Infrared study of the vibrational behavior of CrO4

Journal of Molecular Structure 738 (2005) 211–215 www.elsevier.com/locate/molstruc

Infrared study of the vibrational behavior of CrO2K guest ions 4 matrix-isolated in metal (II) sulfates (MeZCa, Sr, Ba, Pb) D. Stoilovaa,*, M. Georgievb, D. Marinovaa a

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev” Str., bl. 11, 1113 Sofia, Bulgaria b Department of Inorganic Chemistry, University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Str., 1756 Sofia, Bulgaria Received 26 July 2004; revised 29 November 2004; accepted 2 December 2004

Abstract Infrared spectra of matrix-isolated CrO2K guest ions in host sulfate matrices - CaSO4$2H2O, SrSO4, BaSO4 and PbSO4 are reported and 4 discussed with respect to the Cr–O stretching and O–Cr–O bending modes. An adequate measure for the CrO2K 4 guest ion distortion is the site group splitting Dnas and Dnmax (the difference between the highest and the lowest wavenumbered components of the stretching and bending 2K modes). When the smaller SO2K 4 ions are replaced by the larger CrO4 ions the mean frequencies of the asymmetric stretching and bending modes (n3 and n4 ) as well as the frequencies of n1 of the CrO2K guest ions are shifted to higher wavenumbers as compared to those in the 4 respective neat chromates due to the larger repulsion potential at the host lattice sites (smaller values of the unit-cell volumes of the neat sulfates than those of the neat chromates). The CrO2K 4 guest ions exhibit three bands corresponding to the n3 modes as deduced from the site group analysis (C2 site symmetry in CaSO4$2H2O and Cs site symmetry in SrSO4, BaSO4 and PbSO4). However, the bending modes n4 and n2 of the CrO2K 4 guest ions in SrSO4, BaSO4 and PbSO4 show an effectively higher local symmetry than the ‘rigorous’ crystallographic one (two bands for n4 and one band for n2 instead of a triplet and a doublet expected, respectively). Such different apparent site symmetries observed in various spectral regions may be attributed to the different influence of energetic and geometrical distortions of the polyatomic entities at particular site on various modes. q 2005 Elsevier B.V. All rights reserved. Keywords: IR matrix-spectroscopy; Host sulfate matrices; Matrix-isolated CrO2K guest ions; Distortion of CrO2K guest ions 4 4

1. Introduction The crystal matrix-spectroscopy provides important information about the local potential at the lattice site and the chemical nature of the ligand environment in the lattice. When polyatomic ions are doped in host lattices at low concentration 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 origin) are neglected. Thus, the vibrational spectra of the guest ions are essentially determined by the site symmetry, which is assumed to be the same as that of the respective host ions (substitutionally mixed crystals). Furthermore, the spectra

* Corresponding author. Tel.: C359 2 979 35 66; fax: C359 2 870 50 24. E-mail address: [email protected] (D. Stoilova). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.12.016

of matrix-isolated polyatomic ions are an excellent probe of the local potential of the host crystal at the respective site, as shown by the extent of distortion of the matrixisolated unit. The matrix-isolated XOnK guest ions in m different host matrices undergo structural distortions involving changes in the both X–O bond lengths and O– X–O bond angles as compared to those in the neat compounds. The distortion of the matrix-isolated entities at the various lattice sites (as compared to the free entities in solution or in gas) established by spectroscopic studies (infrared and Raman) is called an energetic distortion in order to distinguish it from the geometrical distortion revealed by structural data. Both the site group splitting of the asymmetric modes (Dnas) and Dnmax (the differences between the highest and the lowest wavenumbered components of the stretching and bending modes, respectively) are an adequate measure for the energetic distortion of the matrix-isolated polyatomic ions [1–6].

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D. Stoilova et al. / Journal of Molecular Structure 738 (2005) 211–215

In the present paper the infrared spectra of matrixisolated CrO2K guest ions in host sulfate matrices CaSO4$ 4 2H2O, SrSO4, BaSO4 and PbSO4 are reported and discussed in the regions of the Cr–O stretching and O–Cr–O bending modes. The sulfates and chromates of calcium, strontium, barium and lead crystallize in different space groups as 6 follows: CaSO4$2H2O—in I21/c ðC2h Þ (the SO2K 4 ions in C2 site symmetry) [7], CaCrO4$2H2O—in Pbcm ðD11 2h Þ (the CrO2K 4 ions in C1 site symmetry) [8], SrSO4, BaSO4, PbSO4 2K and BaCrO4—in Pnma ðD16 ions in Cs site 2h Þ (the XO4 5 symmetry) [9–12], SrCrO4 and PbCrO4—in P21/n ðC2h Þ (the 2K CrO4 ions in C1 site symmetry) [13,14]. The different space groups of the sulfates and chromates determine the formation of different solid solution types—the incorporation of the CrO2K guest ions in the BaSO4 matrix is due to 4 isomorphous substitution, while that of the CrO2K guest 4 ions in the CaSO4$2H2O, SrSO4 and PbSO4 matrices—to isodimorphous substitution.

2. Experimental Metal sulfates and chromates (MeZCa, Sr, Ba, Pb) were prepared by precipitation from the corresponding acetate solutions of calcium, strontium, barium and lead with K2SO4 and K2CrO4 solutions, respectively. The sulfates containing matrix-isolated CrO2K (about 5–8%) were 4 obtained using the same precipitation procedure in the presence of chromate ions. The samples were filtered, washed with alcohol and dried at 40–50 8C. All the reagents used were ‘p.a.’ (Merck). The infrared spectra in the region of 4000–400 cmK1 were recorded on a Bruker model IFS 25 Fourier transform interferometers (resolution !2 cmK1) and those in the region 400–200 cmK1—on a Philips PU 9712 spectrometer. KBr discs were used as matrices. Ion exchange or other reactions with KBr have not been observed.

3. Results and discussion The free XO2K ions (XZS,Cr) under perfect Td 4 symmetry exhibit the following internal modes: n1(A1), the symmetric stretching mode, n2(E), the OXO symmetric bending mode, n3(F2) and n4(F2), the asymmetric stretching and bending modes, respectively. Due to the site symmetry the triply degenerate modes n3 and n4 should be observed in the vibrational spectra as either a single band, a doublet, or a triplet and n2—as a single band or doublet. Furthermore, the bands could split additionally into more components as a result of correlation field splittings (factor group effects). Infrared spectra of the neat sulfates and chromates as well as those of matrix-isolated CrO2K guest ions in the host 4 sulfates in the regions of the X–O stretching and O–X–O bending modes are presented in Figs. 1 and 2 (see also Tables 1 and 2). The site group analysis for the neat chromates

Fig. 1. Infrared bands of the X–O stretching modes (XZS, Cr) and those of the matrix-isolated CrO2K guest ions in host sulfate lattices. 4

predicts that in C1 site symmetry (CaCrO4$2H2O, SrCrO4 and PbCrO4) and Cs (BaCrO4) the degeneracy of E and F2 is removed, thus resulting in the appearance of two bands for n2 (2A for CaCrO4$2H2O, SrCrO4 and PbCrO4, and A 0 CA 00 for BaCrO4) and three bands for n3 and n4 (3A for CaCrO4$ 2H2O, SrCrO4 and PbCrO4 and 2A 0 CA 00 for BaCrO4). The factor group splitting leads to: CaCrO4$2H2O (D2h factor group symmetry)—six IR active components for n2 (2B1uC 2B2uC2B3u) and nine IR active components (3B1uC3B2uC 3B3u) for n3 and n4; SrCrO4 and PbCrO4 (C2h factor group symmetry)—four IR active components for n2 (2AuC2B2u) and six IR active components for n3 and n4 (3AuC3B2u); BaCrO4 (D2h factor group symmetry)—three IR active components for n2 (B1uCB2uCB3u) and five IR active components for n3 and n4 (2B1uCB2uC2B3u). The correlation diagrams between Td point group, site symmetry and factor group symmetry for the respective neat chromates are presented in Figs. 3–5. The relatively large half-widths of the unit-cell group modes in the neat compounds are due to the LO/TO splitting of these bands with high oscillator strength [15]. Therefore, the band frequencies in the neat compounds cannot be recorded with sufficient precision in order to compare them with the band frequencies of the matrix-isolated CrO2K 4 guest ions (Figs. 1 and 2).


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Table 1 Mean frequencies of the asymmetric modes (n3 and n4 ), separations (n3 -n1 and n4Kv2), Dnmax (the difference between the highest and the lowest wavenumbered components of the stretching and bending modes, respectively) and site group splitting (Dn3 and Dn4) in cmK1 for the neat sulfates and chromates Compounds CaSO4$2H2O SrSO4 BaSO4 PbSO4 CaCrO4$2H2O SrCrO4 BaCrO4 PbCrO4

Stretching modes n3

n1

n3 -n1

Dn3

Dnmax

V/z

1141 1148 1131 1112 902 901 904 854

1002 995 983 968 850 843 857 754

139 153 148 144 52 58 47 100

45 108 109 92 54 50 79 105

161 215 212 186 74 82 92 154

123.8 76.8 86.2 79.6 125.1 86.1 92.4 87.7

Compounds

Bending modes

CaSO4$2H2O SrSO4 BaSO4 PbSO4 CaCrO4$2H2O SrCrO4 BaCrO4 PbCrO4

n4

n2

n4 -n2

Dn4

Dnmax

636 629 625 614 371 406 401 381

– – – – – 344 336 330

– – – – – 62 65 51

66 33 29 25 30 58 47 55

– – – – – 90 90 80

the respective neat chromates, while that of Dn3 for the guest ions in SrSO4 is larger than that of the CrO2K 4 same ions in SrCrO4, thus indicating a stronger distortion of the CrO2K guest ions in SrSO4 as compared to that of 4 Fig. 2. Infrared bands of the O–X–O bending modes (XZS, Cr) and those of the matrix-isolated CrO2K guest ions in host sulfate lattices. 4

The site group analysis (C1 site symmetry in CaSO4$ 2H2O and Cs site symmetry in SrSO4, BaSO4 and PbSO4, see Figs. 6 and 7) predicts three infrared bands for the n3 and n4 modes of the CrO2K guest ions matrix-isolated in the 4 sulfate lattices. The three components are denoted as na, nb and nc (nc being the lowest wavenumbered component). Two infrared bands corresponding to the n2 modes of the CrO2K guest ions are expected—two components of A 4 symmetry and two components of A 0 4 A 00 symmetry for n2 in CaSO4$2H2O and in SrSO4, BaSO4 and PbSO4, respectively. Due to the low site symmetry of the guest ions the n1 modes are activated. Indeed, three bands corresponding to n3 of the matrix-isolated CrO2K guest 4 ions in sulfate matrices are seen in the spectra (Fig. 1). The differences Dnab (na–nb) and Dnbc (nb–nc) of the matrixisolated CrO2K guest ions have close values (13 and 4 12 cmK1 in CaSO4$2H2O, 45 and 27 cmK1 in SrSO4, 36 and 22 cmK1 in BaSO4, and 44 and 32 cmK1 in PbSO4). The low intensities of the bands corresponding to n1 are an evidence for small distortions of the guest ions. The values of Dn3 (site group splitting) for the CrO2K guest ions in 4 CaSO4$2H2O, BaSO4 and PbSO4 are smaller than those in

Table 2 Infrared wavenumbers (cmK1) of the asymmetric modes (n3 and n4), mean values of the asymmetric modes (n3 and n4 ), site group splitting (Dn3 and Dn4), symmetric modes (n1 and n2) and Dnmax (the difference between the highest and the lowest wavenumbered components of the stretching and bending modes) for matrix-isolated CrO2K guest ions in sulfate matrices 4 Host compounds CaSO4$2H2O

SrSO4

BaSO4

PbSO4

Host compounds

SrSO4 BaSO4 PbSO4

Stretching modes n3

n3

n1

Dn3

Dnmax

928 915 903 963 918 891 941 905 883 924 880 848

915

878

25

50

924

870

72

93

910

863

58

78

884

–

76

–

Bending modes n4

n4

n2

Dn4

Dnmax

429 400 427 397 395 370

415

370

29

59

412

357

30

70

383

350

25

45

214


Fig. 6. Correlation diagram between the molecular point group and the crystallographic site group of CrO2K ions isolated in CaSO4$2H2O. 4 Fig. 3. Correlation diagram between Td point group, site symmetry (C1) and factor group symmetry (D2h) (CrO2K ions in CaCrO4$2H2O). 4

Fig. 4. Correlation diagram between Td point group, site symmetry (C1) and factor group symmetry (C2h) (CrO2K ions in SrCrO4 and PbCrO4). 4

Fig. 7. Correlation diagram between the molecular point group and the crystallographic site group of CrO2K ions isolated in SrCrO4, BaSO4 and 4 PbCrO4.

the same ions in the corresponding chromate (compare Tables 1 and 2). However, it is interesting to note that in the region of appearance of the bands due to the bending modes two bands corresponding to n4 and one band corresponding to n2 are seen in the spectra instead of a triplet and a doublet expected, respectively (Fig. 2, Table 2). The bands corresponding to the bending modes of the chromate doped calcium sulfate dihydrate could not be resolved since they overlap with the bands due to the water librations of the host compound. According to Jayasooriya et al. [16] the spectral appearances in the cases of neat SrSO4, BaSO4 and PbSO4 may be better explained assuming pseudosymmetry space group (so called latent space group) Imma

16 ðD28 2h Þ instead of Pnma ðD2h Þ. Imma space group is obtained from Pnma space group by a slight distortion. Thus, assuming the pseudo-symmetry space group concept, the pseudo site symmetry of the CrO2K guest ions is C2v. The 4 correlation between the molecular point group Td and C2v site symmetry is presented in Fig. 8. As can be seen, the n3 region would be the same, except that the symmetries of the site group components are different. Then the appearance of only one band for n2 is reasonable (site group component of A1 symmetry). While the n2 bending region resembles an effective C2v symmetry, the region of n4 resembles an even more symmetrical pattern (characteristic for S4, D2d or C3v symmetry). Thus, the force fields corresponding to the dopant bending modes n2 and n4 show an effectively higher symmetry than those of the stretching modes. The same

Fig. 5. Correlation diagram between Td point group, site symmetry (Cs) and factor group symmetry (D2h) (CrO2K ions in BaCrO4). 4

Fig. 8. Correlation diagram between the molecular point group and the latent site group of CrO2K ions isolated in SrCrO4, BaSO4 and PbCrO4. 4


phenomenon, i.e. the significantly smaller site group splitting of n4 modes of the matrix-isolated tetrahedral anions in comparison of the splitting of n3 ones has been observed for other tetrahedral ions matrix-isolated in various matrices—for example, ClOK doped KMnO4 4 [17,18], SO2K doped CaCrO4.2H2O, SrCrO4 and BaCrO4 4 [19]. Such different apparent site symmetries observed in various spectral regions may be attributed to the different influence of energetic and geometrical distortions of the polyatomic entities at particular site on various normal modes. When the smaller SO2K ions are replaced by the larger 4 CrO2K ions the mean frequencies of the asymmetric modes 4 (n3 and n4 ) of the guest ions are shifted to higher frequencies as compared to those in the neat chromates (see Tables 1 and 2) due to the higher compression that the guest ions undergo at the host lattice sites (smaller values of the unit-cell volumes of the host sulfates, i.e. larger repulsion potential at the lattice sites as compared to the neat chromates). The n1 modes of the CrO2K guest ions exhibit the same behavior, 4 i.e. they are shifted to higher frequencies (878, 870 and 863 cmK1 for n1 of the matrix-isolated CrO2K guest ions 4 and 950, 843 and 857 cmK1 for n1 of the CrO2K ions in the 4 neat chromates of calcium, strontium and barium, respectively (see Fig. 1).

4. Conclusion The present investigations show that the CrO2K guest 4 ions matrix-isolated in host sulfate lattices CaSO4$2H2O, SrSO4, BaSO4 and PbSO4 exhibit three bands corresponding to n3 as deduced from the site group analysis and the differences Dnab (na–nb) and Dnbc (nb–nc) have close values. However, the force fields corresponding to the dopant bending modes n2 and n4 of the CrO2K guest ions in 4

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the SrSO4, BaSO4 and PbSO4 matrices show an effectively higher symmetry than those of the stretching modes (two bands for n4 and one band for n2 instead of a triplet and a doublet expected, respectively). The values of site group splitting (Dn3 and Dn4) and Dnmax as well as the spectral guest ions undergo appearances indicate that the CrO2K 4 stronger distortions with respect to the Cr–O bond lengths as compared to those with respect to the O–Cr–O bond angles.

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