Vibrational Spectroscopy 55 (2011) 188–194
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Vibrational spectra of Cs2 CaCl4 ·2H2 O a,b ˇ Filip Svonson a , Bojan Soptrajanov , Ljupco Pejov a , Viktor Stefov a,∗ a b
Institut za hemija, PMF, Univerzitet “Sv. Kiril i Metodij”, P.O. Box 162, 1001 Skopje, Macedonia Makedonska akademija na naukite i umetnostite, Skopje, Macedonia
a r t i c l e
i n f o
Article history: Received 17 August 2010 Received in revised form 15 November 2010 Accepted 18 November 2010 Available online 1 December 2010 This work is dedicated to Academician Gligor Jovanovski, the leader of the structural chemistry in Macedonia, on the occasion of his 65th birthday.
a b s t r a c t The Fourier transform infrared spectra of Cs2 CaCl4 ·2H2 O as well as those of a series of its partially deuterated analogues were recorded at room and at liquid-nitrogen temperature (RT and LNT, respectively). The RT Raman spectra of the protiated form and of its almost completely deuterated analogue were also studied. The combined results from the analysis of the spectra were used to assign the observed bands. The mechanical anharmonicity of the OH(D) stretching and bending motions were further analyzed by computing the corresponding anharmonicity constants by several algorithms. The obtained trends in the series of structurally similar compounds were discussed. © 2010 Elsevier B.V. All rights reserved.
Keywords: Cs2 CaCl4 ·2H2 O Vibrational spectra Infrared Raman Anharmonicity constants
1. Introduction With this contribution we continue the study of the vibrational spectra of aquahalogeno complexes which have been a subject of our interest for the last two decades [1–15]. The complex Cs2 CaCl4 ·2H2 O and its deuterated analogues were prepared and their room temperature (RT) infrared spectra, the infrared spectra recorded at the boiling temperature of liquid nitrogen (LNT) and the RT Raman spectra were recorded. To the best of our knowledge, the vibrational spectra of Cs2 CaCl4 ·2H2 O have not been studied as yet. Our attention was directed mainly towards the internal and external vibrations of the water molecules and we attempted to carry out a thorough assignment of the corresponding bands. The crystal structure of the title compound has been determined by X-ray diffraction [16]. It crystallizes in the triclinic system (space ¯ with a = 690.4 pm, b = 751.3 pm, c = 587.7 pm, ˛ = 92.28◦ , group P1), ˇ = 96.13◦ , = 65.23◦ and one formula unit per unit cell. In the unit cell, the Ca2+ ions lie on positions with Ci symmetry, while all other ions occupy general positions.
Only one type of water molecules, trigonally coordinated and located on general positions, are present in the structure. Each water molecule is surrounded by one Ca2+ ion (an electron acceptor) and two chloride ions (potential proton acceptors). The water molecules form hydrogen bonds of the Ow · · ·Cl(1) and Ow · · ·Cl(2), types with Ow · · ·Cl length 315.2 and 326.7 pm, respectively. It should be pointed out that the structure of this compound is very similar to the structure of the isostructural compounds MI 2 [MII Cl4 (H2 O)2 ] (MI = Rb, Cs; MII = Mn, Ni) [17,18]. Not only all the above-mentioned structures do belong to the same space groups, but their structural units also lay on positions with the same symmetry. So we can say that all these compounds are mutually isostructural. However, there is a subtle point worth mentioning. In the above-mentioned group of MI 2 [MII Cl4 (H2 O)2 ] compounds, MII is a d element, whereas Ca is not. As a consequence, we shall write the formula of the presently studied compound as Cs2 CaCl4 ·2H2 O rather than as Cs2 [CaCl4 (H2 O)2 ]. It should be further pointed out that the MII –O length in Cs2 CaCl4 ·2H2 O is greater than the MII –O length found in the other isostructural compounds [16–18].
2. Experimental ∗ Corresponding author. Tel.: +389 2 3249 942; fax: +389 2 3226 865. E-mail addresses:
[email protected],
[email protected] (V. Stefov). 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.11.006
Throughout this work, commercial p.a. chemicals were used. The title compound was prepared using the method described by
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Table 1 Unit-cell group analysis of the three internal modes and librations of the water molecules in Cs2 CaCl4 ·2H2 O.
Fig. 1. Fourier transform infrared spectra of Cs2 CaCl2 ·2H2 O recorded at RT (lower curve) and at LNT (upper curve). LNT spectrum is offset with respect to the RT one.
Evans et al. [16] by crystallization from an aqueous solution containing CsCl and CaCl2 in the molar ratio 2:1. The solution is left to slowly evaporate at room temperature until prismatic crystals are formed. The deuterated analogues were prepared by dissolving a small amount of the protiated compound in H2 O–D2 O mixtures of appropriate composition. The crystallization of the obtained solutions was carried out in a dry box. All infrared spectra were obtained using the Perkin-Elmer System 2000 infrared interferometer in the 5100–370 cm−1 region with a resolution of 2 cm−1 . They were recorded from both pressed KBr disks and Nujol mulls at RT and LNT, 32 spectra being accumulated and averaged. A low-temperature cell Graseby Specac P/N 21525 with KBr windows was used for obtaining LNT spectra. The Raman spectra were recorded (with a resolution of 2 cm−1 ) using a LabRam 300 (Horiba Jobin–Yvon) micro Raman spectrometer equipped with a frequency-doubled Nd:YAG laser operating at 532 nm with an excitation power of 6.7 mW at the sample. 3. Results and discussion The infrared spectra of Cs2 CaCl4 ·2H2 O recorded at RT and LNT are shown in Fig. 1 and the Raman spectra recorded at RT are presented in Fig. 2. The infrared spectra of this compound are very similar to those of the isostructural compounds MI 2 [MII Cl4 (H2 O)2 ] (MI = Rb, Cs; MII = Mn, Ni), which were studied before [3,4,7]. 3.1. Internal vibrations of the water molecules According to the factor-group analysis (see Table 1), in each of the infrared and Raman spectra of the investigated compound three bands due to the internal vibrations of the water molecules are expected. As was mentioned above, there is only one crystallographic type of water molecules located on general positions. Thus, under the
Fig. 2. Raman spectrum of Cs2 CaCl2 ·2H2 O recorded at RT.
influence of static field the stretching and bending modes become modes which belong to the A symmetry type, regardless of their previous symmetry. Under the influence of the correlation field, each such mode splits into two components belonging to the gerade (Ag ) symmetry type (Raman active) or the ungerade (Au ) type (infrared active). Hence, we expect three Raman and three infrared active components, not necessarily coincident, due to the internal vibrations of the single type of water molecules (there is only one formula unit in the spectroscopic unit cell). 3.1.1. Stretching vibrations In the region of the stretching modes of water molecules in the infrared spectra of the protiated form recorded at LNT and the Raman spectra recorded at RT (see Figs. 1 and 2) as well as in those of the almost completely deuterated analogue (see Figs. 3 and 4) at least three bands are found (two strong and one weak), and not two as expected. In the LNT infrared spectrum of Cs2 CaCl4 ·2H2 O these bands are found at 3407, 3370 and 3230 cm−1 , and at 2539, 2465 and 2378 cm−1 in the corresponding spectrum of the deuterated analogue (see Figs. 1 and 3). In the RT Raman spectrum of the protiated
Fig. 3. Fourier transform infrared spectra of partially deuterated analogues of Cs2 CaCl2 ·2H2 O recorded at LNT in the region of the OH and OD stretching vibrations (the content of deuterium increases from bottom to top curve, offset spectra are presented).
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Fig. 4. Raman spectra of Cs2 CaCl2 ·2H2 O (lower curve) and its almost completely deuterated analogue (upper curve, offset) recorded at RT.
compound, the analogous bands are found at 3407, 3353 and 3229 cm−1 whereas in the RT Raman spectrum of the almost completely deuterated compound they appear at 2547, 2473 and 2382 cm−1 (see Figs. 2 and 4). Similar bands were observed in the spectra of the MI 2 [MII Cl4 (H2 O)2 ] compounds (MI = Rb, Cs; MII = Mn, Ni) [3,7]. In the presently studied spectra, the band at the lowest frequency in both O–H and O–D stretching regions has lower intensity and is sharper than the two higher-frequency bands. The difference in the intensity and the slight temperature dependence of the lowest-frequency band points to different origins of the pair of stronger bands, on the one hand, and the additional weak one, on the other. With reasonable certainty the latter can be attributed to a second-order mode involving the HOH (or DOD) bending mode. The absence of an analogous third band in the infrared spectra of isotopically isolated HDO molecules (in the O–H stretching region in the spectra of the highly deuterated compounds and in the O–D stretching region in the compounds with low deuterium content, see Figs. 3 and 5) supports this conclusion. It is highly probable that this vibration is intensified by a weak interaction of the Fermi-resonance type with the fundamental H2 O/D2 O stretching vibrations. The vibrational interaction would have been more obvious if the difference in frequencies between the above mentioned modes were small and not, as in our case, larger than 100 cm−1 . Adams and Lock [19] assigned the bands around 3210 cm−1 in the RT infrared spectra of Rb2 [MnCl4 (H2 O)2 ] and Cs2 [MnCl4 (H2 O)2 ] to OH stretching modes, whereas Shankle and Bates [18], while
Fig. 5. Fourier transform infrared spectra of slightly deuterated (≈3% D) analogue of Cs2 CaCl2 ·2H2 O recorded at RT (lower curve) and LNT (upper curve, offset) in the region of the OD stretching vibrations.
analyzing the vibrational spectra of Rb2 [NiCl4 (H2 O)2 ] recorded at 15 K, assumed that the band around 3220 cm−1 is the result of the “appearance of additional vibrational degree of freedom”, with no further explanation. In accordance with the arguments presented by Lutz [20], we attributed the band at 3407 cm−1 (in the protiated form of the title compound) to the antisymmetric H–O–H stretch and that at 3370 cm−1 to the symmetric H–O–H one. The bands above 2450 cm−1 (in the highly deuterated form) were attributed to the antisymmetric D–O–D (higher frequency band) and to the symmetric D–O–D (lower frequency band) stretching modes, respectively. The presence of two intense bands at 2523 and 2487 cm−1 in the RT infrared spectrum and at 2514 and 2477 cm−1 in the spectrum recorded at LNT in the region of the O–D stretching modes of the isotopically isolated HOD and DOH molecules (see Fig. 5) is in accordance with the presence of non-equivalent hydrogen bonds, i.e. the presence of asymmetric water molecules in the structure of the title compound. As in the spectra of the MI 2 [MII Cl4 (H2 O)2 ] (MI = Rb, Cs; MII = Mn, Ni) compounds [3,7] both O–D stretching modes of the isotopically isolated HDO molecules have a positive temperature coefficient (d/dT > 0) which, according to Falk et al. [21], is an evidence for the presence of linear or slightly bent hydrogen bonds in the structure of the given compound. Since in studies of many crystalline hydrates this assumption has been confirmed as correct [20], it can be said that in our case, linear or slightly bent hydrogen bonds are present. 3.1.2. Bending vibrations The bands due to the bending vibrations of the H2 O, HOD and D2 O molecules in the gas phase are found at 1594, 1403, and 1178 cm−1 , respectively [22]. In the condensed phases they usually have higher frequency values [20]. Based on studies of a large number of crystalline hydrates, Falk [23] found that their average values were 1636, 1426 and 1194 cm−1 . In accordance with the results of the factor group analysis (see Table 1), only one band at 1631 and 1635 cm−1 is found in the H–O–H bending region in the infrared and Raman spectra of Cs2 CaCl4 ·2H2 O recorded at RT (see Figs. 1, 2 and 6). In the lowtemperature infrared spectrum, the corresponding band appears at 1633 cm−1 , and a band at 1201 cm−1 is found in the D–O–D bending region (see Fig. 6). The appearance of one H–O–H and one D–O–D band in the corresponding bending regions implies the presence of one type of water molecules in the examined compound. In the infrared spectra of the compound with the highest deuterium con-
Fig. 6. Fourier transform infrared spectra of partially deuterated analogues of Cs2 CaCl2 ·2H2 O recorded at LNT in the region of the HOH, HDO and DOD bending vibrations (the content of deuterium increases from bottom to top curve, offset spectra are presented).
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tent, several bands (at 1650, 1633, 1622 and 1570 cm−1 ) were found in the HOH bending region. With a reasonable certainty the band at 1633 cm−1 can be attributed to the fundamental H–O–H bending mode, whereas the origin of the other three bands is considered to be analogous to the bands at 2240, 2206 and 2139 cm−1 and can be attributed to combinations of librations and/or translations with HOH bending modes. In the infrared spectra of the partially deuterated analogues in the HOD (DOH) bending region two new bands (at 1446 and 1437 cm−1 ) are observed (see Fig. 6), which indicates that the protons in the structure are not equivalent, a conclusion which is in accordance with the structural data of the examined compound.
3.2. External vibrations of the water molecules The hindered rotations and translations are included in the external vibrations of the water molecules. The bands due to H2 O librations are usually found in the region from 1000 to 300 cm−1 [24]. The hindered translations of water molecules coordinated to metal atoms are expected in the region from 500 to 300 cm−1 , whereas for non-coordinated water molecules the hindered translations are found below 300 cm−1 [20]. An important feature of the bands due to librations is the significant temperature effect on their shape and frequency [24]. On lowering the temperature, namely, the librational bands shift to higher frequencies and apparently increase in intensity. Upon deuteration (depending on the extent of the isotopic change), bands due to HDO (DHO) and D2 O librations are found at lower frequencies. The sensitivity of the librations to deuteration and the lowering of the temperature can assist in their assignment although absolutely certain criteria for this do not seem to exist [20,24]. Two bands were found in the corresponding region of the RT infrared spectra of the related compounds Rb2 [MnCl4 (H2 O)2 ] and Cs2 [MnCl4 (H2 O)2 ] [19] and assigned to the wagging and the rocking H2 O librational modes, whereas in the LNT infrared and Raman spectra of Rb2 [NiCl4 (H2 O)2 ] again two librational bands were observed but were left without a more detailed attribution [18]. In our studies [4,7] of the infrared spectra of the partially deuterated analogues of the isomorphic compounds MI 2 [MII Cl4 (H2 O)2 ] (MI = Rb, Cs; MII = Mn, Ni) bands due to librational modes were also observed and assigned with the frequency order twisting > wagging > rocking. It should be pointed out that this order is not in accordance with the model calculations for trigonally coordinated water molecule presented by Eriksson and Lindgren [25] (where the calculated order is rocking > twisting > wagging), but is in agreement with Stefov’s calculations [26] showing that for trigonally coordinated water molecules it is possible for the rocking mode to be with the lowest frequency. As can be seen in Fig. 7, in the 800–400 cm−1 region of the LNT infrared spectra of Cs2 CaCl4 ·2H2 O, at least three bands (at 585, 557 and 495 cm−1 ) are found, the intensity of the band at the highest frequency being the lowest. Because of their temperature dependence, these three bands can be assigned to librational modes. At least two unusually intense bands in this region at 592 and 486 cm−1 can also be seen in the Raman spectra recorded at room temperature (see Fig. 8). The infrared spectra of the partially deuterated Cs2 CaCl4 ·2H2 O compounds recorded at LNT (Fig. 9) were also studied. By examining these spectra it can be noticed that on increased deuterium content the two bands at 585 and 557 cm−1 continuously decrease in intensity and the band at 495 cm−1 behaves similarly while simultaneously a new band appears at 475 cm−1 and gains intensity. In the spectra of the analogues with low deuterium content, new bands at 397 and 391 cm−1 are seen with intensity which at first increases and then decreases with increasing the deuterium
Fig. 7. Fourier transform infrared spectra of Cs2 CaCl2 ·2H2 O recorded at RT (lower curve) and at LNT (upper curve) in the region of the external modes of water molecules. LNT spectrum is offset with respect to the RT one.
content. Such a behavior suggests that the latter two bands are due to HDO librational modes. On the basis of their frequency isotopic ratio (1.23), it is obvious that the bands at 397 and 391 cm−1 are HDO analogues of the band at 495 cm−1 in the spectra of the protiated compound, whereas, the D2 O analogue of this band should be expected at around 367 cm−1 (the H2 O/D2 O frequency ratio is 495 cm−1 /367 cm−1 = 1.35). Furthermore, the band at 475 cm−1 can be the HDO analogue of that at 585 cm−1 (the isotopic frequency ratio is 1.23) or, at the same time, of the band at 557 cm−1 (frequency isotopic ratio = 1.17). If the band at 475 cm−1 were indeed the HDO analogue of both mentioned bands, it would mean that they are due to two strongly mixed libration modes (most probably, out-of-plane ones). In that case, the band at 495 cm−1 can be attributed to the rocking mode and according to the frequency isotopic ratios (H2 O/HDO and H2 O/D2 O) and the change in intensity of this band on increasing the deu-
Fig. 8. Raman spectrum of Cs2 CaCl2 ·2H2 O recorded at RT in the region of the external modes of water molecules.
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3.3. Second-order transitions involving external modes of the water molecules and the corresponding isotopomers
Fig. 9. Fourier transform infrared spectra of partially deuterated analogues of Cs2 CaCl2 ·2H2 O recorded at LNT (upper curve) in the region of the external modes of water molecules (the content of deuterium increases from bottom to top curve, offset spectra are presented).
Several not very strong bands around 2200 and 2140 cm−1 are found in the infrared spectra of Cs2 CaCl4 ·2H2 O recorded at RT and LNT (see Figs. 1 and 3), as in the spectra of Rb2 [MnCl4 (H2 O)2 ], Cs2 [MnCl4 (H2 O)2 ], and Rb2 [NiCl4 (H2 O)2 ] [3,7]. The appearance of such bands is common for different crystalline hydrates [28] and they are attributed to combinations of librations and/or translations with HOH bending modes. The bands found around 1300–1000 cm−1 in the lowtemperature infrared and room temperature Raman spectra of the protiated compound (see Figs. 1 and 2) can be attributed to secondorder transitions of the external modes of the water molecules. As mentioned before, the bands found around 3230 cm−1 in the vibrational spectra of the protiated compound can be attributed to the second-order HOH bending modes (the first overtones of the bending modes), and, accordingly, the bands at 2380 cm−1 in the spectra of the deuterated analogues (see Figs. 3 and 4) would be due to second-order DOD modes. In the infrared spectra of the partially deuterated analogues of this compound, as in the other three isostructural compounds [3,7], two very weak bands at 2869 and 2852 cm−1 and one at 1910 cm−1 are observed (see Fig. 3). With increasing the deuterium content in the sample, the intensity of these bands at first increases and then decreases so that they can definitely be attributed to second-order HOD and DOH bending modes, the asymmetric bands at 1910 cm−1 being probably due to combinations of librations and HOD and DOH bending modes. 3.4. Anharmonicity constants of the isotopically isolated isotopomers of water molecules
terium content such an assignment seems reasonable. The bands at 585 and 557 cm−1 in the infrared spectra of Cs2 CaCl4 ·2H2 O would be due to the out-of-plane twisting and wagging libration modes (pure or, more probably, mixed), respectively. The D2 O analogues of the two above-mentioned bands should be those at 433, 416 and 412 cm−1 observable in the spectra of the highly deuterated compound (see Fig. 9). It should be pointed out that in the Raman spectrum of Cs2 CaCl4 ·2H2 O (see Fig. 2), in the region of the water librations the strongest band is that at 486 cm−1 . This could mean that it is a result of a twisting not a wagging mode. Namely, it is assumed that in Raman spectra the band from the twisting mode would be with highest intensity [27]. However, since the symmetry of the water molecules is low, it is reasonable to assume that their librational modes are more or less mixed. In any case, it can be stated with a reasonable certainty that all librational modes are more or less mixed with each other and their exact assignment is still open. The band at 289 cm−1 in the Raman spectrum (see Fig. 8) can be attributed to the Ca–Cl stretching mode, as assigned by Adams and Lock [19] in the spectra of Rb2 [MnCl4 (H2 O)2 ] and Cs2 [MnCl4 (H2 O)2 ]. True, on the basis of the larger mass of Mn, one would expect the corresponding stretching band to be slightly redshifted in the manganese compound (with respect to the Ca one), at ≈280 cm−1 , assuming harmonic vibrations and identical form of the normal modes in both compounds. However, keeping in mind that Mn is a transition metal, it is expected that the bond order of the Mn–Cl bonds is somewhat larger than that of the Ca–Cl bonds, thus bringing a blue shift in the frequency upon substitution of Ca with Mn. The overall effect may be such (as a result of cancellation of both effects) that the band frequencies in both compounds are about the same. Additionally, because of the long Ca–O distance (236.7 pm), this band or part of it can be due to Ca–O stretching mode.
Vibrations, which include motions of hydrogen atoms, are strongly anharmonic even in free water molecules. The frequencies of the internal vibrational modes of the free water molecules are nearly 200 cm−1 (for the stretching ones) and 50 cm−1 (for the bending ones) lower than their corresponding harmonic values [20,22,29]. The mechanical anharmonicity of the stretching modes in comparison to the mechanical anharmonicity of the bending modes is far more expressed in the solid hydrates [20,30]. As is known [31], the observed frequencies (i ) of the stretching modes of the isotopically isolated HDO molecules are directly correlated to their harmonic frequencies ωe,i , where ωe xe is the anharmonicity constant. To calculate the anharmonicity constants of the stretching modes of the isotopically isolated HDO molecules, we have applied the equations of Huong et al. [31]. The calculated anharmonicity constants of the isotopically isolated isotopomers of water molecules of Cs2 CaCl4 ·2H2 O and those of the isostructural compounds MI 2 [MII Cl4 (H2 O)2 ] (MI = Rb, Cs; MII = Mn, Ni) [3,26] are given in Table 2. As can be seen in Table 2, the anharmonicity constants in a series of compounds do not increase evenly with the decrease of the frequencies of the OH and OD stretching modes of the isotopically isolated HDO molecules. This is not in accordance with the correlations found by Berglund et al. [30] but is not unusual for crystalline hydrates [32–34]. This experimental fact can be due to the change of the anharmonicity force constants (i.e. the cubic and quartic force constants in the vibrational potential energy expression) as a result of hydrogen bonding. These constants can also depend on the local crystal field which is different in the different compounds of this series in spite of them being isostructural. An alternative method for calculating the anharmonicity constants of O–H(D) stretching modes in hydrogen bonded systems was suggested by Sceats and Rice [35]. The main assumption in
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Table 2 Frequencies of the stretching and bending modes, and anharmonicity constants for the isotopically isolated isotopomers of water molecules in Rb2 [MnCl4 (H2 O)2 ], Cs2 [MnCl4 (H2 O)2 ], Rb2 [NiCl4 (H2 O)2 ] and Cs2 CaCl4 ·2H2 O. Type of vibration
(O–H1 )a (O–D1 ) (O–H2 ) (O–D2 ) ı(HOH) ı(HOD) ı(DOH) ı(DOD) a
Rb2 [MnCl4 (H2 O)2 ]
Cs2 [MnCl4 (H2 O)2 ]
Rb2 [NiCl4 (H2 O)2 ]
Cs2 CaCl4 ·2H2 O
(cm−1 )
(cm−1 )
(cm−1 )
(cm−1 )
3375 2501 3322 2465 1622 1433 1426 1191
ωe xe (cm−1 ) O–H
O–D
130.5
69.1
137.0
72.4
16.0 9.5 9.5 9.0
3412 2527 3316 2461 1620 1437 1421 1191
ωe xe (cm−1 ) O–H
O–D
129.0
68.5
139.0
73.3
15.0 10.5 10.0 9.0
3393 2519 3300 2457 1623 1435 1428 1194
ωe xe (cm−1 ) O–H
O–D
126.5
67.0
140.5
74.6
14.0 9.0 10.5 9.0
3398 2514 3345 2477 1633 1446 1437 1201
ωe xe (cm−1 ) O–H
O–D
121.6
64.2
125.3
65.9
18.0 11.5 11.0 12.0
H1 and D1 , proton and deuterium atoms in the HOD molecule; H2 and D2 , proton and deuterium atoms in the DOH molecule.
this model is that the diagonal anharmonicity terms in the expression of the vibrational potential energy (cubic and quartic force constants) are independent of the strength of the hydrogen bonds, whereas the non diagonal terms are insignificantly small. According to this model, obviously, the increase of the anharmonicity of the O–H(D) stretching modes is due to the higher amplitude of the O–H(D) stretching motion. The anharmonic constants calculated according to this model (see Table 3) are in an exceptional compliance with those in Table 2, which confirms the accuracy of Sceats and Rice model [35]. For the calculation of the anharmonicity constants of the bending modes of the isotopically isolated H2 O, HDO and D2 O molecules, the following equation was used [20,32]: xe ωe =
1 (2ω1 − ω2 ) 2
where the frequencies of the appropriate bending modes and their second-order transition modes were marked by ω1 and ω2 , respectively. From Table 2, it can be seen that the values of corresponding anharmonicity constants of the bending modes for the appropriate isotopomers of the water molecules are very similar in all 4 isostructural compounds and are similar to those found in other crystalline hydrates [36]. However, it should be pointed out that the anharmonicity constants for Cs2 CaCl4 ·2H2 O have higher values in comparison to Rb2 [MnCl4 (H2 O)2 ], Cs2 [MnCl4 (H2 O)2 ] and Rb2 [NiCl4 (H2 O)2 ]. Finally, based on the calculated anharmonicity constants, an attempt was made to assign the bands due to higherorder transitions (overtones and combinations in the appropriate spectral region) including O–H(D) stretching motions. Previously, single crystal polarized spectra of Rb2 [MnCl4 (H2 O)2 ] and Cs2 [MnCl4 (H2 O)2 ] in the near infrared region have been studied in detail by Walker and McCarthy [37]. The appropriate spectral region of the protiated compound of the title compound is shown in Fig. 10. The bands at 4822 and 4775 cm−1 are most likely due to 2(OD) transitions. The bands Table 3 Anharmonicity constants of the stretching O–H vibrations calculated according to Sceats and Rice [35] model for the whole series of compounds. −1
Compound
(cm
Cs2 CaCl4 ·2H2 O
3398 3345 3375 3322 3412 3316 3393 3300
Rb2 [MnCl4 (H2 O)2 ] Cs2 [MnCl4 (H2 O)2 ] Rb2 [NiCl4 (H2 O)2 ]
)
−1
ωe xe (cm 115.7 123.2 118.9 126.6 113.8 127.5 116.4 129.9
)
Fig. 10. Fourier transform infrared spectra of partially deuterated analogues of Cs2 CaCl2 ·2H2 O recorded at LNT in the region from 5100 to 4700 cm−1 (the content of deuterium increases from bottom to top curve, offset spectra are presented).
at 5022 and 4964 cm−1 , whose intensity decreases with increasing the deuterium content, can be assigned to combinations which include fundamental (HOH) vibrations. The 2(OH) transitions, i.e. the (OH) overtones, were not observed in the infrared spectra of the examined compound as a result of the low value of the dipole moment change of this transition. This result is analogous to the previous results obtained for the series of isomorphic metal saccharinate hexahydrates by Pejov et al. [38,39]. Acknowledgement The financial support of the Ministry of Education and Science of the Republic of Macedonia is gratefully acknowledged. References ˇ [1] V. Stefov, B. Soptrajanov, V. Petruˇsevski, Bull. Chem. Technol. Macedonia 7 (1989) 151. ˇ [2] V. Stefov, B. Soptrajanov, V. Petruˇsevski, Vestn. Slov. Kem. Drus. 37 (1990) 181. ˇ [3] V. Stefov, B. Soptrajanov, V. Petruˇsevski, Croat. Chem. Acta 65 (1992) 151. ˇ [4] V. Stefov, B. Soptrajanov, V. Petruˇsevski, J. Mol. Struct. 266 (1992) 211. ˇ [5] V. Stefov, B. Soptrajanov, V. Petruˇsevski, J. Mol. Struct. 267 (1992) 203.
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