Study on the Hydrates of Beryllium Sulfate and Selenate: Thermal Analysis, X-Ray Diffraction and Infrared Spectroscopy

Home , Beryllium sulfate

Journal of the UniversityM. Georgiev, of Chemical V. Karadjova, Technology D. Marinova, and Metallurgy, D. Stoilova 43, 1, 2008, 139-148

STUDY ON THE HYDRATES OF BERYLLIUM SULFATE AND SELENATE: THERMAL ANALYSIS, X-RAY DIFFRACTION AND INFRARED SPECTROSCOPY

M. Georgiev1, V. Karadjova1, D. Marinova2, D. Stoilova2

1 University of Chemical Technology and Metalurgy Received 05 October 2007 8 Kl. Ohridski, 1756 Sofia, Bulgaria Accepted 23 January 2008 E-mail:[email protected] 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 Some properties of different hydrates of beryllium selenate and sulfate as determined by means of TG, DTA, DSC, infrared spectroscopy and X-ray diffraction are reported. The crystal and molecular structure of BeSeO .4H O is solved 4 2 by means of single crystal X-ray diffraction. The lattice parameters of BeSeO .2H O, BeSeO and BeSO .2H O are calcu- 4 2 4 4 2 lated on the basis of X-ray powder diffraction data. Infrared spectra of beryllium compounds (hydrates and anhydrous salts) are discussed with respect to the hydrogen bond strength, the normal vibrations of the SO 2- and SeO 2- ions as well 4 4 as to the [BeO ] skeleton modes. The formation of strong hydrogen bonds in the beryllium salts under study is caused by 4 the large ionic potential of the Be2+ ions, i.e. by the strong synergetic effect. The DSC measurements evidence that the value of the enthalpy of dehydration (∆H ) of BeSeO .4H O is larger as compared to that of BeSO .4H O (234.0 kJ mol- deh 4 2 4 2 1 for the former compound and 171.9 kJ mol-1 for the latter one) due to the stronger hydrogen bonds in the selenate compound than those in the respective sulfate. The solubility diagram of the BeSeO - H SeO - H O system at 25 °C is 4 2 4 2 presented as well. Keywords: Hydrates of beryllium sulfate and selenate; crystal structure of BeSeO .4H O; lattice parameters of 4 2 BeSeO .2H O, BeSeO and BeSO .2H O; infrared spectroscopy; matrix isolated HDO; hydrogen bond strength; enthalpy 4 2 4 4 2 of dehydration.

INTRODUCTION and very strong hydrogen bonds formed in hydrated beryllium salts (short O ···O bond distances and con- w The chemistry of the beryllium salts is not well siderable red-shifts of the OD stretches of matrix-iso- studied, irrespective of the promising physical proper- lated HDO molecules), even if the hydrogen bond ac- ties of the beryllium compounds. This is due probably ceptor strengths of the corresponding oxygen acceptors to the great latent toxicity, which the beryllium ions are small [1-4]. exhibit to biological systems. The existence of several crystal hydrates of be- The small beryllium ions are known to display ryllium sulfate is discussed in the literature [5-7]. The strong Be-OH interactions (synergetic effect), i.e. a tetrahydrate and dihydrate are found to crystallize in 2 strong increase of the hydrogen bond strength of water the three component BeSO - H SO - H O system stud- 4 2 4 2 molecules coordinated to beryllium ions due to the large ied in wide temperature and concentration intervals (up ionic potential of Be2+. This is revealed by the strong to 95° C and 80 mass % H SO ). X-Ray powder diffrac- 2 4

139 Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008 tion data (d-values) are presented for BeSO .2H O and ces. Ion exchange or other reactions with KBr have not 4 2 BeSO but lattice parameters have not been calculated been observed (infrared spectra using Nujol mulls were 4 [5]. The literature data for the existence of BeSO .H O also measured). The X-ray powder diffraction analysis 4 2 are contradictory [5-7]. For example, Bear and Turnbull was carried out with a DRON-3 diffractometer using [6] reported d- values of the monohydrate. The multi- Cu Kα radiation at a scanning speed of 1°min-1. The stage thermal dehydration of BeSO .4H O as investi- lattice parameters were calculated using the programs 4 2 gated by simultaneous thermal analysis (TG, DTA, DTG) ITO and LSUCR. The thermal dehydration processes is described in [7]. The available literature data on be- were studied using a derivatograph Paulik-Paulik-Erdey ryllium selenate hydrates are scanty. Selivanova et al. MOM OD-102 at a heating rate of 10°C min-1 (sample [8] reported powder diffraction patterns (d-values) of mass 200 mg). The DSC measurements were recorded BeSeO .4H O, BeSeO .2H O and BeSeO . on a Perkin-Elmer DSC7 instrument in argon atmo- 4 2 4 2 4 In the present paper we report our studies on the sphere up to 400 °C at a heating rate of 5°C min-1 using hydrates of beryllium sulfate and selenate by means of standard Al-pans (sample mass 15 mg). Temperature, thermal analysis (TG, DTA and DSC measurements), heat and sensitivity were carefully calibrated before ex- single crystal and powder X-ray diffraction and vibra- periments using indium (purity > 99.9 %) as a standard tional spectroscopy. substance. The experimental error for the enthalpy of dehydration (∆H ) was about 2-2.5 %. deh EXPERIMENTAL RESULTS AND DISCUSSION BeSeO .4H O was prepared by neutralization of 4 2 beryllium oxide with dilute selenic acid solutions at 50- Crystal structures 60°C. Then the solutions were filtered, concentrated at 60-70°C, and cooled to room temperature. The crystals According to [9] BeSO ·4H O crystallizes in the 4 2 were filtered, washed with alcohol and dried in air. tetragonal space group I-4c2 (a = 8.012, c = 10.712(1) BeSeO .2H O was prepared by dehydration over P O , V = 687.6 3, Z = 4). The lattice is built up from 4 2 2 5 Å Å for two weeks. Commercial BeSO .4H O was used. Be(H O) and SO tetrahedra which possess slight angu- 4 2 2 4 4 BeSO .2H O was obtained according to the solubility in lar distortion from T symmetry. The water molecule 4 2 d the three-component BeSO - H SO - H O system at (one crystallographical type) is in C site symmetry, thus 4 2 4 2 1 25 °C [5]. Anhydrous compounds, BeSeO and BeSO forming two strong hydrogen bonds O ···O of 2.624 and 4 4, w were prepared from the corresponding tetrahydrates by 2.690 Å. heating at 300 and 400°C, respectively. The reagents The crystal structure of BeSeO ·4H O as deter- 4 2 used were p.a. quality (Merck). mined by single crystal X-ray diffraction method is dis- The solubility in the three-component BeSeO - cussed in our previous paper [10]. It crystallizes in the 4 H SeO - H O system at 25 °C was studied by the method orthorhombic space group Cmca (a = 11.920(1), b = 2 4 2 of isothermal decrease of supersaturation. Preliminary 11.449(1), c = 10.764(1) Å, V = 1468.8 Å3, Z = 2). The experiments show that the equilibrium between the liq- crystal structure is composed of isolated Be(H O) and 2 4 uid and solid phases was reached in two days. The com- SeO tetrahedra which are interconnected by strong hy- 4 positions of the liquid phases and the wet solid phases drogen bonds with O ···O lengths between 2.619 and w were determined as follows: the selenate ions were de- 2.661 Å. Three crystallographically different water mol- termined gravimetrically after precipitation as PbSeO ecules exist in the lattice two molecules in C symme- 4 s and the concentration of the selenic acid was determined try and one molecule in C site symmetry. The struc- 1 by neutralization with NaOH. The concentration of the ture is closely related with that of acentric tetragonal Be2+ ions was calculated by difference. BeSO ·4H O but differs in pronounced polyhedral ro- 4 2 The infrared spectra were recorded on the Bruker tations and a partial rearrangement of the hydrogen model IFS 25 Fourier transform interferometers (reso- bonding scheme. The crystal structures of BeSO ·4H O 4 2 lution < 2 cm-1) at ambient using KBr discs as matri- and BeSeO ·4H O are shown in Fig. 1. 4 2

140 M. Georgiev, V. Karadjova, D. Marinova, D. Stoilova

Fig. 1. Projections of the crystal structures of BeSeO4.4H2O and BeSO4.4H2O along the a1-axis.

Dehydration processes of beryllium selenate and sulfate BeSO .4H O BeSO .3H O BeSO .2H O 4 2 → 4 2 → 4 2 → BeSO .H O BeSO 4 2 → 4 DTA, DTG, TG and DSC curves of BeSO ·4H O The thermal dehydration of BeSeO .4H O in air 4 2 4 2 and BeSeO ·4H O are presented in Fig. 2 and 3. Ac- is discussed in details in our previous paper [11]. Based 4 2 cording to DTG and TG curves the thermal dehydra- on DTA, DTG and DSC measurements the following tion of BeSO ·4H O begins at about 50 °C and com- scheme is proposed: 4 2 pletes at about 270 °C, thus forming an anhydrous beryllium sulfate. The latter is stable in a comparatively wide temperature interval of 270 550 °C. Four endoeffects centered at 115, 148, 225 and 265 °C on the DTG curve reflect the stepwise separation of the water molecules (Fig. 2 b). The mass loss calcula- tions show that the two endoeffects at 115 and 148 °C correspond to the separation of two water molecules and those at 225 and 265 °C to the last two molecules. No intervals of stability of intermediate hydrates are observed on the TG curve, thus indicating that the dehydration of BeSO ·4H O occurs si- 4 2 multaneously under the conditions of DTA and TG measurements due to both the heating rate and the sample mass. The stepwise dehydration of BeSO ·4H O is confirmed by the DSC curve (peaks 4 2 at 110.2, 116.03, 219.3 and 235.0 °C, see Fig. 3b). However, in the case of DSC measurements an interval of stability of BeSO ·2H O is well-distinguished - 4 2 135- 166 °C. On the basis of DTG curve the following scheme of dehydration could be proposed: Fig. 2. DTA, DTG and TG curves of: a, BeSeO4.4H2O; b,

BeSO4.4H2O.

141 Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008

BeSO .2H O isolated from the three component 4 2 BeSO - H SO - H O system at 25 °C crystallizes in the 4 2 4 2 orthorhombic system (a = 5.752(1) Å; b = 9.605(2) Å; c = 4.520(1) Å; V = 249.7(6) Å3). hkl- and d-values are summarized in Table 1. BeSeO .2H O is isostructural 4 2 with the respective sulfate and crystallizes in the orthorhombic system (a = 5.843(2) Å; b = 9.790(3) Å; c = 4.692(1) ; V = 268.4 3) and BeSeO in the tetrago- Å Å 4 nal system (a = 4.648(1) Å; c = 7.084(3) Å; V = 153.1(1) Å3) [11]. Unfortunately, our attempts to obtain stable phases monohydrate and semi hydrate by heating of the tetrahydrate in air were unsuccessfully. Thus, our efforts were concentrated on the study of dehydration processes in the selenic acid solutions. No literature data are available in the literature about the solubility in the BeSeO - H SeO - H O system. 4 2 4 2 The solubility diagram of the above system at 25 °C is shown in Fig. 4 (the respective experimental data are Fig. 3. DSC curves of: a, BeSeO .4H O; b, BeSO .4H O. summarized in Table 2). It is seen that BeSeO ·4H O 4 2 4 2 4 2 crystallizes up to concentration of 71.94 mass % H SeO , 2 4 BeSeO .4H O BeSeO .2H O BeSeO .H O i.e. no dehydration processes occur. The comparison of 4 2 → 4 2 → 4 2 → BeSeO .0.5H O BeSeO the solubility of beryllium sulfate and beryllium sel- 4 2 → 4 The DSC measurements show that ∆H of the enate in the respective acids shows that the dissolving deh dehydration process BeSeO ·4H O BeSeO + 4H O effect of the selenic acid on the beryllium selenate is 4 2 → 4 2 has value of 234.0 kJ mol-1, while ∆H of the respec- much greater than that of the sulfuric acid on the beryl- deh tive process for the BeSO ·4H O has value of 171.9 kJ lium sulfate (about 11 mass % of beryllium selenate 4 2 mol-1. The larger value of ∆H in the case of the sel- and about 0.64 mass % of beryllium sulfate at about deh enate as compared to that in the case of the respective 72-74 mass % of the acids [5]). The higher solubility of sulfate is due to the formation of stronger hydrogen the beryllium selenate is probably related to the strong bonds in the former compound. The formation of stron- interactions between the entities in the solutions ger hydrogen bonds in the beryllium selenate is owing [Be(H O) ]2+ and SeO 2-. Both the strong synergetic ef- 2 4 4 to the stronger proton acceptor capability of the SeO 2- fect of the Be2+ ions and the strong hydrogen bond ac- 4 ions than that of the SO 2- ions [4, 12-14]. ceptor strength of the SeO 2- ions result in the forma- 4 4

Table 1. hkl, d-values ( ) and relative intensities for BeSO ·2H O. Å 4 2

d obs hkl I/Io d obs hkl I/Io 4.939 110 47 2.3784 131 6 4.806 020 40 2.3520 211 < 5 4.090 011 12 2.1666 221 10 3.5560 101 100 2.1204 041 < 5 3.3331 111 7 2.0543 112 < 5 3.2926 021 < 5 1.9323 231 6 2.8762 200 8 1.8429 240 < 5 2.8568 121 9 1.7765 202 < 5 2.7965 130 10 1.7602 301, 132 < 5 2.6133 031 < 5 1.6674 222 < 5

142 M. Georgiev, V. Karadjova, D. Marinova, D. Stoilova

Table 2. Solubility in the BeSeO - H SeO - H O system at 25°C. 4 2 4 2 Composition of Liquid phase, % Liquid phase, % mass mass the solid phase BeSeO4 H2SeO4 BeSeO4 H2SeO4 36.22 - - BeSeO4.4H2O 32.69 5.35 58.59 1.78 “ - ” 30.29 8.80 62.60 2.01 “ - ” 25.85 15.93 63.30 2.01 “ - ” 19.99 30.03 62.48 3.13 “ - ” 15.00 46.50 64.58 3.32 “ - ” 11.71 53.77 54.49 12.57 “ - ” 11.58 60.56 65.11 3.05 “ - ” 10.93 68.50 60.01 9.26 “ - ” 10.81 71.94 61.48 7.51 “ - ”

Fig. 4. Solubility diagram of the BeSeO4 - H2SeO4 - H2O system at 25 °C. tion of strong hydrogen bonds between [Be(H O )]2+ and centrations of selenic acid in order to obtain lower hy- 2 4 SeO 2- ions in solutions and explain the higher solubil- drates failed. 4 ity of the beryllium selenate in selenic acid. On the other hand, the lower vapor pressure over the selenate solu- Infrared spectroscopy tions due to the higher solubility of beryllium selenate explains the formation of one hydrate only - The free tetrahedral ions (XO n-) under perfect 4 BeSeO ·4H O. Because of experimental difficulties our T symmetry exhibit four internal vibrations: ν (A ), the 4 2 d 1 1 efforts to study the solubility isotherms at higher con- symmetric X-O stretching mode, ν (E), the symmetric 2

143 Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008

XO bending modes, ν (F ) and ν (F ), the asymmetric 4 3 2 4 2 734 X-O stretching and XO bending modes, respectively. 660 4 The free SO 2- and SeO 2- ions are reported to display 4 4 internal vibrations as follows: SO 2- ions - ν at 983 cm-1, 1656 4 1 1687 776 ν - 450 cm-1, ν - 1105 cm-1 and ν 611 cm-1 and a 617 2 3 4

977 SeO 2- ions - ν at 833 cm-1, ν - 335 cm-1, ν - 875 cm-1 690 4 1 2 3 and ν 432 cm-1, respectively [15]. The complex ions, 4 1125 [Be(H O) ]2+, are known to exhibit in aqueous solutions 2 4 432 617 and in solid lattices the vibrations (skeleton vibrations) 1087 776 - ν ¢, ν ¢, ν ¢ and ν ¢ in the range of 540, 250-300, 1 2 3 4 700-900 and 350 cm-1, respectively [16, 17]. b 490 Infrared spectra of hydrates of beryllium sulfate

1242 and selenate as well as those of deuterated samples (ca 592 855 663 80% D O) and the anhydrous compounds are presented 2 in Fig. 5 and 6. The Raman spectrum of BeSeO .4H O 1125 4 2 1094 is shown in Fig. 7. The BeO skeleton vibrations, the 4 536 water librations and some of the normal vibrations of the sulfate and selenate ions appear in the infrared re- 1656 -1 c 421 gion below 1000 cm . Hence, a strong coupling of these 777 vibrations is expected to occur, thus leading to overlap-

#" 615

730 ping of the infrared bands. Consequently, the assign- 908 ments of the bands must be done with a great deal of

1148 caution comparing the spectra of protiated, deuterated 1096 421 and anhydrous compounds. 829 740 According to the structural data of BeSO .4H O 4 2 the SO 2- ions occupy site symmetry D in the unit-cell 1656 1478 4 2 of D symmetry [8]. Thus, the factor group analysis pre- d 908 2d dicts two vibrations for ν and four vibrations for ν 536 1 2

772 (both of A and B symmetry), which are Raman active 1 1 1054 615 only. The infrared and Raman active modes ν and ν 3 4 (B and E symmetry) are expected to exhibit two bands, 1143 2 1096 respectively. The correlation diagram between T mo- d lecular point group, site symmetry D and factor group 515 2 736 symmetry D is shown in Fig. 7. The intensive bands at 2d 1125 and 1087 cm-1 in the spectrum of BeSO .4H O 4 2 e (Fig. 5a) are attributed to two components of ν of the 3 SO tetrahedra. The comparison of the spectrum of 4 BeSO .4H O and that of the highly deuterated sample 768 4 2 1144 1130 574 (compare Fig. 5a and b) allow us to assign the bands at 660 and 617 cm-1 to ν . As was mentioned above the 2000 1600 1200 800 400 4 bands corresponding to the symmetric stretching and Wavenumber, cm-1 bending modes could be observed in Raman spectra only. BeSO .2H O (Fig. 5c) exhibits three bands in the 4 2 Fig. 5. Infrared spectra of beryllium sulfate hydrates in the stretching mode region (1150 1050 cm-1). The inten- region of the internal vibrations of the SO 2- ions, BeO skeleton 4 4 sive bands at 1148 and 1096 cm-1 are attributed to ν vibrations and water librations (a, BeSO .4H O; b, BeSO .4D O 3 4 2 4 2 and that at the lower frequency (1054 cm-1) to the sym- (ca 80% D2O); c, BeSO4.2H2O; d, BeSO4.2D2O (ca 60% D2O); e,

BeSO4).

144 M. Georgiev, V. Karadjova, D. Marinova, D. Stoilova

metric stretching mode ν of the SO tetrahedra. The 1 4 slightly asymmetric band at 615 cm-1 is due to the asymmetric bending mode ν and the band at 421 cm-1 to the 4 670 1682 corresponding symmetric bending mode ν . Thus, the 1509

2 spectroscopic findings allow us to assume that the SO 436 4 a 990 tetrahedra in the dihydrate are probably regular with 760 respect to the O-S-O bond angles (one asymmetric band 410 -1 1164 at 615 cm ) and distorted with respect to the S-O bond 870 lengths (∆n = 52 cm-1). The anhydrous compound ex- as hibits a broad band centered at 1144 cm-1 and a shoul- der at 1130 cm-1, which are attributed to ν of the SO 3 4 1236 1133 530 tetrahedra. The corresponding asymmetric bending vi- b brations ν occur at 574 cm-1 (Fig. 5e). The comparison 770 440 4 of the spectra of BeSO .4H O, BeSO .2H O and BeSO 855 4 2 4 2 4 reveals that the mean values of the asymmetric stretch- 880 ing modes ν are shifted to higher frequencies on going 3 from the tetrahydrate to the anhydrous compound (1106, -1 1125 and 1137 cm , respectively) due to the increasing 440 repulsion potential of the lattices, i.e. to the decreasing c 666 unit-cell volumes of the respective compounds. 703 The unit-cell theoretical treatment for the SeO 2- 913 4 ions in BeSeO .4H O (C (x) site symmetry and D fac- 4 2 2 2h 2000 1600 1200 800 400 tor group symmetry) yields:

Wavenumbers, cm-1 Ãν = A (Raman) + A (inactive) + B (Raman) + B (IR) 1 g u 3g 3u Fig. 6. Infrared spectra of beryllium selenate hydrates in the region of the internal vibrations of the SeO 2- ions, BeO skeleton Ãν =2A (Raman) + 2A (inactive)+2B (Raman)+2B (IR) 4 4 2 g u 3g 3u vibrations and water librations (a, BeSeO .4H O; b, BeSeO .4D O Ãν , ν = A (Raman) + A (inactive) + B (Raman) + B 4 2 4 2 3 4 g u 3g 3u (ca 80% D2O); c, BeSeO4) (IR) + 2B (Raman) + 2B (IR) + 2B (Raman) + 2B (IR) 1g 1u 2g 2u responds to that expected from the site symmetry of the Fig. 6a shows that the ν and ν components of SeO 2- ions, thus indicating a slight distortion of the 3 4 4 SeO tetrahedra with respect to the O-Se-O bond angles the triplet expected according to the site group analysis 4 for the SeO 2- ions coalesce into a single band at 870 in accordance with the structural data (similar Se-O bond 4 cm-1 and a doublet at 436 and 410 cm-1, respectively. lengths and small differences in the O-Se-O bond angles, No band corresponding to is observed in the infrared [10]). The anhydrous compound, BeSeO , exhibits nar- 1 4 spectrum of BeSeO .4H O. The Raman measurements row bands at 913 and 440 cm-1 assigned to ν and ν of 4 2 3 4 show that BeSeO .4H O exhibits a strong band at 854 the SeO tetrahedra, respectively (Fig. 6c). BeSeO .2H O 4 2 4 4 2 cm-1, which is attributed to symmetric stretching mode proves to be unstable in air and transforms partly into ν and three bands at 460, 443 and 433 cm-1, which are BeSeO .4H O. Our efforts to measure the spectrum of 1 4 2 assigned to asymmetric bending modes ν of the SeO dihydrate failed (a strong overlapping of the bands is 4 4 tetrahedra (Fig. 7). The ν modes appear as a doublet at observed). 2 366 and 343 cm-1. Thus, the analysis of the infrared The unit-cell group theoretical treatment for the complex [Be(H O) ] in BeSeO .4H O (Be2+ ions in C (yz) spectra reveals that the local molecular symmetry of 2 4 4 2 s the SeO tetrahedra is close to T , i.e. higher than the site symmetry, D factor group symmetry) yields: 4 d 2d Ãν = A (Raman) + B (IR) + B (Raman) + B (IR) crystallographic one (effective spectroscopic symmetry) 1 g 1u 3g 2u Ãν = A (Raman) + B (IR) + B (Raman) + B (IR)+ if the Se-O stretches are considered. However, the num- 2 g 1u 3g 2u A (inactive) + B (Raman) + B (Raman) + B (IR) ber of the bending modes in the Raman spectrum cor- u 1g 2g 2u

145 Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008

776 cm-1 transforms into two bands at 768 and 736 cm-1 in the spectrum of the anhydrous compound corresponding to ν of BeO complex (compare Fig. 5a and 5e). 3 4 BeSO .2H O exhibits two bands in the region of 780 4 2 730 cm-1, which could be attributed to the asymmetric stretches Be-O (Fig. 5c). However, the lower frequency band decreases in intensity upon deuteration and consequently this band is due to water librations (Fig. 5d). Then the band at 777 cm-1 is assigned to ν and that at 3 536 cm-1 to ν of the BeO complex formed in the dihy- 1 4 Fig. 7. Raman spectrum of BeSeO .4H O 4 2 drate, respectively. The comparison of the spectra of the selenates and sulfates (BeSO .4H O and BeSeO .4H O Ãν , ν = 2A (Raman) + 2B (IR) + 2B (Raman) + 4 2 4 2 3 4 g 1u 3g on one hand, and BeSO and BeSeO on the other) shows 2B (IR)+A (inactive)+B (Raman)+B (Raman) + B (IR) 4 4 2u u 1g 2g 2u that the bands corresponding to ν in the sulfate com- The ν and ν modes of the beryllium aqua com- 3 3 1 pounds appear at higher frequencies, thus indicating a plex occur at 760 cm-1 and in the region of 550-500 cm-1, comparatively different strength of the Be-O bonds (Be- respectively (see Fig. 6a). The band at 760 cm-1 shifts to OH and Be-OXO , X = S, Se) in the selenates and sul- higher frequency upon deuteration (compare Fig. 5a and b) 2 3 due to the coupling with D O librations. The infrared fates, i.e. the respective bonds in the latter compounds 2 spectrum of the anhydrous beryllium selenate exhibits are stronger than those in the former ones. The smaller unit-cell volume of BeSO favors additionally the shifts strong bands at 703 and 666 cm-1, which are assigned to 4 Be-O asymmetric stretching modes - ν of BeO skel- of these bands to larger wavenumbers. 3 4 eton (Fig. 6c).

According to the structural data of BeSO .4H O 4 2 the Be2+ ions occupy D site symmetry and the correla- 2 tion diagram for the [Be(H O) ]2+ complex is identical 2 4 with that of the SO 2- ions in BeSO .4H O (see Fig. 8). 4 4 2 The band at 776 cm-1 remains its intensity upon deu- a 2230 teration (Fig. 5b) and consequently the band could be 3380 2940 attributed to ν of [Be(H O) ]2+ complex. No infrared 2360 3 2 4 3130 band corresponding to ν is expected to appear accord- 1 ing to the site symmetry of the Be2+ ions. The band at

Td D2 D2d b

point group site symmetry factor group symmetry

3184 2404 ν1(A1) A1 (R)

ν2(E)- B1 (R) c A2 (inactive) 2323 B1 3113 2957 2244 ν3(F) B2 (IR, Raman) // 3500 3000 2400 2100 B2 -1 ν4(F) E (IR,Raman) Wavenumbers, cm B3 Fig. 9. Infrared bands of OH and OD stretches (matrix-isolated

Fig. 8. Correlation diagram between Td point group, site HDO molecules) in BeSeO4.4H2O, BeSO4.4H2O and BeSO4.2H2O 2- symmetry and factor group symmetry (SO4 ions and [Be(H2O)4] (a, BeSO4.4H2O; b, BeSO4.2H2O; c, BeSeO4.4H2O) complex in BeSO44H2O.

146 M. Georgiev, V. Karadjova, D. Marinova, D. Stoilova

The water molecules (two types in C site sym- shifts and small values of isotopic ratios upon deutera- s metry and one type in C site symmetry) in BeSeO .4H O tion [2]. The comparison of the spectra of BeSeO .4H O 1 4 2 4 2 are expected to exhibit four bands corresponding to four and BeSeO .4D O (Fig. 6a and b) allow us to claim that 4 2 uncoupled OD stretches of matrix-isolated HDO mol- the band at 990 cm-1 corresponds to rocking modes of ecules (four different OH oscillators). However, two H O. The band at 855 cm-1 in the spectrum of the highly 2 bands at 2323 and 2244 cm-1 are observed only in the deuterated sample (Fig. 6b) is assigned to rocking modes spectra of the isotopically dilute sample (Fig. 9c). Since of D O (isotopic ratio has value of 1.16). The broad 2 the four O ···O hydrogen bonds are of similar lengths band centered at 530 cm-1 is attributed to wagging and w the assignments of the bands could not be made pre- twisting modes of D O (the corresponding wagging modes 2 cisely. The band at the lower frequency (2244 cm-1) is of H O occur at 670 cm-1, isotopic ratio has value of 2 due probably to hydrogen bonds formed by both the 1.26). The bands at 977, 734 and 690 cm-1 in the spec- H O(1) and H O(2) molecules (the O ...O hydrogen bond trum of BeSO .4H O shift to 855, 592 and 490 cm-1 in 2 2 w 4 2 lengths have values of 2.619 and 2.635 Å, respectively, the spectrum of deuterated tetrahydrate (compare Fig. according to the structural data). The higher frequency 5a and b). Consequently, these bands could be assigned band at 2323 cm-1 is attributed to hydrogen bonds to rocking, twisting and wagging modes of H O and D O, 2 2 formed by the water molecules in C site symmetry and, respectively. The respective isotopic ratios have values 1 hence, they are assumed to be symmetrically hydrogen of 1.14, 1.24 and 1.41. The band centered at 908 cm-1 in bonded (O ···O are 2.649 and 2.659 ). The water mol- the spectrum of BeSO .2H O decreases in intensity and w Å 4 2 ecules in BeSO .4H O (one type in C site symmetry) shifts to lower frequencies upon deuteration (Fig. 5c 4 2 1 are expected to exhibit two bands corresponding to D- and d) and, hence, it is assigned to rocking modes of O vibrations of matrix-isolated HDO molecules (two water molecules (isotopic ratio has value of 1.10). different OD oscillator). Indeed, two bands at 2230 and 2360 cm-1 (bond lengths are 2.624 and 2.69 Å, respec- CONCLUSIONS tively) are observed in the spectrum of BeSO .4H O (see 4 2 Fig. 9a). However, BeSO .2H O displays one band only The single crystal X-ray investigation of the 4 2 at higher frequencies (2404 cm-1) as compare to orthorhombic BeSeO .4H O reveals on one hand close 4 2 BeSO .4H O, thus indicating the formation of weaker structural and chemical relationships with tetragonal 4 2 hydrogen bonds in the former compound (Fig. 9b). The BeSO .4H O, while on the other hand essential polyhe- 4 2 appearance of only one band for the D-O vibrations dral reorientations and a partial rearrangement of the allow us to assume that the molecular symmetry of the hydrogen bonding scheme is found. On the basis of the water molecules in the dihydrate is close to C . The X-ray powder diffraction data the lattice parameters of 2v formation of strong hydrogen bonds reflects on the shape some lower crystal hydrates of beryllium selenate and of the spectra in the high frequency region (2000-4000 sulfate are calculated (BeSeO .2H O, BeSeO and 4 2 4 cm-1) where the H O stretching modes occur - the bands BeSO .2H O). The analysis of the infrared spectra of 2 4 2 exhibit large red-shifts and large half-widths. This phe- beryllium compounds under study shows that very strong nomenon is pronounced in a greater degree in the hydrogen bonds are formed in these compounds due to tetrahydrates, thus reflecting the formation of stronger the large ionic potential of the Be2+ ions, i.e. to the strong hydrogen bonds in these compounds. synergetic effect. The formation of strong hydrogen The formation of comparatively strong hydrogen bonds leads to the very small values of the isotopic ra- bonds in the beryllium compounds under study is con- tio for the rocking vibrations of the water molecules firmed if the wavenumbers of the water librations (rock- (1.16 and 1.14 for the tetrahydrates of selenate and sul- ing, twisting and wagging) are considered. In the case of fate, respectively). It has been established that the sel- very strong hydrogen bonds and of trigonal planar envi- enate tetrahydrate forms stronger hydrogen bonds as ronments of water molecules as in the case of compared to those formed in the respective sulfate if tetrahydrates of beryllium selenate and sulfate the wa- the wavenumbers of the O-D vibrations of matrix iso- ter librations are reported to display considerable blue lated HDO molecules are considered owing to the stron-

147 Journal of the University of Chemical Technology and Metallurgy, 43, 1, 2008 ger proton acceptor capability of the selenate ions. Thus, Soc., 73, 1951, 2831-2835. the formation of hydrogen bonds of different strength 6. I.J. Bear, A.G.Turnbull, Austral. J. Chem., 19, 1966, reflects on the other properties of the beryllium salts: 751-757. (i) The value of the enthalpy of dehydration (∆H ) of 7. B. Braun, H. Rossbach, H. Herberg, K. Henkel, deh BeSeO .4H O is larger as compared to that of Thermochim. Acta, 92, 1985, 121-124. 4 2 BeSO .4H O (234.0 kJ mol-1 for the former compound 8. N.M. Selivanova, V.A. Shnayder, I.S. Streltsov, Zhur. 4 2 and 171.9 kJ mol-1 for the latter one). (ii) The beryl- Neorg. Khim., 5, 1960, 2272-2279. lium selenate exhibits a higher solubility in selenic acid 9. T. Kellersohn, R.G. Delaplane, I. Olovsson, Acta solutions than the beryllium sulfate (the ions [Be(H O) ]2+ Crystallogr., B50, 1994, 316-326. 2 4 and SeO 2- are hold in solutions via strong hydrogen 10. M. Wildner, D. Stoilova, M. Georgiev, V. Karadjova, 4 bonds). (iii) The higher solubility of the beryllium sel- J. Mol. Struct., 707, 2004, 123-130. enate causes a lower vapor pressure over the solutions, 11. V.G. Koleva, V.Karadjova, M. Georgiev, Cryst. Res. thus resulting in the formation of one hydrate only - Tehnol., 39, 2004, 1020-1023. BeSeO ·4H O, i.e. no dehydration processes occur. 12. D. Stoilova, H.D. Lutz, J. Mol. Struct., 450, 1998, 4 2 101-106. REFERENCES 13. M. Georgiev, D. Stoilova, M. Wildner, V. Karadjova, J. Mol. Struct., 752, 2005, 158-165. 1. H.D. Lutz, Struct. Bond. (Berlin) 69, 1988, 97-123. 14. M.P. Georgiev, D.G. Stoilova, D.M. Marinova, V.A. 2. H.D. Lutz, N. Lange, M. Maneva, M. Georgiev, Z. Karadjova, Cryst. Res. Technol., 42, 2007, 54-58. Anorg. Allg. Chem., 594, 1991, 77-86. 15. K. Nakamoto, Infrared spectra of inorganic and 3. H.D. Lutz, J. Mol. Sturct., 646, 2003, 227-236. coordination compounds, Izd. Mir., 1966, Moscow. 4. M. Georgiev, M. Wildner, D. Stoilova, V. Karadjova, 16. F. Bertin, J. Derouault, Can. J. Chem., 57, 1979, J. Mol. Struct., 753, 2005, 104-112. 913-919. 5. A.N. Campbell, A.J. Sukava, J. Koop, J. Amer. Chem. 17. C. Pigenet, J. Raman Spectrosc., 13, 1982, 262-269.

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