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The NIR absorption spectrum of water in FeCl2·4H2O single crystals
J. Phys. Chem. Solids Vol. 53, No. 9, pp. 1237-1243, 1992 Printed in Great Britain.
0022-3697/92 $5.00+ 0.00 © 1992 Pergamon Press Ltd
THE NIR ABSORPTION SPECTRUM OF WATER IN FeCI2.4H20 SINGLE CRYSTALS SILVIA M. DOGLIA, M A R C O MARTINI, GIORGIO SPINOLO and A N N A M . VILLA Dipartimento di Fisica,Universita'di Milano, via Celoria 16, 20133 Milano, Italy (Received 2 October 1991; accepted in revisedform 30 March 1992) Abstract--The polarized near i.r. absorption spectrum of water in FcCI2.4H20 single crystals has been measured at I0 K in the 3600-7200cm-t region. The observed bands can be assigned to overtones and combinations of the fundamental and librational modes of the crystal water molecules.Polarization helps in attributing the contribution of the two different types of water present in the crystal and in understanding which fundamental vibrations combine. This analysis gives support to the assignment of the fundamental spectrum bands which are not experimentally accessible on single crystals and are of difficult interpretation, particularly in the stretching and librational regions.
Keywords: Overtones spectra, near i.r. absorption, water, hydrogen bond, hydrates.
1. INTRODUCTION I.r. and Raman spectra of fundamental vibrations of liquid H20 are characterized by rather broad bands whose structures are analysed with some difficulty; as a consequence, the description of the interactions that water has with its environment is still a matter of debate. The effects of the hydrogen bond on the O-H stretching vibrations of water have been widely discussed [1], hut their quantitative understanding is still controversial [2]. Crystalline hydrates [3] may be better systems on which carry on spectroscopic studies on the structure and interactions of water, since the molecules in these systems are frozen in a well-defined environment [4]. Unfortunately, the abundant i.r. data are obtained on crystals crushed and embedded in pellets and therefore the spectra are characterized by broad unpolarized bands. Raman studies, on the other hand, can be performed on single crystals and the information given by polarization conveniently exploited. In our laboratory a program focused on Raman [5] and i.r. studies [6] of the fundamental vibrations of water in crystalline hydrates is going on; it is clear however that, due to the high oscillator strength in the fundamental i.r. region, one is still compelled to use pellets, losing in such a way the information that oriented spectra on single crystals can give. Single crystal spectra are still far from being reached, due to the difficulty of obtaining single crystals as thin as 10tim. However, in the near i.r. region (NIP-,), where overtones and combinations of fundamental modes of water occur, polarized i.r. single crystal spectra of high quality can be obtained from samples with
optical path of few 102/~m: here we want to report on such types of data. The NIR overtone spectrum of water in vapour, liquid and ice has been extensively studied by several authors [1, 7-15], but even in ice it displays broad and structureless bands down to low temperatures [12]. On the contrary, the low temperature NIR spectrum of hydrated single crystals shows well resolved and sharp bands, that can be assigned with confidence to specific combinations of fundamental and librational modes of the water molecules [16, 17]. Its analysis is therefore a useful complement to the discussion for the assignment of modes in the fundamental spectrum which is not experimentally accessible on single crystals. In the following we will show, besides an updating of i.r. data on FeCI2"4H:O fundamental absorptions obtained on pellets, NIR polarized absorption results obtained on FeCl 2•4H 20 single crystals at cryogenic temperatures. In the discussion of the spectra we put forward in evidence the contribution of the two types of water and the selection rule according to which the couplings for binary combinations are allowed when the direction of the electric dipole associated with each of the components is the same. The 2vs and 2va that appear both as harmonic and anharmonic overtones offer us the possibility of a few comments on the anharmonicity of the OH oscillator and on its overtones in different environments. 2. EXPERIMENTAL
Crystal structure o f FeC12 •4 H 2 0 The unit cell of FeCI2-4H20 is monoclinic and contains two formula units [18, 19]. Its space group is
1237
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SILVIAM. DOGLIAet aL
P21c. The dimensions of the cell are a = 5.885, b = 7.180, c = 8.514 A, and fl = 111.09°, ~ = y = 90 °. The crystal consists of discrete octahedra of Fe(H20)4CI 2 linked by O-H . . . CI hydrogen bonds (Fig. 1). The orientation of the octahedra in the crystal is such that one of their O - F e - O axes points along the crystallographic c-axis, and they are disposed in layers perpendicular to the c-axis and parallel to the cleavage plane. Within each octahedron, two water molecules have a bond angle of 107.3° (type 1 molecules) and two of 103.6° (type 2). Note that this latter value is close to the bond angle of free water (104°). In Table 1 bond lengths, angles and distances are reported, putting forward evidence of a slight distortion of the octahedron. A neutron diffraction study [19] has revealed the presence of an amount of 11-20% of disordered Fe atoms and consequently of water geometries different from those listed in Table 1. Since the Fe-O(1) bond is along the C2-axis of the water molecule, while the Fe--O(2) bond is almost along one of the two oxygen lone pairs, water molecules of type 1 lie in the plane passing through O(1), Fe and the two chlorine ions, while those of type 2 are forced out of the plane O(l)-Fe-O(2), maintaining the coplanarity with C1- ions to which they are hydrogen bonded. Materials and methods
Pale green crystals of FeCI2"4H20 were obtained by slow evaporation of an over-saturated aqueous solution of tetrahydrated iron chloride at room temperature. For partially deuterated samples, a D20 solution of the salt was used. In order to obtain spectra in the NIR region, crystals were thinned down to a few hundred # m by passing them over blotting paper soaked in H20, or D20 for partially deuterated samples.
Table 1. Bond distances (A) and bond angles (degrees) for FeC12•4H2Ot Bond Bond distances (A) angles Fe--CI Fe--O(1) Fe-O(2) O(1)-O(2) O(1)-O(2)
2.514 2.121 2.086 2.96 3.05
H(1)-O(1)-H(1) H(2)-O(2)-H(2) O(I)-Fe-CI O(2)-Fe~l O(l)-Fe-O(2)
107.3° 103.6° 89.86° 90.24° 88.21°
tFrom Refs 18 and 19. In the NIR (3600-8000cm-1), the spectra were obtained at 10 K by a Cary-2300 spectrophotometer. Polarized spectra were obtained by placing i.r. polarizing filters both on the sample and on the reference beam of the spectrophotometer. The polarized absorption has been obtained by light propagating in direction parallel to the b-axis, the electric vector being oriented in two different directions perpendicular one to the other and lying in the plane parallel to the crystal surface (see Fig. 1). Polarization I has been chosen to maximize the signal on the peak at 5634 cm-1, which instead is minimized in polarization II. Further, polarization I is parallel to the vs and 6 associated dipoles of type 2 water and to a component of va and rocking (p), associated dipoles of type 1 water; polarization II is parallel to the c-axis and to the vs and 6 dipoles of water 1 and to va and p dipoles of water 2. Because of the high intensity of water fundamental pellets of dried KC1 with concentrations of 1-5% in weight of FeC12"4H20 were used, in order to get sufficient transmission. The spectra of water fundamental bands were recorded at liquid nitrogen temperature on a Nicolet FX-1 FTIR spectrophotometer. 3. RESULTS AND DISCUSSION F u n d a m e n t a l v i b r a t i o n s (4000-400 c m - i)
~=111.09 ~
~2)
(I)
,~
af
O @0 @CI ~H .... H - b o n d
Fig. 1. Structure of FeCI2.4H20 from Ref. 18. Two octahedra are represented and the c- and b-axis directions indicated. The two types of water are labelled (I) and (2), respectively. The directions of light propagation and polarization a r e s h o w n a s well.
The i.r. absorption spectrum of FeCI2"4H20 in KC1 pellets was recorded between 4000 and 650 cm- l at room temperature by Gamo in the 1960s [20]. The lack of optical data in the librations range (700-400cm -~) and of precise structural information, more recently obtained by neutron diffraction studies [19], did not allow a detailed analysis of the observed frequencies. For these reasons, it is now worthwhile to update the absorption spectrum of FeCI2. 4H20 using measurments with a FT instrument, extending the spectral range down to 400cm -~ arid lowering the temperature down to 77 K (Fig. 2a). These new experimental results allow us to reconsider the assignments of the observed peaks along lines currently accepted in the literature.
The NIR absorption spectrum of water in FeC12.4H20 single crystals As can be seen, the stretching absorption appears as a large and composite band with a maximum at 3388cm -] and an evident shoulder at 3450crn-L Moreover, the bending overtone, which always characterizes the water absorption in this region, can be recognized at 3220 cm- ], with an intensity arising from the Fermi resonance interaction between the water bending overtone and the symmetric (see below) stretching vs. However, for the FeC12"4H20 crystal, the energy difference between the bending overtone and the symmetric stretching is larger than 100 cm -l, and this can be taken as an indication [21] that the Fermi resonance is weak, not appreciably affecting the energy position of the interacting modes. In order to push further the analysis, we also studied the absorption spectrum in the OH stretching region of a partially deuterated crystal to take advantage from the narrowing of OH bands, four peaks can be recognized at about 3390, 3400, 3420 and 3450 cm-1, as illustrated in Fig. 2b. An assignment of each component to water 1 and water 2 symmetric and antisymmetric stretchings can be made on the grounds of the following considerations. Since the water antisymmetric stretching is normally at higher frequency than the symmetric one [21], we assign the bands at 3450cm -t and at 3390 cm -t to va and vs, respectively. In particular, the
2.4
b)
1.8
3~oo
35oo
O.,o /\
,/ ",,
33oo
ill
a)
~>
E, 0.8 rr 0.6
/ /
/ ~ ........ ~ / Y ' ~ - '
0.2 4000
3400
2800 2200 1600 WAVENUMBERS (cm -1)
1000
400
Fig. 2. (a) Liquid nitrogen absorption spectrum of FeC12"4H20 in KC1 pellet and (b) liquid nitrogen absorption spectrum of a partially deuterated crystal in KC1 pellet. In the insert one sees enlarged the top of the OH stretching absorption.
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waters of type 2, which have a geometry closer to that of free water, are expected to have stretching frequencies at higher energy than molecules of type 1. Therefore, the peak at 3450 cm -~ may be associated with the antisymmetric stretching of water of type 2, while that at 3390cm -~ is probably due to type 1 molecules symmetric stretching. The remaining two bands at 3420 cm -~ and at 3400 cm- t are tentatively assigned to the Va of water 1 and to the vs of water 2, taking into account that this frequency separation is in agreement with what suggested in the literature [21] for symmetric and antisymmetric stretchings. As already pointed out, these assignments are only tentative. However, their use in the analysis of the overtone region will fit the experimental spectra rather well, as will be seen in the following section, giving an a posteriori indication that such assignments are correct. The bending absorption band shows two wellresolved peaks at 1640 and 1610 cm -~, due to the two different types of water molecules. By comparing the results obtained for the water molecules in the crystal with the results reported for the water molecules in vapor and in liquid form [see Refs 7-10, 15], we could therefore assign the two components of the bending absorption band at 1640 and 1610 cm -~ to the water type 1 and type 2, respectively, since water 2 has a smaller bond angle [19] similar to liquid water. This assignment is supported by the study of Lindgren [4] on the increase of the bending frequency upon the bond angle. In the spectral region investigated here, also the external modes relative to the hindered rotations of water can be seen from 1000 to 400 cm -l. Two peaks at 588 and 484 cm -~ are evident; however, complete Raman and FTIR results [5, 6] are available in this region and the i.r. data are reported in Table 2 together with the proposed attributions. In Fig. 2a one may also observe a band with two resolved components at 2226 and 2196 cm -l which can be assigned to the combination of bending and rocking modes of water, that have been also observed for liquid water at 2125 cm -~ [7, 10]. The excellent agreement found for the combinations 1640+ 588 cm -] and 1610 + 588 cm -~ is perhaps fortuitous; indeed there should be two rocking frequencies for the two types of water, as is suggested from the spectrum shown in Fig. 2 of Ref. 6. In the spectrum shown in Fig. 2a we cannot exclude the combinations 5 + t and 5 + w but certainly their presence is hardly detectable. Such a situation may be due to the fact that the electric dipoles associated with ~ and p lie in the H20 plane while t and w dipoles are out of such a plane.
SILVIAM. DOGUAet al.
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Table
2.
Fundamental
vibrational
frequencies
of
and 7200crn -~ at 1 0 K is presented in Fig. 3; it displays a pattern common to other hydrated crystals [16, 17, 22] and is due to overtones and combination bands of crystal water. Three spectral regions can be recognized, from 3600 to 4800 cm -l, from 4800 to 6000 cm -~, and from 6000 to 7200 c m - L Each region contains bands due to binary or ternary combinations of modes in which the OH stretchings are always involved and combine with different water fundamental modes. For the hydrated crystals previously investigated, however, a precise assignment of the observed bands could not be given, since the fundamental vibration energies were not known with sufficient precision, also due to strong Fermi resonance effects [16, 17]. In the case of the FeC12.4H20, knowledge of the fundamental vibration energies has allowed us to discuss with greater detail the assignment of the single bands proposed in Table 3. In the first (3600-4800 crn - l ) region, we have, at 4042 c m - 1, the binary combination of va + p. This assignment is supported by the consideration that the antisymmetric stretching and the rocking modes of water are both polarized in the same direction, perpendicular to the C2~ axis of the molecules. This binary excitation can occur within the same water molecule or in two neighbouring ones [23]. In this second case polarization requirements are also matched, since neighbouring waters of the same type are parallel oriented in the crystal. We recall that this combination mode has been also observed by inelastic neutron scattering in liquid water by Ricci et al. [24, 25]. These authors suggested that this spectral feature could be related to the breaking of the H-bond, occurring when stretching and rocking vibrations are simultaneously excited within the same
FeCI2"4H20 (cm-t) taken at 77 K. H20 vapour frequencies are also
H20
reported for comparison (from Ref. 15)
OH?
OD~
Vapour
Assignment
3756 3657 3450
3450 3420 3400 3390
3390 3220 2226 2196 1640 1610
Va Vs
va(2)
2535 2510 2484 2477 2368
v,O) v,(2)
v,(1) 26
6(l)+p 6(2) + p 1437 1427
60)
I190 I182
6(2) 6 Libration (t) Libration (p) Libration (w) M-O M-O M-X OID OID OID OID OID
1595 632 588 484 386 348 228 206 197 172 149 142
1"OH and HOD bands in a partially deuterated sample. :~OD and D20 bands in a partially deuterated sample. v, = symmetric stretching. va = antisymmetric stretching. 6 = bending. p = rocking. w = wagging. t = twisting. M-O = Fe-oxygen stretching. M-X = Fe-chlorine stretching. OID = octahedron internal deformation. Librations and low energy vibrations are from Refs 6 and 7. Overtone
and
combination
bands
(3600-
7200 c m - I) The polarized absorption spectrum in the N I R region of FeC12.4H20 single crystal between 3600
200
'E 100
0 3600
,
I
44OO
I
I
I
/
5200 6000 WAVENUMBERS (crn"I)
I
I
6800
Fig. 3. NIR polarized absorption spectrum of FeCI2"4H20 single crystal at 10 K. Orientation of the polarizing filter: I (3_ c), II (11 c).
The NIR absorption spectrum of water in FeCI2-4H20 single crystals
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Table 3. Observed and calculated overtones and combination bands (era-t) of FeCi2-4H20 at 10 K. H 2 0 vapour bands are also reported for comparison (from Ref. 15) Obs.
Cal.
Pol. I
PoE II
4042s 4337w
4039w
Vapour
4348w 4388w 4442w 4529w 4684w 5010s
4670w 5035s
5235 5331 (residue) 5182sh
5107s 5209sh
5634m 6489s 6566w (residue) 6623w 6693w
5647w (residue) 6566w 6579m 6689w 6794w
6810sh 7202 6845s 6887m
6826w 6906m 7445
Combinations Binary
Ternary
4038 v,(2) + p 4336 4356 4394 4429 4527 4675 5010 vs(2)+6(2) 5030 v~(1)+6(1) v~+ 6 + Xn(Xi2 ~ - 17) va "4-6 q- X23(X23~ --20) 5104 5174 5204 5638 5620 2 x v,(1)+ 2X33(X33--.--178) 2x 6622 6699 6780 2 x 6800 2 x 2x 6840 2 x
v~(2) + p + M - 0(348) va(l) + p + M - 0(348) v~(1) + p + M - 0(386) v~(2) + p + M - 0(386) v,(1) + p + w va(2) + p + t
v,(1) + v~(1) + v~(1) + v,,(2) + vs(l) +
60) + 60) + 6(1) + 6(2) + 6(1) +
OL(72) OID(142) OID(172) w p
va(2) + 2X33(Xa3 -~ - 163.5) v+ 2x 6 v+ 2x 6 v,(l) v,(2) v,+ 2 X . ( X , "" -56) v.(1)
6900 2 x v,(2) 2 x v, + 2X33(X33_ -33.5)
OL = octahedron libration, from Raman measurements, but also expected in i.r. m = medium, s = strong, sh = shoulder, w = weak. water molecule. The above analysis of the combination band at 4042 cm-1 which involves necessarily the rocking mode, supports the assignments previously proposed for the water librational mode in the crystal [6]. Combinations va + t, vs + p, v, + w and vs + w are also possible, although with a drop in probability, if the two oscillations are excited on different types of next nearest neighbour H 2 0 molecules. F r o m 4200 to 4800 c m - ~ (see also Table 3), a series of less probable transitions is present in the spectrum: ternary combinations of stretchings, librations and occasional participation of low frequency ( < 4 0 0 c m -~) modes. In the first subregion the absorption coefficient ranges from 40 to 170 cm -~ and in the second from 10 to 3 0 c m -~. In the region 480045000 cm -~ it is again easy to separate the strong lower energy binary combinations vs + 6 from the lower probability ternary combinations; our proposals are shown in Table 3. In the binary combinations region, the polarized spectra, as discussed in section 2, offer the possibility of choosing which peak is due to vs(2) + 6 (2), polarization I, and which to v s ( l ) + 6 ( 1 ) , polarization II. The energies calculated starting from our tentative attributions o f Table 2 satisfactorily match the measured values, adding strength to those proposals. PCS 5 3 ~ G
In the third absorption region a complex band structure is present. A broad absorption (between 6000 and 8000 c m - ~), which is particularly evident in polarization I, is superimposed to a pattern of sharp peaks. N I R crystal field studies [26] on FeCI2 suggest that the broad band centered around 7000 cm-~ can be attributed to the transition 3Tt (H) o f t b e Fe z+ ion in an octahedric ligand field. In polarization II at 6906 and in I at 6845 crn-~ two sharp peaks are observed at the double frequencies of the fundamental antisymmetric stretchings v,(2) and v,(1) (within the uncertainties of the fundamental vibration values) of the two types of water molecules of the crystal. Again the analysis o f the polarized N I R absorption supports our assignment of the antisymmetric stretchings of the different water moleeules that are hidden in the broad fundamental band (see Table 2). In the same range a number of less intense bands are observed and may be assigned to the combinations of the stretching modes with the two bending overtones as proposed in Table 3. At 6489 crn -1 (polarization I) and 6579 cm -1 (polarization II) intense peaks are present, that cannot be assigned with reasonably good precision to any harmonic combination or overtone of the stretching modes. We propose to assign them to the anharmonic overtones o f the antisymmetric stretchings o f water
1242
SILVIAM. DOGLIAet aL
type 1 and type 2, respectively, following the sugges, tion of Refs 11 and 12. Here the anharmonic shifts from the harmonic sums (whose frequencies in the present case can be evaluated with good precision) are 2X = - 3 5 6 cm -1 and 2X = - 3 2 7 cm -1 which correspond to anharmonicity constants X = - 1 7 8 c m - I and X = - 163.5 cm-1. These values are in agreement with the indications given in the literature [17]. By inspection of the table, one can see that some features remain unexplained; among those we may mention the double structure of the two absorptions between 6480 and 6580cm -1 and the peak at 6887 cm-1 that can hardly be attributed to a residue of the 6906 cm- 1 band. The characteristic feature of this last absorption region is then the presence of the stretching summation bands occurring at the exact double frequency of the fundamentals and of the anharmonic overtone shifted toward lower frequency by anharmonicity. To summarize, we identify three different spectral regions in the N I R adsorption spectrum of FeCl:.4H20. In these regions the following combination bands can be assigned: (1) combinations between the water stretching bands and the low frequency modes of the crystal (water librations and octahedra internal stretchings) in the region between 3600 and 4800 cm-l; (2) intense binary combinations of water stretching and bending modes in the region between 4800 and 6000cm-I; (3) binary combinations and overtones of water stretching modes in the region between 6000 and 7200 cm-l. Finally, we would like to remark the presence in the third region of both the anharmonic overtones and the harmonic sums of water stretching modes. The former modes are the well-known internal overtones of the water molecule that can also be observed in vapor and that are affected by mechanical anharmonicity. The latter modes, instead, are due to a different phenomenon: the simultaneous excitation of two adjacent water molecules in the crystal. A brief comparison with literature data for the same type of transitions in variously condensed, ordered and interacting H20 molecules is in order. In non-interacting (vapour) H20, only anharmonic overtones have been observed [7-10, 15], proving the mechanical anharmonicity of the H20 potential; the same situation occurs in liquid water. In ice, Ron and Hornig [11] have assigned bands at 6050cm -1 and 6550 cm- 1 to anharmonic and "harmonic overtone", respectively; strictly speaking the latter transition should correspond to the simultaneous singly excited stretching transition in two adjacent interacting water molecules. This conclusion is reached because in the interacting H20 molecules of ice the potential should
be even more anharmonic than in vapour and therefore harmonic overtones within a single molecule should be excluded. This type of assignment has been furthermore confirmed by measurements of HOD in D20 [13]. By varying the concentration of HOD it has been possible to show that the "harmonic overtone" of the H20 stretchings disappears at low concentrations for HOD molecules isolated in D20 matrix. The "harmonic overtone" is therefore absent in diluted (including vapour) or disordered (liquid water) systems. Monohydrated crystals such as K2 SnC14" H : O [16], where each formula unit contains only one water molecule and neighbouring waters are far apart in the crystal are "diluted" system: overtones occur always at anharmonic frequencies; no harmonic modes have been observed. On the contrary, in the dihydrates (M2CuC14"2H20 and CuCI2.H20 ) [17] and in the tetrahydrate presented here, where waters are in ordered arrays and the distance between neighbouring molecules is lower (see Fig. 1 and Table 1) one can recognize both the anharmonic and the "harmonic overtones" for the stretching modes. In other ordered and densely packed hydrogen bonded molecular crystals, such as solid HCI at 77 K, Ron and Horning [11] have observed both the "harmonic" and anharmonic overtones of the internal stretching mode (vn~o). Again it is suggested that the anharmonic mode at 5313 cm -1 is due to the overtone excitation within a single molecule, whereas the "harmonic" mode at 5464 cm- 1is due to the simultaneous excitation of adjacent HCI molecules. 4. CONCLUSION We have shown that the near i.r. absorption spectrum of FeCI2.4H20 single crystal at low temperature can be interpreted as being due to the overtones and combinations of the fundamental vibrational modes of the two types of H 20 present in the crystal. Interpretation of the binary combinations and first stretching overtones is quite satisfactory; ternary combination interpretation is certainly more debatable. The analysis of the polarized combination spectra enlightens in few cases the attribution of the bands due to the fundamental vibrations. Such attribution is often difficult due to the poor quality of the fundamental spectra taken on pellets, which unavoidably give broad bands and no polarization information. In particular, the stretching overtones are well resolved and allow the evaluation of the stretching frequencies of the different types of water in the crystal. Furthermore, the analysis of the combination
The NIR absorption spectrum of water in FeC12.4H20 single crystals bands between antisymmetric stretching and rocking modes supports the assignment of the librational modes previously proposed [6]. In many cases the polarization of the incident light has made easier the attributions by allowing the correlation between the transition dipoles and the direction of the incident electric vector. A n interesting feature of the near i.r. spectrum of water in FeC12.4H20, c o m m o n also to water in ice [12-14], is the presence of the "harmonic stretching sums" of the two types of water in the crystal. These binary excitations occurring at the exact double frequencies of the fundamental mode indicate that simultaneous excitations occurs in neighbouring interacting molecules, a behaviour already reported for several hydrogen bonded molecular crystals [11, 12].
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Acknowledgement--We are glad to acknowledge the continuous interest and useful discussions with our colleague Prof. F. Cariati.
17. 18. 19.
REFERENCES 1. Walrafen G. E., Raman and infrared spectral investigations of water structure. In Water. A Comprehensive Treatise, 2nd ed., Vol. 1, chap. 5. Plenum, New York (1973). 2. Falk M., Spectrochim. Acta 40A, 43 (1984). 3. Falk M. and Knop O., Water in stoichiometric hydrates. In Water. A Comprehensive Treatise, 2nd ed., Vol. 2, chap. 2. Plenum, New York (1973) 4. Lindgren J. and Tegenfeldt J., J. Mol. Struct. 20, 335 (1974). 5. Cariati F., Masserano F., Martini M. and Spinolo G., J. Raman Spectr. 20, 773 (1989); Cariati F., Bruni S.,
20. 21. 22. 23. 24. 25. 26.
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Martini M. and Spinoio G., J. Raman Spectr. 22, 397 (1991). Diatto P., Martini M. and Spinolo G., J. Phys. Chem. Solids 49, 1469 (1988); 49, 1139 (1988). Herzberg G., Molecular Spectra and Molecular Structure. Van Nostrand, New York (1950). Curcio J. A. and Petty C. C., J. Opt. Soc. America 41, 302 (1951). Luck W. A. P., Ber. Bunsenges. physik. Chem. 69, 626 (1965). Eisenberg D. and Kauzmann W., The Structure and Properties of Water. Oxford University Press, Oxford (1969). Ron A. and Hornig D. F., J. Chem. Phys. 39, 1129 (1963). Sceats M. G. and P,dce S. A., J. Chem. Phys. 71, 973 (1979). Kroh D. and Ron A., Chem. Phys. Lett. 36, 527 0975). Andaloru G. and Leone M., Il Nuovo Cimento 5D, 133 (1985). Bykov A. D., Makushkin Yu. S. and Ulenikov O. N., J. Mol. Spectr. 99, 221 (1983). Walker I. M. and McCarthy P. J., Can. J. Chem. 64, 1012 (1986). Walker I. M. and McCarthy P. J., J. Phys. Chem. 93, 2230 (1989). Penfold B. R. and Grigor J. A., Acta Cryst. 12, 850 (1959). Verbist J. J., Hamilton W. C., Koetzle T. F. and Lehmann M. S., J. Chem. Phys. 56, 3257 (1972). Gamo I., Bull. Chem. Soc. Japan 34, 1433 (1961). Erikssonn A. and Lindgren J., J. Mol. Struct. 53, 97 (1979). McCarthy P. J. and Walker I. M., Spectrochim. Acta 39A, 827 (1983). Sandorfy C., in Infrared and Raman Spectroscopy of Biological Molecules (Edited by T. M. Theophanides), pp. 305-318. D. Reidel, Dordrecht. Ricci M. A., Chen S. H., Price D. L., Loong C.-K., Toukan K. and Teixeira J., Physica 136B, 190 (1986). Ricei M. A. and Chen S.-H., Phys. Rev. A34, 1714 (1986). Weakliem H. A., J. Chem. Phys. 36, 2117 (1962).
0022-3697/92 $5.00+ 0.00 © 1992 Pergamon Press Ltd
THE NIR ABSORPTION SPECTRUM OF WATER IN FeCI2.4H20 SINGLE CRYSTALS SILVIA M. DOGLIA, M A R C O MARTINI, GIORGIO SPINOLO and A N N A M . VILLA Dipartimento di Fisica,Universita'di Milano, via Celoria 16, 20133 Milano, Italy (Received 2 October 1991; accepted in revisedform 30 March 1992) Abstract--The polarized near i.r. absorption spectrum of water in FcCI2.4H20 single crystals has been measured at I0 K in the 3600-7200cm-t region. The observed bands can be assigned to overtones and combinations of the fundamental and librational modes of the crystal water molecules.Polarization helps in attributing the contribution of the two different types of water present in the crystal and in understanding which fundamental vibrations combine. This analysis gives support to the assignment of the fundamental spectrum bands which are not experimentally accessible on single crystals and are of difficult interpretation, particularly in the stretching and librational regions.
Keywords: Overtones spectra, near i.r. absorption, water, hydrogen bond, hydrates.
1. INTRODUCTION I.r. and Raman spectra of fundamental vibrations of liquid H20 are characterized by rather broad bands whose structures are analysed with some difficulty; as a consequence, the description of the interactions that water has with its environment is still a matter of debate. The effects of the hydrogen bond on the O-H stretching vibrations of water have been widely discussed [1], hut their quantitative understanding is still controversial [2]. Crystalline hydrates [3] may be better systems on which carry on spectroscopic studies on the structure and interactions of water, since the molecules in these systems are frozen in a well-defined environment [4]. Unfortunately, the abundant i.r. data are obtained on crystals crushed and embedded in pellets and therefore the spectra are characterized by broad unpolarized bands. Raman studies, on the other hand, can be performed on single crystals and the information given by polarization conveniently exploited. In our laboratory a program focused on Raman [5] and i.r. studies [6] of the fundamental vibrations of water in crystalline hydrates is going on; it is clear however that, due to the high oscillator strength in the fundamental i.r. region, one is still compelled to use pellets, losing in such a way the information that oriented spectra on single crystals can give. Single crystal spectra are still far from being reached, due to the difficulty of obtaining single crystals as thin as 10tim. However, in the near i.r. region (NIP-,), where overtones and combinations of fundamental modes of water occur, polarized i.r. single crystal spectra of high quality can be obtained from samples with
optical path of few 102/~m: here we want to report on such types of data. The NIR overtone spectrum of water in vapour, liquid and ice has been extensively studied by several authors [1, 7-15], but even in ice it displays broad and structureless bands down to low temperatures [12]. On the contrary, the low temperature NIR spectrum of hydrated single crystals shows well resolved and sharp bands, that can be assigned with confidence to specific combinations of fundamental and librational modes of the water molecules [16, 17]. Its analysis is therefore a useful complement to the discussion for the assignment of modes in the fundamental spectrum which is not experimentally accessible on single crystals. In the following we will show, besides an updating of i.r. data on FeCI2"4H:O fundamental absorptions obtained on pellets, NIR polarized absorption results obtained on FeCl 2•4H 20 single crystals at cryogenic temperatures. In the discussion of the spectra we put forward in evidence the contribution of the two types of water and the selection rule according to which the couplings for binary combinations are allowed when the direction of the electric dipole associated with each of the components is the same. The 2vs and 2va that appear both as harmonic and anharmonic overtones offer us the possibility of a few comments on the anharmonicity of the OH oscillator and on its overtones in different environments. 2. EXPERIMENTAL
Crystal structure o f FeC12 •4 H 2 0 The unit cell of FeCI2-4H20 is monoclinic and contains two formula units [18, 19]. Its space group is
1237
1238
SILVIAM. DOGLIAet aL
P21c. The dimensions of the cell are a = 5.885, b = 7.180, c = 8.514 A, and fl = 111.09°, ~ = y = 90 °. The crystal consists of discrete octahedra of Fe(H20)4CI 2 linked by O-H . . . CI hydrogen bonds (Fig. 1). The orientation of the octahedra in the crystal is such that one of their O - F e - O axes points along the crystallographic c-axis, and they are disposed in layers perpendicular to the c-axis and parallel to the cleavage plane. Within each octahedron, two water molecules have a bond angle of 107.3° (type 1 molecules) and two of 103.6° (type 2). Note that this latter value is close to the bond angle of free water (104°). In Table 1 bond lengths, angles and distances are reported, putting forward evidence of a slight distortion of the octahedron. A neutron diffraction study [19] has revealed the presence of an amount of 11-20% of disordered Fe atoms and consequently of water geometries different from those listed in Table 1. Since the Fe-O(1) bond is along the C2-axis of the water molecule, while the Fe--O(2) bond is almost along one of the two oxygen lone pairs, water molecules of type 1 lie in the plane passing through O(1), Fe and the two chlorine ions, while those of type 2 are forced out of the plane O(l)-Fe-O(2), maintaining the coplanarity with C1- ions to which they are hydrogen bonded. Materials and methods
Pale green crystals of FeCI2"4H20 were obtained by slow evaporation of an over-saturated aqueous solution of tetrahydrated iron chloride at room temperature. For partially deuterated samples, a D20 solution of the salt was used. In order to obtain spectra in the NIR region, crystals were thinned down to a few hundred # m by passing them over blotting paper soaked in H20, or D20 for partially deuterated samples.
Table 1. Bond distances (A) and bond angles (degrees) for FeC12•4H2Ot Bond Bond distances (A) angles Fe--CI Fe--O(1) Fe-O(2) O(1)-O(2) O(1)-O(2)
2.514 2.121 2.086 2.96 3.05
H(1)-O(1)-H(1) H(2)-O(2)-H(2) O(I)-Fe-CI O(2)-Fe~l O(l)-Fe-O(2)
107.3° 103.6° 89.86° 90.24° 88.21°
tFrom Refs 18 and 19. In the NIR (3600-8000cm-1), the spectra were obtained at 10 K by a Cary-2300 spectrophotometer. Polarized spectra were obtained by placing i.r. polarizing filters both on the sample and on the reference beam of the spectrophotometer. The polarized absorption has been obtained by light propagating in direction parallel to the b-axis, the electric vector being oriented in two different directions perpendicular one to the other and lying in the plane parallel to the crystal surface (see Fig. 1). Polarization I has been chosen to maximize the signal on the peak at 5634 cm-1, which instead is minimized in polarization II. Further, polarization I is parallel to the vs and 6 associated dipoles of type 2 water and to a component of va and rocking (p), associated dipoles of type 1 water; polarization II is parallel to the c-axis and to the vs and 6 dipoles of water 1 and to va and p dipoles of water 2. Because of the high intensity of water fundamental pellets of dried KC1 with concentrations of 1-5% in weight of FeC12"4H20 were used, in order to get sufficient transmission. The spectra of water fundamental bands were recorded at liquid nitrogen temperature on a Nicolet FX-1 FTIR spectrophotometer. 3. RESULTS AND DISCUSSION F u n d a m e n t a l v i b r a t i o n s (4000-400 c m - i)
~=111.09 ~
~2)
(I)
,~
af
O @0 @CI ~H .... H - b o n d
Fig. 1. Structure of FeCI2.4H20 from Ref. 18. Two octahedra are represented and the c- and b-axis directions indicated. The two types of water are labelled (I) and (2), respectively. The directions of light propagation and polarization a r e s h o w n a s well.
The i.r. absorption spectrum of FeCI2"4H20 in KC1 pellets was recorded between 4000 and 650 cm- l at room temperature by Gamo in the 1960s [20]. The lack of optical data in the librations range (700-400cm -~) and of precise structural information, more recently obtained by neutron diffraction studies [19], did not allow a detailed analysis of the observed frequencies. For these reasons, it is now worthwhile to update the absorption spectrum of FeCI2. 4H20 using measurments with a FT instrument, extending the spectral range down to 400cm -~ arid lowering the temperature down to 77 K (Fig. 2a). These new experimental results allow us to reconsider the assignments of the observed peaks along lines currently accepted in the literature.
The NIR absorption spectrum of water in FeC12.4H20 single crystals As can be seen, the stretching absorption appears as a large and composite band with a maximum at 3388cm -] and an evident shoulder at 3450crn-L Moreover, the bending overtone, which always characterizes the water absorption in this region, can be recognized at 3220 cm- ], with an intensity arising from the Fermi resonance interaction between the water bending overtone and the symmetric (see below) stretching vs. However, for the FeC12"4H20 crystal, the energy difference between the bending overtone and the symmetric stretching is larger than 100 cm -l, and this can be taken as an indication [21] that the Fermi resonance is weak, not appreciably affecting the energy position of the interacting modes. In order to push further the analysis, we also studied the absorption spectrum in the OH stretching region of a partially deuterated crystal to take advantage from the narrowing of OH bands, four peaks can be recognized at about 3390, 3400, 3420 and 3450 cm-1, as illustrated in Fig. 2b. An assignment of each component to water 1 and water 2 symmetric and antisymmetric stretchings can be made on the grounds of the following considerations. Since the water antisymmetric stretching is normally at higher frequency than the symmetric one [21], we assign the bands at 3450cm -t and at 3390 cm -t to va and vs, respectively. In particular, the
2.4
b)
1.8
3~oo
35oo
O.,o /\
,/ ",,
33oo
ill
a)
~>
E, 0.8 rr 0.6
/ /
/ ~ ........ ~ / Y ' ~ - '
0.2 4000
3400
2800 2200 1600 WAVENUMBERS (cm -1)
1000
400
Fig. 2. (a) Liquid nitrogen absorption spectrum of FeC12"4H20 in KC1 pellet and (b) liquid nitrogen absorption spectrum of a partially deuterated crystal in KC1 pellet. In the insert one sees enlarged the top of the OH stretching absorption.
1239
waters of type 2, which have a geometry closer to that of free water, are expected to have stretching frequencies at higher energy than molecules of type 1. Therefore, the peak at 3450 cm -~ may be associated with the antisymmetric stretching of water of type 2, while that at 3390cm -~ is probably due to type 1 molecules symmetric stretching. The remaining two bands at 3420 cm -~ and at 3400 cm- t are tentatively assigned to the Va of water 1 and to the vs of water 2, taking into account that this frequency separation is in agreement with what suggested in the literature [21] for symmetric and antisymmetric stretchings. As already pointed out, these assignments are only tentative. However, their use in the analysis of the overtone region will fit the experimental spectra rather well, as will be seen in the following section, giving an a posteriori indication that such assignments are correct. The bending absorption band shows two wellresolved peaks at 1640 and 1610 cm -~, due to the two different types of water molecules. By comparing the results obtained for the water molecules in the crystal with the results reported for the water molecules in vapor and in liquid form [see Refs 7-10, 15], we could therefore assign the two components of the bending absorption band at 1640 and 1610 cm -~ to the water type 1 and type 2, respectively, since water 2 has a smaller bond angle [19] similar to liquid water. This assignment is supported by the study of Lindgren [4] on the increase of the bending frequency upon the bond angle. In the spectral region investigated here, also the external modes relative to the hindered rotations of water can be seen from 1000 to 400 cm -l. Two peaks at 588 and 484 cm -~ are evident; however, complete Raman and FTIR results [5, 6] are available in this region and the i.r. data are reported in Table 2 together with the proposed attributions. In Fig. 2a one may also observe a band with two resolved components at 2226 and 2196 cm -l which can be assigned to the combination of bending and rocking modes of water, that have been also observed for liquid water at 2125 cm -~ [7, 10]. The excellent agreement found for the combinations 1640+ 588 cm -] and 1610 + 588 cm -~ is perhaps fortuitous; indeed there should be two rocking frequencies for the two types of water, as is suggested from the spectrum shown in Fig. 2 of Ref. 6. In the spectrum shown in Fig. 2a we cannot exclude the combinations 5 + t and 5 + w but certainly their presence is hardly detectable. Such a situation may be due to the fact that the electric dipoles associated with ~ and p lie in the H20 plane while t and w dipoles are out of such a plane.
SILVIAM. DOGUAet al.
1240
Table
2.
Fundamental
vibrational
frequencies
of
and 7200crn -~ at 1 0 K is presented in Fig. 3; it displays a pattern common to other hydrated crystals [16, 17, 22] and is due to overtones and combination bands of crystal water. Three spectral regions can be recognized, from 3600 to 4800 cm -l, from 4800 to 6000 cm -~, and from 6000 to 7200 c m - L Each region contains bands due to binary or ternary combinations of modes in which the OH stretchings are always involved and combine with different water fundamental modes. For the hydrated crystals previously investigated, however, a precise assignment of the observed bands could not be given, since the fundamental vibration energies were not known with sufficient precision, also due to strong Fermi resonance effects [16, 17]. In the case of the FeC12.4H20, knowledge of the fundamental vibration energies has allowed us to discuss with greater detail the assignment of the single bands proposed in Table 3. In the first (3600-4800 crn - l ) region, we have, at 4042 c m - 1, the binary combination of va + p. This assignment is supported by the consideration that the antisymmetric stretching and the rocking modes of water are both polarized in the same direction, perpendicular to the C2~ axis of the molecules. This binary excitation can occur within the same water molecule or in two neighbouring ones [23]. In this second case polarization requirements are also matched, since neighbouring waters of the same type are parallel oriented in the crystal. We recall that this combination mode has been also observed by inelastic neutron scattering in liquid water by Ricci et al. [24, 25]. These authors suggested that this spectral feature could be related to the breaking of the H-bond, occurring when stretching and rocking vibrations are simultaneously excited within the same
FeCI2"4H20 (cm-t) taken at 77 K. H20 vapour frequencies are also
H20
reported for comparison (from Ref. 15)
OH?
OD~
Vapour
Assignment
3756 3657 3450
3450 3420 3400 3390
3390 3220 2226 2196 1640 1610
Va Vs
va(2)
2535 2510 2484 2477 2368
v,O) v,(2)
v,(1) 26
6(l)+p 6(2) + p 1437 1427
60)
I190 I182
6(2) 6 Libration (t) Libration (p) Libration (w) M-O M-O M-X OID OID OID OID OID
1595 632 588 484 386 348 228 206 197 172 149 142
1"OH and HOD bands in a partially deuterated sample. :~OD and D20 bands in a partially deuterated sample. v, = symmetric stretching. va = antisymmetric stretching. 6 = bending. p = rocking. w = wagging. t = twisting. M-O = Fe-oxygen stretching. M-X = Fe-chlorine stretching. OID = octahedron internal deformation. Librations and low energy vibrations are from Refs 6 and 7. Overtone
and
combination
bands
(3600-
7200 c m - I) The polarized absorption spectrum in the N I R region of FeC12.4H20 single crystal between 3600
200
'E 100
0 3600
,
I
44OO
I
I
I
/
5200 6000 WAVENUMBERS (crn"I)
I
I
6800
Fig. 3. NIR polarized absorption spectrum of FeCI2"4H20 single crystal at 10 K. Orientation of the polarizing filter: I (3_ c), II (11 c).
The NIR absorption spectrum of water in FeCI2-4H20 single crystals
1241
Table 3. Observed and calculated overtones and combination bands (era-t) of FeCi2-4H20 at 10 K. H 2 0 vapour bands are also reported for comparison (from Ref. 15) Obs.
Cal.
Pol. I
PoE II
4042s 4337w
4039w
Vapour
4348w 4388w 4442w 4529w 4684w 5010s
4670w 5035s
5235 5331 (residue) 5182sh
5107s 5209sh
5634m 6489s 6566w (residue) 6623w 6693w
5647w (residue) 6566w 6579m 6689w 6794w
6810sh 7202 6845s 6887m
6826w 6906m 7445
Combinations Binary
Ternary
4038 v,(2) + p 4336 4356 4394 4429 4527 4675 5010 vs(2)+6(2) 5030 v~(1)+6(1) v~+ 6 + Xn(Xi2 ~ - 17) va "4-6 q- X23(X23~ --20) 5104 5174 5204 5638 5620 2 x v,(1)+ 2X33(X33--.--178) 2x 6622 6699 6780 2 x 6800 2 x 2x 6840 2 x
v~(2) + p + M - 0(348) va(l) + p + M - 0(348) v~(1) + p + M - 0(386) v~(2) + p + M - 0(386) v,(1) + p + w va(2) + p + t
v,(1) + v~(1) + v~(1) + v,,(2) + vs(l) +
60) + 60) + 6(1) + 6(2) + 6(1) +
OL(72) OID(142) OID(172) w p
va(2) + 2X33(Xa3 -~ - 163.5) v+ 2x 6 v+ 2x 6 v,(l) v,(2) v,+ 2 X . ( X , "" -56) v.(1)
6900 2 x v,(2) 2 x v, + 2X33(X33_ -33.5)
OL = octahedron libration, from Raman measurements, but also expected in i.r. m = medium, s = strong, sh = shoulder, w = weak. water molecule. The above analysis of the combination band at 4042 cm-1 which involves necessarily the rocking mode, supports the assignments previously proposed for the water librational mode in the crystal [6]. Combinations va + t, vs + p, v, + w and vs + w are also possible, although with a drop in probability, if the two oscillations are excited on different types of next nearest neighbour H 2 0 molecules. F r o m 4200 to 4800 c m - ~ (see also Table 3), a series of less probable transitions is present in the spectrum: ternary combinations of stretchings, librations and occasional participation of low frequency ( < 4 0 0 c m -~) modes. In the first subregion the absorption coefficient ranges from 40 to 170 cm -~ and in the second from 10 to 3 0 c m -~. In the region 480045000 cm -~ it is again easy to separate the strong lower energy binary combinations vs + 6 from the lower probability ternary combinations; our proposals are shown in Table 3. In the binary combinations region, the polarized spectra, as discussed in section 2, offer the possibility of choosing which peak is due to vs(2) + 6 (2), polarization I, and which to v s ( l ) + 6 ( 1 ) , polarization II. The energies calculated starting from our tentative attributions o f Table 2 satisfactorily match the measured values, adding strength to those proposals. PCS 5 3 ~ G
In the third absorption region a complex band structure is present. A broad absorption (between 6000 and 8000 c m - ~), which is particularly evident in polarization I, is superimposed to a pattern of sharp peaks. N I R crystal field studies [26] on FeCI2 suggest that the broad band centered around 7000 cm-~ can be attributed to the transition 3Tt (H) o f t b e Fe z+ ion in an octahedric ligand field. In polarization II at 6906 and in I at 6845 crn-~ two sharp peaks are observed at the double frequencies of the fundamental antisymmetric stretchings v,(2) and v,(1) (within the uncertainties of the fundamental vibration values) of the two types of water molecules of the crystal. Again the analysis o f the polarized N I R absorption supports our assignment of the antisymmetric stretchings of the different water moleeules that are hidden in the broad fundamental band (see Table 2). In the same range a number of less intense bands are observed and may be assigned to the combinations of the stretching modes with the two bending overtones as proposed in Table 3. At 6489 crn -1 (polarization I) and 6579 cm -1 (polarization II) intense peaks are present, that cannot be assigned with reasonably good precision to any harmonic combination or overtone of the stretching modes. We propose to assign them to the anharmonic overtones o f the antisymmetric stretchings o f water
1242
SILVIAM. DOGLIAet aL
type 1 and type 2, respectively, following the sugges, tion of Refs 11 and 12. Here the anharmonic shifts from the harmonic sums (whose frequencies in the present case can be evaluated with good precision) are 2X = - 3 5 6 cm -1 and 2X = - 3 2 7 cm -1 which correspond to anharmonicity constants X = - 1 7 8 c m - I and X = - 163.5 cm-1. These values are in agreement with the indications given in the literature [17]. By inspection of the table, one can see that some features remain unexplained; among those we may mention the double structure of the two absorptions between 6480 and 6580cm -1 and the peak at 6887 cm-1 that can hardly be attributed to a residue of the 6906 cm- 1 band. The characteristic feature of this last absorption region is then the presence of the stretching summation bands occurring at the exact double frequency of the fundamentals and of the anharmonic overtone shifted toward lower frequency by anharmonicity. To summarize, we identify three different spectral regions in the N I R adsorption spectrum of FeCl:.4H20. In these regions the following combination bands can be assigned: (1) combinations between the water stretching bands and the low frequency modes of the crystal (water librations and octahedra internal stretchings) in the region between 3600 and 4800 cm-l; (2) intense binary combinations of water stretching and bending modes in the region between 4800 and 6000cm-I; (3) binary combinations and overtones of water stretching modes in the region between 6000 and 7200 cm-l. Finally, we would like to remark the presence in the third region of both the anharmonic overtones and the harmonic sums of water stretching modes. The former modes are the well-known internal overtones of the water molecule that can also be observed in vapor and that are affected by mechanical anharmonicity. The latter modes, instead, are due to a different phenomenon: the simultaneous excitation of two adjacent water molecules in the crystal. A brief comparison with literature data for the same type of transitions in variously condensed, ordered and interacting H20 molecules is in order. In non-interacting (vapour) H20, only anharmonic overtones have been observed [7-10, 15], proving the mechanical anharmonicity of the H20 potential; the same situation occurs in liquid water. In ice, Ron and Hornig [11] have assigned bands at 6050cm -1 and 6550 cm- 1 to anharmonic and "harmonic overtone", respectively; strictly speaking the latter transition should correspond to the simultaneous singly excited stretching transition in two adjacent interacting water molecules. This conclusion is reached because in the interacting H20 molecules of ice the potential should
be even more anharmonic than in vapour and therefore harmonic overtones within a single molecule should be excluded. This type of assignment has been furthermore confirmed by measurements of HOD in D20 [13]. By varying the concentration of HOD it has been possible to show that the "harmonic overtone" of the H20 stretchings disappears at low concentrations for HOD molecules isolated in D20 matrix. The "harmonic overtone" is therefore absent in diluted (including vapour) or disordered (liquid water) systems. Monohydrated crystals such as K2 SnC14" H : O [16], where each formula unit contains only one water molecule and neighbouring waters are far apart in the crystal are "diluted" system: overtones occur always at anharmonic frequencies; no harmonic modes have been observed. On the contrary, in the dihydrates (M2CuC14"2H20 and CuCI2.H20 ) [17] and in the tetrahydrate presented here, where waters are in ordered arrays and the distance between neighbouring molecules is lower (see Fig. 1 and Table 1) one can recognize both the anharmonic and the "harmonic overtones" for the stretching modes. In other ordered and densely packed hydrogen bonded molecular crystals, such as solid HCI at 77 K, Ron and Horning [11] have observed both the "harmonic" and anharmonic overtones of the internal stretching mode (vn~o). Again it is suggested that the anharmonic mode at 5313 cm -1 is due to the overtone excitation within a single molecule, whereas the "harmonic" mode at 5464 cm- 1is due to the simultaneous excitation of adjacent HCI molecules. 4. CONCLUSION We have shown that the near i.r. absorption spectrum of FeCI2.4H20 single crystal at low temperature can be interpreted as being due to the overtones and combinations of the fundamental vibrational modes of the two types of H 20 present in the crystal. Interpretation of the binary combinations and first stretching overtones is quite satisfactory; ternary combination interpretation is certainly more debatable. The analysis of the polarized combination spectra enlightens in few cases the attribution of the bands due to the fundamental vibrations. Such attribution is often difficult due to the poor quality of the fundamental spectra taken on pellets, which unavoidably give broad bands and no polarization information. In particular, the stretching overtones are well resolved and allow the evaluation of the stretching frequencies of the different types of water in the crystal. Furthermore, the analysis of the combination
The NIR absorption spectrum of water in FeC12.4H20 single crystals bands between antisymmetric stretching and rocking modes supports the assignment of the librational modes previously proposed [6]. In many cases the polarization of the incident light has made easier the attributions by allowing the correlation between the transition dipoles and the direction of the incident electric vector. A n interesting feature of the near i.r. spectrum of water in FeC12.4H20, c o m m o n also to water in ice [12-14], is the presence of the "harmonic stretching sums" of the two types of water in the crystal. These binary excitations occurring at the exact double frequencies of the fundamental mode indicate that simultaneous excitations occurs in neighbouring interacting molecules, a behaviour already reported for several hydrogen bonded molecular crystals [11, 12].
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Acknowledgement--We are glad to acknowledge the continuous interest and useful discussions with our colleague Prof. F. Cariati.
17. 18. 19.
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20. 21. 22. 23. 24. 25. 26.
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Martini M. and Spinoio G., J. Raman Spectr. 22, 397 (1991). Diatto P., Martini M. and Spinolo G., J. Phys. Chem. Solids 49, 1469 (1988); 49, 1139 (1988). Herzberg G., Molecular Spectra and Molecular Structure. Van Nostrand, New York (1950). Curcio J. A. and Petty C. C., J. Opt. Soc. America 41, 302 (1951). Luck W. A. P., Ber. Bunsenges. physik. Chem. 69, 626 (1965). Eisenberg D. and Kauzmann W., The Structure and Properties of Water. Oxford University Press, Oxford (1969). Ron A. and Hornig D. F., J. Chem. Phys. 39, 1129 (1963). Sceats M. G. and P,dce S. A., J. Chem. Phys. 71, 973 (1979). Kroh D. and Ron A., Chem. Phys. Lett. 36, 527 0975). Andaloru G. and Leone M., Il Nuovo Cimento 5D, 133 (1985). Bykov A. D., Makushkin Yu. S. and Ulenikov O. N., J. Mol. Spectr. 99, 221 (1983). Walker I. M. and McCarthy P. J., Can. J. Chem. 64, 1012 (1986). Walker I. M. and McCarthy P. J., J. Phys. Chem. 93, 2230 (1989). Penfold B. R. and Grigor J. A., Acta Cryst. 12, 850 (1959). Verbist J. J., Hamilton W. C., Koetzle T. F. and Lehmann M. S., J. Chem. Phys. 56, 3257 (1972). Gamo I., Bull. Chem. Soc. Japan 34, 1433 (1961). Erikssonn A. and Lindgren J., J. Mol. Struct. 53, 97 (1979). McCarthy P. J. and Walker I. M., Spectrochim. Acta 39A, 827 (1983). Sandorfy C., in Infrared and Raman Spectroscopy of Biological Molecules (Edited by T. M. Theophanides), pp. 305-318. D. Reidel, Dordrecht. Ricci M. A., Chen S. H., Price D. L., Loong C.-K., Toukan K. and Teixeira J., Physica 136B, 190 (1986). Ricei M. A. and Chen S.-H., Phys. Rev. A34, 1714 (1986). Weakliem H. A., J. Chem. Phys. 36, 2117 (1962).