- Email: [email protected]
Bond orientations in uranyl nitrate hexahydrate using attenuated total reflection
Spectrochimios Acta,1966, Vol.22,pp.1253 to 1260. Pergamon PressLtd.Printed inNorthern Ireland
Bond orientations in uranyl nitrate hexahydrate using attenuated total reflection A. M. DEANE, E. W. T. RICHARDS and I. G. STEPHEN Spectroscopy
Group, Chemistry Division, AERE,
Harwell
(Received 23 November 1965) Ah&&-Bond orientations in uranyl nitrate hexahydrate have been measured using polarized infra-red radiation and the technique of attenuated total reflection. The technique of using polarized radiation with attenuated total reflection is discussed and the results used to produce a model in agreement with X-ray data.
SPECTROSCOPICbond orientation data is usually obtained from dichroic ratio measurements on the reflection spectrum or on the absorption spectrum of an optically thin crystal [l] ; the reflection spectrum is frequently less rich in data than the absorption spectrum and the exposed crystal surface may alter during the measurements, but it is frequently extremely difficult to find crystal flakes a few microns thick for absorption measurements. The attenuated total reflection technique (ATR) provides an infra-red spectrum resembling the absorption spectrum of a specimen in intimate contact with the ATR prism [2]. It has been found that the combination of the ATR technique with use of polarized radiation yields useful information on bond orientation using optically thick crystals. POYNTINQ [3] has shown from electromagnetic theory that, under the conditions of total internal reflection, the perturbation in the medium of lower refractive index is propagated in the direction of the incident beam, and has described the changes of polarization resulting at the interface. More recently the validity of these equations was elegantly demonstrated by CULSHAW and JONES [4] using large components and centimetric wavelengths. We have found, as expected, that the crystal plane examined in our ATR experiments is that perpendicular to the incident beam, but have found much higher dichroic ratios than we anticipated, despite the considerable beam convergence. Measurements with the polarizer before or after the sample and also with uncrossed polarizers, one before and one after the sample gave identical results, so no effects of the total reflection upon the polarization data obtained could be detected. Further consideration shows that this is not surprising; POYNTING showed that under the condition of total internal reflection there can be no net energy absorption [l] [2] [3] [4]
S. S. MITRA, Solid&ate Phye. 13,21 (1962). J. FAERENFORT,Spectrochim. Acta 17,698 (1961). J. H. POYNTINU, Phil. Trans. 175,343 (1884), and standard textbooks. W. CULSHAW and D. S. JONES, Proc. P&e. Sot. B. 66, 859 (1953).
1263
1254
A. M.
DEANE,E. W. T. RICHARDS and I. G. STEPHEN
in the medium of lower refractive index, but in the ATR spectrum absorption demonstrably occurs. It is well known that the “best” spectra result when the mean angle of incidence at the specimen is close to the minimum for total reflection in nonabsorbing regions. The cone angle of the incident beam is about 45’. As the refractive index of the specimen varies with wavelength in the vicinity .of an absorption band varying proportions of the incident cone will be at less than the critical angle, will pass into the sample and be lost to the reflected beam. At the wavelength of maximum refractive index of the specimen near the absorption maximum, the largest proportion of the light cone will be at less than the critical angle, the reflected beam is most reduced and an absorption type spectrum is observed. The dependence of the spectrum on cone angle can easily be demonstrated by appropriate masking of the light incident on the sample. If the refractive index of the specimen rises very high, ordinary reflection at the interface will occur and reduce the apparent extinction. When polarized light is used, the refractive index variations appropriate to the polarization used and in the plane perpendicular to the incident beam attenuate the beam to produce a spectrum. Figure 1 shows spectra we have obtained using a single crystal of uranyl nitrate hexahydrate mounted in a TR5 micro ATR attachment [5] used with a silver chloride polarizer, also a reflection spectrum off one face for comparison; some difficulty was experienced in obtaining the latter spectrum as the crystal surface (presumably) dehydrated during the measurements. EXPERIMENTAL Crystals of uranyl nitrate hexahydrate grown from aqueous solution show two faces at right angles, described by VDOVENKO [6] as the (001) and (100) faces. A crystal was cut to a cube of about 5 mm side and another face cut at 45’, as in Fig. 2. The natural faces of a freshly grown crystal were suitable for direct examination, cut faces were lightly polished on fine emery paper using saturated uranyl nitrate solution as a lubricant. The crystal was contacted with a KRS 5 ATR prism using methylene iodide saturated with sulphur as an immersion fluid and spectra run at a mean angle of incidence of about 40”. Manipulation of a model helps to show that measurements made on the three faces indicated in Fig. 2, appropriately oriented, are an examination of three mutually perpendicular planes when the angle of incidence is 45”. Since the crystal is orthorhombic and 45’ planes do not coincide with simple crystal planes, we have designated the planes we studied as a, /I and y planes. Using natural faces or hand polishing we were not able to dispense with the immersion fluid, but the fluid may be an aid to keeping down the surface temperature and preventing dessication. We were able to repeat spectra on one face after several hours without difficulty, but there is a slow reaction of the nitrate with the prism material. Polarization was effected with a silver chloride pile of plates polarizer which was [5] Research and Industrial Instrument Co., London. [6] V. M. VDOVENKO, et al. Radiokhimya 2, 24 (1960).
Bond orientations
in many1 nitrate
hexahydrate
a plane
/3 plane
Fig.
1. Spectra
of uranyl nitrate hexahydrate crystal using polarised B, C, D, ATR spectra. A. Reflection spectrum. ---Electric vector at 90’. -__ Electric vector at 0’.
Fig. 2. The shape of the crystal on which measurements were made, showing the faces used and the planes examined in the crystal. Light incident at 45“ on the (001) face perpendicular to the Q plane. Light incident at 45” on the (010) face perpendicular to the /I plane. Light incident at 45O on the face cut out 45’, perpendicular to the y plane.
radiation.
1256
A. M. DEANE, E. W. T. RICHARDS and I. 0. STEPHEN
normally mounted after the ATR sodium chloride prism.
attachment
on an H.800
spectrometer
with
DISCUSSION
Two structures for the hexahydrate, based on X-ray diffraction studies, have been suggested by VDOVENKO [6] and by LYNTON [7]. The former proposed a hexa-aquo ion, [U0,(H,0),]2+2N0,-, but this seems incompatible with infra-red data [S]. LYNTON’S structure, two covalent nitrates and two water molecules in a puckered equatorial plane round a nearly linear uranyl group with four more water molecules filling holes in the lattice, is more consistent with the data obtained from an infra-red absorption spectrum of the powder. The relatively small doubling of individual nitrate absorption bands is less than might be expected from the different sized nitrate groups in LYNTON’S model. The observed absorption bands of uranyl nitrate hexahydrate are listed in Table 1 with assignments based on the polarization data in Fig 1. Projections of a model which fits the polarization data on to the planes examined are shown in Fig. 3. Table
1. Frequencies observed in the absorption spectrum of many1 nitrate hexahydrate, and their assignments Frequency 3500* IX,.3100 1634 1675 1495 1339 1304 1050 1041 962 938 867 c&. 820 807 741
OnXp
Assignment v, &v, VI &v,
V2 VZ VZ VI V4 Vl Vl % % 11 libration V6 %
Species
A, & B, .4,&B, A, A1 A41 B, B, A1 -41 CU+ CU+ cs+ B, A1
* On cooling the 3600cm-’ band is resolved into componentsat 3470, 3550cm-‘.
The uranyl group is an essentially linear triatomic group with the asymmetric stretching frequency polarized parallel to the C, symmetry axis. This means that the group must be set in the crystal with the C, axis parallel to the y plane and symmetrically between the a and p planes as shown. The planar nitrate groups, covalent and possessing C’s, symmetry, have three A, class vibrations polarized parallel to the symmetry axis, two B, class bands polarized perpendicular to the axis in the plane of the group and one B, band, an out-of-plane deformation polarized perpendicular to the plane of the group, The two strongest nitrate bands, vs and vp in Fig. 1, show distorted envelopes in the ATR spectrum by comparison with reflection or absorption bands; similar effects are seen in the ATR spectra of other compounds and the polarization data can be deduced despite the distortion. [7] H. LYNTON and J. E. FLEMING, AERE EMR/PR 1110 (6). [S] J. G. ALLPRESS and A. N. HAMBLY, Au&. J. Chem. 12, 569 (1959).
1257
Bond orientations in many1 nitrate hexahydrate
The results suggest that the two nitrates on each uranium have their symmetry axes parallel, but this leaves two possible arrangements with the nitrate groups monodentate or bidentate. If the bond were monodentate, one might expect the perpendicular band v4 to be the highest observed frequency. Since the parallel vibration, vz, is the highest observed frequency it seems reasonable to assume bidentate linking, with the oxygen on the symmetry axis not involved in the bond. We Y
Q
B done
done
plane
@
0
Uranium Oxygen
0
Nitrogen
0
Hydrogen
O” 900
Direction of the electric vector for spectra in Fig. I,_
Fig. 3. Diagrammatic projections of the positions of atoms of a many1 nitrate hexahydrate molecule into three mutually perpendicular planes. The orientation (but not the position) of one of the four loosely bound water molecules is shown relative to the rest of the molecule.
believe that the uranium-oxygen bands in the 300-200 cm-l region support this assumption, [9, lo], but this region is still inadequately catalogued and some ambiguity remains. If the plane of the nitrate groups were perpendicular to the uranyl axis, the B, vibration vp should not be seen in the y plane spectra. Its presence may be due to the beam convergence, or to the nitrate groups being inclined to the equatorial plane of The latter cause is assumed in our model (Fig. 3), since LYNTON’S the uranyl group. X-ray data suggest this and a similar puckering of groups round the equatorial plane is present in some other uranyl compounds [Ill. [9] G. TOPPING, Spectrochim. Acta 21, 1743 (1965). [lo] A. M. DEANE (to be published). [ll] E. RABINOWITCH and R. L. BELFORD, Spectroscopy pounds. p. 335 Pergamon Press, Oxford (1964).
and Photochemistry
of Uralzyl Com-
1258
A. M. DEANE, E. W. T. RICHARDSand I. G. STEPHEN
The positions of the water molecules in the structure are more difficult to determine. Water absorptions are seen near 3000 cm-l, about 1620 cm-l and as a general broad absorption near 820 cm-l on which the nitrate absorptions are superimposed. Two possible correlations, not yet fully substantiated, are that sharp bands in the 3400-3600 cm-l region can be assigned to co-ordinated water and that the broad absorption about 820 cm-l is a libration associsted with water only loosely bound in the crystal. (It is, for example, absent in the spectrum of uranyl nitrate dihydrate.) Both types of water contribute to the Ye vibration about 1620 cm-l, so this is of restricted value in deciding the orientation of the water molecules. A larger reproduction of the 3000 cm-l region, Fig. 4, shows the polarization of the stretching vibrations in detail. The doubling of the sharp 3500 cm-l band csn be attributed to small differences of site symmetry of co-ordinated water molecules, or to the resolution of y1 and vs. With the latter assumption, and reversing the usual rule that the asymmetric vibration has the higher frequency, because the lower frequency component is stronger the polarization requirements are met by placing two co-ordinated water molecules with the plane of the molecules slightly inclined to the 010 plane and the symmetry axis parallel to the 001 plane and almost perpendicular to the 100 plane. The second water molecule is arbitrarily placed in the truss position; additional polarization measurements on the y plane at various polarizer angles show that maximum absorption intensity of vS lies along the 001 plane shown in Fig. 3. The co-ordination requirements of the uranyl group are met by the two bidentate nitrstes and two water molecules, so we have assumed that four water molecules occupy holes in the lattice. The large dichroic ratio of the libration bands in the tcand /? planes and of the 1620 cm-l band in the y plane suggests a highly ordered arrangement. The lib&ion band seems to be a doublet in the y plane with each component seen in one of the other planes. This may be due to molecules occupying two dissimilar sites, or to the near-degeneracy of two librational modes. We have been unable to distinguish between the two possibilities because the 3000 cm-l stretching bands are broad and unresolved, and we are uncertain which librational modes are involved. A model in which the molecules are placed parallel to the y plane with their symmetry axes on the diagonal (as shown in Fig. 3) seems the simplest which fits the data, assuming that the principal contribution to the intensity of the 3000 cm-l band is from the perpendicular asymmetric vibration. The spectral data used gives only relative orientations, not information about relative positions, and the four water molecules discussed may be placed anywhere relative to the rest of the molecule. Frequency shifts are observed with varying experimental conditions in ATR spectra as would be expected from the earlier discussion ; any attempt to calculate, for example, oxygen-oxygen distances from hydrogen bond strengths [12] must be made from other data. CONCLUSIONS The example
of uranyl
nitrate hexahydrate
demonstrates
that, qualitatively,
[I21 K. NAKAMOTO,J. Am. Chem. Sot. 77, 6480 (1965). E. HARTERT and 0. GLEMSER. 2. Elektrochem. 60, 746 (1966).
Bond orient&ions
in uranyl nitrate hexahydrate
1259
bond orientation data can be obtained relatively easily, using polarized radiation and the ATR technique. The model proposed on the basis of spectroscopic evidence is in agreement with that proposed by LYNTONfrom X-ray data, and in addition gives information on the
Fig. 4. Detailed reproduction
of the 3000 cm-l region of spectra shown in Fig. 1. B: a plane C: #?plane D: y plane.
probable orientation of water molecules in the structure, though measurements are not so precise as those made by diffraction techniques. Further experience with crystals of other materials and an investigation of the influence of beam convergence on the results are desirable for the fullest use of the technique. Acknowledgements-We would like to thank Drs. E. WAIT and E. JACKSON, identi&ation of the crystal faces using an X-ray diffractometer.
who confirmed our
1260
A.M.DEANE,E.W.T.RICHARDS
and
I.G.STEPHEN
ADDENDUM Since this paper was prepared, TAYLOR and MUELLOR [13] have published the results of a neutron diffraction study on many1 nitrate hexahydrate. Their work refines the structure proposed by LYNTON and in addition gives the position of water molecules in the lattice. Two water molecules co-ordinated to the uranium are found in their structure, inclined approximately as indicated in our model, but the additional water is present in several orientations. Our assumption of only one type of uncoordinated water was incorrect, and the splitting of the libration band about 820 cm-l is a consequence of the varying water positions in the lattice. The (Okl) Fourier synthesis given by TAYLOR and MUELLOR shows clearly that the nitrate groups and water round the uranyl group are not staggered as we indicated in Fig. 3, but that the nitrate groups are very slightly bent in a “butterfly” mode. Consequently the presence of the B, vibration v4 in our y plane spectrum is not due to inclination of the nitrate groups but to beam convergence, the alternative possibility which we discussed. [13]
J.C.TAYLOR and M.H.M~~~~~~,ActaCryst.19,536 (1965).
Bond orientations in uranyl nitrate hexahydrate using attenuated total reflection A. M. DEANE, E. W. T. RICHARDS and I. G. STEPHEN Spectroscopy
Group, Chemistry Division, AERE,
Harwell
(Received 23 November 1965) Ah&&-Bond orientations in uranyl nitrate hexahydrate have been measured using polarized infra-red radiation and the technique of attenuated total reflection. The technique of using polarized radiation with attenuated total reflection is discussed and the results used to produce a model in agreement with X-ray data.
SPECTROSCOPICbond orientation data is usually obtained from dichroic ratio measurements on the reflection spectrum or on the absorption spectrum of an optically thin crystal [l] ; the reflection spectrum is frequently less rich in data than the absorption spectrum and the exposed crystal surface may alter during the measurements, but it is frequently extremely difficult to find crystal flakes a few microns thick for absorption measurements. The attenuated total reflection technique (ATR) provides an infra-red spectrum resembling the absorption spectrum of a specimen in intimate contact with the ATR prism [2]. It has been found that the combination of the ATR technique with use of polarized radiation yields useful information on bond orientation using optically thick crystals. POYNTINQ [3] has shown from electromagnetic theory that, under the conditions of total internal reflection, the perturbation in the medium of lower refractive index is propagated in the direction of the incident beam, and has described the changes of polarization resulting at the interface. More recently the validity of these equations was elegantly demonstrated by CULSHAW and JONES [4] using large components and centimetric wavelengths. We have found, as expected, that the crystal plane examined in our ATR experiments is that perpendicular to the incident beam, but have found much higher dichroic ratios than we anticipated, despite the considerable beam convergence. Measurements with the polarizer before or after the sample and also with uncrossed polarizers, one before and one after the sample gave identical results, so no effects of the total reflection upon the polarization data obtained could be detected. Further consideration shows that this is not surprising; POYNTING showed that under the condition of total internal reflection there can be no net energy absorption [l] [2] [3] [4]
S. S. MITRA, Solid&ate Phye. 13,21 (1962). J. FAERENFORT,Spectrochim. Acta 17,698 (1961). J. H. POYNTINU, Phil. Trans. 175,343 (1884), and standard textbooks. W. CULSHAW and D. S. JONES, Proc. P&e. Sot. B. 66, 859 (1953).
1263
1254
A. M.
DEANE,E. W. T. RICHARDS and I. G. STEPHEN
in the medium of lower refractive index, but in the ATR spectrum absorption demonstrably occurs. It is well known that the “best” spectra result when the mean angle of incidence at the specimen is close to the minimum for total reflection in nonabsorbing regions. The cone angle of the incident beam is about 45’. As the refractive index of the specimen varies with wavelength in the vicinity .of an absorption band varying proportions of the incident cone will be at less than the critical angle, will pass into the sample and be lost to the reflected beam. At the wavelength of maximum refractive index of the specimen near the absorption maximum, the largest proportion of the light cone will be at less than the critical angle, the reflected beam is most reduced and an absorption type spectrum is observed. The dependence of the spectrum on cone angle can easily be demonstrated by appropriate masking of the light incident on the sample. If the refractive index of the specimen rises very high, ordinary reflection at the interface will occur and reduce the apparent extinction. When polarized light is used, the refractive index variations appropriate to the polarization used and in the plane perpendicular to the incident beam attenuate the beam to produce a spectrum. Figure 1 shows spectra we have obtained using a single crystal of uranyl nitrate hexahydrate mounted in a TR5 micro ATR attachment [5] used with a silver chloride polarizer, also a reflection spectrum off one face for comparison; some difficulty was experienced in obtaining the latter spectrum as the crystal surface (presumably) dehydrated during the measurements. EXPERIMENTAL Crystals of uranyl nitrate hexahydrate grown from aqueous solution show two faces at right angles, described by VDOVENKO [6] as the (001) and (100) faces. A crystal was cut to a cube of about 5 mm side and another face cut at 45’, as in Fig. 2. The natural faces of a freshly grown crystal were suitable for direct examination, cut faces were lightly polished on fine emery paper using saturated uranyl nitrate solution as a lubricant. The crystal was contacted with a KRS 5 ATR prism using methylene iodide saturated with sulphur as an immersion fluid and spectra run at a mean angle of incidence of about 40”. Manipulation of a model helps to show that measurements made on the three faces indicated in Fig. 2, appropriately oriented, are an examination of three mutually perpendicular planes when the angle of incidence is 45”. Since the crystal is orthorhombic and 45’ planes do not coincide with simple crystal planes, we have designated the planes we studied as a, /I and y planes. Using natural faces or hand polishing we were not able to dispense with the immersion fluid, but the fluid may be an aid to keeping down the surface temperature and preventing dessication. We were able to repeat spectra on one face after several hours without difficulty, but there is a slow reaction of the nitrate with the prism material. Polarization was effected with a silver chloride pile of plates polarizer which was [5] Research and Industrial Instrument Co., London. [6] V. M. VDOVENKO, et al. Radiokhimya 2, 24 (1960).
Bond orientations
in many1 nitrate
hexahydrate
a plane
/3 plane
Fig.
1. Spectra
of uranyl nitrate hexahydrate crystal using polarised B, C, D, ATR spectra. A. Reflection spectrum. ---Electric vector at 90’. -__ Electric vector at 0’.
Fig. 2. The shape of the crystal on which measurements were made, showing the faces used and the planes examined in the crystal. Light incident at 45“ on the (001) face perpendicular to the Q plane. Light incident at 45” on the (010) face perpendicular to the /I plane. Light incident at 45O on the face cut out 45’, perpendicular to the y plane.
radiation.
1256
A. M. DEANE, E. W. T. RICHARDS and I. 0. STEPHEN
normally mounted after the ATR sodium chloride prism.
attachment
on an H.800
spectrometer
with
DISCUSSION
Two structures for the hexahydrate, based on X-ray diffraction studies, have been suggested by VDOVENKO [6] and by LYNTON [7]. The former proposed a hexa-aquo ion, [U0,(H,0),]2+2N0,-, but this seems incompatible with infra-red data [S]. LYNTON’S structure, two covalent nitrates and two water molecules in a puckered equatorial plane round a nearly linear uranyl group with four more water molecules filling holes in the lattice, is more consistent with the data obtained from an infra-red absorption spectrum of the powder. The relatively small doubling of individual nitrate absorption bands is less than might be expected from the different sized nitrate groups in LYNTON’S model. The observed absorption bands of uranyl nitrate hexahydrate are listed in Table 1 with assignments based on the polarization data in Fig 1. Projections of a model which fits the polarization data on to the planes examined are shown in Fig. 3. Table
1. Frequencies observed in the absorption spectrum of many1 nitrate hexahydrate, and their assignments Frequency 3500* IX,.3100 1634 1675 1495 1339 1304 1050 1041 962 938 867 c&. 820 807 741
OnXp
Assignment v, &v, VI &v,
V2 VZ VZ VI V4 Vl Vl % % 11 libration V6 %
Species
A, & B, .4,&B, A, A1 A41 B, B, A1 -41 CU+ CU+ cs+ B, A1
* On cooling the 3600cm-’ band is resolved into componentsat 3470, 3550cm-‘.
The uranyl group is an essentially linear triatomic group with the asymmetric stretching frequency polarized parallel to the C, symmetry axis. This means that the group must be set in the crystal with the C, axis parallel to the y plane and symmetrically between the a and p planes as shown. The planar nitrate groups, covalent and possessing C’s, symmetry, have three A, class vibrations polarized parallel to the symmetry axis, two B, class bands polarized perpendicular to the axis in the plane of the group and one B, band, an out-of-plane deformation polarized perpendicular to the plane of the group, The two strongest nitrate bands, vs and vp in Fig. 1, show distorted envelopes in the ATR spectrum by comparison with reflection or absorption bands; similar effects are seen in the ATR spectra of other compounds and the polarization data can be deduced despite the distortion. [7] H. LYNTON and J. E. FLEMING, AERE EMR/PR 1110 (6). [S] J. G. ALLPRESS and A. N. HAMBLY, Au&. J. Chem. 12, 569 (1959).
1257
Bond orientations in many1 nitrate hexahydrate
The results suggest that the two nitrates on each uranium have their symmetry axes parallel, but this leaves two possible arrangements with the nitrate groups monodentate or bidentate. If the bond were monodentate, one might expect the perpendicular band v4 to be the highest observed frequency. Since the parallel vibration, vz, is the highest observed frequency it seems reasonable to assume bidentate linking, with the oxygen on the symmetry axis not involved in the bond. We Y
Q
B done
done
plane
@
0
Uranium Oxygen
0
Nitrogen
0
Hydrogen
O” 900
Direction of the electric vector for spectra in Fig. I,_
Fig. 3. Diagrammatic projections of the positions of atoms of a many1 nitrate hexahydrate molecule into three mutually perpendicular planes. The orientation (but not the position) of one of the four loosely bound water molecules is shown relative to the rest of the molecule.
believe that the uranium-oxygen bands in the 300-200 cm-l region support this assumption, [9, lo], but this region is still inadequately catalogued and some ambiguity remains. If the plane of the nitrate groups were perpendicular to the uranyl axis, the B, vibration vp should not be seen in the y plane spectra. Its presence may be due to the beam convergence, or to the nitrate groups being inclined to the equatorial plane of The latter cause is assumed in our model (Fig. 3), since LYNTON’S the uranyl group. X-ray data suggest this and a similar puckering of groups round the equatorial plane is present in some other uranyl compounds [Ill. [9] G. TOPPING, Spectrochim. Acta 21, 1743 (1965). [lo] A. M. DEANE (to be published). [ll] E. RABINOWITCH and R. L. BELFORD, Spectroscopy pounds. p. 335 Pergamon Press, Oxford (1964).
and Photochemistry
of Uralzyl Com-
1258
A. M. DEANE, E. W. T. RICHARDSand I. G. STEPHEN
The positions of the water molecules in the structure are more difficult to determine. Water absorptions are seen near 3000 cm-l, about 1620 cm-l and as a general broad absorption near 820 cm-l on which the nitrate absorptions are superimposed. Two possible correlations, not yet fully substantiated, are that sharp bands in the 3400-3600 cm-l region can be assigned to co-ordinated water and that the broad absorption about 820 cm-l is a libration associsted with water only loosely bound in the crystal. (It is, for example, absent in the spectrum of uranyl nitrate dihydrate.) Both types of water contribute to the Ye vibration about 1620 cm-l, so this is of restricted value in deciding the orientation of the water molecules. A larger reproduction of the 3000 cm-l region, Fig. 4, shows the polarization of the stretching vibrations in detail. The doubling of the sharp 3500 cm-l band csn be attributed to small differences of site symmetry of co-ordinated water molecules, or to the resolution of y1 and vs. With the latter assumption, and reversing the usual rule that the asymmetric vibration has the higher frequency, because the lower frequency component is stronger the polarization requirements are met by placing two co-ordinated water molecules with the plane of the molecules slightly inclined to the 010 plane and the symmetry axis parallel to the 001 plane and almost perpendicular to the 100 plane. The second water molecule is arbitrarily placed in the truss position; additional polarization measurements on the y plane at various polarizer angles show that maximum absorption intensity of vS lies along the 001 plane shown in Fig. 3. The co-ordination requirements of the uranyl group are met by the two bidentate nitrstes and two water molecules, so we have assumed that four water molecules occupy holes in the lattice. The large dichroic ratio of the libration bands in the tcand /? planes and of the 1620 cm-l band in the y plane suggests a highly ordered arrangement. The lib&ion band seems to be a doublet in the y plane with each component seen in one of the other planes. This may be due to molecules occupying two dissimilar sites, or to the near-degeneracy of two librational modes. We have been unable to distinguish between the two possibilities because the 3000 cm-l stretching bands are broad and unresolved, and we are uncertain which librational modes are involved. A model in which the molecules are placed parallel to the y plane with their symmetry axes on the diagonal (as shown in Fig. 3) seems the simplest which fits the data, assuming that the principal contribution to the intensity of the 3000 cm-l band is from the perpendicular asymmetric vibration. The spectral data used gives only relative orientations, not information about relative positions, and the four water molecules discussed may be placed anywhere relative to the rest of the molecule. Frequency shifts are observed with varying experimental conditions in ATR spectra as would be expected from the earlier discussion ; any attempt to calculate, for example, oxygen-oxygen distances from hydrogen bond strengths [12] must be made from other data. CONCLUSIONS The example
of uranyl
nitrate hexahydrate
demonstrates
that, qualitatively,
[I21 K. NAKAMOTO,J. Am. Chem. Sot. 77, 6480 (1965). E. HARTERT and 0. GLEMSER. 2. Elektrochem. 60, 746 (1966).
Bond orient&ions
in uranyl nitrate hexahydrate
1259
bond orientation data can be obtained relatively easily, using polarized radiation and the ATR technique. The model proposed on the basis of spectroscopic evidence is in agreement with that proposed by LYNTONfrom X-ray data, and in addition gives information on the
Fig. 4. Detailed reproduction
of the 3000 cm-l region of spectra shown in Fig. 1. B: a plane C: #?plane D: y plane.
probable orientation of water molecules in the structure, though measurements are not so precise as those made by diffraction techniques. Further experience with crystals of other materials and an investigation of the influence of beam convergence on the results are desirable for the fullest use of the technique. Acknowledgements-We would like to thank Drs. E. WAIT and E. JACKSON, identi&ation of the crystal faces using an X-ray diffractometer.
who confirmed our
1260
A.M.DEANE,E.W.T.RICHARDS
and
I.G.STEPHEN
ADDENDUM Since this paper was prepared, TAYLOR and MUELLOR [13] have published the results of a neutron diffraction study on many1 nitrate hexahydrate. Their work refines the structure proposed by LYNTON and in addition gives the position of water molecules in the lattice. Two water molecules co-ordinated to the uranium are found in their structure, inclined approximately as indicated in our model, but the additional water is present in several orientations. Our assumption of only one type of uncoordinated water was incorrect, and the splitting of the libration band about 820 cm-l is a consequence of the varying water positions in the lattice. The (Okl) Fourier synthesis given by TAYLOR and MUELLOR shows clearly that the nitrate groups and water round the uranyl group are not staggered as we indicated in Fig. 3, but that the nitrate groups are very slightly bent in a “butterfly” mode. Consequently the presence of the B, vibration v4 in our y plane spectrum is not due to inclination of the nitrate groups but to beam convergence, the alternative possibility which we discussed. [13]
J.C.TAYLOR and M.H.M~~~~~~,ActaCryst.19,536 (1965).