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TGA-DTA, infrared and x-ray powder diffraction studies on strontium nitroprusside and its hydrates
Journal of Molecular Structure, 70 (1981) 241-254 Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
TGA-DTA, INFRARED AND X-RAY POWDER DIFFRACTION ON STRONTIUM NITROPRUSSIDE AND ITS HYDRATES
C. 0. DELLA
VEDOVA,
J. H. LESK,
E. L. VARETTI**
STUDIES
and P. J. AYMONINO*‘“*
Departamenfo de Quimica, Facultad de CiencMs Exactas, Uniuersidad Nacionai de La Plats, 47 esquina 115, 1900 La Plata (Repriblica Argentina) 0. E. PIRO, B. E. RIVERO***
and E. E. CASTEILLANOg
Departamento de Fikica, Facultad de Ciencias Exactas, Universidad Nacionul de La Plats, 49 esquina 115, 1900 La Plata (Repriblica Argentina)
(Received27 May 1980) ABSTRACT Dehydration data for the tetra-, di- and monohydrate of strontium nitroprusside and the vibrational behaviour of the watermolecules in normal and deuterated samples agree with crystal data previously reported (for the di- and tetrahydrate as single crystals) and in this paper (monohydrate as a powder). Four distinct types of water molecules are present in the three hydrates with one type (oxygen bonded to W+) in common. Other types are a non hydrogen-bonded molecule (dihydrate), a molecule bonded by a single
hydrogen atom to the nitrogen atom of a cyanide group and a set of two molecules similarly bonded to the oxygen atom of the previous type (tetrahydrate). The spatiai orientation of the NO groups of the nitroprusside ions in the tetrahydrate is confirmed by the observed dichroism of the corresponding stretching band.
INTRODUCTION Strontium nitroprusside crystallizes from aqueous solutions as a monoclinic tetrahydrate [ 1, 21 (space group C:,., - C 1 2/m 1 [ 21) with four formulas in the unit cell. The water molecules are distributed among three sets of different symmetries and chemical environments. One of these sets, composed of two identical molecules, is located in general positions and the other two, in special positions (m). The molecules of the first type (W4, its oxygen atoms designated as Ow(4) in ref. 2) are hydrogen bonded to the oxygen atoms of water molecules of the second type (W2 (Ow(Z)), d(O0) = 2.85 R ) and to the oxygen atom of the nitroprusside ions 0) = 3.44 A). The two Ow(4) atoms are related by a center (O(l), d(Oof inversion and they are in close contact with each other, at a Van der Waals’ distance of 2.87 A. Strong repulsion between the lone pairs of these l
l
l
l
*To whom correspondence should be addressed. **Member of the Can-era de1 Investigador Cientifico, CONICET, R. Argentina. ***Member of the Carrera de1 Investigador Cientifico, CIC, R. Argentina. 5 Present address: Instituto de Fisica e Quimica, Sao Carlos (USP), Brasii. OOZZ-2860/81/0000-0000/$02.50
0 1981 Elsevier
Scientific
Publishing
Company
342
oxygen atoms and lability of the water molecules to which they belong should therefore be expected. The second-type molecules (WZ) are bonded through one of their hydrogen atoms to the axial nitrogen atom of nitroprusside ions (N(4), 0) = 2.85 A) while molecules of the third type (W3 (Ow(3)) [23 ), d(Owhich are electrostatically bonded to the strontium ions, do not form hydrogen bonds. It is to be noted that the W4 water molecules are unsymmetrical, i.e. the two OH bonds in each case are nonequivalent, while crystallographic data alone on W2 and W3 molecules are not sufficient for the assignment of their symmetries. The different environments of the water molecules in the tetrahydrate and the dihydrate are reflected in their thermal behav-iour, and the differences are big enough to allow the isolation of the dihydrate and also the monohydrate by regulating the dehydration conditions, as described below. Interestingly, we have been able to obtain from a single crystal of the tetrahydrate a mosaic of crystals of the dihydrate oriented nearly parahel to one another, which allowed a complete structural determination by X-ray diffraction methods [ 3]_ The water molecules lost in this dehydration process are the W4. The dihydrate is structurally closely related to the tetrahydrate, belonging to a space group of higher symmetry (D:B - C 2/c 2/m 2 Jm, orthorhombic) which contains the space group of the tetrahydrate as a subgroup_ The W3 water molecules are still coordinated to Sr*+ in the dihydrate as in the tetrahydrate, but the W2 molecules are slightly shifted from their original positions in the tetrahydrate. Consequently, the hydrogen bond found between Ow(2) and N(4) in the tetrahydrate is lost in the dihydrate, and no other acceptor atom is now close enough to Ow(2) to give rise to hydrogen bonding. The two water molecules are symmetrical because they are in special positions mm. The monohydrate, as well as the anhydrous form of strontium nitroprusside have been obtained only as microcrystalline powders. The X-ray diffractograms of these powders show some lines in common with the diagrams of the higher hydrates; these lines suggest a structural relationship between the four compounds (see below). The water molecule in the monohydrate behaves vibrationally as the W3 molecules in the higher hydrates. Nitroprusside ions are located in mirror planes m in the tetrahydrate [2] and in mm positions in the dihydrate. Both in the tetrahydrate and the dihydrate the axes of the nitroprusside ions lay on (010) planes and are oriented nearly parallel to the a axis, in the tetrahydrate, and exactly coincident with it, in the dihydrate. l
l
EXPERIMENTAL
The dihydrate was obtained from the tetrahydrate [ 1,2] by storage in a desiccator over phosphorus pentoxide until constancy of weight was
243
achieved. Under vacuum, the dehydration went further to give the monohydrate. Finally, the anhydrate was obtained by heating any of the hydrates for 2 h at 100°C under high vacuum. Weight changes in the different dehydration steps always matched to a few tenths of percent with values expected from the formulas assigned to the different hydrates (cf. TGA results reported below). Mean percent values of several runs found are (calculated vaIues in parentheses): tetrahydrate to dihydrate: 9.53 (9.59), dihydrate to monohydrate: 5.09 (5.30), monohydrate to anhydrate: 5.18 (5.60). Deuterated samples were obtained beginning with the tetrahydrate crystallized from appropriate mixtures of common and heavy water. Samples of the lower hydrates and the anhydrous form were always handled in a dry bag. X-ray
diffraction
data
Powder diffractograms of samples mounted on a rotating support covered with two layers of a thin polyethylene film were recorded as in [l] using Cu(Kar) radiation_ TGA-DTA
data
Thermograms of the tetrahydrate were obtained using a Rigaku Denki YLDG/CN 8002 L2 Thermoflex apparatus under flowing dry nitrogen (for experimental details see [4a, b] ). IR spectra The IR absorption spectra of Nujol and Halocarbon mulls between CsI plates were recorded as in [l] . Calibration in the OD stretching region was improved by recording the HBr spectrum [ 51. For the same purpose, in the CN stretching region advantage was taken of the relative sharpness of peaks of sodium nitroprusside dihydrate and the coincidence (within 1 cm-‘) of the wavenumbers reported by three different groups of spectroscopists [Sa-c]. Wavenumbers selected were 2173,2161,2156, and 2143 cm-‘. The accuracy of wavenumbers reported below for sharp bands in those regions is believed to be f 1 cm- ‘. In the other regions of the spectrum it is _+2 cm-‘, also for sharp bands, and &3 cm-’ for broad bands. A RIIC VLT-2 cell was used to obtain spectra at the boiling temperature of nitrogen. Polarized single crystal spectra of very thin plates (shortest edge parallel to the crystahographic a axis) of the tetrahydrate were recorded as for barium nitroprusside dihydrate [7] . Unfortunately, absorbance was too strong in the stretching region of water, even for the thinner samples, to obtain useful information. This was not the case in the CN and NO stretching regions of the nitroprusside ion.
244
Fig. 1. TGA-DTA RESULTS
of Sr[Fe(CN),NO]
- 4HI0.
AND DISCUSSION
TGA-DTA Figure 1 reproduces a thermogram showing four distinct decomposition stages. DTA-peak temperatures are mean values of several runs. The first three steps in the thermogram corresponded merely to successive dehydration stages because the IR spectrum of the residue obtained after the third step in a separate run, which was interrupted at 2OO"C, showed no significant modifications in the CN and NO stretching regions (the spectrum was in fact similar to the spectrum of the anbydrate obtained at 100°C in vacua (see below))_ For the first stage of the dehydration process in the thermobalance, which should correspond to the conversion of the tetrabydrate into the dihydrate, the weight changes found were rather lower than expected. This contrasts with results of the dehydration experiments performed in a desiccator over phosphorus pentoxide (see above). For the other stages, results were in good accord with expectancy. The last step of the thermogram involves the loss of the NO group as shown by IR spectroscopy (cf. ref. 1 where it was erroneously reported that strontium nitroprusside decomposes at 147°C). The weight change at this stage indicated that a CN group was also involved. The residue was perhaps a mixture of diiron hexacyanoferrate(II), distrontium hexacyanoferrate(I1) and strontium dicyanide (cf. [4a] ).
245
X-ray powder
diffraction
data
Results are presented in Table 1 and Fig. 2, where data reported previously for the tetrahydrate [l] are also included for the sake of comparison. Indexing of the diffraction lines of the dihydrate was performed taking
advantage of results reported in [3] for the mosaic crystal. Lack of single crystal data and of comparable diffractograms prevented complete indexing of the diffraction patterns of the monohydrate and anhydrate. In any case, some features of these diffractograms seem to be related to indexed lines of the diffiactograms of the higher hydrates i.e., lines (200), (400), (600) and (800) [ 1,3]. The strong (400) lines of the tetrahydrate and the dihydrate are mainly due to Bragg reflections on the electron-rich “planes” defined by the heavy iron and strontium atoms. The high intensity of the (400) lines results from the fact that these “planes” are at approximately l/4 of celledge length from each other in the direction of the a axis [2, 31. The perTABLE
1
Powder
diffraction
Line
d/n (A)
(a) Sr[Fe(CN),NO] (200) 9.980
(111) WOO) (311) (002) (311) (go01 (602) (710) (313) (80_0) (131) (249)
5.426 4.958 4.495 4.133 4.076 3.308 2.796 2.655 2.549 2.484 2.395 1.755
(b) Sr[Fe(CN),NO] (200) 9.772
(111) (310) (400) (002) (401) (600) (421) (203) (602)
5.347 4.801 4.725 4.122 4.118 3.162 2.760 2.680 2.540
aA(Cu KY)=1.5418
data for strontium sin 0 /X (A -‘)a
nitroprusside
(Z/Z,) x 100
and its hydrates
Line
sin 8 IX (a-‘)”
(I/Z,)
(A\)
2.370 2.281 2.244 2.098 1.943 1.821
0.2109 0.2192 0.2228 0.2383 0.2573 0.2745
15 8 8 6 2 2
d/n
- 4H,O
0.0501 0.0953 0.1008 0.1112 0.1210 0.1227 0.1511 0.1788 0.1883 0.1962 0.2012 0.2088 0.2849
10 15 100 20 10 10 25 20 10 4 10 5 5
(8’30) (801) (331) (0041 (404) (803)
(c) Sr[ Fe(CN),NO] (200) 10.311
(400)
- 2H,O
0.0512 0.0935 0.1041
0.105i3 0.1213 0.1214 0.1581 0.1812 0.1866 0.1969 A.
23 15 45 100 21 12
15 40 3 8
(800)
5.553 5.045 4.835 4.123
(800)
0.0485 0.0900 0.0991
0.1147 0.1213
3.986
0.1254
2.855 2.557 2.500
0.1751 0.1956 0.2002
(d) Sr[Fe(CN),NO] (200) 10.246
(400)
. H,O
5.045 4.568 2.903 2.496 2.262
0.0488
0.0991 0.1094 0.1722 0.2003 0.2210
65 55 100 26 ii 35 25 17
3 100 65 17 39 30
x 100
246
3.05
Fig.
2.
0.10
0.15
0.20
0.25
SC”
e/J-
Powder X-ray diffraction patternsof strontium nitroprussideand its hydrates.
sistence of the highly intense (400) line in the diffractograms of the monohydrate and the anhydrate suggests a similar distribution of the heavy atoms in these compounds. These “planes” also contribute to the (800) lines, part of whose intensity is also due to reflections on other “planes” defined by the axial ligands of the nitroprusside ions which are interposed between them at ca. l/8 of a (see Fig. 2 and ref. 2). The correlation of the (400) and (800) lines among the four compounds is indicated in Fig. 2, together with the corresponding correlation of the (ZOO) and (600) lines. The vanishing of the latter line in passing from the dihydrate to the monohydrate is related to the disappearance of the W2 water molecule. In the same way, the almost complete disappearance of the (200) line during the last dehydration step can be related to the elimination of the W3 water molecule (see below). The particular crystallographic arrangement in the higher hydrates along the a axis discussed above (see also [2, 3]), and the correlation noted for the lines (200), (400), (600) and (800) among the entire series of compounds, make it interesting to compare the changes in the projections of the electron densities along the a axis, as the dehydration proceeds from the tetrahydrate to the anhydrate. As pointed out above, the space group of the tetrahydrate (C 1 2/m 1) [ 21 is a subgroup of the space group corresponding to the dihydrate (C 2/c 2/m 2,/m) [ 33. In particular, the respective unit cells can be related by a slight distortion (scarcely 8” in the cell parameter P and even minor relative changes in the other cell parameters), without changing the relative position of the center of symmetry present in both
247
cells. This fact is reflected in the similarities in the packing of both crystalline structures. In particular, the sign of the structure factors F(hO0) (h = 2,4, 6,8) are the same. Assuming that the same applies to the monohydrate and the anhydrate, it is easy to synthesize the projection of the electron density along the a axis for these latter compounds. The amplitudes of the four structure factors IF(hOO)l (h = 2, 4,6, 8) were obtained from the integrated intensities as measured on the corresponding powder X-ray diffractograms. The intensities were corrected for Lorentz and polarization effects only, using the data reduction formula [8]
lwwl,wd.
=
(1 [
Ji,“,“,T2, B )
112
X Integr. intens. 1
where eB is the angle corresponding to the Bragg reflection of Miller indices (hkl). Comparisons were made for the tetrahydrate and the dihydrate between results obtained for the structure factors IF(hOO)l,,ti and IJw0wlrnono,st_ (these latter expressed in an absolute scale after being obtained from the diffraction intensities collected on Weissenberg photographs, and corrected for Lorentz and polarization effects but not for absorption or extinction [2, 31). The comparison served a double purpose: in an approximated absolute scale, and (b) to ascer(a) to put F(~OO)I,, tain the degree of accuracy of the data reduction procedure using the powder X-ray diffractograms. The lF(hOO)lpoti (h = 2, 4, 6, 8) factors proved to be in good agreement with the corresponding IF(hOO)l,,,,,,_ (mean relative errors less than 10%). The data pertaining to the monohydrate and the anhydrate were scaled to make the modulus of the F(400) factor for these hydrates equal to the corresponding value for the dihydrate. The results of the computed Fourier syntheses are plotted in Fig. 3 for the lower hydrates and the anhydrate. The F(OOO) term has not been included. Figure 3 has been labelled with the symbols of the atoms and water molecules which are expected to project on the different regions. Comparing the relative electronic densities in the regions about 3c= 0 and x = l/4 (see Fig. 3a, b), the loss of the W2 water molecule can be followed during the second step of dehydration (to which is related the disappearance of the (600) line when going from the dihydrate to the monohydrate (see Fig. 2b, c)). In the same way, the loss of the W3 water molecule can be envisaged in the last dehydration step (a process which brings about the almost complete disappearance of the (200) line present in all the hydrates (see Fig. 3b, c))_ The (600) line is also absent in the anhydrous compound (see Fig. 2). The above interpretation concerning the sequence in which the two water molecules are lost during the last two dehydration steps agrees with the TGA-DTA and spectroscopic results (see below for discussion of IR spectra). In Fared spectra Room temperature mull spectra of the hydrates and the anhydrate are shown in Fig. 4, where bands due to Nujol are not included. Table 2 presents
248 Sr + Fe + Equatorial
Sr + Fe + Equatorial
CN
CN
a b
Fig. 3. Plots of (p(x)electron (CN),NO] displaced
z -
F(OOO))
(electrons)
corresponding
to the projection
of the
density p(x) (em A-‘) along the a axis for: (a) Sr[Fe(CN),NO] .2H,O; (b) Sr[Fe- H,O and (c) Sr[Fe(CN),NO]. For the sake of clarity, the curves have been 500 electrons
from each other.
The Fourier
syntheses:
(p(x)-
z -
F(OO0))
= 2~:F(h00)-cos(27rhx) include only the terms with h = 2,4,6 and 8, which have been scaled to an approximately absolute scale. F(OOO) is equal to 508 electrons in the case of the dihydrate. V is the volume of the unit cell and A is the area determined by the basic b and c vectors.
wavenumbers and tentative assignments for the absorption peaks. It includes information already reported for the tetrahydrate [l] to facilitate comparisons. All spectra have in common obvious features, due to the nitroprusside ion, which are assigned as in ref. 1. The splitting of the NO stretching band clearly observed in the dihydrate spectrum should be compared with a similar feature reported for barium nitroprusside dihydrate, which was assigned to a dynamic interaction between the NO groups in the unit cell (factor group splitting) [7]. Th is could also be the case for the dihydrate (factor group: D&. A splitting of the NO stretching band is also seen in the anhydrate. The spatial orientation of the polar axis of the nitroprusside ion in the tetrahydrate found by X-ray diffraction methods [Z] is confirmed by the polarized spectrum in the NO stretching region of a plate cut from a single crystal with faces parallel to.the crystallographic (100) planes. Figure 5 shows the spectra obtained with the plate oriented parallel to the slit of the spectrophotometer and making an angle of approximately 45” with its normal_ The broken line reproduces the spectrum recorded with the analyser parallel to the b axis and therefore normal to the a axis and the NO groups, and the full line, in a direction normal to it, i.e., tilted with respect to the NO groups.
249
Sr [Fe(CN)5N0]-4H,
0
I
sf [F~cNI,NO]-~H,O
Sr [FEKNI,NO]-H,O
sr [F:F~(cN),NO] 10
35bo
1 3100
1 8 1 1 8 2300 2000’ Wavenumber
Fig. 4. Room
temperature
.
I
1500 800
I
I-,
500’
(cm-‘)
IR mull spectra of strontium
0 WavenumDer
nitroprusside
(cm-’
and its hydrates.
Fig. 5. Polarized spectra of a thin (100) plate of Sr[Fe(CN),NO] -4H,O placed with the b axis parallel to the slit of the spectrophotometer and making a 45” angle with the light ray direction: (a) analyser parallel to the b axis; (b) analyser parallel to the c axis.
)
250 TABLE
2
Infrared spectra of strontium nitroprusside and its hydrates in Nujol mulls (room temperature spectra)a f;C~egXWJOI
Sr[Fe(CN),NO] - 2H,O
3877 3621 (W3) 3560 (W3) 3420 (W2.
3871 3624 (W2. 3559 (W2. -
W3) W3)
2155 2114 sh 1965 1939
2148 2108 sh 1960
1910
W4)
SzF;(CN),NO 2
sh
1
3867 3621 (W3) 3559 (W3) -
3867
2190 2158 2116
2188 2159 2125 sh 1957 sh 1940 1930
1960 1936
sh
1609 661 643 560 504 465 sh 439 422 375 sh 328
664 649 560 sh 506 470 sh 441 420
662 649 sh
659 644 sh -
509 470 sh 443 sh 425
507 -
380 325
390 sh 326
assigned
to water
vibrations
NO stretching HOH HOH HOH FeNO
>
438 428 409 -
show
(W2) (W3) (W3.
bending bending W4) bendings
deformation
FeN fzO
stretching libration
H,O
hbration
b
j H,O b
326
%h: shoulder. b FeC stretching s and FeCN deformations Bands
H,O antisymmetric stretching Hi0 symmetric stretching H,O (H-bonded) stretching CN (axial) stretching CN (equatorial) stretching ‘“CN stretching
-
1622 -
1620 -
%I0
-
-
1636
Assignments
Sr[Fe(CN),NOl
libration
[cf. 7 1.
features
expected
for the dif-
(and chemical) molecular types. In the tetrahydrate 3420 cm-’ and 1636 cm-’ must be due to the strongly hydrogen bonded W2 water molecules. Partial deuteration allows some detailed assignments of the other water bands. Table 3 gives the wavenumbers and assignments for bands observed in the low-temperature spectra of the partially deuterated hydrates at high and low deuterium concentration. The spectrum of the deuterated monohydrate shows peaks at 2644 cm-’ and 1426 cm-’ which must be due to the OD stretching and HOD bending of a water molecule which seems to be symmetric and crystallographically unique. The high wavenumber of the OD stretching band (and concomitant low value of the HOD bending) indicates that very weak, if any, hydrogen bonding is operating in the monohydrate. In this respect, it should be compared with values reported for the least bonded water molecule of barium nitroprusside dihydrate (2665 and 2647 cm-’ for the non-equivalent OD bonds [7] ) and for the water molecules of sodium nitroprusside dlhydrate (2654 and 2630 cm-‘) [6b]. No acceptor atoms have been found properly located around the least bound water molecule in BaNpr 2H,O to give place to well defined hydrogen bonds [7 ] . In Na,Npr 2H20 one of the ferent
crystallographic
the broad
bands
at
l
l
251 TABLE3 Water
vibrations
perature
spectra)
Tetrahydrate=
Dihydrate
b
3636 3623 3609 3592 3572 3560
3629 b 3594 b 3557 3504 3355 b 2700 FB2 2650 b 2625 2597 2586 2527 2480 =br:
2723 2694
of strontium
Monohydrate
3624 3590 3559
2694
2678 2644 2632
2644
2592
2601
broad.
bSee
nitroprusaide
hydrates
Assignments
Stretchings H,O(W2) antisymmetric H20(W3) antisymmetric OH(W2:HDO) OH(W3:HDO) H,O(W2) symmetric H,O(W3) svmmetric OH(W4:DOH-*Ow(2)) OH(W2:DOH..N(4)) D,O
at high
isotopic
Tetrahydratea
1664 1610 1597 1450 1424 1411 1213 1190 1176
dilution
Dihydrate
(low
Monohydrate
br 1616 1609
1618
1425 1420
1426
1194
1200
br
br
tem-
Assignments
Bendings HOH(W2) HOH(W3) HOH(W2) HOH(W4) HOD(W2) HOD(W3) HOD(W2) HOD(W4) DOD(W2) DOD(W3) DOD(W4)
text.
OH bonds is considered as essentially not hydrogen bonded, giving place to the highest stretching wavenumber [6b]. The other OH group is supposed to be weakly bonded to a N atom at a distance of 3.27 -4. It is to be remembered that coordination of water to cations lowers the stretching frequencies [lo, ll] due to an electron polarization effect which depends on the chargeto-radius ratio for the cation. Sr*’ should therefore be the strongest polarizer as compared with Ba*+ and Na+ thus giving place to the greatest shift in the stretching wavenumbers of water. This effect therefore decreases the stretching wavenumber of water in the strontium compound more than in the sodium and barium ones. It hence reinforces the supposition that water in SrNpr* H,O is essentially not hydrogen bonded. At high enough deuterium concentrations two peaks are seen at each side of the OD stretching peak of HOD (at 2694 and 2601 cm-‘) and a peak also appears at 1200 cm-‘. These features are respectively due to the stretching vibrations (antisymmetric and symmetric) and to the bending vibration of fully deuterated water molecules (D,O). Again, the wavenumbers found for the stretching vibrations of DzO in the deuterated monohydrate are similar to values reported for Na?Npr - 2H20 [lo] and BaNpr - 2H20 173 (least bonded water). The OH stretching of HOD shows it up at 3590 cm-‘, the ratio 3590/2644 = 1.358 being as expected [cf. refs. 6b, 71. Th e wavenumber of the OH stretching of HOD is practically equal to the mean value of the antisymmetric (3624 cm-‘) and symmetric (3553 cm-‘) stretchings of the corresponding (non-deutemted) molecule (mean: 3591 cm-‘). The coincidence of these values is even better than for
252
the OD stretchings. The deformation band of HZ0 at low protium concentration appears at 1618 cm-‘. The spectra of the deuterated dihydrate show in the stretching region peaks which can be directly compared with the OD stretchings of the partially deuterated monohydrate and assigned to the W3 water molecule, as proposed in Table 3. Again the mean value of the D20 stretching wavenumbers of the W3 molecules (2643 cm-‘) is nearly equal to the wavenumber of the isolated OD-stretching (2644 cm-’ ). In the spectra of the dihydrate there also appears a new set of three peaks which must be related to the additional water molecule (WZ). Wavenumbers of these peaks are higher than wavenumbers of the corresponding peaks of the first set. This fact confirms that the W2 molecules are less bound than the W3 molecules. Interestingly, the OD stretching wavenumbers of the W2 molecule in the dihydrate are higher than any of the figures reported for the long list of hydrates referred to in [lo] and for the A-type water of BaNpr - 2H20 which is supposed not to be hydrogen bonded [7] (th is is also the case for the OH stretchings as noted below). Moreover, OD-stretching wavenumbers of W2 are only 39-65 cm-’ below values reported for the corresponding &etchings of HDO and D20 in the gas phase [9]. In the OH stretching region, the OD stretchings find their exact counterparts. The fact that only six features are found in each stretching region substantiates the equivalence of the OH (OD) bonds in each type of water molecule. The HOH, DOH and DOD bending modes appear in the low temperature spectra as doublets as expected for two non-equivalent but symmetrical water molecules. The peak of higher wavenumber in each set should be assigned to W3, while the other peak should correspond to W2 because of the polarization effect operating upon W3. In the tetrahydrate spectra features at 2700, 2650 and 2597 cm-’ can be assigned straightforwardly by comparison with the spectra of the monohydrate and the dihydrate to the W3 water molecule, which is not hydrogen bonded. The features of highest and lowest wavenumber must correspond respectively to the antisymmetric and symmetric stretching vibrations of the wholly deuterated W3. The center peak, which must be due to the isolated OD stretching, shows no splitting, pointing to a high symmetry for the W3 molecule, which should therefore lie normally to the crystallographic mirror plane (m), this acting as a bisector_ The D atoms should therefore be in equivalent sites as required by the space group. This assignment does not preclude that the peak at 2650 cm-’ is a composite one, resulting from the superimposition of the discussed W3 isolated OD-stretching and similar stretchings due to the fully deuterated W2 and W4 water molecules. In fact, these latter molecules have one of their OD (OH) bonds practically free from hydrogen-bonding and therefore the corresponding stretching vibrations could fall in the same narrow spectral region as W3. For the bonded OD groups, from graphs of v OD versus the distance between deuterium bonded
253
oxygen atoms [12], a wavenumber of about 2500 cm-’ should be expected from the 0 - - -0 distance of 2.85 J$ found between W4 and W2 in the tetrahydrate. The strong peak at 2586 cm-’ lies precisely in that spectral region confirming its assignment to the bonded OD stretching of W4. The third strong peak at 2480 cm-’ must therefore be due to W2 which is deuterium bonded to a nitrogen atom of the nitroprusside anion. The 2.85 a distance also found between 0w(2) and N(4) suggests a value of 2400 cm-’ for the corresponding OD-stretching wavenumber [ 121. The assignment of the peak at 2480 cm-’ to this vibration is again straightforward. The DzO stretching vibrations of W2 and W4 are difficult to assign because of strong band overlapping, shifts produced by hydrogen-bonding and also possible correlation effects. This precludes an unambiguous assignment of these vibrations. The isolated OD stretching bands find their expected counterparts in the OH region as shown in Table 3, the v OH/~OD ratio being in the range 1.355 i- 0.002 in every case. In the HzO, HOD and D20 bending regions the same pattern is observed, i.e. a broad band at higher wavenumbers which should be assigned to the heavily hydrogen bonded W2 water and two sharp peaks at lower wavenumbers of which the one at the highest wavenumber could be ascribed to the W3 molecule by comparison with the monohydrate and dihydrate spectra; the remaining peak must therefore be assigned to the W4 molecule. SUMMARY
AND CONCLUSIONS
The three strontium nitroprusside hydrates have in common a water molecule (W3) electrostatically bonded to the Sr”’ cation by its oxygen atom and with its hydrogen atoms free from hydrogen-bonding, a fact which is confirmed by the high wavenumbers of its stretching vibrations, which are practically the same in the three hydrates. This water molecule is obviously the tightest bound and requires the most stringent dehydration conditions to be eliminated. In the higher hydrates there are other types of water molecules (one in the dihydrate (W2) and two in the tetrahydrate (W2 and W4)) which are different and uncorrelated because in the tetrahydrate both types are differently hydrogen bonded while in the dihydrate there is no such bonding. The tightest hydrogen-bonded water molecule in the tetrahydrate (W2) is heavily bonded by one of its hydrogen atoms to the nitrogen atom of a CN group while the other hydrogen atom seems to be free. At the same time its oxygen atom accepts a hydrogen bond from a water molecule of the other type (W4). This is the only bond formed by these latter molecules. The stretching bands of the bonded OH groups are displaced, as expected, towards lower wavenumbers in comparison with the &etchings of the free groups (estimated displacements: 100, 150 cm-’ for W4 and W2, respectively). The shift is obviously greater for the more heavily hydrogen bonded OH group. These spectroscopic facts are them-
254
selves reflected in the different thermal lability of the two types of water molecules, the least hydrogen bonded W4 molecules (destabilized by lone-pairs repulsion) being evolved first, giving place to the dihydrate. The dichroic behaviour of the NO stretching band in the tetrahydrate is in accordance with the spatial orientation of this group in the crystal found by X-ray diffraction [ 21. Spectroscopic and thermal properties of the strontium nitroprusside hydrates are therefore in good accord with expectation from the crystallographic data previously reported for the di- and the tetrahydrate (as single crystals) [Z, 31 and data now reported for the monohydrate (as a powder) _ ACKNOWLEDGEMENTS To Consejo National de Investigaciones Cientificas y Tknicas, R. Argentina, for financial help and to the Alexander von Humboldt Foundation, F.R. Germany, for the gift of some equipment. REFERENCES 1 A. G. Alvarez, P. J. Aymonino, E. J. Baran, L. A. Gentii, A. H. Lafranconi and E. L. Varetti, J. Inorg. Nucl. Chem., 38 (1976) 221. 2 E. E. Casteliano, 0. E. Piro and B. E. Rivero, Acta Crystailogr., Sect. B, 33 (1977) 1725. 3 E. E. Castellano, 0. E. Piro, A. D. Podjarny, B. E. Rivero, P. J. Aymonino, J. H. Lesk and E. L. Varetti, Acta Crystallogr., Sect. B, 34 (1978) 2673. 4 (a) L. A. Gentil, J. A. Olabe, E. J. Baran and P. J. Aymonino, J. Therm. Anal., 7 (1975) 279. (b) J. A. Olabe, L. A. Gentil, E. J. Baran and P. J. Aymonino, Monatsh. Chem., 106 (1975) 941. 5 Tables of Wavenumbers for the Calibration of Infrared Spectrometers, Pure Appl. Chem., l(l961) 537. 6 (a) R. K. Khanna, C. W. Brown and L. H. Jones, Inorg. Chem., 8 (1969) 2195. (b) M. Holzbecher, 0. Knop and M. Faik, Can. J. Chem., 49 (1971) 1413. (c) L. Tosi, Spectrochim. Acta, Part A, 29 (1973) 353. 7 E. L. Varetti and P. J. Aymonino, Inorg. Chim. Acta, 7 (1973) 597. 8 H. H. Zachariasen, Theory of X-Ray Diffraction in Crystals, John Wiley, New York, 1945. 9 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd edn., John Wiley, New York, 1978. 10 M. Faik and 0. Knop, in F. Franks (Ed.), Water. A Comprehensive Treatise, Vol. II, Plenum Press, New York, 1973, Chap. II. 11 G. Sartori, C. Furlani and A. Damiani, J. Inorg. Chem., 8 (1958) 119. 12 W. C. Hamilton and J. A. Ibers, Hydrogen Bonding in Solids, W. A. Benjamin, New York, 1968.
Printed in The Netherlands
TGA-DTA, INFRARED AND X-RAY POWDER DIFFRACTION ON STRONTIUM NITROPRUSSIDE AND ITS HYDRATES
C. 0. DELLA
VEDOVA,
J. H. LESK,
E. L. VARETTI**
STUDIES
and P. J. AYMONINO*‘“*
Departamenfo de Quimica, Facultad de CiencMs Exactas, Uniuersidad Nacionai de La Plats, 47 esquina 115, 1900 La Plata (Repriblica Argentina) 0. E. PIRO, B. E. RIVERO***
and E. E. CASTEILLANOg
Departamento de Fikica, Facultad de Ciencias Exactas, Universidad Nacionul de La Plats, 49 esquina 115, 1900 La Plata (Repriblica Argentina)
(Received27 May 1980) ABSTRACT Dehydration data for the tetra-, di- and monohydrate of strontium nitroprusside and the vibrational behaviour of the watermolecules in normal and deuterated samples agree with crystal data previously reported (for the di- and tetrahydrate as single crystals) and in this paper (monohydrate as a powder). Four distinct types of water molecules are present in the three hydrates with one type (oxygen bonded to W+) in common. Other types are a non hydrogen-bonded molecule (dihydrate), a molecule bonded by a single
hydrogen atom to the nitrogen atom of a cyanide group and a set of two molecules similarly bonded to the oxygen atom of the previous type (tetrahydrate). The spatiai orientation of the NO groups of the nitroprusside ions in the tetrahydrate is confirmed by the observed dichroism of the corresponding stretching band.
INTRODUCTION Strontium nitroprusside crystallizes from aqueous solutions as a monoclinic tetrahydrate [ 1, 21 (space group C:,., - C 1 2/m 1 [ 21) with four formulas in the unit cell. The water molecules are distributed among three sets of different symmetries and chemical environments. One of these sets, composed of two identical molecules, is located in general positions and the other two, in special positions (m). The molecules of the first type (W4, its oxygen atoms designated as Ow(4) in ref. 2) are hydrogen bonded to the oxygen atoms of water molecules of the second type (W2 (Ow(Z)), d(O0) = 2.85 R ) and to the oxygen atom of the nitroprusside ions 0) = 3.44 A). The two Ow(4) atoms are related by a center (O(l), d(Oof inversion and they are in close contact with each other, at a Van der Waals’ distance of 2.87 A. Strong repulsion between the lone pairs of these l
l
l
l
*To whom correspondence should be addressed. **Member of the Can-era de1 Investigador Cientifico, CONICET, R. Argentina. ***Member of the Carrera de1 Investigador Cientifico, CIC, R. Argentina. 5 Present address: Instituto de Fisica e Quimica, Sao Carlos (USP), Brasii. OOZZ-2860/81/0000-0000/$02.50
0 1981 Elsevier
Scientific
Publishing
Company
342
oxygen atoms and lability of the water molecules to which they belong should therefore be expected. The second-type molecules (WZ) are bonded through one of their hydrogen atoms to the axial nitrogen atom of nitroprusside ions (N(4), 0) = 2.85 A) while molecules of the third type (W3 (Ow(3)) [23 ), d(Owhich are electrostatically bonded to the strontium ions, do not form hydrogen bonds. It is to be noted that the W4 water molecules are unsymmetrical, i.e. the two OH bonds in each case are nonequivalent, while crystallographic data alone on W2 and W3 molecules are not sufficient for the assignment of their symmetries. The different environments of the water molecules in the tetrahydrate and the dihydrate are reflected in their thermal behav-iour, and the differences are big enough to allow the isolation of the dihydrate and also the monohydrate by regulating the dehydration conditions, as described below. Interestingly, we have been able to obtain from a single crystal of the tetrahydrate a mosaic of crystals of the dihydrate oriented nearly parahel to one another, which allowed a complete structural determination by X-ray diffraction methods [ 3]_ The water molecules lost in this dehydration process are the W4. The dihydrate is structurally closely related to the tetrahydrate, belonging to a space group of higher symmetry (D:B - C 2/c 2/m 2 Jm, orthorhombic) which contains the space group of the tetrahydrate as a subgroup_ The W3 water molecules are still coordinated to Sr*+ in the dihydrate as in the tetrahydrate, but the W2 molecules are slightly shifted from their original positions in the tetrahydrate. Consequently, the hydrogen bond found between Ow(2) and N(4) in the tetrahydrate is lost in the dihydrate, and no other acceptor atom is now close enough to Ow(2) to give rise to hydrogen bonding. The two water molecules are symmetrical because they are in special positions mm. The monohydrate, as well as the anhydrous form of strontium nitroprusside have been obtained only as microcrystalline powders. The X-ray diffractograms of these powders show some lines in common with the diagrams of the higher hydrates; these lines suggest a structural relationship between the four compounds (see below). The water molecule in the monohydrate behaves vibrationally as the W3 molecules in the higher hydrates. Nitroprusside ions are located in mirror planes m in the tetrahydrate [2] and in mm positions in the dihydrate. Both in the tetrahydrate and the dihydrate the axes of the nitroprusside ions lay on (010) planes and are oriented nearly parallel to the a axis, in the tetrahydrate, and exactly coincident with it, in the dihydrate. l
l
EXPERIMENTAL
The dihydrate was obtained from the tetrahydrate [ 1,2] by storage in a desiccator over phosphorus pentoxide until constancy of weight was
243
achieved. Under vacuum, the dehydration went further to give the monohydrate. Finally, the anhydrate was obtained by heating any of the hydrates for 2 h at 100°C under high vacuum. Weight changes in the different dehydration steps always matched to a few tenths of percent with values expected from the formulas assigned to the different hydrates (cf. TGA results reported below). Mean percent values of several runs found are (calculated vaIues in parentheses): tetrahydrate to dihydrate: 9.53 (9.59), dihydrate to monohydrate: 5.09 (5.30), monohydrate to anhydrate: 5.18 (5.60). Deuterated samples were obtained beginning with the tetrahydrate crystallized from appropriate mixtures of common and heavy water. Samples of the lower hydrates and the anhydrous form were always handled in a dry bag. X-ray
diffraction
data
Powder diffractograms of samples mounted on a rotating support covered with two layers of a thin polyethylene film were recorded as in [l] using Cu(Kar) radiation_ TGA-DTA
data
Thermograms of the tetrahydrate were obtained using a Rigaku Denki YLDG/CN 8002 L2 Thermoflex apparatus under flowing dry nitrogen (for experimental details see [4a, b] ). IR spectra The IR absorption spectra of Nujol and Halocarbon mulls between CsI plates were recorded as in [l] . Calibration in the OD stretching region was improved by recording the HBr spectrum [ 51. For the same purpose, in the CN stretching region advantage was taken of the relative sharpness of peaks of sodium nitroprusside dihydrate and the coincidence (within 1 cm-‘) of the wavenumbers reported by three different groups of spectroscopists [Sa-c]. Wavenumbers selected were 2173,2161,2156, and 2143 cm-‘. The accuracy of wavenumbers reported below for sharp bands in those regions is believed to be f 1 cm- ‘. In the other regions of the spectrum it is _+2 cm-‘, also for sharp bands, and &3 cm-’ for broad bands. A RIIC VLT-2 cell was used to obtain spectra at the boiling temperature of nitrogen. Polarized single crystal spectra of very thin plates (shortest edge parallel to the crystahographic a axis) of the tetrahydrate were recorded as for barium nitroprusside dihydrate [7] . Unfortunately, absorbance was too strong in the stretching region of water, even for the thinner samples, to obtain useful information. This was not the case in the CN and NO stretching regions of the nitroprusside ion.
244
Fig. 1. TGA-DTA RESULTS
of Sr[Fe(CN),NO]
- 4HI0.
AND DISCUSSION
TGA-DTA Figure 1 reproduces a thermogram showing four distinct decomposition stages. DTA-peak temperatures are mean values of several runs. The first three steps in the thermogram corresponded merely to successive dehydration stages because the IR spectrum of the residue obtained after the third step in a separate run, which was interrupted at 2OO"C, showed no significant modifications in the CN and NO stretching regions (the spectrum was in fact similar to the spectrum of the anbydrate obtained at 100°C in vacua (see below))_ For the first stage of the dehydration process in the thermobalance, which should correspond to the conversion of the tetrabydrate into the dihydrate, the weight changes found were rather lower than expected. This contrasts with results of the dehydration experiments performed in a desiccator over phosphorus pentoxide (see above). For the other stages, results were in good accord with expectancy. The last step of the thermogram involves the loss of the NO group as shown by IR spectroscopy (cf. ref. 1 where it was erroneously reported that strontium nitroprusside decomposes at 147°C). The weight change at this stage indicated that a CN group was also involved. The residue was perhaps a mixture of diiron hexacyanoferrate(II), distrontium hexacyanoferrate(I1) and strontium dicyanide (cf. [4a] ).
245
X-ray powder
diffraction
data
Results are presented in Table 1 and Fig. 2, where data reported previously for the tetrahydrate [l] are also included for the sake of comparison. Indexing of the diffraction lines of the dihydrate was performed taking
advantage of results reported in [3] for the mosaic crystal. Lack of single crystal data and of comparable diffractograms prevented complete indexing of the diffraction patterns of the monohydrate and anhydrate. In any case, some features of these diffractograms seem to be related to indexed lines of the diffiactograms of the higher hydrates i.e., lines (200), (400), (600) and (800) [ 1,3]. The strong (400) lines of the tetrahydrate and the dihydrate are mainly due to Bragg reflections on the electron-rich “planes” defined by the heavy iron and strontium atoms. The high intensity of the (400) lines results from the fact that these “planes” are at approximately l/4 of celledge length from each other in the direction of the a axis [2, 31. The perTABLE
1
Powder
diffraction
Line
d/n (A)
(a) Sr[Fe(CN),NO] (200) 9.980
(111) WOO) (311) (002) (311) (go01 (602) (710) (313) (80_0) (131) (249)
5.426 4.958 4.495 4.133 4.076 3.308 2.796 2.655 2.549 2.484 2.395 1.755
(b) Sr[Fe(CN),NO] (200) 9.772
(111) (310) (400) (002) (401) (600) (421) (203) (602)
5.347 4.801 4.725 4.122 4.118 3.162 2.760 2.680 2.540
aA(Cu KY)=1.5418
data for strontium sin 0 /X (A -‘)a
nitroprusside
(Z/Z,) x 100
and its hydrates
Line
sin 8 IX (a-‘)”
(I/Z,)
(A\)
2.370 2.281 2.244 2.098 1.943 1.821
0.2109 0.2192 0.2228 0.2383 0.2573 0.2745
15 8 8 6 2 2
d/n
- 4H,O
0.0501 0.0953 0.1008 0.1112 0.1210 0.1227 0.1511 0.1788 0.1883 0.1962 0.2012 0.2088 0.2849
10 15 100 20 10 10 25 20 10 4 10 5 5
(8’30) (801) (331) (0041 (404) (803)
(c) Sr[ Fe(CN),NO] (200) 10.311
(400)
- 2H,O
0.0512 0.0935 0.1041
0.105i3 0.1213 0.1214 0.1581 0.1812 0.1866 0.1969 A.
23 15 45 100 21 12
15 40 3 8
(800)
5.553 5.045 4.835 4.123
(800)
0.0485 0.0900 0.0991
0.1147 0.1213
3.986
0.1254
2.855 2.557 2.500
0.1751 0.1956 0.2002
(d) Sr[Fe(CN),NO] (200) 10.246
(400)
. H,O
5.045 4.568 2.903 2.496 2.262
0.0488
0.0991 0.1094 0.1722 0.2003 0.2210
65 55 100 26 ii 35 25 17
3 100 65 17 39 30
x 100
246
3.05
Fig.
2.
0.10
0.15
0.20
0.25
SC”
e/J-
Powder X-ray diffraction patternsof strontium nitroprussideand its hydrates.
sistence of the highly intense (400) line in the diffractograms of the monohydrate and the anhydrate suggests a similar distribution of the heavy atoms in these compounds. These “planes” also contribute to the (800) lines, part of whose intensity is also due to reflections on other “planes” defined by the axial ligands of the nitroprusside ions which are interposed between them at ca. l/8 of a (see Fig. 2 and ref. 2). The correlation of the (400) and (800) lines among the four compounds is indicated in Fig. 2, together with the corresponding correlation of the (ZOO) and (600) lines. The vanishing of the latter line in passing from the dihydrate to the monohydrate is related to the disappearance of the W2 water molecule. In the same way, the almost complete disappearance of the (200) line during the last dehydration step can be related to the elimination of the W3 water molecule (see below). The particular crystallographic arrangement in the higher hydrates along the a axis discussed above (see also [2, 3]), and the correlation noted for the lines (200), (400), (600) and (800) among the entire series of compounds, make it interesting to compare the changes in the projections of the electron densities along the a axis, as the dehydration proceeds from the tetrahydrate to the anhydrate. As pointed out above, the space group of the tetrahydrate (C 1 2/m 1) [ 21 is a subgroup of the space group corresponding to the dihydrate (C 2/c 2/m 2,/m) [ 33. In particular, the respective unit cells can be related by a slight distortion (scarcely 8” in the cell parameter P and even minor relative changes in the other cell parameters), without changing the relative position of the center of symmetry present in both
247
cells. This fact is reflected in the similarities in the packing of both crystalline structures. In particular, the sign of the structure factors F(hO0) (h = 2,4, 6,8) are the same. Assuming that the same applies to the monohydrate and the anhydrate, it is easy to synthesize the projection of the electron density along the a axis for these latter compounds. The amplitudes of the four structure factors IF(hOO)l (h = 2, 4,6, 8) were obtained from the integrated intensities as measured on the corresponding powder X-ray diffractograms. The intensities were corrected for Lorentz and polarization effects only, using the data reduction formula [8]
lwwl,wd.
=
(1 [
Ji,“,“,T2, B )
112
X Integr. intens. 1
where eB is the angle corresponding to the Bragg reflection of Miller indices (hkl). Comparisons were made for the tetrahydrate and the dihydrate between results obtained for the structure factors IF(hOO)l,,ti and IJw0wlrnono,st_ (these latter expressed in an absolute scale after being obtained from the diffraction intensities collected on Weissenberg photographs, and corrected for Lorentz and polarization effects but not for absorption or extinction [2, 31). The comparison served a double purpose: in an approximated absolute scale, and (b) to ascer(a) to put F(~OO)I,, tain the degree of accuracy of the data reduction procedure using the powder X-ray diffractograms. The lF(hOO)lpoti (h = 2, 4, 6, 8) factors proved to be in good agreement with the corresponding IF(hOO)l,,,,,,_ (mean relative errors less than 10%). The data pertaining to the monohydrate and the anhydrate were scaled to make the modulus of the F(400) factor for these hydrates equal to the corresponding value for the dihydrate. The results of the computed Fourier syntheses are plotted in Fig. 3 for the lower hydrates and the anhydrate. The F(OOO) term has not been included. Figure 3 has been labelled with the symbols of the atoms and water molecules which are expected to project on the different regions. Comparing the relative electronic densities in the regions about 3c= 0 and x = l/4 (see Fig. 3a, b), the loss of the W2 water molecule can be followed during the second step of dehydration (to which is related the disappearance of the (600) line when going from the dihydrate to the monohydrate (see Fig. 2b, c)). In the same way, the loss of the W3 water molecule can be envisaged in the last dehydration step (a process which brings about the almost complete disappearance of the (200) line present in all the hydrates (see Fig. 3b, c))_ The (600) line is also absent in the anhydrous compound (see Fig. 2). The above interpretation concerning the sequence in which the two water molecules are lost during the last two dehydration steps agrees with the TGA-DTA and spectroscopic results (see below for discussion of IR spectra). In Fared spectra Room temperature mull spectra of the hydrates and the anhydrate are shown in Fig. 4, where bands due to Nujol are not included. Table 2 presents
248 Sr + Fe + Equatorial
Sr + Fe + Equatorial
CN
CN
a b
Fig. 3. Plots of (p(x)electron (CN),NO] displaced
z -
F(OOO))
(electrons)
corresponding
to the projection
of the
density p(x) (em A-‘) along the a axis for: (a) Sr[Fe(CN),NO] .2H,O; (b) Sr[Fe- H,O and (c) Sr[Fe(CN),NO]. For the sake of clarity, the curves have been 500 electrons
from each other.
The Fourier
syntheses:
(p(x)-
z -
F(OO0))
= 2~:F(h00)-cos(27rhx) include only the terms with h = 2,4,6 and 8, which have been scaled to an approximately absolute scale. F(OOO) is equal to 508 electrons in the case of the dihydrate. V is the volume of the unit cell and A is the area determined by the basic b and c vectors.
wavenumbers and tentative assignments for the absorption peaks. It includes information already reported for the tetrahydrate [l] to facilitate comparisons. All spectra have in common obvious features, due to the nitroprusside ion, which are assigned as in ref. 1. The splitting of the NO stretching band clearly observed in the dihydrate spectrum should be compared with a similar feature reported for barium nitroprusside dihydrate, which was assigned to a dynamic interaction between the NO groups in the unit cell (factor group splitting) [7]. Th is could also be the case for the dihydrate (factor group: D&. A splitting of the NO stretching band is also seen in the anhydrate. The spatial orientation of the polar axis of the nitroprusside ion in the tetrahydrate found by X-ray diffraction methods [Z] is confirmed by the polarized spectrum in the NO stretching region of a plate cut from a single crystal with faces parallel to.the crystallographic (100) planes. Figure 5 shows the spectra obtained with the plate oriented parallel to the slit of the spectrophotometer and making an angle of approximately 45” with its normal_ The broken line reproduces the spectrum recorded with the analyser parallel to the b axis and therefore normal to the a axis and the NO groups, and the full line, in a direction normal to it, i.e., tilted with respect to the NO groups.
249
Sr [Fe(CN)5N0]-4H,
0
I
sf [F~cNI,NO]-~H,O
Sr [FEKNI,NO]-H,O
sr [F:F~(cN),NO] 10
35bo
1 3100
1 8 1 1 8 2300 2000’ Wavenumber
Fig. 4. Room
temperature
.
I
1500 800
I
I-,
500’
(cm-‘)
IR mull spectra of strontium
0 WavenumDer
nitroprusside
(cm-’
and its hydrates.
Fig. 5. Polarized spectra of a thin (100) plate of Sr[Fe(CN),NO] -4H,O placed with the b axis parallel to the slit of the spectrophotometer and making a 45” angle with the light ray direction: (a) analyser parallel to the b axis; (b) analyser parallel to the c axis.
)
250 TABLE
2
Infrared spectra of strontium nitroprusside and its hydrates in Nujol mulls (room temperature spectra)a f;C~egXWJOI
Sr[Fe(CN),NO] - 2H,O
3877 3621 (W3) 3560 (W3) 3420 (W2.
3871 3624 (W2. 3559 (W2. -
W3) W3)
2155 2114 sh 1965 1939
2148 2108 sh 1960
1910
W4)
SzF;(CN),NO 2
sh
1
3867 3621 (W3) 3559 (W3) -
3867
2190 2158 2116
2188 2159 2125 sh 1957 sh 1940 1930
1960 1936
sh
1609 661 643 560 504 465 sh 439 422 375 sh 328
664 649 560 sh 506 470 sh 441 420
662 649 sh
659 644 sh -
509 470 sh 443 sh 425
507 -
380 325
390 sh 326
assigned
to water
vibrations
NO stretching HOH HOH HOH FeNO
>
438 428 409 -
show
(W2) (W3) (W3.
bending bending W4) bendings
deformation
FeN fzO
stretching libration
H,O
hbration
b
j H,O b
326
%h: shoulder. b FeC stretching s and FeCN deformations Bands
H,O antisymmetric stretching Hi0 symmetric stretching H,O (H-bonded) stretching CN (axial) stretching CN (equatorial) stretching ‘“CN stretching
-
1622 -
1620 -
%I0
-
-
1636
Assignments
Sr[Fe(CN),NOl
libration
[cf. 7 1.
features
expected
for the dif-
(and chemical) molecular types. In the tetrahydrate 3420 cm-’ and 1636 cm-’ must be due to the strongly hydrogen bonded W2 water molecules. Partial deuteration allows some detailed assignments of the other water bands. Table 3 gives the wavenumbers and assignments for bands observed in the low-temperature spectra of the partially deuterated hydrates at high and low deuterium concentration. The spectrum of the deuterated monohydrate shows peaks at 2644 cm-’ and 1426 cm-’ which must be due to the OD stretching and HOD bending of a water molecule which seems to be symmetric and crystallographically unique. The high wavenumber of the OD stretching band (and concomitant low value of the HOD bending) indicates that very weak, if any, hydrogen bonding is operating in the monohydrate. In this respect, it should be compared with values reported for the least bonded water molecule of barium nitroprusside dihydrate (2665 and 2647 cm-’ for the non-equivalent OD bonds [7] ) and for the water molecules of sodium nitroprusside dlhydrate (2654 and 2630 cm-‘) [6b]. No acceptor atoms have been found properly located around the least bound water molecule in BaNpr 2H,O to give place to well defined hydrogen bonds [7 ] . In Na,Npr 2H20 one of the ferent
crystallographic
the broad
bands
at
l
l
251 TABLE3 Water
vibrations
perature
spectra)
Tetrahydrate=
Dihydrate
b
3636 3623 3609 3592 3572 3560
3629 b 3594 b 3557 3504 3355 b 2700 FB2 2650 b 2625 2597 2586 2527 2480 =br:
2723 2694
of strontium
Monohydrate
3624 3590 3559
2694
2678 2644 2632
2644
2592
2601
broad.
bSee
nitroprusaide
hydrates
Assignments
Stretchings H,O(W2) antisymmetric H20(W3) antisymmetric OH(W2:HDO) OH(W3:HDO) H,O(W2) symmetric H,O(W3) svmmetric OH(W4:DOH-*Ow(2)) OH(W2:DOH..N(4)) D,O
at high
isotopic
Tetrahydratea
1664 1610 1597 1450 1424 1411 1213 1190 1176
dilution
Dihydrate
(low
Monohydrate
br 1616 1609
1618
1425 1420
1426
1194
1200
br
br
tem-
Assignments
Bendings HOH(W2) HOH(W3) HOH(W2) HOH(W4) HOD(W2) HOD(W3) HOD(W2) HOD(W4) DOD(W2) DOD(W3) DOD(W4)
text.
OH bonds is considered as essentially not hydrogen bonded, giving place to the highest stretching wavenumber [6b]. The other OH group is supposed to be weakly bonded to a N atom at a distance of 3.27 -4. It is to be remembered that coordination of water to cations lowers the stretching frequencies [lo, ll] due to an electron polarization effect which depends on the chargeto-radius ratio for the cation. Sr*’ should therefore be the strongest polarizer as compared with Ba*+ and Na+ thus giving place to the greatest shift in the stretching wavenumbers of water. This effect therefore decreases the stretching wavenumber of water in the strontium compound more than in the sodium and barium ones. It hence reinforces the supposition that water in SrNpr* H,O is essentially not hydrogen bonded. At high enough deuterium concentrations two peaks are seen at each side of the OD stretching peak of HOD (at 2694 and 2601 cm-‘) and a peak also appears at 1200 cm-‘. These features are respectively due to the stretching vibrations (antisymmetric and symmetric) and to the bending vibration of fully deuterated water molecules (D,O). Again, the wavenumbers found for the stretching vibrations of DzO in the deuterated monohydrate are similar to values reported for Na?Npr - 2H20 [lo] and BaNpr - 2H20 173 (least bonded water). The OH stretching of HOD shows it up at 3590 cm-‘, the ratio 3590/2644 = 1.358 being as expected [cf. refs. 6b, 71. Th e wavenumber of the OH stretching of HOD is practically equal to the mean value of the antisymmetric (3624 cm-‘) and symmetric (3553 cm-‘) stretchings of the corresponding (non-deutemted) molecule (mean: 3591 cm-‘). The coincidence of these values is even better than for
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the OD stretchings. The deformation band of HZ0 at low protium concentration appears at 1618 cm-‘. The spectra of the deuterated dihydrate show in the stretching region peaks which can be directly compared with the OD stretchings of the partially deuterated monohydrate and assigned to the W3 water molecule, as proposed in Table 3. Again the mean value of the D20 stretching wavenumbers of the W3 molecules (2643 cm-‘) is nearly equal to the wavenumber of the isolated OD-stretching (2644 cm-’ ). In the spectra of the dihydrate there also appears a new set of three peaks which must be related to the additional water molecule (WZ). Wavenumbers of these peaks are higher than wavenumbers of the corresponding peaks of the first set. This fact confirms that the W2 molecules are less bound than the W3 molecules. Interestingly, the OD stretching wavenumbers of the W2 molecule in the dihydrate are higher than any of the figures reported for the long list of hydrates referred to in [lo] and for the A-type water of BaNpr - 2H20 which is supposed not to be hydrogen bonded [7] (th is is also the case for the OH stretchings as noted below). Moreover, OD-stretching wavenumbers of W2 are only 39-65 cm-’ below values reported for the corresponding &etchings of HDO and D20 in the gas phase [9]. In the OH stretching region, the OD stretchings find their exact counterparts. The fact that only six features are found in each stretching region substantiates the equivalence of the OH (OD) bonds in each type of water molecule. The HOH, DOH and DOD bending modes appear in the low temperature spectra as doublets as expected for two non-equivalent but symmetrical water molecules. The peak of higher wavenumber in each set should be assigned to W3, while the other peak should correspond to W2 because of the polarization effect operating upon W3. In the tetrahydrate spectra features at 2700, 2650 and 2597 cm-’ can be assigned straightforwardly by comparison with the spectra of the monohydrate and the dihydrate to the W3 water molecule, which is not hydrogen bonded. The features of highest and lowest wavenumber must correspond respectively to the antisymmetric and symmetric stretching vibrations of the wholly deuterated W3. The center peak, which must be due to the isolated OD stretching, shows no splitting, pointing to a high symmetry for the W3 molecule, which should therefore lie normally to the crystallographic mirror plane (m), this acting as a bisector_ The D atoms should therefore be in equivalent sites as required by the space group. This assignment does not preclude that the peak at 2650 cm-’ is a composite one, resulting from the superimposition of the discussed W3 isolated OD-stretching and similar stretchings due to the fully deuterated W2 and W4 water molecules. In fact, these latter molecules have one of their OD (OH) bonds practically free from hydrogen-bonding and therefore the corresponding stretching vibrations could fall in the same narrow spectral region as W3. For the bonded OD groups, from graphs of v OD versus the distance between deuterium bonded
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oxygen atoms [12], a wavenumber of about 2500 cm-’ should be expected from the 0 - - -0 distance of 2.85 J$ found between W4 and W2 in the tetrahydrate. The strong peak at 2586 cm-’ lies precisely in that spectral region confirming its assignment to the bonded OD stretching of W4. The third strong peak at 2480 cm-’ must therefore be due to W2 which is deuterium bonded to a nitrogen atom of the nitroprusside anion. The 2.85 a distance also found between 0w(2) and N(4) suggests a value of 2400 cm-’ for the corresponding OD-stretching wavenumber [ 121. The assignment of the peak at 2480 cm-’ to this vibration is again straightforward. The DzO stretching vibrations of W2 and W4 are difficult to assign because of strong band overlapping, shifts produced by hydrogen-bonding and also possible correlation effects. This precludes an unambiguous assignment of these vibrations. The isolated OD stretching bands find their expected counterparts in the OH region as shown in Table 3, the v OH/~OD ratio being in the range 1.355 i- 0.002 in every case. In the HzO, HOD and D20 bending regions the same pattern is observed, i.e. a broad band at higher wavenumbers which should be assigned to the heavily hydrogen bonded W2 water and two sharp peaks at lower wavenumbers of which the one at the highest wavenumber could be ascribed to the W3 molecule by comparison with the monohydrate and dihydrate spectra; the remaining peak must therefore be assigned to the W4 molecule. SUMMARY
AND CONCLUSIONS
The three strontium nitroprusside hydrates have in common a water molecule (W3) electrostatically bonded to the Sr”’ cation by its oxygen atom and with its hydrogen atoms free from hydrogen-bonding, a fact which is confirmed by the high wavenumbers of its stretching vibrations, which are practically the same in the three hydrates. This water molecule is obviously the tightest bound and requires the most stringent dehydration conditions to be eliminated. In the higher hydrates there are other types of water molecules (one in the dihydrate (W2) and two in the tetrahydrate (W2 and W4)) which are different and uncorrelated because in the tetrahydrate both types are differently hydrogen bonded while in the dihydrate there is no such bonding. The tightest hydrogen-bonded water molecule in the tetrahydrate (W2) is heavily bonded by one of its hydrogen atoms to the nitrogen atom of a CN group while the other hydrogen atom seems to be free. At the same time its oxygen atom accepts a hydrogen bond from a water molecule of the other type (W4). This is the only bond formed by these latter molecules. The stretching bands of the bonded OH groups are displaced, as expected, towards lower wavenumbers in comparison with the &etchings of the free groups (estimated displacements: 100, 150 cm-’ for W4 and W2, respectively). The shift is obviously greater for the more heavily hydrogen bonded OH group. These spectroscopic facts are them-
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selves reflected in the different thermal lability of the two types of water molecules, the least hydrogen bonded W4 molecules (destabilized by lone-pairs repulsion) being evolved first, giving place to the dihydrate. The dichroic behaviour of the NO stretching band in the tetrahydrate is in accordance with the spatial orientation of this group in the crystal found by X-ray diffraction [ 21. Spectroscopic and thermal properties of the strontium nitroprusside hydrates are therefore in good accord with expectation from the crystallographic data previously reported for the di- and the tetrahydrate (as single crystals) [Z, 31 and data now reported for the monohydrate (as a powder) _ ACKNOWLEDGEMENTS To Consejo National de Investigaciones Cientificas y Tknicas, R. Argentina, for financial help and to the Alexander von Humboldt Foundation, F.R. Germany, for the gift of some equipment. REFERENCES 1 A. G. Alvarez, P. J. Aymonino, E. J. Baran, L. A. Gentii, A. H. Lafranconi and E. L. Varetti, J. Inorg. Nucl. Chem., 38 (1976) 221. 2 E. E. Casteliano, 0. E. Piro and B. E. Rivero, Acta Crystailogr., Sect. B, 33 (1977) 1725. 3 E. E. Castellano, 0. E. Piro, A. D. Podjarny, B. E. Rivero, P. J. Aymonino, J. H. Lesk and E. L. Varetti, Acta Crystallogr., Sect. B, 34 (1978) 2673. 4 (a) L. A. Gentil, J. A. Olabe, E. J. Baran and P. J. Aymonino, J. Therm. Anal., 7 (1975) 279. (b) J. A. Olabe, L. A. Gentil, E. J. Baran and P. J. Aymonino, Monatsh. Chem., 106 (1975) 941. 5 Tables of Wavenumbers for the Calibration of Infrared Spectrometers, Pure Appl. Chem., l(l961) 537. 6 (a) R. K. Khanna, C. W. Brown and L. H. Jones, Inorg. Chem., 8 (1969) 2195. (b) M. Holzbecher, 0. Knop and M. Faik, Can. J. Chem., 49 (1971) 1413. (c) L. Tosi, Spectrochim. Acta, Part A, 29 (1973) 353. 7 E. L. Varetti and P. J. Aymonino, Inorg. Chim. Acta, 7 (1973) 597. 8 H. H. Zachariasen, Theory of X-Ray Diffraction in Crystals, John Wiley, New York, 1945. 9 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd edn., John Wiley, New York, 1978. 10 M. Faik and 0. Knop, in F. Franks (Ed.), Water. A Comprehensive Treatise, Vol. II, Plenum Press, New York, 1973, Chap. II. 11 G. Sartori, C. Furlani and A. Damiani, J. Inorg. Chem., 8 (1958) 119. 12 W. C. Hamilton and J. A. Ibers, Hydrogen Bonding in Solids, W. A. Benjamin, New York, 1968.