Intercalation of 1-Alkanols and 1,ω-Alkanediols into NbOPO4and NbOAsO4

Article abstract of DOI:10.1006/jssc.1998.7916

Intercalation compounds NbOXO₄· 2C_nH_2n+1OH (X=P, As; andn=2-18) and NbOXO₄· C_nH_2n(OH)₂(n=2-10) have been prepared. The molecules of alcohols are placed between the host layers in a bimolecular way, being anchored to them by donor-acceptor bonds between the oxygen atom of an OH group and a niobium atom as well as by hydrogen bonds. The molecules of diols are arranged in a monomolecular way and form bridges linking the adjacent layers of the host. The aliphatic chains of the guest molecules possess theall-transconfiguration and are inclined with respect to the host layers. The stability of the intercalates of NbOPO₄with 1-tetradecanol and 1,10-decanediol has been studied by XRD as a function of temperature and by thermogravimetry.

Full text of DOI:10.1006/jssc.1998.7916

JOURNAL OF SOLID STATE CHEMISTRY 141, 64—69 (1998)
ARTICLE NO. SC987916
Intercalation of 1-Alkanols and 1,x-Alkanediols
into NbOPO4 and NbOAsO4
Ludvık Benes,ꢀ Klara Melanova, and Vıtezslav Zima
Joint Laboratory of Solid State Chemistry of the Academy of Sciences of the Czech Republic and University of Pardubice, Studentska& 84, 530 09 Pardubice,
Czech Republic
and
Jirı Votinsky
Department of General and Inorganic Chemistry of the University of Pardubice, Na& m. Legiı& 565, 532 10 Pardubice, Czech Republic
Received January 13, 1998; in revised form June 3, 1998; accepted June 4, 1998
sufficient amounts of the guest molecules, the chains in the
intercalate possess the all-trans configuration (1,18) and
and n؍ 2–18) and NbOXO4 · CnH2n(OH)2 (n ؍ 2–10) have been their axes form either an acute angle (usually 55—60°) (11, 16)
Intercalation compounds NbOXO4 · 2CnH2n؉1OH (X ؍ P, As;
prepared. The molecules of alcohols are placed between the host
layers in a bimolecular way, being anchored to them by do-
nor–acceptor bonds between the oxygen atom of an OH group
and a niobium atom as well as by hydrogen bonds. The molecules
of diols are arranged in a monomolecular way and form bridges
linking the adjacent layers of the host. The aliphatic chains of the
guest molecules possess the all-trans configuration and are in-
clined with respect to the host layers. The stability of the interca-
lates of NbOPO4 with 1-tetradecanol and 1,10-decanediol has
been studied by XRD as a function of temperature and by
thermogravimetry. ( 1998 Academic Press
or a right angle (1, 13, 14, 17) with the host layers. The
arrangement of the guest molecules in the interlayer space of
the host is most often bimolecular (10, 14, 17). Mono-
molecular arrangement is usually found for guest molecules
with two functional groups (12, 14).
Recently, we have prepared the VOPO intercalates with
ꢁ
1-alkanols (up to 1-octadecanol) and 1,u-alkanediols (up to
1,10-decanediol) using microwave heating (13). Having de-
termined the lattice parameters of all of these complexes, we
suggest the alcohol molecules are placed between the host
layers in a bimolecular way. Molecules are anchored by
donor—acceptor bonds between the oxygen atom of an OH
group and a vanadium atom of the layer as well as by
hydrogen bonds. The molecules of diols are in mono-
INTRODUCTION
Intercalation of various molecules into inorganic layered molecular arrangement and form bridges linking the adjac-
structures leads to new materials with interesting properties ent layers of the host using similar bonds. The aliphatic
and structures. A number of recently published papers have chains of both intercalated alcohol and diol molecules pos-
reported attempts to find the structural principles governing sess all-trans configurations, and their axes are perpendicu-
the deposition of aliphatic molecules with terminal func- lar to the host layers.
tional groups between the layers of hosts. With this aim, the
This paper presents the results of intercalation of 1-al-
intercalations of, for example, aliphatic alcohols, diols, kanols (up to 1-octadecanol) and 1,u-alkanediols (up to
amines, and carboxylic acids with unbranched carbon 1,10-decanediol) into isostructural tetragonal niobyl phos-
chains into some natural silicates (1—4), a-zirconium phos- phate and niobyl arsenate.
phate (5, 6), calcium niobates (7), vanadyl or niobyl phos-
phates, arsenates, and sulfates (8—13), and vanadyl
alkylphosphonates (14, 15) have been investigated.
As follows from the results obtained, the intercalated
molecules are always anchored to the host layers by their
functional groups (16, 17). At room temperature and with
EXPERIMENTAL
1. Preparations
Niobyl phosphate trihydrate was obtained by the method
of Chernorukov (19) as modified by Bruque (20). Nb O
(10 g) was added to 70 ml of HF (40% w/w) and refluxed
ꢂ ꢃ
ꢀTo whom correspondence should be addressed.
under a Teflon-coated condenser for 2 days. Concentrated
64
0022-4596/98 $25.00
Copyright ( 1998 by Academic Press
All rights of reproduction in any form reserved.

INTERCALATED NbOPO AND NbOAsO
65
ꢁ
ꢁ
H PO (41 ml, 85% w/w) was then added and the mixture a heating rate of 5°C/min. The weight of the samples was
ꢄ
ꢁ
was heated to evaporate HF. A constant volume of the 100 mg. In some cases, particularly for complexes of solid
mixture was maintained by addition of distilled water. The alcohols and diols, the composition was determined by
resultant precipitate was filtered, and washed with 5 M elemental analysis (C, H).
HNO and with distilled water several times to remove free
The content of water in the intercalate of both hosts with
acid. The solid was dried at room temperature. Niobyl propanol was determined in the following way: About 1 g of
arsenate tetrahydrate was prepared in a similar way.
the intercalate was sealed in an evacuated double ampoule.
ꢄ
The layered complexes with alcohols with short aliphatic The part of the ampoule containing the intercalate was
chains (up to butanol) were prepared by suspending heated to 200°C while the other part was cooled to !80°C.
microcrystalline NbOXO ) yH O (ca. 0.25 g) in dry liquid After distillation of the liquid from the intercalate, the parts
ꢁ
ꢂ
alcohol (ca. 7—10 g). This reaction mixture was placed in a of the ampoule were separated and the amount of water in
ꢄ
15-cm glass flask equipped with a reflux condenser and put the condensed liquid was determined by titration according
into the waveguide of a microwave generator where the to the Fischer method (Karl Fischer AF8 Autotitrator,
mixture was heated under stirring (about 10 min). Interca- Orion, U.S.A.).
lates with diols and alcohols with longer carbon chains were
Infrared spectra were recorded on a Bio-Rad FTS spec-
\ꢀ
prepared by reintercalation reaction. The solid propanol- trometer with a spectral range 4000—500 cm using a dry
intercalated hosts were prepared in advance and used as KBr powder containing 10% of the intercalate. The result-
a starting material for the reactions with other alcohols. ant reflectance spectra were converted into Kubelka—Munk
After cooling, the solid product formed was filtered off. format.
When starting from a solid alcohol, the suspension was
separated from the melt by hot filtration. All samples for
X-ray diffraction analyses were left with a small residue of
the respective alcohol. The samples for TG—DTA and el-
RESULTS AND DISCUSSION
Only ethanol and propanol can be intercalated directly
emental analyses were washed with dry acetone or toluene into niobyl phosphate and only ethanol, propanol, and
and dried at room temperature.
butanol into niobyl arsenate. In contrast to vanadyl phos-
The microwave apparatus used for preparing the com- phate dihydrate (13), direct intercalation of longer alcohols
plexes was specially constructed by Radan Ltd. Pardubice, and diols into NbOXO was not successful. The intercalates
ꢁ
the Czech Republic. It operates at a frequency of 2450$30 with these longer alcohols and all diols had to be prepared
MHz with a total generator output of 800 W of which 30—50 by reintercalation of propanol intercalate with both hosts.
W is absorbed by the reaction mixture. The metal wave- To avoid formation of cointercalates containing propanol
guide consists of a square cavity (5.2;5.2 cm) with an and longer alcohol, propanol intercalate with an excess of
opening of 2-cm diameter, which serves for locating the the given alcohol was exposed to microwave radiation for
reaction vessel in the waveguide axis.
10 min and then filtered off. The solid obtained was mixed
with a new dose of given alcohol and then exposed to
a microwave field for another 10 min.
2. Characterization of Intercalates
The intercalates prepared were crystalline solids and their
The powder data of the intercalates with a minor surplus diffractograms showed a series of relatively sharp (00l) re-
of the guest alcohol were obtained with an X-ray diffrac- flections (some examples of the diffractograms are given in
tometer (HZG-4, Germany) using CuKa radiation Figs. 1 and 2). The a parameter of the tetragonal lattice was
ꢀ
(j"1.54051 A) with discrimination of the CuKb by an determined from the diffraction lines (110) and (200). The
Ni-filter. The CuKa intensities were removed from the values found for the lattice parameters a and c are presented
ꢂ
original data. Silicium (a"5.43055 A) was used as internal in Tables 1 and 2 for all the intercalates prepared. The
standard. Diffraction angles were measured from 1.5° to 50° results of thermogravimetry of NbOPO intercalates to-
ꢁ
(2#). The obtained data were refined by a least-squares gether with several results of elemental analyses showed that
ꢂ
program minimizing (2# !2# ) . Temperature meas- 1-alkanols form complexes with 1.93$0.09 mole of alkanol
%ꢀꢁ
#!ꢂ#
urements (from 22 to 310°C) were carried out on a heated per mol of NbOPO ; hence their composition roughly cor-
corundum plate with a thermocouple. The diffractogram responds to the formula NbOPO ) 2C H
was then measured at this temperature, which was kept intercalates with 1,u-alkanediols have stoichiometric co-
constant. The temperature was then increased and the pro- efficient x"0.96$0.06, corresponding to the formula
cedure was repeated. Each cycle of heating and measuring NbOPO ) C H (OH) . The composition of intercalates of
ꢁ
OH. The
ꢁ
L ꢂL>ꢀ
ꢁ
L ꢂL
ꢂ
lasted about 20 min.
NbOAsO with alkanols and alkanediols determined by
ꢁ
The TGA’s of the intercalates were performed with an elemental analyses is almost the same as in the case of niobyl
MOM Derivatograph (Hungary), the measurements being phosphate intercalates. Thermogravimetry could not be
carried out in the temperature interval of 30—600°C in air at used for determination of the composition because the host

66
BENES ET AL.
TABLE 1
Lattice Parameters of 1-Alkanol-Intercalated Niobyl Phosphate
and Arsenate
NbOPO ) 2C H
OH
NbOAsO ) 2C H
OH
ꢁ
L
ꢂL>ꢀ
ꢁ
L
ꢂL>ꢀ
n
a [A]
c [A]
a [A]
c [A]
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
12.94
14.75
17.83
19.23
21.84
23.54
27.16
29.79
32.67
34.30
37.75
39.70
42.70
45.06
46.65
49.50
51.40
6.65
6.66
6.66
6.66
6.65
6.66
6.66
6.65
6.65
6.65
6.66
6.66
6.66
6.65
6.66
6.65
6.67
12.85
6.43
6.43
6.42
6.42
6.44
6.44
6.42
6.44
6.42
6.43
6.44
6.45
6.44
6.44
6.43
6.44
14.36
17.49
19.44
22.32
23.78
27.43
29.73
32.59
34.88
37.53
40.10
42.86
45.40
47.44
—
41.73?
43.50?
45.83?
47.56?
49.52?
42.01?
44.68?
47.20?
49.50?
FIG. 1. Diffractograms of the intercalates of NbOPO with 1-tetra-
decanol and 1,10-decanediol.
ꢁ
—
4
is decomposed at about 500°C. Probably, As is reduced to
'''
As by the rest of the organic guest during heating and
? Basal spacings of freshly prepared intercalates.
As O formed escapes.
ꢂ ꢄ
By thermal decomposition of the intercalates of both
Infrared spectra were measured to elucidate interactions
hosts with propanol, it was found that these intercalates between the guest molecules and the host lattice. The val-
contain less than 1 mol% of water. This amount is small in ence vibration band of the M"O group (M"V, Nb) in this
comparison with that found for amine intercalates (8, 9) and type of compound is sensitive to the donor ability of the
does not imply regular participation of water molecules in ligand which coordinates the metal atom at the position
the interlayer arrangement of the guest molecules. As all opposite to the oxygen atom (21). In anhydrous niobyl
\ꢀ
other alcohol intercalates were prepared by reintercalation arsenate, the l(Nb"O) vibration is found at 1020 cm (22).
from the propanol intercalates, one can assume that the role In NbOAsO ) 4H O, the wavenumber of this band is 998
ꢁ
ꢂ
\ꢀ
cm but intercalation of basic molecules leads to the shift
of water in these intercalates is also negligible.
\ꢀ
of this band to lower wavenumbers (990 cm for amides,
\ꢀ
950—970 cm
for n-alkylamines) (9, 23). In the case of
NbOPO , this region is overlapped by the intense bands of
ꢁ
the PO tetrahedron. The IR spectra of all of the interca-
ꢁ
lates of NbOAsO with alcohols and diols contain the
ꢁ
TABLE 2
Lattice Parameters of 1,x-Alkanediol-Intercalated Niobyl
Phosphate and Arsenate
NbOPO ) C H (OH)
NbOAsO ) C H (OH)
ꢁ
L
ꢂL
ꢂ
ꢁ
L
ꢂL
ꢂ
n
a [A]
c [A]
a [A]
c [A]
2
3
4
5
6
7
8
9
10
6.45
6.44
6.42
6.44
6.43
6.44
6.43
6.42
6.42
8.14
9.89
6.64
6.65
6.65
6.65
6.65
6.64
6.65
6.66
6.65
8.07
9.67
10.57
12.12
12.9
14.59
15.54
16.45
17.56
10.83
12.26
13.56
14.48
15.77
16.87
18.36
FIG. 2. Diffractograms of the intercalates of NbOAsO with 1-octa-
ꢁ
nol, 1-heptanol, and 1-hexanol.

INTERCALATED NbOPO AND NbOAsO
67
ꢁ
ꢁ
\ꢀ
l(Nb"O) band in the 995 to 1000 cm region. This indi-
cates the presence of a donor—acceptor bond between an
oxygen atom of the alcohol or diol and the niobium atom
analogous to the bond of water molecules. A medium-in-
\ꢀ
tense band was found at about 3500 cm , which probably
corresponds to the valence vibration of the OH group of the
guest molecule coordinated to the host. A very broad band
\ꢀ
with a maximum at about 3100 cm belongs most prob-
ably to the interaction of the guest with the host lattice
resulting in the hydrogen bond between oxygen atoms of the
host lattice and the OH group of the guest. All the interca-
\ꢀ
lates show C—H stretching bands in the 2850 to 2960 cm
region.
Figures 3 and 4 show the dependencies of the basal
spacing of the intercalates prepared on the number of car-
bon atoms in the aliphatic chain of the guest molecule. The
layered complexes of 1-alkanols and 1,u-alkanediols with
both niobyl phosphate and niobyl arsenate show a linear
dependence of their c parameters on the number of carbon
atoms n. Parameters of individual straight lines together
with their correlation coefficients are given in Table 3.
Assuming that the alkyl chains of 1-alkanols are in the
all-trans configuration, the slope of 2.49 A for niobyl phos-
phate and 2.54 A for niobyl arsenate indicates a bilayer
arrangement of the alkyl chains. The alcohol bilayers are
tilted at an angle 77° in niobyl phosphate intercalates and
83° in niobyl arsenate intercalates. Both values are close to
those for the perpendicular arrangement which was ob-
served for vanadyl phosphate intercalated with 1-alkanols
(13). In the case of alkanediols, the slopes are less than half
FIG. 4. Dependence of basal spacing of NbOAsO intercalates with
alcohols (circles) and diols (squares) on the number of carbon atoms in the
aliphatic chain.
ꢁ
of those for alcohol intercalates. This indicates a mono-
molecular arrangement of diol molecules, where the alkyl
chains are oriented at 74° to the NbOPO layers and 67° to
ꢁ
the niobyl arsenate layers. The bilayer arrangement of alco-
hol molecules and monolayer arrangement of diol molecu-
les in the interlayer space of both hosts agrees with the
difference found between the stoichiometries of these inter-
calates.
The values of basal spacing of freshly prepared interca-
lates of both hosts with longer alcohols are distinctly lower
than those corresponding to the straight lines found (see
Figs. 3 and 4, dark circles). When the intercalate is left at
room temperature for a longer time (1 month), the values of
basal spacing increase and reach the values corresponding
to the linear dependence. This deviation is probably caused
by a disorder of the all-trans configuration accompanied by
a kink-block or a gauche-block formation. The presence of
these disorders was observed during heating of bimolecular
films of silicate layers and discussed in detail by Lagaly (18).
TABLE 3
Parameters of Straight Lines Calculated from the Depend-
ences of Basal Spacings on the Number of Carbon Atoms in the
Guest Chain
Intercalate
Slope [A] y intercept [A] Rꢂ-squared
NbOPO ) 2C H
OH
2.49$0.03
2.54$0.02
1.23$0.02
1.18$0.03
7.2$0.3
7.0$0.2
6.0$0.2
6.0$0.2
0.998
0.999
0.997
0.995
ꢁ
L
ꢂL>ꢀ
NbOAsO ) 2C H
OH
ꢁ
L
ꢂL>ꢀ
FIG. 3. Dependence of basal spacing of NbOPO intercalates with
NbOPO ) C H (OH)
ꢁ
ꢁ
L
ꢂL
ꢂ
alcohols (circles) and diols (squares) on the number of carbon atoms in the
NbOAsO ) C H (OH)
ꢁ
L
ꢂL
ꢂ
aliphatic chain.

68
BENES ET AL.
As the synthesis of the intercalates in the microwave field
proceeds at relatively high temperature, in molten alcohols,
these disorders are retained during cooling of the melt and
are removed by long standing at room temperature.
An isolated kink defect in an otherwise all-trans aliphatic
chain causes it to shorten by *l"l sin b, where
!!
l "1.537 A is the C—C bond distance and b"56.3° is half
!!
of the CCC angle. In the case of the chain in the perpendicu-
lar position, this shortening is equal to 1.28 A and decreases
with increasing angle of tilting of the chain. In the case of
a kink-block, this shortening is 1.0—1.2 A (18). In the case of
freshly prepared NbOPO intercalates, a deviation of
ꢁ
0.8—1.9 A was found for alcohols from C to C . This
ꢀꢁ
ꢀꢅ
indicates the presence of either one kink per pair of chains
(i.e., two chains one above the other in the bimolecular film)
or two kinks per pair of chains (either each chain of the pair
contains one kink or one of the chains contains both kinks).
The decrease of basal spacing for NbOAsO intercalates is
ꢁ
2.8 and 3.4 A for pentadecanol and hexadecanol, respective-
FIG. 5. Thermal decomposition of NbOPO .2C
H
OH followed by
ꢁ
ꢀꢁ ꢂꢆ
ly, which indicates the formation of a block containing XRD and thermogravimetry.
probably three kinks per pair of chains or a gauche block.
The formation of kink or gauche defects is more sterically
demanding; therefore it occurs in the intercalates of basal spacing, connected with the deintercalation of the
NbOXO , which have higher lattice parameter a, and the second molecule of alcohol, reaches a value of 9.5 A at
ꢁ
area in which one chain is anchored to the host is larger. In 200°C and remains constant up to the end of the measure-
the case of VOPO , which has the smallest parameter a of ment (310°C). The sample turns gray to black in this temper-
ꢁ
its tetragonal lattice, the formation of the intercalate with ature region due to the pyrolysis of alcohol. White
any basal spacing deviation has not been observed (13).
The results of thermogravimetric analysis and the ther- nation at 600°C in air. The slow decrease of weight in the
anhydrous niobyl phosphate (c"4.12 A) is formed by calci-
mal dependence of the basal spacing of NbOPO ) region 300—600°C corresponds to this process. The
ꢁ
2C H OH intercalate are given in Fig. 5. As can be seen NbOPO ) C H OH intercalate was prepared by heating
ꢀꢁ ꢂꢆ
ꢁ
ꢁ
ꢀꢁ ꢂꢆ
ꢀꢁ ꢂꢆ
from the TG analysis, tetradecanol is released in two steps NbOPO ) 2C H OH in vacuo to 170°C for 4 h. The alco-
at 240 and 280°C. The total weight loss corresponds to the hol content was checked by thermogravimetry (weight loss:
presumed stoichiometric composition. The decrease of the calculated for NbOPO ) C H OH 51.25%, found
ꢁ
ꢀꢁ ꢂꢆ
basal spacing is observed at considerably lower temper- 50.9%). Lattice parameters of this intercalate are a"6.40 A
ature. The first small decrease at 80—85°C corresponds to and c"23.94 A.
the disturbing of the all-trans configuration. At 90°C, alco-
The results of the thermogravimetric analysis and the
hol is released from the interlayer space as evidenced by thermal dependence of the basal spacing of NbOPO )
ꢁ
a steep decrease of the basal spacing. This phenomenon is C H (OH) intercalate are given in Fig. 6. This diol inter-
ꢀꢇ ꢂꢇ
ꢂ
accompanied by broadening of the (001) diffraction lines, calate is more stable than the previous alcohol intercalate
which is caused by disorder of the regular stacking of the studied as follows from the XRD results and its basal
layers along the c-axis. In the temperature region between spacing is almost constant up to 120°C. Higher stability of
145 and 175°C, basal spacing is almost constant, with the diol intercalates is probably due to the fact that the diol
a value around 23 A. The most probable composition of the molecules are anchored to the layers by two functional
intercalate is NbOPO ) C H OH in this temperature re- groups. A small decrease of basal spacing (about 0.6 A) is
ꢁ
ꢀꢁ ꢂꢆ
gion. If the basal spacing of anhydrous niobyl phosphate is observed in the region 120—158°C. A steep decrease of basal
4.12 A and that of the intercalate with bimolecular alcohol spacing accompanied by a considerable broadening of the
layers is 42.70 A, then one molecule of tetradecanol in the (001) diffraction lines between 158 and 185°C is given by
intercalate posseses 19.29 A of the interlayer distance. The deintercalation of a part of the diol molecules. A stable
basal spacing of the intercalate with alcohol in mono- phase with a basal spacing of 14.1 A and with sharp (001)
molecular arrangement should be, under the same inclina- diffraction lines is present in the region 185—230°C. It can be
tion of the carbon chain, equal to 4.12#19.29"23.41 A. presumed from the first weight loss of the sample that this
This value is in good agreement with that found experi- phase contains about 0.6 mol of diol per mol of niobyl
mentally (24.0—22.6 A). The consequent steep decrease of the phosphate. In this phase, the angle at which the chain is

INTERCALATED NbOPO AND NbOAsO
69
ꢁ
ꢁ
ACKNOWLEDGMENTS
This work was supported by the Grant Agency of the Czech Republic
(Grants 203/97/1321 and 203/97/1010) and by the Academy of Sciences of
the Czech Republic, Key Project No. 12.
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ꢂ
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19. N. G. Chernorukov, N. P. Egorov, and I. R. Mochalova, Zh. Neorg.
Khim. 23, 2931 (1978).
tilted decreases to a value of 43°. This region is followed by
a considerable decrease of basal spacing to 9.7 A at 245°C.
The thermal behavior of the sample at higher temperature is
similar to that of the tetradecanol intercalate. The
NbOPO ) yC H (OH) intercalate was prepared by
ꢁ
ꢀꢇ ꢂꢇ
ꢂ
heating NbOPO ) C H (OH) to 210°C in vacuo for 16 h.
ꢁ
ꢀꢇ ꢂꢇ
ꢂ
The stoichiometric coefficient y"0.55 was determined by
thermogravimetry. Lattice parameters of this intercalate are
a"6.42 A and c"14.51 A.
As follows from the comparison of TG and powder XRD
measurements, the temperatures of deintercalation found by
these methods can be very different in the case of the guests
with high boiling point. This difference is caused by the
insensitivity of TG to crystal structure changes which occur
during releasing of guest molecules from the interlayer space
before their evaporation from the sample.
20. S. Bruque, M. Martinez-Lara, L. Moreno-Real, A. Jimenez-Lopez,
B. Casal, and E. Ruiz-Hitzky, Inorg. Chem. 26, 847 (1987).
21. J. W. Johnson, A. J. Jacobson, J. F. Brody, and S. M. Rich, Inorg. Chem.
21, 3820 (1982).
22. N. G. Chernorukov, N. P. Egorov, and I. A. Korshunov, Neorg. Mater.
15, 335 (1979).
23. A. L. Garcia-Ponce, L. Moreno-Real, and A. Jimenez-Lopez, J. Incl.
Phenom. 9, 355 (1990).