INFRARED SPECTROSCOPIC STUDY OF TUTTON COMPOUNDS. I ...

M.ofGeorgiev, D.Technology Marinova, D. Journal of the University Chemical andStoilova Metallurgy, 45, 1, 2010, 75-82

INFRARED SPECTROSCOPIC STUDY OF TUTTON COMPOUNDS. I. VIBRATIONAL BEHAVIOR OF SO42- IONS INCLUDED IN Me2’Me’’(SeO4)2·6H2O (Me’= K, NH4+; Me’’= Mg, Co, Ni, Cu, Zn) M. Georgiev1, D. Marinova2, D. Stoilova2 1

University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected]

Received 05 December 2009 Accepted 15 February 2010

2

Institute of General and Inorganic Chemistry Bulgarian Academy of Sciences Akad. G. Bonchev str., bl.11, 1113, Sofia, Bulgaria E-mail: [email protected]

ABSTRACT Infrared spectra of the Tutton compounds Me2’ Me’’(XO4)2·6H2O (Me’ = K, NH4+; Me’’ = Mg, Co, Ni, Cu, Zn; X = S, Se) as well as those of SO42- ions included in the selenate matrices are presented and discussed in the region of the stretching modes ν3 and ν1. The SO42- ions matrix-isolated in the selenate matrices (approximately 2 mol %) exhibit three bands for ν3 and one band of small intensity for ν1 in good agreement with the low site symmetry C1 of the SeO42- host anions. The extent of energetic distortion of the isomorphously included ions as deduced from the values of ∆ν3 (sitegroup splitting for the SO42- guest ions) and ∆νmax (the difference between the highest and the lowest components of the stretching modes for the sulfate ions) is commented. The spectroscopic experiments show that the degree of energetic distortion of the guest ions is not affected by the guest ion concentrations up to 15-20 mol %. It has been established that the SO42- guest ions are weaker distorted in the potassium selenates as compared to the same ions in the neat potassium sulfates due to the larger unit-cell volumes of the selenate compounds, i.e. to the smaller repulsion potential of the host lattices. However, the SO42- guest ions included in the ammonium selenates display larger values of ∆ν3 and ∆νmax than those in the potassium selenates owing to the formation of hydrogen bonds between the SO42- guest ions and NH4+ host ions, thus leading to the increase in the electrostatic field strength at the lattice sites where the guest ions are located. Keywords: Tutton compounds, crystal matrix-infrared spectroscopy, matrix-isolated SO42- ions, extent of energetic distortion of the guest ions.

INTRODUCTION Recently, our scientific interest and efforts have been concentrated on Tutton salts, since these compounds can be considered as potential proton conductors due to the existence of comparatively strong hydrogen bonds determined by the strong proton acceptor capabilities of the sulfate and selenate ions. For example, in our previous papers infrared spectra of NH4+ and

SO42- ions included in the structures of copper and magnesium Tutton salts are presented and discussed [1,2]. The compounds under study belong to a large number of isomorphous compounds with a general formula Me2’Me’’(XO4)2.6H2O (Me’ = K, NH4+, Rb, Cs; Me’’ = Mg, Mn, Co, Ni, Cu, Zn; X = S, Se) known as Tutton salts. They crystallize in the monoclinic space group P21/c (C2h5) with two formula units in the unitcell. The crystal structures of these compounds are built

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Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010

up from isolated octahedra, [Me’’(H2O)6] (three crystallographically different water molecules are coordinated to the Me’’ cations) and tetrahedra XO4. The polyhedra are linked by hydrogen bonds and Me’ cations. All atoms and polyatomic units, except the divalent metal ions Me’’, which lies at the centre of inversion Ci, are located at general positions C1 [3-19]. As an example the crystal structures of K 2 Mg(SO 4 ) 2 .6H 2 O and (NH4)2Mg(SO4)2.6H2O are shown in Fig. 1. Numerous papers are also devoted to vibrational spectra (infrared and Raman) of Tutton salts (see, for example, Refs. [2023] and Refs. therein). Continuing our study on the vibrational behavior of matrix-isolated ions in different matrices, in this paper we report infrared spectra of SO42- ions included in the structures of potassium and ammonium selenates in the region of ν3 and ν1.The spectral regions of ν4 and ν2 of the SO42- guest ions could not be analyzed precisely due to the overlapping of these motions with motions of other entities in the structures (water librations). The method of crystal matrix-spectroscopy provides important information about both the electrostatic field strength at the lattice sites where the guest ions are located as deduced by their extent of distortion and the chemical nature of the ligand environment in the lattice. When polyatomic ions are doped in host lattices at low concentrations (up to 7-10 mol %) the correlation field splitting, the dispersion of phonon curves (due to the interactions between identical oscillators) and LO/TO splitting effects (due to the long-range forces of electrostatic character) are neglected. Thus, the number of the normal vibrations of the matrix-isolated ions is essentially determined by the site symmetry, which is assumed to be the same as that of the respective host ions (substitutional mixed crystals). The XOmn- guest ions incorporated in host lattices undergo structural distortions involving changes in both the X − O bond lengths and the O − X − O bond angles as compared to the neat compounds. The distortion of the guest ions as established from spectroscopic experiments (infrared and Raman) is called energetic distortion in order to distinguish it from that revealed from structural data (geometric distortion) [24, 25]. Both the site group splitting of the asymmetric modes ∆νas and ∆νmax (the differences between the highest and the lowest wavenumbered components of the stretching and bending modes, re-

76

spectively, [24-28]) are used as an adequate measure for the degree of energetic distortion of matrix-isolated polyatomic ions. EXPERIMENTAL The simple metal selenates, K2SeO4, (NH4)2SeO4 and Me’’SeO4.nH2O (Me’’ = Mg, Co, Ni, Cu, Zn) were prepared by neutralization of the respective carbonates and hydroxide carbonates with dilute selenic acid solutions at 60-70°C, and cooled to room temperature. The crystals were filtered, washed with alcohol and dried in air. Commercial sulfates were used. The reagents used were ‘p.a.’ (Merck). The Tutton salts were obtained by crystallization from the saturated solutions of the respective simple sulfates and selenates. The samples containing included SO42- ions were prepared using the same crystallization procedure in the presence of the guest ions. The infrared spectra were recorded on a Bruker model IFS 25 Fourier transform interferometers (resolution < 2 cm-1) at ambient temperature using KBr discs as matrices. Ion exchange or other reactions with KBr have not been observed (infrared spectra using Nujol mulls were also measured). RESULTS AND DISCUSSION The free tetrahedral ions, SO42-, and SeO42, under perfect Td symmetry exhibit four internal vibrations: ν1(A1), the symmetric X − O stretching modes, ν2(E), the symmetric XO4 bending modes, ν3(F2) and ν4(F2), the asymmetric X − O stretching and XO4 bending modes, respectively. The normal vibrations of the free tetrahedral ions in aqueous solutions are reported to appear at: for the SeO42- ions: ν1 = 833 cm-1, ν2 = 335 cm-1, ν3 = 875 cm-1 and ν4 = 432 cm-1; for the SO42- ions: ν1 = 983 cm-1,ν2 = 450 cm-1, ν3 = 1105 cm-1 and ν4 = 611 cm-1 [29]. On going into solid state the normal modes are expected to shift to higher or lower frequencies due to different intraand intermolecular interactions. The low site symmetry C1 of the tetrahedral ions will cause a removal of the degeneracy of both the doubly degenerate ν2 modes and the triply degenerate ν3 and ν4 modes, thus resulting in the appearance of two

M. Georgiev, D. Marinova, D. Stoilova

Table 1. Some structural and spectroscopic characteristics for the XO42- ions in the neat Tutton salts (V/n, unit-cell volumes divided by the numbers of the XO42- ions; X − O, mean values of the X − O bond lengths; ∆r(XO4), the difference between the longest and the shortest X − O bond lengths in the respective tetrahedra; ∆νmax, the difference between the highest and the lowest wavenumbered components of the stretches of the XO42- ions; the structural data are taken from Refs. [3-19]).

ν−ν33

Å

∆r(XO4) Å

ν3 cm-1

164

1.474

0.065

1147, 1108, 1098

cm 1115

(NH4)2Mg(SO4)2⋅6H2O

174

1.473

0.021

1147, 1108, 1098

K2Mg(SeO4)2⋅6H2O

174

1.634

0.018

(NH4)2Mg(SeO4)2⋅6H2O

183

1.635

K2Co(SO4)2⋅6H2O

165

(NH4)2Co(SO4)2⋅6H2O

ν1 cm-1

∆ν3 cm-1

984

49

163

1118

984

49

163

899, 877

888

835

22

64

0.016

901, 875

888

835

26

66

1.474

0.020

1144, 1100

1122

984

44

160

175

1.510

0.065

1146, 1102

1124

982

44

164

K2Co(SeO4)2⋅6H2O

174

1.633

0.021

899, 877

888

831

22

68

(NH4)2Co(SeO4)2⋅6H2O

182

1.638

0.017

899, 873

886

833

26

66

K2Ni(SO4)2⋅6H2O

162

1.473

0.016

1144, 1111, 1108

1119

982

44

164

(NH4)2Ni(SO4)2⋅6H2O

171

1.476

0.020

1144, 1108, 1100

1117

982

44

164

K2Ni(SeO4)2⋅6H2O

172

1.633

0.015

896, 880

888

830

16

66

(NH4)2Ni(SeO4)2⋅6H2O

180

1.637

0.021

899, 873

886

831

26

68

K2Cu(SO4)2⋅6H2O

164

1.471

0.024

1144, 1102, 1097

1114

984

47

160

(NH4)2Cu(SO4)2⋅6H2O

173

1.473

0.022

1144, 1102, 1095

1114

981

49

163

K2Cu(SeO4)2⋅6H2O

173

1.628

0.013

894, 881sh

888

837

13

57

(NH4)2Cu(SeO4)2⋅6H2O

181

1.628

0.030

893, 882sh, 877

884

835

16

58

K2Zn(SO 4)2⋅6H2O

164

1.470

0.018

1141, 1108, 1102

1117

982

39

159

(NH4)2Zn(SO 4)2⋅6H2O

173

1.474

0.021

1144, 1105

1124

984

39

160

K2Zn(SeO 4)2⋅6H2O

174

1.633

0.020

896, 879

888

833

17

63

(NH4)2Zn(SeO 4)2⋅6H2O

182

1.639

0.017

898, 875

887

833

23

65

V/n Å3

X

K2Mg(SO4)2⋅6H2O

Compounds

O

bands for ν2 (2A) and three bands for ν3 and ν4 (3A), respectively. The ν1 mode is activated. The factor group analysis (C2h factor group symmetry) predicts a splitting of each species of A symmetry into four components – Au + Ag + Bu + Bg (related to interactions of identical oscillators, correlation field effects). The species Au and Bu are infrared active, while Ag and Bg are

-1

∆νmax cm-1

Raman active (mutual exclusion principle). Consequently, 18 infrared bands (9Au + 9Bu) and 18 Raman bands (9Ag + 9Bg) will correspond to the normal motions of the tetrahedral ions. The correlation diagram between Td point group, site symmetry of the XO42- ions and factor group symmetry is shown in Fig. 2.

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Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010

Table 2. Some spectroscopic characteristics of the SO42-guest ions matrix-isolated in selenate matrices (for the assignments see Table 1). Host compounds

SO42- guest ions (approximately 2 mol %) ν3 cm -1

ν-13

ν1 cm -1

∆ν3 cm -1

∆νmax cm -1

980

34

154

K2Mg(SeO4)2⋅6H2O

1134, 1119, 1100

cm 1118

(NH4)2Mg(SeO4)2⋅6H2O

1134, 1097, 1089,

1107

980

45

154

K2Co(SeO4)2⋅6H2O

1133, 1121, 1097

1117

982

36

151

(NH4)2Co(SeO4)2⋅6H2O

1133, 1117, 1095

1115

981

38

152

K2Ni(SeO4)2⋅6H2O

1132, 1113, 1102

1116

982

30

150

(NH4)2Ni(SeO4)2⋅6H2O

1137, 1119, 1086

1114

979

51

158

K2Cu(SeO4)2⋅6H2O

1130, 1198, 1098

1112

981

32

149

(NH4)2Cu(SeO4)2⋅6H2O

1132, 1092, 1079

1101

981

53

151

K2Zn(SeO 4)2⋅6H2O

1133, 1119, 1100

1117

982

33

151

(NH4)2Zn(SeO4)2⋅6H2O

1135, 1120, 1086

1114

982

49

153

K2Mg(SO4)2.6H2O

(NH4)2Mg(SO4)2.6H2O

Fig. 1. Crystal structures of K2Mg(SO4)2·6H2O and (NH4)2Mg(SO4)2·6H2O. Infrared spectra of neat Tutton salts as well as those of mixed crystals K 2Me’’(SeO 4) 2-x(SO 4) x.6H 2O and (NH4)2Me’’(SeO4)2-x(SO4)x.6H2O are presented in Figs. 3 and 4. Some structural and spectroscopic parameters for the neat compounds are summarized in Table 1.

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The double sulfates and selenates, respectively, exhibit similar spectra due to the isostructureness of the salts. The three site symmetry components of ν3 of the sulfate ions appear in the region of 1150-1095 cm-1. The ν1 modes are observed at close values - in the region of

M. Georgiev, D. Marinova, D. Stoilova

Fig. 2. Correlation diagram between Td point symmetry, C1 site symmetry and C2h factor group symmetry (SO42- and SeO42- ions in Tutton compounds). 984-982 cm-1. In the case of the cobalt Tutton salts and (NH4)2Zn(SO4)2·6H2O the three expected site symmetry components of ν3 of the SO42- ions coalesce into two bands (see Table 1 and Figs. 3 and 4). The selenate ions in all neat selenates show two infrared bands in the region of 900-870 cm-1 instead of three bands expected corresponding to ν3 and one band corresponding to ν1

(837-830 cm-1). The extent of energetic distortion of the SO42- ions in the double sulfates as deduced from the values of ∆ν3 and ∆max is similar - the values of ∆ν3 vary from 39 cm-1 for the potassium zinc compounds to 49 cm-1 for the magnesium compounds and the values of ∆max - from 159 cm-1 for the zinc compounds to 164 cm-1 for the nickel compounds. The SeO42- ions in the Tutton selenates exhibit values of ∆ν3 and ∆max in the spectral regions of 26-13 cm-1 and 68-63 cm-1, respectively, i.e. these ions are remarkably weaker distorted as compared to the SO42- ions in the Tutton sulfates. The small values of ∆ν3 are in agreement with the similar geometric distortion ∆r of the sulfate and selenate ions (∆r is calculated as a difference between the longest and the shortest X − O bond lengths, see Table 1). The larger extent of energetic distortion of the SO42- ions as compared to that of the SeO42- ions could be explained with the smaller unit-cell volumes of the sulfate compounds (larger repulsion potential of the sulfate lattices). When the respective potassium and ammonium compounds are compared it is seen that the SO42- and SeO42- ions exhibit a close degree of energetic distortion. The smaller unit-cell volumes of the potassium salts are ex-

Fig. 3. Infrared spectra of the neat Tutton compouns and mixed crystals K2Me’’(SeO4)2-x(SO4)x·6H2O (Me’’ = Mg, Co, Ni, Cu, Zn).

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Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010

Fig. 4. Infrared spectra (NH4)2Me’’(SeO4)2-x(SO4)x·6H2O (Me’’= Mg, Co, Ni, Cu, Zn). pected to cause stronger distortions of the tetrahedral ions. On the other hand, the SO42- and SeO42- ions in the ammonium compounds are involved in hydrogen bonds with NH4+ ions additionally to those with water molecules - this fact is expected to facilitate the distortion of these ions. Thus, we believe that as a result of this competitive effect the SO42- and SeO42- ions in the potassium and ammonium compounds exhibit similar values of ∆ν3 and ∆νmax. The asymmetric bending modes ν4 in the neat sulfate compounds appear in the interval of 635-610 cm-1 (two bands corresponding to ν4 are observed in the spectra instead of three expected). The values of ∆ν4 vary from 10 to 20 cm-1, thus indicating that the sulfate tetrahedra are weaker distorted with respect to the O − S − O bond angles as compared to the S − O bond lengths. The bands in the region below 800 cm-1 are attributed to water librations and will be commented in a forthcoming paper. Figs. 3 and 4 show that the matrix-isolated SO42ions (approximately 2 mol % guest ions) display three infrared bands corresponding to ν3 in perfect agreement with the low site symmetry of the SeO42- host ions (see also Table 2). Bands of small intensity around 980 cm-1 appear in the spectra of the selenate samples containing about 15-20 mol % sulfate ions and they are assigned to

80

ν1 of the guest sulfate ions (in the case of the cobalt, nickel and zinc selenate matrices containing sulfate ions spectra are recorded also at higher concentrations of the samples in KBr in order to distinguish the ν1 mode, the spectra are not shown). When the sulfate ion concentrations increase more than about 5-10 mol % the three site symmetry components of ν3 coalesce into two bands of similar intensity. However, the increase in the guest ion concentrations causes no changes in the extent of energetic distortion of the guest ions as deduced from the values of ∆ν3 and ∆νmax. When the larger SeO42- ions in the selenate matrices are replaced by the smaller SO42ions the mean values of the asymmetric stretching modes ν 3 of the included SO42- ions are slightly shifted to lower

frequencies as compared to those of the same ions in the neat sulfate compounds due to the smaller repulsion potential of the matrices. When the degree of energetic distortion of the sulfate ions in the neat potassium sulfates and that of the same ions included in the potassium selenate matrices is compared it is readily seen that the values of ∆ν3 and ∆max for the matrix-isolated ions are smaller as compared to those in the neat potassium sulfates (compare Tables 1 and 2). These observations are owing to the larger unit-cell volumes of the

M. Georgiev, D. Marinova, D. Stoilova

selenate matrices. However, the spectroscopic experiments reveal that the electrostatic field around the SO42guest ions in the ammonium matrices is stronger as compared to that in the respective potassium selenates, i.e. larger values of ∆ν3 and ∆max. The formation of hydrogen bonds between the SO42- guest ions and both the water molecules of the host compound and the NH4+ host ions obviously increases the field strength at the lattice sites where the guest ions are situated and as a result the extent of energetic distortion of the guest ions increases (for example, ∆ν3 of the sulfate guest ions have values of 30 and 51 cm-1 in the nickel potassium and ammonium compounds, and 33 and 49 cm-1 in the zinc potassium and ammonium compounds, respectively). CONCLUSIONS The analysis of the infrared spectra reveals: (i) The energetic distortion of the SO42- ions in the neat sulfates is remarkably stronger than that of the SeO42ions in the respective neat selenates owing to the smaller unit-cell volumes of the sulfates, i.e. to the larger repulsion potential of the sulfate lattices. (ii) The SO42- ions matrix-isolated in the selenate matrices (approximately 2 mol %) exhibit three infrared bands corresponding to site symmetry components of ν3 in good agreement with the low site symmetry of the SeO42- host ions. The ν1 modes are observed at concentrations of the sulfate ions larger than approximately 10 mol %. (iii) The SO42- ions included in the potassium selenates are weaker distorted than the same ions in the neat potassium sulfates as revealed from the values of ∆ν3 and ∆νmax due to the larger unit-cell volumes of the selenate matrices. (iv) The formation of hydrogen bonds between the SO42- guest ions and the NH4+ host ions increases the electrostatic field strength around the guest ions, thus leading to the larger values of ∆ν3 of the sulfate guest ions in the ammonium selenates as compared to those in the potassium ones. REFERENCES 1. D. Marinova, M. Georgiev, D. Stoilova, J. Mol. Struct., ' ', 2009, 67-72. 2. D. Marinova, M. Georgiev, D. Stoilova, J. Mol. Struct., '!&, 2009, 179-184.

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