Vibrational spectra of hexaaqua complexes V. The water bending bands in the infrared spectra of Tuton salts

Journal

of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 408/409 (1997) 283-286

Vibrational spectra of hexaaqua complexes V. The water bending bands in the infrared spectra of Tuton salts Bojan Soptrajanov *, Vladimir M. PetruSevski lnstirutzahem&,

PMF,

Univerzirer

“Sv. Kin’1 i Meiodij”,

PO Box 162, 91001 Skopje, Mucedonin

Received 26 August 1996; accepted 4 November 1996

Abstract Multiple overlapped bands forming a broad complex which extends over several hundreds of reciprocal centimeters appear in the water bending region of the spectra of Tuton salts whose general formula is M~M”(XY&6Hz0. The essential characteristics of the complex persist when one univalent cation is replaced by another, when the nature of the divalent cation is changed or when a sulfate salt is compared with an analogous selenate compound. This shows that the feature is related to water vibrations only and this is supported by its behaviour on deuteration. Since neither the number of components nor the spread of their frequencies can be attributed solely to the fundamental vibrations of different types of water molecules, anharmonic couplings of the 6(HOH) modes with second-order transitions must be responsible for the observed spectral picture. 0 1997 Elsevier Science B.V. Keywords:

Infrared spectra; Tuton salts; Hexaaqa complexes;

1. Introduction The Tuton salts are a large class of compounds (double salts) with the general formula M:M” (XY4)2.6Hz0 where M’ = K, NH4, Rb, Cs; M” = Mg, Ni, Co, Fe, Cu, Zn; X = S, Se, Be; Y = 0 when X = S or Se and F when X = Be. In some way they are related to the alums, another large group of double salts whose general formula is M’M”‘(XY4)2*12H20. Analogous univalent cations and identical anions can be found both in the Tuton salts and the alums so that there are sulfate, selenate and tetrafluoroberyllate Tuton salts as well as sulfate, selenate and tetrafluoroberyllate aiums. Another common feature of

* Corresponding author 0022.2860/97/$17.00 0 PI/ SOO22-2860(96)09660-3

Water bending vibrations

the two groups of compounds is the existence of hexaaquacations in their structure. For a more detailed discussion of the parallels (and differences) between these two classes of double salts, see Ref. [I] and the references given therein. Continuing our studies on hexaaqua complexes [25], we now report the partial results of our studies on the infrared spectra of the group of sulfate and selenate Tuton salts, i.e. of compounds with the formula M:M”(X04)2.6H20 (where X = S or Se). This time the emphasis is put on the water bending region which, as will be discussed, is probably the most interesting one from the spectroscopic point of view. In this respect, the present study is a direct continuation of the work devoted to the same spectral region in alums [5]. Despite numerous studies of the vibrational spectra of Tuton salts (see, for example, Refs. [6-IO]

1997 Elsevier Science B.V. All rights reserved

284

B. Soptrajanov, V.M. PerruSevski/Journal of Molecular

I 4000

I

I

3000

Structure

I

2000

4081409 (1997) 283-286

I

I600

I200

WAVENUMBER

/

800

400

cm-i

Fig. 1. RT and LNT infrared spectra of K$~I(SO~)~.~H~O.

and the earlier papers which have been cited there) this region has received practically no attention.

2. Experimental The Tuton salts were prepared by standard methods. Mixtures of aqueous solutions of the appropriate M:X04 and M”X04 compounds (the salts of the univalent and divalent cations being taken in an equimolar ratio) were left to slowly evaporate in air. The infrared spectra were recorded, at room temperature (RT) and at the boiling temperature of liquid nitrogen (LNT), on a Perkin-Elmer 580 spectrophotometer. The measurements at low temperatures were accomplished using an RIIC variable-temperature cell VLT-2.

given an interpretation, whereas the rest were disregarded completely, together with the unusual breadth (several hundreds of reciprocal centimeters) of the feature. Thus, Sekar et al. [9] attributed three of their infrared and Raman bands to the HOH bendings of the three existing types of water molecules and only listed (without explanation) a considerable number of Raman bands, some of which are reported as strong. The comparison of the IR spectra of various sulfate and selenate compounds shows that the feature as a whole is related only to water motions. Thus, irrespective of the nature of the metal ion (Fig. 3), M*+

3. Results and discussion The RT and LNT spectra of a representative member of the series, K2C~(S04)2*6H20 are shown in Fig. 1. The feature which is by far the most unusual in these spectra is the broad and irregularly structured complex of overlapped band centred at = 17001600 cm-‘. As seen in Fig. 1 and in Fig. 2, the structure of the complex is more clearly visible at low temperatures. In some of the previous work, only the maxima which were intuitively suitable candidates for assigning to the 6(HOH) vibrations were

1900

1700 WAVENUMBER

Fig. 2. Effect of temperature K2Mg(S0&6H20.

1500

1300

/cm“

in the 6(HOH) region of the spectra of

B. .%prrajanov,

of Molecular Structure 4081409 (1997) 283-286

V.M. PerruSevskiIJournal

285

\

/ 1900

,

I

I

1700

I

1

1500

WAVENUMBER

Fig. 3. The water bending region (S04)2,6Hz0 (a) and KzNi(S04)2.6H20

I

/

I cm-’

in the (b).

I

I

1900

1300

I

1700

I

/

1.500

I

1300

WAVENUMBER /cm-’

spectra

of Rb2Ni

cation (Fig. 4) or the anion (Fig. 5) the feature, with some minor variations, persists. Its behaviour on deuteration (Fig. 6) also leads to the conclusion that water modes are responsible for the appearance of the complex. An important point worth remembering is the above-mentioned finding of Sekar et al. [9] of multiple bands in the water bending region in the Raman spectra so that this must be a common characteristic of both types of vibrational spectra. Since the structural differences between the three types of water molecules are not such as to explain the number of the submaxima (which by far exceeds the

Fig. 5. The water bending region in the KzNi(Se04)2,6Hz0 (a) and KzNi(SOa)2.6H20 (b).

spectra

of

expectations) and their spread both in our spectra and in the reported [9] Raman ones, it must be concluded that one deals with a case of vibrational interactions of some kind between the fundamental 6(HOH) modes and overtones or combinations which originate from external modes of water molecules. The first candidates among the latter which come to mind are the Hz0 librations which appear between 950 and 700 cm-’ and can be easily recognized on the basis of their temperature sensitivity (on cooling they shift to higher frequencies and become more intense).

a i;

‘ =\ b \ b

I

1

1 WAVENUMBER

/ cm-’

Fig. 4. The water bending region in the K$o(SO,)~~6H~O (a) and K2Fe(S04)2.6H20 (b).

I

1600

1200

WAVENUMBER

spectra

of

Fig. 6. Spectra of partially KzMg(SeOJ2.6H20 (b).

deuterated

I

I cm -’

K2Mg(S04)2.6H20

(a) and

286

B. Soptrajanov, V.M. PetruLvski/Joumal

However, it is likely that the second-order transitions of some low-lying fundamentals (probably the ones which could be described as modes of the hydrogen bonds) are also involved in the interactions, perhaps via a mechanism which is similar (but can not be identical) to that described by Bertie and Falk [ 111. The spectral picture found in the water bending region of Tuton salts resembles that reported by us in alums [5]. Since the hydrogen bonds (at least some of them) in the alums are strong, whereas those formed by the water molecules in the investigated compounds must be judged as only mediumstrong, the above-mentioned finding shows that the strength of the hydrogen bonds is not the essential factor to be taken seriously into consideration as long as the H-bonds are not weak. There is no doubt that the essential precondition for the vibrational interactions of the described kind (and, for that matter, of any type) is the presence of vibrational anharmonicity. In all Tuton salts the water molecules are peculiarly bonded, with the metal cations being placed in the general direction where one of the oxygen orbitals occupied by the lone-pair electrons is expected. The resulting asymmetry of the force field around the Hz0 molecules certainly makes the water vibrations (especially the bending ones)

of Molecular Structure 408/409 (1997) 283-286

anharmonic, although this may not be the only factor causing the anharmonicity of the 6(HOH) fundamental. Thus, the preconditions for the appearance of the peculiar bands studied here are fulfilled.

References [l] V.M. PetruSevski, Acta Crystallogr., B50 (1994) 625. [2] V. PetruSevski and B. Soptrajanov, J. Mol. Struct., 219 (1990) 67. [3] V. Stefov, B. Soptrajanov and V. PetruSevski, J. Mol. Struct., 267 (1992) 203. [4] V. Stefov, V.M. PetruSevski and B. Soptrajanov, J. Mol. Struct., 293 (1993) 97. [5] B. Soptrajanov, V.M. Petrusevski, J. Mol. Struct., 293 (1993) 101. [6] LA. Oxton and 0. Knop, J. Mol. Struct., 49 (1978) 309. [7] S.P. Gupta, B. Singh and B.N. Khanna, J. Mol. Struct., 112 (1984) 41. [8] G. Sekar, V. Ramakrishnan and G. Aruldhas, J. Solid State Chem., 66 (1987) 235. [9] Cl. Sekar, V. Ramakrishnan and G. Aruldhas, J. Solid State Chem., 74 (1988)424. [lo] B. Beagley, A. Eriksson, J. Lindgren, I. Persson, L.G.M. Pettersson, M. SandstrSm, U. Wahlgren and E.W. White, J. Phys.: Condens. Matter, I (1989) 2395. [ll] J.E. Bertie and M.V. Falk, Can. J. Chem., 51 (1973) 1713.