hereafter abbreviated as Cu(OAc),) was dissolved in distilled water(aq), equivolume mixture of water and
4
glycerol(H2Qtgly), and ethanol(C H OH) until their saturation. Sample solutions were sealed in cells and then immerse4 3m liquid nitrogen. After vitrification, they were kept at this or lower temperature. EXAFS and XANES were measured at BLlOB station of the Photon Factory using a Si(311) channel-cut monochromator in a transmission mode.
111. RESULTS AND DISCUSSION
In the crystalline state, copper(1I)acetate
- -
monohydrate has the binuclear structure bridged by four acetate groups. According to the neutron diffraction study by Brown and Chidambaram, r(Cu-0)= 194.15, 195.24, 198.98, 199.18 pm, ~(CU-OH ) =2
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868116
C8-620 JOURNAL DE PHYSIQUE
216.13 pm, and r(Cu
...
Cu)= 261.43 pm 121. Its XANES and Fourier transform of EXAFS are shown in Fig. 1. Two shoulders in the XANES, which are due to 1s-p+
shakedowntransition 131, and the second and third peaks in the Fourier transform of the EXAFS are the typical feature of the binuclear structure 141. So, if the solute in a solution crystallyze during vitrification, these signs must be observed.
EleV
Fig. 1 XANES (a) and Fourier transform of EXAFS (b) on crystalline copper(I1)acetate at room temperature.
On the other hand, the solute in the aqueous solution has mononuclear structure as shown in Fig. 2b and the EXAFS spectra can be interpreted by the single shell model that four oxygen atoms exist at 194pm apart from a copper atom 141. Its XANES spectrum (Fig. 2a) is also different from that of crystalline state, and thus
supports mononuclear structure.
EleV
Fig. 2 XANES (a) and Fourier transform of EXAFS (b) on copper(II)acetate/
water-glycerol solution.
The glassy state of copper(I1)acetate aqeous soluiton was not obtained owing to crystallization,so equivolume mixture of water and glycerol was used as the solvent.
This solution and the aqueous solution showed identical XANES and EXAFS at room temperature as shown in Fig.2. The XANES of the glassy solution was identical to that of the solution at room temperature (Fig.2). This identity indicates that the soxute did not crystallize during vitrification. However, the EXAFS for the glassy
pm) can explain the EXAFS spectra very well. Owing to the smaller thermal vibration in the glassy state than in the liquid state, weaker interactions due to the Jahn-Teller effect are. clarified.
Structural parameters determined from the EXAFS spectra are summarised in Table 1, compared with the data on copper(I1)-aqua complex. The shorter and longer Cu-0 interaction indicates the equatorial and axial bonds, respectively. Rather short axial bond length compared with those found in a copper(I1)perchlorate aqueous solutions is due to coordination of acetates in the axial sites. Because of lower symmetry of ligands coordination in this complex than in the case of aqua-complex, the Jahn-Teller effect may be weak, then the axial bond lengths shorten. The Debye-Waller type factor(@-) of equatorial Cu-0 bonds does not decrease by vitrification as expected from pure thermal motion. This fact indicates that bond lengths of equatorial Cu-0 is not so uniform, making distorted square in aqueous solution.
The XANES spectrum of copper(1I)acetate monohydrate in ethanolic solution is different from that of aqueous solution. It is rather similar to that of the crystalline state. As shown in Fig. 3 , about 60% of the solute forms the binuclear structure in ethanolic solution. The first peak is due to the Cu-0 interactions of acetate groups, the second is due to Cu-0 of ethanol or water and Cu...Cu, and the third faint peak is due to Cu...C of carbonyl groups. The magnitude of the third peak is smaller than that for crystalline copper(II)acetate, which indicates that there is little intermolecular interaction (Cu...O of neighboring molecule) in the solution. The small peaks around 360 pm is derived from longer intramolecular Cu...O and Cu...C interactions. When the solution is vitrified, XANES does not change.
However, the Fourier transform of EXAFS changes in the region from 200 to 300 pm.
Three peaks in this region are due to Cu-0, Cu. ..Cu, and Cu.. .C interactions.
Structural parameters determined from these EXAFS are given in Table 1.
Fig. 3 XANES (a) and Fourier transform of EXAFS (c) on copper(II)acetate/ethanol solution.
The change of the EXAFS spectra from liquid to glassy state can be interpreted as the result of smaller thermal vibration in glassy state, because the XANES spectra of solutions at room tempaerature are identical to those of glassy state. It should be emphasized that the EXAFS study of solute structure in glassy solutions is one of the most reliable methods to detect metal-metal interaction, weak interatomic interaction, and longer-range ordering in solution.
C8-622 JOURNAL
DE
PHYSIQUETable 1. Structural parameters of solute in copper(I1)acetate aqueous and ethanolic solution in liquid and glassy states compared with those of copper(I1)perchlorate aquous solution.
P r/pm 0. /pm sample method Ref.
Cu-0 4 194 5 CU(OAC)~ aq EXAFS this work Cu-0 4 194 7 Cu(0Ac) 2/H20+glycerol EXAFS this work Cu-0 4 195 6 C~(OAc)~/H~0Cgl~cerol EXAFS this work Cu-0 2 228 7 glass
Cu-0 195.5 4 Cu(C10)
4 2 aq EXAFS / 5 /
Cu-0 260 12
Cu-0 4 195 7 CU(OAC)~/C~H~OH EXAFS this work CU-0 1.3 237 13
Cu...Cu 0.6 258 8
Cu-0 4 197 2 CU(OAC)~/C~H~OH glass EXAFS this work Cu-0 1.2 212 5
This work was performed as a part of proposal number 85021 at the Photon Factory.
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