γ-HIO3 - A metastable, centrosymmetric polymorph of iodic acid

Article abstract of DOI:10.1002/zaac.200500099

From solutions of chromium(III) perchlorate and periodic acid, single crystals of γ-HIO₃ were obtained and characterized by single-crystal X-ray diffraction, Raman spectroscopy and thermal analysis. The compound crystallizes in the orthorhombic crystal system, space group Pbca (a = 563.92, b = 611.10, c = 1507.16 pm). The structure is built up by dimers (HIO₃)₂, which are formed by hydrogen bonds. The crystals are metastable and transform into the stable modification, α-HIO ₃, within a couple of weeks.

Full text of DOI:10.1002/zaac.200500099

γ-HIO3 ؊ a Metastable, Centrosymmetric Polymorph of Iodic Acid
Andreas Fischer* and Martin Lindsjö
Stockholm / Sweden, Inorganic Chemistry, Royal Institute of Technology (KTH)
Received March 3rd, 2005.
Abstract. From solutions of chromium(III) perchlorate and pe-
riodic acid, single crystals of γ-HIO3 were obtained and characteri-
zed by single-crystal X-ray diffraction, Raman spectroscopy and
thermal analysis. The compound crystallizes in the orthorhombic
crystal system, space group Pbca (a ϭ 563.92, b ϭ 611.10, c ϭ
1507.16 pm). The structure is built up by dimers (HIO3)2, which
are formed by hydrogen bonds. The crystals are metastable and
transform into the stable modification, α-HIO3, within a couple
of weeks.
Keywords: Iodic acid, Polymorphism
Introduction
product of H5IO6, HIO3, crystallized as thin, hexagonal,
slightly elongated plates. A determination of the lattice con-
stants on the single-crystal diffractometer showed that the
structure was not represented in the ICSD. We therefore
decided to perform a characterization, using single-crystal
X-ray diffraction and Raman spectroscopy. Additionally,
we attempted to study the thermal behaviour.
Details of the single-crystal structure determination are
given in Table 1, Table 2 lists the atomic parameters, Table 3
some important interatomic distances and bond angles.
γ-HIO3 crystallizes in the orthorhombic space group Pbca.
Iodic acid and oxoiodates(V) have been object of research
for a long period of time. This is due to the interesting op-
tical properties of the acid itself, which is being used for
second harmonic generation [1], and the potential similar
properties of salts derived from this acid. The structure of
iodic acid is therefore well investigated. It was for the first
time determined in 1941 [2] and was investigated more thor-
oughly on a later occasion [3, 4]. Iodic acid crystallizes in
the non-centrosymmetric space group P212121, which ap-
parently is a prerequisite of the exceptional optical proper-
ties. The structure is built by hydrogen-bonded chains of
HIO3 molecules. There is some evidence in the literature
of a possible polymorphism of iodic acid [5 and references
therein]. However, no structural characterization of a se-
cond polymorph has been described so far.
Table 1 Crystallographic data and results of the structure deter-
mination of γ-HIO3. Standard deviations are given in parantheses.a)
γ-HIO3
In our investigation of different transition metal/per-
iodate systems, we have studied mixtures of solutions of
chromium(III) perchlorate and periodic acid. In these mix-
tures, chromium(III) is rapidly oxidized to chromium(VI).
Upon evaporation of one of these solutions, single crystals
of the title compound could be isolated. The compound
was characterized by single-crystal X-ray diffraction and
Raman spectroscopy and the results are presented here.
Sum formula
Lattice constants/pm
HIO3
aϭ563.92(3)
bϭ611.10(4)
cϭ1507.16(9)
519.38(5)
4.50
175.91
8
Cell volume/106 pm3
density (calc.)/g·cmϪ3
formula weight
number of formula units
Crystal system, space group
Temperature of measurement
Diffractometer
orthorhombic, Pbca (No. 61)
299 K
Bruker-Nonius KappaCCD
Radiation, wavelength
absorption coefficient/cmϪ1
Crystal size/mm
absorption correction
number of reflections
unique reflections, Rint
number of parameters
Extinction correction
Extinction coefficient
R values
Mo-Kα, 71.073 pm
104.5
0.01ϫ0.10ϫ0.15
numerical, Tminϭ0.189, Tmaxϭ0.886
5758
588, 0.0310
Results and Discussion
A reaction mixture of chromium(III) perchlorate and an
excess of iodic acid yielded immediately a yet unidentified
chromium(VI) species. Upon evaporation, the reduction
38
SHELXL97
0.0021(2)
wR2ϭ0.0348b)
Rϭ0.0215 for all reflections
Rϭ0.0176 for 538 reflections with I>2σ(I)
1.178
Goodness of fit
Largest electron density
difference peak/hole
* Dr. A. Fischer
0.71 / Ϫ0.57
Inorganic Chemistry
Royal Institute of Technology (KTH)
100 44 Stockholm
a) Crystallographic data for the structures have been deposited with the
FIZ Karlsruhe, D-76344 Eggenstein-Leopoldshafen/Germany, e-mail
crysdata@fiz-karlsruhe.de. Copies of the data can be retrieved free of charge
by citing the deposition numbers CSD-415140.
Sweden
Phone: ϩ46(0)8 790-7987
Fax: ϩ46(0)8 212 626
E-mail: andif@inorg.kth.se
b) wϭ[σ2Fo2ϩ 1.0085 P]Ϫ1 where Pϭ(Fo2ϩ2Fc2)/3
1574
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
DOI: 10.1002/zaac.200500099
Z. Anorg. Allg. Chem. 2005, 631, 1574Ϫ1576

γ-HIO3 Ϫ a Metastable, Centrosymmetric Polymorph of Iodic Acid
Table 2 Atomic positions and equivalent isotropic displacement
parameters B/104 pm2 of the atoms in the structure of γ-HIO3.
Standard deviations are given in parentheses.
Atom
x
y
z
B
I(1)
0.00770(3)
0.2483(4)
Ϫ0.1504(4)
0.1604(4)
0.1793
0.33394(3)
0.4788(4)
0.5543(4)
0.2357(4)
0.3460
0.15461(1)
0.20725(14)
0.10325(14)
0.05171(14)
0.0117
0.907(9)
1.43(4)
1.56(4)
1.56(4)
1.89
O(1)
O(2)
O(3)
H(3)
Table 3 Selected interatomic distances and bond angles in the
structure of γ-HIO3. Standard deviations are given in parentheses.
Symmetry operations: i: Ϫ0.5Ϫx, Ϫ0.5ϩy, z; ii: 0.5Ϫx, Ϫ0.5ϩy, z;
iii: Ϫ0.5ϩx, y, 0.5Ϫz; iv: Ϫx, 1Ϫy, Ϫz.
atoms
distance / pm
bonds
bond angle / °
I(1)ϪO(1)
I(1)ϪO(2)
I(1)ϪO(3)
I(1)ϪO(1i)
I(1)ϪO(1ii)
I(1)ϪO(2iii)
O(3)ϪH(3)
O(2iv)ϪH(3)
180.4(2)
191.1(2)
187.3(2)
268.9(2)
269.4(2)
275.3(2)
91.1(2)
O(1)ϪI(1)ϪO(2)
O(2)ϪI(1)ϪO(3)
O(3)ϪI(1)ϪO(1)
I(1)ϪO(3)ϪH(3)
101.3(1)
96.4(1)
100.1(1)
111.4(2)
Fig. 2 The unit cell of γ-HIO3 in a view along the a axis. The H
bonds in one of the (HIO3)2 dimers are indicated by dashed lines.
184.4(2)
There is one HIO3 molecule in the asymmetric unit, in a
position close to an inversion centre. Thus, a second mol-
ecule is generated by symmetry, which is connected to the
first one by two hydrogen bonds (Fig. 1). This makes the
structure remarkably different from that of the α modifi-
cation, where one molecule acts as an H-bond donor
towards one neighbour molecule and as an acceptor
towards another, yielding infinite chains. Figure 2 shows the
packing of the HIO3 molecules in the crystal structure.
The geometry of the HIO3 molecule is essentially the
same as the α modification (IϪO bonds: 179.1-187.3 pm;
α: 178.3-189.6 pm). The H bond is significantly longer than
in the α form (184.4 pm as compared to 172.5 pm).
In conformity with α-HIO3, each iodine atom is coordi-
nated by three oxygen atoms from neighbouring acid mol-
ecules, yielding a distorted octahedral surrounding (IϪO:
268.9Ϫ275.3 pm; α: 250.4Ϫ288.9 pm). The spread of the
long I-O contacts is thus much larger in the stable α modifi-
cation. It can be noted that the density of the title
compound is significantly lower than that of α-HIO3 (α:
4.63 g/cm3, γ: 4.50 g/cm3).
Fig. 3 Raman spectra, 130-1000 cmϪ1, of α-HIO3 (bottom) and
γ-HIO3 (top).
If crystals of γ-HIO3 are set aside in the reaction me-
dium, they transform slowly, within a couple of weeks, into
the stable polymorph. Thermal investigations of γ-HIO3
turned out to be difficult, since only very small amounts
could be isolated. The calculation of transformation en-
thalpies was therefore not possible. DSC investigations
show an exothermic reaction at 160°, which is in good
agreement with previous studies [5].
In those studies, an X-ray investigation (powder diffrac-
tion) of “β-HIO3” was undertaken. However, the diffraction
data are not at all in agreement with those calculated for the
title compound. Nor do the lattice constants, which were
determined in our investigations, agree with the ratios given
in earlier publications. We decided therefore to call our
metastable polymorph “γ-HIO3”, leaving the question open
whether there really is a β modification.
Fig. 1 The dimer (HIO3)2 in γ-HIO3. Thermal ellipsoids are
drawn at the 70 % probability level. Symmetry operations: i:
Ϫ0.5Ϫx, Ϫ0.5ϩy, z; ii: 0.5Ϫx, Ϫ0.5ϩy, z; iii: Ϫ0.5ϩx, y, 0.5Ϫz.
Raman spectra for solid α-HIO3 and γ-HIO3 are shown
in Figure 3. Raman bands observed for the two modifi-
Z. Anorg. Allg. Chem. 2005, 631, 1574Ϫ1576
zaac.wiley-vch.de
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1575

A. Fischer, M. Lindsjö
Table 4 Observed Raman bands /cmϪ1 for solid α-HIO3 and
γ-HIO3. Assignments for α-HIO3 are taken from reference [6].a)
Structure determination
Details of the structure determination are listed in Table 1.
γ-HIO3
α-HIO3
Assignment [6]
Diffraction data were collected on a Bruker-Nonius KappaCCD
diffractometer. Numerical absorption corrections were applied [7].
A structure model, consisting of I and O atoms, was obtained using
direct methods [8]. The structure was refined on F2 with anisotropic
displacement parameters for all atoms [9]. The hydrogen atom was
then localized in a difference-Fourier synthesis and refined using a
riding model. All figures were prepared using DIAMOND [10].
836, vw
782, s
763, w
714, s
ν8 ϩ lattice mode
ν8, IO2 antisymmetric stretching
ν8 ϩ lattice mode
ν3, IO2 symmetric stretching
ν4, IOЈ stretching
ν5, OЈIO2 rocking
793, m
729, vs
674, vw
392, w
342, m
633, s
380, w
330, m
314, sh
297, w
ν6, IO2 deformation
ν9, OЈIO bending
277, vw
263, vw
Raman spectroscopy
Raman spectra from single crystals of α-HIO3 and γ-HIO3, respec-
tively, were recorded using a Renishaw System 1000 spectrometer,
equipped with a DMLM Leica microscope and a 25 mW He-Ne
laser (633 nm).
220, vw
194, vw
Lattice mode
Lattice mode
a) Abbrevations: s, strong; m, medium; w, weak; sh, shoulder; v, very.
DSC analysis
cations are listed in Table 4. The spectrum of α-HIO3 is in
very good agreement with earlier measurements [6]. On the
other hand, the spectrum of the title compound includes
some apparent differences, particularly in the region of IϪO
A sample of approximately 1 mg of γ-HIO3 was sealed into an Al
capsule. A DSC experiment was done on a PerkinElmer DSC7
differential scanning calorimeter. Due to the high uncertainty of
the mass determination, no attempt was made to obtain quantitat-
ive data.
stretching modes, 600-800 cmϪ1
.
´
´
Acknowledgements. Dr. Zoltan Szabo is acknowledged for his
valuable help with translations from the Hungarian. The Swedish
Research Council (VR) is acknowledged for financial support.
Conclusion
By reaction of chromium(III) perchlorate with an excess of
periodic acid in aqueous solution, iodic acid was obtained.
Slow evaporation of the water yielded crystals of a new,
metastable polymorph of iodic acid. This finding could be
reproduced in a second experiment. Apparently, the compo-
sition of the reaction system hampers the crystallization of
the stable α modification or leads to an increased formation
of dimers (HIO3)2 in solution, which favours the crystalliza-
tion of γ-HIO3.
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A 1-molar solution of chromium(III) perchlorate was prepared
from “Cr(ClO4)3·6 H2O” (Alfa Aesar) and demineralized water. As
was determined later, this hydrate is really a nonahydrate. The chro-
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of γ-HIO3 were obtained. These transformed into the stable α
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 1574Ϫ1576