Infrared and polarized raman spectra of kainite

Spectrochimica Actq Vol. 45A, No. 8, pp. 877-878, Printed in Great Brilain.

1989. 0

TECHNICAL INFRARED

AND POLARIZED

1989 Maxwell

05&58539/89 $3.00+0.00 Pcrgamon Macmillan plc

NOTE

RAMAN SPECTRA

OF KAINITE

(Received 20 May 1988; in final form 21 January 1989; accepted 30 January 1989) Abstract-The i.r. and Raman spectra of KMgClS0,.3H,O and its deuterated compound KMgClS0;3D,O have been recorded and analysed. Due to site symmetry effects the i.r. inactive modes appear as sharp bands in the i.r. spectrum. Water bands indicate strong hydrogen bonding and show the presence of three crystallographically distinct water molecules in the unit cell.

Kainite is a mineral [l] with chemical formula KMgClS0,.3H,O. The space group of this compound is determined as CZ/m [2], but detailed crystal structure data such as bond lengths and bond angles are not available. In the present paper, i.r. and Raman spectroscopic techniques

have been used as a probe to determine the nature of the SOi- ion and hydrogen bond in this compound. Crystals suitable for the polarization study were prepared by the slow evaporation of an aqueous solution containing equimolar ratios of KCI and MgS0,.7H,O at room tem-

Table 1. Spectral data and band assignments (cm-‘) of KMgClS0,.3H,O/D,O KMgClSO,. 3H,O Raman i.r. B* A, 3408 3330 3292

3348 3308

3258 3162 3080

3244 3168 3089

1720 1697 1655

1720 1670 1644

1162 1138 1110 1093

1173 1138 1122 1094

3000-3500

1700b

1140b 1090 b

1048

SAA45:8-G

2500 2470 2445

KMgClSO,. 3D,O i.r.

Assignments

H,O(D,O) H,O(D,O)

I II III

H,O(D,O) “3

22OG2600

2420 2340 2300

“1

H,O(D,O) H,O(D,O) H,O(D,O)

I II III

1270 1250 1218

“2

1225 b

H,O(D,O) H,O(D,O) H,O(D,O)

III II I

1150b

SOa

1175 1150 1125 1080

1080 b

1020 980 m

988

990

945

918

890 815

853 808 784

860 b

740

750 b

700

746 721 705

662 644 633 612

647 630 621

630 m 608 sh

590 525 503

599

585 sh

472 460

470 461 442

440m

420 365

390 m

415 388 315 284 242

Raman

984

980 m

945

WWW)

298 240m

560 530

650 635 600

“1

H,WW)

“4

so4

“2

so4

615 b

435 400

465 445

440m External modes of H,O(D,O) and M-O stretching

265 230 811

878

Technical Note Table 1 KMgCISO;3H,O Raman Ail B* 195 151 125

147 131

88

76

i.r.

Raman

(continued)

KMgCISO,.3D,O i.r.

175 115

Assignments

SO, rotational SO, translational

b, broad; m, medium; sh, shoulder; C, combination; r,t,w, rocking, twisting and wagging modes of water. perature (300 K) [3]. The deuterated compound KMgCIS0,.3D,O was obtained by using heavy water as solvent, and was recrystallized several times in a vacuum desiccator. Only polycrystalline samples were obtained for the deuterated compound. Infrared spectra were recorded on a Perkin-Elmer 580 spectrophotometer with the samples prepared as KBr discs. Polarized Raman spectra were recorded on a Spex Ramalog 1401 double monochromator equipped _ _. with a Spectra-Physics model 165 Ar+ laser. KMgCISO,.3HiO crystallizes in the monoclinic system with mace aroun C2/mC?. and z = 16. As the lattice is’not prim&e, oily eight molecules are considered for the analysis of the spectra. The atoms (K, Mg and Cl), SOi- ion and the water molecule are at C, sites. Factor group analysis [4] at k = 0 gives 408 modes and they split into l- = 102A, + 1028, + 102A, + 102B,. An isolated SOi- ion with Td symmetry has its normal vibrations at 981 (vi(A,)), 451 (vJE)), 1104 (v3(F2)) and 613 (v., (F2)) cm- ’ [S]. All these modes are Raman active, while v1 and vI are i.r. inactive. Observed bands and their assignment to different modes are given in Table 1. Degeneracies of va, vs and vq modes are completely lifted in the Raman spectrum, and the i.r. inactive modes appear as sharp bands in the i.r. spectrum. This is attributed to site symmetry effects, as the symmetry of the anion is lowered from Td to Ci. In the internal mode region one expects 18 Raman active bands to appear in each species (A, and 83. But only 11 bands are observed in both the species. This can occur due to negligible splittings and overlapping of bands. In the i.r. spectrum only a few bands are observed due to poor resolution. A free water molecule absorbs at 3756 (vs), 3652 (vi) and 1595 (va) cm-’ [6]. In the present investigation a number of broad bands with frequencies considerably shifted from the free state values have been observed, indicating strong hydrogen bonding. The strengths of these hydrogen bonds are comparable with those observed in Tutton salts [7-93. The bands observed in the stretching and bending regions show the presence of three crystallographically distinct water molecules in the unit cell. This is also confirmed by the corresponding bands obtained in the deuterated compound with an isotopic ratio of 1.3-1.36. Bands indicating the formation of HDO are not observed in the spectra. The external modes of H,O molecules, SOi- ion and metal-oxygen stretching bands are observed below

400 cm- i. Sulphate bands are tentatively assigned by considering the fact that rotatory modes appear at higher frequencies with larger intensities than translatory modes in the Raman spectrum [lo, 111. The weak bands observed in the 230-420 cm- r region are assigned to the translational modes of water [12] and to the metal&oxygen stretching modes. Department of Physics Deva Matha College Kuravilangad 686 633 Kottayam, Kerala, India

XAVIER MATHEW

Department of Physics University of Kerala Kariavattom, Trivandrum 695581 Kerala, India

V. U.

NAYAR*

REFERENCES

[l] D. HIELTZES,Landbouwk. Tljdschr. 49,844 (1937). [2] H. LINSTEDT,Naturwissenschaften 38,476 (1951). [3] P. SUBRAMANIANand N. HARIHARAN,Pramana Xi, 555 (1986). [4] W. G. FATELY, F. R. DOLLISH, N. T. MCDEVI~T and F. F. BENTLEY,Infrared and Raman selection Rules for Molecular and Lattice Vibrations: The Correlation Method. Wiley, New York (1972). [S] G. HERZBERG,Molecular Spectra and Molecular Structure-11. Infrared and Raman Spectra of Polyatomic Molecules, p. 167. D. Van Nostrand, New York (1945). [6] S. N. VIN&RAD~V and R. H. LINNEL, Hydrogen Bondina. D. 50. Van Nostrand Reinhold. New York (1971). [7] G. SEKAR, V. RAMAKRISHNANand. G. ARULDHAS, j. Solid St. Chem. 66, 235 (1987). -I.

[8] G.

SEKAR, V. RAMAKRISHNANand G. ARULDHAS,.I.

Solid St. Chem. 74, 424 (1988). JAYAKUMAR, G. SEKAR, P. RAJAG~PAL and G. ARULDHASPhys Stat. Solidi (a) 109, 861 (1988). P. DAWSON, M. M. HARGREAVEand G. R. WILKINSON,

[9] V. S. [lo]

Spectrochim. Acta 33A, 83 (1977).

fll] - V. _

RAMAKRISHNAN,V. U. NAYAR and G. ARULDHAS,

Infrared Phys. 25,607 (1985).

[12] J. E. BERTIE,H. J. LABBE and Phys. %I,4501 (1969).

E. WHALLEY, J. them.

*To whom correspondence should be addressed.